Nisr

National Institute of Standards and Technology

Technology Administration, United States Department of Commerce

NIST Technical Note 1500-9

Materials Reliability Series
«
NIST Technical Note 1500-9

Materials Reliability Series

Charpy Verification Program:

Reports Covering 1989-2002

C. N. McCowan

T. A. Siewert

D. P. Vigliotti

Materials Reliability Division, Materials Science and Engineering Laboratory

National Institute of Standards and Technology

September 2003

U.S. DEPARTMENT OF COMMERCE

Donald L. Evans, Secretary

TECHNOLOGY ADMINISTRATION

Phillip J. Bond, Under Secretary for Technology

NATIONAL INSTITUTE OF STANDARDS AND TECHNOLOGY

Arden L. Bement, Jr., Director
National Institute of Standards and Technology Technical Note 1500-9

Natl. Inst. Stand. Technol., Tech. Note 1500-8, 375 pages (September 2003)

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Preface

This is a compendium of various studies on Charpy impact test procedure and specimen preparation variables in which NIST staff has participated in the past decade or so. Some of these

studies had detail sufficient to make them worthy of peer review and publication in archival

journals, others were just simple studies of one or two process parameters, and have never been

published. However, all of them still contain information that is useful to those who produced

reference grade verification specimens or have an interest in understanding how the various

processing and procedure parameters affect absorbed energy. The collection serves as a single,

archival source for these reports and the data that they contain.

This Technical Note includes 23 presentations, grouped into sections on:

1 . Overview and History

2. Materials and Heat Treatment

3. Specimen, Machine, and Procedure Effects

4. Statistical Evaluations of Charpy Impact Data

Since the late 1980s, NIST has furnished standard reference materials for use in verif\'ing the

performance of Charpy impact machine according to the requirements of ASTM Standard E 23.

Some of these reports describe improvements in processing procedures for the specimens (for

reduced scatter), while other reports indicate how changes in the testing procedures affect the

impact energy.

. Acknowledgment: The editor wishes to express his appreciation to many other NIST staff

members and students who contributed to these studies and helped to run the verification

program. These include James Alcorn, Erin Alexander, Jenniffer Caragol. Diane Cyr, Samantha

Dimmick, Lisa Dirling, Cathleen Farrell, Matthew Kuhn, Leslie Leininger, Brian Marsh, Nicole

Neumeyer, Emma Nicoletti, Jason Pepin, Joanna Perez, Alisha Rodriguez, Dominique Shepherd,

Jesse Sycuro and Meigen Thomas.

C.N. McCowan

May 1,2003

iii
Foreword

The Materials Reliability Series of NIST Technical Notes are reports covering significant

research accomplishments of the Materials Reliability Division. The Division develops measure- ment technologies that enable the producers and users of materials to improve the quality and

consistency of materials, for nondestructive evaluation to assure quality of finished materials and

products, and for materials evaluation to assure reliable performance.

This report is the ninth in the series. It documents our publications and studies on Charpy impact

machines for the past decade. Previous reports in this series are:

Technical Note 1500-1 Tensile Testing of Thin Films: Techniques and Results, by D.T.

Read, 1997

Technical Note 1500-2 Procedures for the Electron-Beam Moire Technique, by E.S.

Drexler, 1998

Technical Note 1500-3 High-Energy, Transmission X-ray Diffraction for Monitoring

Turbine-Blade Solidification, by D.W. Fitting, W.P. Dube, and

T.A. Siewert, 1998

Technical Note 1500-4 Nondestructive Characterization of Reactor Pressure Vessel Steels:

A Feasibility Study, by H.I. McHenry and G.A. Alers, 1998

Technical Note 1500-5 Electron-Beam Moire Technique: Advances, Verification,

Application, by E.L. Drexler, 1998

Technical Note 1500-6 Constitutive Behavior Modeling of Steels Under Hot-Rolling

Conditions, by Y.W. Cheng, 1999

Technical Note 1500-7 Structure-Property Relationships in Steel Produced in Hot-Strip

Mills, by P.T. Purtscher, Y.W. Cheng, and C.N. McCowan, 1999

Technical Note 1500-8 Recommended Practice: Installing, Maintaining, and Verifying

Your Charpy Impact Machine, by D.P. Vigliotti, T.A. Siewert, and

C.N. McCowan, 2002

iv
Contents

Part 1: Overview and History 1

The NIST Charpy V-notch Verification Program: Overview and Operating Procedures 3 The History and Importance of Impact Testing 43

For 100 Years, Notched Bar Impact Testing Standards Have Yielded Widespread Benefits 55

Impact Test Methods: Procedures and Analysis 67

Part 2: Materials and Heat Treatment 83

Evaluation of ABS Plastic Impact Verification Specimens 85

Performance Verification of Impact Machines for Testing Plastics 99

Standard and Experimental Heat Treatments for Low-Energy 4340 Steel Verification Specimens 1 1

5

Homogenization and Subcritical Annealing: 4340 Charpy V-notch Verification Specimens 151

Heat Treatment of NIST T-200 CVN Specimens 159

Evaluation of 1 1 . 1 mm Bar for Service in the Charpy V-notch Program 1 83

Investigation of Cracks in CharpyV-notch Specimens 1 89

Part 3: Specimen, Machine, and Procedure Effects 203

Report of Measurement of Three Charpy Impact Machines

at the National Institute of Standards and Technology 205

Maintaining the Accuracy of Charpy Impact Machines 215

Effect of Reduced Notch Radius on Charpy Impact Energy 225

The Effect of Charpy V-Notch Striker Radii on the Absorbed Energy 23

Low Cost Lower Bound Toughness Measurements 243

The Role of Strike Marks on the Reproducibility of Charpy Impact Test Results 261

Effect of Surface Finish of Charpy Anvils and Striking Bits on Absorbed Energy 279

The Influence of Shear Lip Symmetry on the Absorbed Energy of Charpy Impact Specimens . . . 287

Part 4: Statistical Evaluations of Charpy Impact Data 295

International Comparison of Impact Verification Programs 313

Consensus Values and Reference Values Illustrated by the Charpy Machine

Certification Program 331

Proposed Changes to the Charpy V-notch Machine Certification Requirements 349

Uncertainty in Reference Values for the Charpy V-notch Program 362

iii

Part 1: Overview and History

1

The NIST Charpy V-notch Verification Program:

Overview and Operating Procedures

C.N. McCowan, T.A. Siewert, D.P. Vigliotti

NIST, Materials Reliability Division

This report documents the procedures used by NIST in the Charpy V-notch reference material

program. It was prepared to provide outside observers with accurate and detailed information on how the Charpy verification program is conducted, and also to serve as the basis for an internal

record that will be updated to reflect when and why changes were made to the program.

Keywords: Charpy V-notch test, impact testing, mechanical testing, NIST, reference material,

verification program

1. Introduction

1 . 1 Background

Charpy impact testing is often specified as an acceptance test for structural materials, and

companies performing acceptance tests are typically required to verify the performance of their

impact machine periodically. The procedure for verifying the performance of Charpy impact

machines has a physical part and an engineering part. The physical part covers the direct

verification of the impact machine, through a detailed evaluation of the machine dimensions,

alignment, etc. The engineering part covers the indirect verification of the machine performance,

which entails breaking sets of Charpy impact reference specimens. The indirect verification

procedure was added about 40 years ago, because the use of direct verification procedures alone

could not explain some unacceptable differences among the results of the machines tested. Often

these differences could be traced to interactions between the machine components and the

specimens, and only testing with verification specimens could resolve these effects. NIST

supplies the impact reference specimens used to indirectly verify the performance of machines

according to ASTM Standard E 23. The procedures used to conduct this program are the focus

of this report.

Originally, the U.S. Army (Watertown Arsenal, AMMRC) produced and distributed the reference

specimens for the verification of impact machines in the United States. NIST took over the

program from the Army in 1989, and Army personnel helped to transfer the Charpy machines

and their evaluation procedures to NIST. The three Charpy machines owned by the Army, and

now by NIST, have been defined in ASTM Standard E 23 as the "master Charpy impact

machines" for the United States for more than 12 years. Some of these machines have been in

the program for 30 years. Each year, the verification test results for approximately 1000

industrial machines are evaluated. If the results of the industrial machines agree with the results

of the master machines within either 1.4 J or 5 %, the machines are certified for acceptance

testing according to the requirements of ASTM Standard E 23.

3
1 .2 Relationship to National and International Standards

ASTM Standard E 23 requires the indirect verification of Charpy V-notch impact machines

annually. Also, the ASME Boiler and Pressure Vessel Code, many U.S. military procurement

specifications, and several ISO Standards require Charpy impact machine verification. The

European and Japanese verification programs provide traceability to national laboratories and

some intemational agreements are written to require traceability to a national laboratory. The

NIST verification specimens meet ASTM and ISO requirements for indirect verification testing

of Charpy V-notch impact machines and also provide traceability to a national laboratory.

Currently there are four laboratories in the world that certify and distribute reference materials

for the verification of Charpy impact machines: (1) The Institute for Reference Materials and

Measurements (IRMM, Belgium), (2) Laboratoire National d'Essais (LNE, France), (3) The

National Institute of Standards and Technology (NIST, USA), and (4) The National Research

Laboratory of Metrology (NRLM, Japan). These four laboratories supply specimens to verify

the performance of more than 2000 impact machines annually.

1.3 Industrial Needs Met by the Program

The primary purpose of the program is to provide U.S. industry a source for high-quality impact

verification specimens. Because this program also offers an evaluation service for the results of

verification tests, several additional benefits are provided to industry. There are direct benefits

to the customer in the evaluation and interpretation of the test results by NIST, such as a verification letter that auditors acknowledge and rarely question. Indirect benefits, due to the

centralized evaluation of all verification test results include: (1) some assurance that trading

partners and competitors are reporting comparable impact values, and (2) a centralized database

that can support arguments for or against changes to national and intemational impact standards

that affect U.S. industries.

2. Program Design Philosophy and Scope

The NIST Charpy verification program is designed to provide a complete service for our

customers. The program can be divided into three basic parts, as follows: (1) The production of

the verification materials, which includes purchasing the raw materials, contracting for their heat

treatment and machining, batch certifications of verification specimens, and distribution of the

specimens. (2) The evaluation of verification test results, which entails evaluating the data and

specimens received from our customers, entering the data into our database, writing verification

letters that include specific remarks concerning the performance of the machines being evaluated,

and communicating with customers by fax, phone, and email regarding the outcome of tests. (3) The evaluation of data in our database, which serves as a final quality-control tool on our

verification specimens, and as a means to evaluate and track the performance of industrial

machines.

The accurate and consistent certification of the absorbed energy for our verification materials

is the central part of the program, upon which all else is based. As is the case for many

4
measurements, these factors are difficult to address, particularly consistency over long-time

intervals. Our procedures are based on the fact that the master machines at NIST are the

designated reference machines for the United States (by ASTM E 23), and by our own definition

the average value of the three machines is correct.

Our primary control for the program is tracking the performance of each master machine relative to the others. A change in the performance of one machine initiates an evaluation of that

machine and the measurement system in general. Although this approach is a practical solution

to a complex problem, and clearly has shortcomings, it has provided a robust and stable base for our certification procedures over the last 12 years. Additional controls to evaluate the quality of

the verification specimens, such as the testing of control specimens and constant monitoring of

the average energy values from customer verification test results are also used to monitor the

quality of the specimens. In retrospect, we find that over the last 12 years, pooled data from the

three master machines has proven to be a reliable and reasonable target for measuring the

performance of industrial impact machines.

3. Description of Equipment and Personnel

The impact machines used by NIST were purchased from three different commercial suppliers,

not custom built at NIST, and so represent the machines used by industries around the world.

This is true for most of the equipment used in the program.

3 . 1 Impact Energy Measurement

We have five Charpy V-notch impact machines that are used for the measurement of absorbed

impact energy in the program:*

Machine #1 Tokyo Koki Seizosho, "C" type pendulum, S/N 878303

359 Joule Capacity, Reference Machine

Machine #2 Tinius Olsen, Model 74, "U" type pendulum, S/N 130005

358 Joule Capacity, Reference Machine

Machine #3 Satec, Model SI-IC, "U" type pendulum, S/N 1262

325 Joule Capacity, Reference Machine

Machine #4 Satec, Model SI-1K3, "U" type pendulum, S/N 1662

407 Joule Capacity, Research Machine

Machine #5 Tinius Olsen, Model 84, "U" type pendulum, S/N 165153

407 Joule Capacity, Research Machine

*Trade names and names of manufacturers are included in several places in this report to accurately describe NIST activities. Such

inclusion neither constitutes or implies endorsement by NIST or by the U.S. government.

5
Machines 1, 2, and 3 are the primary reference machines (the master machines). Machines 4 and

5 have higher capacity, newer designs that will eventually replace the older machines (as needed, once we are certain of their stability). All of the impact machines are equipped with optical

encoders and digital readouts. Machine 4 has an instrumented striker.

The master machines are used only for assigning certified energy values to lots of verification

specimens, and for occasional participation in measurement development programs and international round robins. The two backup machines are used for research on conventional and

instrumented Charpy V-notch testing.

3.2 Hardness Measurements

Measurements are made on a commercial hardness testing machine. The tester is linked to a

personal computer that is used to acquire and file data for the tests. These data are processed to evaluate the hardness level, and the uniformity in the hardness, of our verification specimens.

3.3 Dimensional Measurements

The notch depth, radius, angle, and centering are measured on a commercial optical comparator

(SOX) prior to impact testing. The squareness is measured with a gage described in ASTM E 23.

The overall specimen dimensions are measured with digital calipers. A second, older optical

comparator is used as a backup system for dimensional measurements. Data from the optical

comparators and the calipers are output to a personal computer.

3.4 Software

Software was developed by NIST personnel to help manage specific tasks that are routinely

performed when evaluating customer test results or certifying a production lot of verification

specimens.

The database of customer data and information is organized as follows: (1) A main panel that

contains fields for the serial number, manufacturer, capacity, and pendulum design of the

machine, along with customer information such as the company name, address, and contact

person, and also contains a comment field and the pass/fail status from the last verification test made on the machine. (2) A data panel that contains fields for the serial number of the machine,

record number, test evaluation date, initials of the NIST operator who evaluated the data, data

fields for the energy data (for four energy levels), the series number of the lot tested, automatically calculated fields for the customer's average energy, the NIST reference value, the

difference between the customer and NIST energy values, and a pass/fail status field. (3) A panel

containing information on test companies (so address information is available to address letters to third parties who conduct verification tests for the customer). (4) A reference value panel that

contains all of the certified energy values for our verification lots. A word processing program is used to help write customer letters. The program uses macros and

Boolean logic to construct the letters according to operator input at various prompts. All of the

pertinent data concerning the customers' test results and address information are accessible in the

6
program, and in addition, approximately 60 ""standardized comments" are available for selection

by the operator to help constmct the letter. A NIST data program is also used to collect and calculate output from the hardness tester and the

dimensional measurement equipment.

3.5 Persomiel

Three people are involved with the Charpy \'erification program in Boulder. One specializes in

the operation and maintenance of the impact machines, and topically handles all of the day-to-da\'

operations of the program (customer evaluations and senice. and pilot lot certifications). The

second specializes in the issues relating to the materials used to produce the verification

materials, and in standards governing impact verification testing I'ASTM and ISO ). The third oversees the program. All the personnel in\'oh'ed m the program are capable of filling in for the

others, which pro^ides adequate backup for the program.

4. Procurement Requirements for Verification Materials

Two matenals are currently used to make the specimens for the indirect verification of Charp\'

impact machines. .An AISI t}pe 434-0 steel is used to make specimens at the low-and-high energy

levels. A t}pe T-200 maraging steel is used to make specimens at the super-high energy level.

4.1 T\-pe 4340 Steel

4.1.1 Compositional and melting requirements

We require .ALSI 4340 steel bars, from a single heat to minimize compositional and microstructural variation. Because steel plants produce steel in different heat sizes i inherent to their

facihties), we have tried to add some flexibilit}' in our contracts b>" bracketing the quantit}' of the

steel to be purchased. We prefer to purchase about 5000 kg (5 ton ) heats. The bids are evaluated

primarily on cost, but we also consider deli\"er." time. The composition for the heat of r.pe 4340

steel that XIST is currently using is gi\"en in Table 1.

Table 1. Composition of 4340 steel imass ^c). c Si Mn Ni Cr Mo S P

0.4 0.28 0.66 0.53 0.28 0.001 0.004

The steel is required to be produced using a double-vacuum-meltmg procedure ( vacuuminduction-melt vacuum-arc-remelt ) and meet the compositional requirements of AISI-SAE alloy

4340. The steel must also meet the stricter requirements of .ANIS 6414. \». hich describes steel

production by a vacuum-melting procedure. In addition, we desire the phosphorus, sulfur,

vanadium, niobium, titanium, and copper contents of the steel to be as lo\'. as possible. The

7
maximum concentrations (in mass %) allowed for these elements are P = 0.010, S = 0.005,

V = 0.030, Nb = 0.005, Ti = 0.003, and Cu = 0.35.'

The composition is certified using standard analytical procedures (such as optical emission

spectroscopy or x-ray fluorescence), and the equipment is calibrated by standards traceable

to NIST. The composition is measured at the top and bottom remelted ingot. These two

measurements must be included in the documentation with the order, meet our compositional

requirements, and meet the limits on residuals given above. Deviations between the two

measurements (in mass percent between the top and bottom of VAR ingot) can not exceed 0.020

for C, Si, and Mo; 0.090 for Mn; 0.030 for Cr; 0.040 for Ni; 0.002 for P; and 0.001 for S.

The compositions of three heats of 4340 steel we have used are given in Table 2. The current

alloy in use, heat number E4261, was processed from six ingots and the label on the bar indicates

the location (top or bottom) in the ingot from which it was produced. This heat yielded 8699 kg

of bar stock from a gross ingot weight of 9221 kg.

4.1.2 Product form

The ingots are forged, hot rolled, then cold finished to 12.7 mm square bars (+3.8 mm, -0.0 mm)

and annealed. The comer radius of the finished bars cannot exceed 0.76 mm. The maximum

acceptable grain size is ASTM number 8. In other attributes (decarburization, surface condition,

etc.), the steel must be suitable for use as 10 mm square Charpy V-notch specimens.

The bar is normalized at 950 °C, and hardened to approximately 35 Rockwell C (HRC). We will accept

alternate heat-treating schedules by mutual agreement.

Our goal here is to produce bars with a minimum of large

carbides in the structure, the most uniform carbide precipitation possible, and a uniform hardness. The bar is required to be machine straightened (for twist and bow),

and shipped in lengths of no less than 2 m and no more

than 4 m.

4.1.3 Packaging

The bar is packaged in bundles identified with reference

to the ingot position from which it was processed. This

identification is used to limit the material used for a given

pilot lot to a single ingot location, which reduces microstructural inhomogeneities between bars. The bundles

must weigh less than 900 kg (4000 lb), which is the

capacity of our fork lift truck.

Table 2. Composition of 4340

alloy, mass %.

Heat # 2397 E4261 E487

B 1 Year 1990 1993 1998

C 0.42 0.40 0.43

Mn 0.75 0.67 0.70

P 0.008 0.004 0.004

S 0.001 0.001 0.002

Si 0.30 0.28 0.30

Cr 0.84 0.83 0.82

Ni 1.83 1.77 1.78

Mo 0.27 0.28 0.24

Cu 0.03 NA 0.09

There has been some question whether this very low sulfur level is the optimum level. Internal data from

one steel producer indicate that sulfur levels of 0.01 to 0.03 may help reduce the variation in impact toughness for 4340 steels.

8
4.2.1 Compositional and melting requirements

We require double-vacuum-melted 1 8 Ni maraging steel bars. The steel must be of a single heat

and the ingot(s) must be adequately forged prior to rolling to minimize compositional and

microstructural variation in the final products. The steel must be produced using a vacuuminduction-melt vacuum-arc-remelt (VIMA'^AR) procedure, and meet the nominal compositional

requirements given in Table 3:

Table 3. Type T-200 steel (mass percent).

Ni Mo Ti Al Si, max Mn, max C, max S, max Co, max P, max

18.5 3.0 0.7 0.1 0.1 0.1 0.01 0.01 0.5 0.01

The composition must be certified using standard analytical procedures, using equipment

calibrated by standards traceable to NIST. The composition is measured at the top and bottom of

the ingot. These two measurements are included in the documentation with the order, and must

meet the requirements given above within reasonable tolerances for an 18 Ni maraging steel. If

the presence of any residual elements (not included in the requirements above) are expected for

the alloy, a maximum allowable concentration for this element must be agreed upon. Deviations

between the two measurements (in mass percent between the top and bottom of VAR ingot) can

not exceed those expected for high-quality VINWAR 18 Ni ingots (by current steel making

standards).

Information on the current T-200 material we are using is given in Table 4. The three columns of data represent

results of samples taken from the top and bottom of the

VAR ingot, and the melt used to make the ingots. The alloy was melted in a vacuum induction furnace and cast into an

electrode mold approximately 432 mm in diameter which

weighed 3630 kg (4 ton). The electrode was remelted into an ingot with a diameter of 508 mm, which was cropped

(3175 kg) and 100 % conditioned prior to chemical analysis

(top and bottom). The 508 mm ingot was forged to a 432

mm octagon, then to 350 mm square, then to 250 mm

square, and cut into 6 equal lengths. The 250 mm square was then forged to 152 mm square (billet) and air cooled.

Each 152 mm billet was cut into three lengths, resulting in a

total of 18. These 18 billets are coded and each bundle of

bar that NIST received is from a single billet. The 152 mm

billets were direct rolled on a mill to 57 mm and cut to lengths of approximately 660 mm prior to final rolling.

Table 4. T-200 composition.

Top of

Ingot

Bottom of Melt

Ingot

c 0.003 0.002 0.004 Mn 0.03 0.03 0.02 P 0.007 0.007 0.005 S 0.003 0.003 0.003

Si 0.01 0.01 0.01 Cr 0.20 0.21 0.20 Ni 18.79 18.77 18.77 Mo 3.01 2.97 2.89 W 0.02 0.01 <0.01 V 0.01 0.01 0.01 Co 0.47 0.47 0.51 Cu 0.01 0.01 0.01

Ti 0.79 0.78 0.80 Al 0.11 0.11 0.128 B 0.0008 0.0008 <0.002 Zr 0.005 0.005 NA

9
4.2.2 Product form

The heat is processed to 12.7 mm (+3.8 mm, -0.0 mm) square bar. The comer radius of the

finished bars cannot exceed 0.76 mm. The maximum average grain size accepted is ASTM

number 10. In other attributes (surface condition, etc), the steel is required to be suitable for use

as 10 mm square Charpy V-notch specimens. The bars are delivered in the as-rolled condition.

The bar is machine straightened (for twist and bow), and shipped in lengths of no less than 2 m

and no more than 4 m.

4.2.3 Packaging

The bar is required to be packaged in bundles identified with reference to the ingot and which

portion of the ingot it was processed from. If possible, the bundles consist of bar rolled from

individual billets used for the rolling operation. The bundles must weigh less than 1815 kg (2

ton).

5. Specimen Production

5.1 Heat Treatment

5.1.1 Type 4340 steel

The 4340 steel is heat treated to produce low- and high-energy verification specimens. Typically,

as indicated in Figure 1, low energy levels are attained by tempering at temperatures between

300 and 400 °C. The high energy specimens are tempered near 600 °C. The microstructure of

the specimens must be 100 % tempered martensite.

The heat treatments originally recommended by the Army Materials Technology Laboratory are shown in Table 5.

Although the heat treatment of 4340 steel is straightforward for most commercial applications, it

is not easy to produce the quality required for the impact verification specimens, particularly for

production lots of approximately 1200 specimens.

One reason for this is that the transition behavior, shown in Figure 2, is not ideal for 4340 steel:

at -40 °C the upper shelf of the high-energy specimens, and the lower shelf of the low-energy

specimens are not flat. This can result in increasing the scatter during testing. Added to this are

the effects of slight differences in heat treating between specimens, slight inhomogeneities in the

steel, and other considerations. So, for our case, where a maximum range in hardness of less

than 0.5 HRC is needed for a production lot, slight differences in the thermal history of the

specimens can quickly present problems. Our experience has shown that the heat treatments

recommended by the Army can give good results for small lot sizes. For example, we had two

heat treating shops follow these recommendations to produce two low-energy lots for impact

testing. No additional heat treatment specifications were added to our instructions, so different

quench oils, etc. were used by the shops.

10
100

2C0 300 400 5CC 600 Tempemg TefTiperatiiire- C

Figure 1. Data for 4340 verification

material. 2001.

12C

S8 -

1C I ' 1

' ' ' -2CC -15C -ICO -«0 C SO ICO 150

TestTer.ise-raU's. C

Figure 2. Transition ciir\"es for 4340

steel that has been heat treated for

low- and high-energ}' venflcation

specimens.

Table 5. Example heat treatments for low- and high-energ}- level r\pe 4340 impact specimens.

Low-energ}^ specimens,

hardness 46 HRC =1 HRC

High-energ}' specimens,

hardness 32 HRC =1 HRC

Nonnalize 900 'C 1 1650 'Fi for 1 h. air cool Normalize 900 "'C > 1650 -"F for i h. air cool Harden 8"^! 1 1600 '- F. for 1 h. oil quench Harden S'"l 'C . 1600 ' F- for 1 h. oil quench

Temper 400 =C i''50 '- F) for 1.5 h. oil quench Temper 593 'C : 100 F^ for 1,25 h. oil quench

The variation in energies for both lots was low: One lot had a coefficient of variation of 0,04 (an

acceptable variation for impact verification specimen).

and the other lot had a coefficient of variation of 0.02 i a ver}' low vanation). However, to attain

results of this qualit}- for production lots of appro.ximateh l200 specimens, extreme!}- well

controlled processing is necessar}'. and r>picalh' double tempering, stress relief cr\ o-treatment,

and other steps are used to fme-mne the process for a given heat treating shop.

It is our experience that the specifics of the heat treatment should not be dictated to the shop.

Each heat-treatment shop is different and needs leeway to adjust the process to best suit the

equipment. Currently we use three shops and each uses a different process. All are capable of

attaining similar quality specimens (after climbing difficult learning cur\'es ). A t}pical qualit}'

for impact verification specimens is characterized by a coefficient of variation (the ratio of the

standard deviation to the average absorbed energy) of less than 0.04. The highest quahty

specimens approach coefficients of variation near 0.02.

11
Mestastable

1000 r

' ^ 1800 Equilibrium

10001

0 I 1 1 1 1 I I I 0 I I II I I i_ 0 5 10 15 20 25 30 0 5 10 15 20 25 30 Ni content % Ni content %

• Figure 3: Phase diagrams for the Fe- Ni system.

5.1.2 Type T-200 steel

The T-200 steel is an 18 Ni, cobalt-strengthened maraging steel. This alloy can be solutiontreated at 900 to 925 °C, control-cooled, grain-refined using multiple heating and cooling cycles near 760 to 815 °C, then aged to attain the appropriate strength/toughness combination. We age

to produce a low-strength, high-toughness material. Recommended aging for a hardness of 30

HRC is 315 °C for 6 h. In general, the aging reactions are more sensitive to temperature than

time.

The phase transformations for the T-200 steel that are of most interest are the martensite

transformation on cooling, and the formation of austenite on heating (holding at temperature).

As shown in Figure 3, the martensite in 18 Ni alloys is quite stable during heating to temperatures approaching 540 °C (1000 °F), which makes the aging of the martensite possible.

However, substantial amounts of reverted austenite can form in Co-free maraging steels (and in

other maraging steels) during aging treatments at temperatures of less than 540 °C (1000 °F), and

it is not clear whether reverted or retained austenite would adversely affect the scatter in the

Charpy impact energy.

12
For our alloy, we find that annealing temperatures of about 815 to 870 °C (1500 and 1600 °F)

are high enough to avoid the two-phase region and produce a fully annealed structure, and low

enough to avoid significant grain growth.

Most research does not include aging data for temperatures as low as 315 °C (600 °F), because

it is not of commercial interest. There has been some indication, however, that different precipitates are formed when the alloys are aged at low temperatures. A study on an 18 Ni Cocontaining 350 grade maraging steel showed distinct differences in the precipitates formed above

and below 450 °C (845 °F). NijTi precipitates are formed in T-200 alloys at high aging temperatures, but actual precipitation probably doesn't occur at low aging temperatures (315 °C for

3 h). It is likely that clusters of Ni and Ti atoms cause the strengthening at low aging temperatures, and the toughness is lower for these under-aged clusters in maraging steels than it is for

peak-aged steels (apparently because clusters or coherent precipitates restrict cross-slip in the

matrix and Ni3Ti precipitates allow more homogeneous slip).

Maraging steel can become embrittled during high-temperature solution treatments. The embrittlement is caused by precipitation of Ti(C,N) at grain boundaries during cooling, and can be retained even following re-annealing. Quenching from high temperature prevents the

precipitation and subsequent embrittlement.

We have also found that quenching from high temperatures results in higher toughness (lower

hardness) for our alloy, as indicated by the data in Figure 4. Quenching from the annealing

temperature clearly results in a softer material, and the difference between the hardness of the

air- cooled and water-quenched material is retained after aging. We found a difference of about

40

35

O Air Cooled From 845 C

II 20 1 ^ \ ^ ^ ^ ^ I 0 1 2 3 4 5 6 7

Aging Time at 315 C, Hr

Figure 4. Hardness for air-cooled and water-quenched

samples.

13
(

{

5 HRC, which is expected to result in a significant

increase in the toughness of the material.

Initial heat treatments on the new heat of T-200

material provide a general understanding of the

energy levels that might be expected from the

material. The mechanical test results for various

heat treatments are shown in Figures 5 through 7. In Figures 5 and 6, the samples were annealed at 954 °C (1750 °F) for one hour and air-cooled, then

re-annealed at 760 °C (1400 °F) for 1 h and aircooled. These samples were then divided into

five groups and aged at 260, 290, 315, 345, and

370 °C (500, 550, 600, 650, and 700 °F) for

three hours and air cooled. The data show the

relationship between the impact toughness and

the hardness of the material for these heat

treatment conditions. The data in Figure 7 are similar to those in Figure 5, but these samples were annealed at 900 °C (1650 °F) for 1 h and

water- quenched, then reheated twice to 675 °C

(1250 °F) and water quenched as a grain

refinement treatment, and re-annealed at 815 °C

(1500 °F) for 1 h and air-cooled prior to aging at 315 and 370 °C (600 and 700 °F) for 3 h. Other

variations of these two heat treatment schedules

produced similar results. Overall, it appears that

this T-200 material can be aged to produce Charpy

specimen having impact energies of near 215 J (160fflbf).

5.2 Sampling

A production lot of approximately 1200 specimens

are heat treated together as a single furnace load.

A spatial (not random) sample of at least 100

specimens is removed from the heat treating

baskets for pilot-lot evaluations. As shown in

Figure 8, a spatial sample allows us to evaluate

and minimize any correlation between the variation

in energy of the samples to their position in the

heat treatment baskets. If the pilot-lot sample

is acceptable, the remaining specimens in the

production lot are completed. An additional 30

random samples are removed from the production lot (following delivery to NIST). These

250 300 350

Aging Temperature, C

400

Figure 5. Impact energy of samples

annealed at 954 °C and 760 °C, then aged.

o

01X

«

Mc

•HX

4140

39

38

37

36

32

3130

1

1 1 250 300 350

Aging Temperature, C

400

Figure 6. Hardness of samples plotted in

figure 5.

260-

240 ^ 220

0} iS 200 u

toE 180

160

140

f

100 200 300

Aging Temperature, C

400

Figure 7. Impact energy of samples

annealed at 900 °C, 675 °C and 815 °C prior

to aging.

14
specimens are either used for evaluations of the

production lot or held as control samples for future

Figure 8. 4340 data showing the primar\-

variation in the specimens correlates to the

position of the specimen in the heattreatment basket.

testing.

5.3 Machining

5.3.1 Process

Prior to heat treating, the square bars are cut to approximately 56 mm long blanks and ground to finished length. Then one end of the specimen

blank is stamped with 'NIST', and other with a

series nimiber and a serial number. The series

number identifies the production lot and the energy

level (LL for low energy, HH for high energ>'. and

SH for super high energy). The serial numbers

range between one and the total number of specimens in the production lot. For the specimens

made with 4340 steel, the surfaces are all ground

to nominal size to remove surface flaws that might

result m quench cracking during the heat- treatment

operations.

From the production lot of heat-treated specimen

blanks. 100 are machined to final dimensions for pilot

lot testing (Figure 9).

5.3.2 Machining requirements

The dimensional requirements for NIST verification

specimens, given in Table 6, meet or exceed the

ASTM E 23 specifications. This minimizes vanations

in impact energ>' due to physical variations in the

specimens. Also, the notch centering and the length

tolerance for NIST specimens are equivalent to the ISO Standard 164. which permits the specimens to be used in impact machines with end-centering devices. The NIST requirement for

surface finish is also equivalent to the ISO 164 requirement. AU of these dimensional requirements can be met with standard machining practices.

Specimen notches are form ground on a surface grinder (machining with a fl\- cutter or multitooth cutter is not permitted). To avoid "burning" or cold working the material at the base of the

notch, the next to the last cut is required to remove more than 0.25 mm and less than 0.38 mm

and the final cut must not remove more than 0, 12 mm. When the specimens are finished and

ready for shipment, they are given a protective coating of oil.

Figure 9. A Charpy V-notch sample

with dimensions labeled in reference to Table 6.

15
Table 6. Dimensional requirements for NIST Charpy impact verification specimens.

Height (H) 10 mm, ±0.03 mm, with adjacent sides square within 90° ±9 min

Width (W) 10 mm, ±0.03 mm.

Length (L) 55 mm, +0.00 mm, -0.3 mm

Notch position L/2 27.5 mm ±0.2 mm, perpendicular to the longitudinal axis of specimen within

90° ±9min

Notch radius 0.25 mm, ±0.025 mm, with radius tangent to the notch angle

Notch depth (dl) 2 mm, ±0.025 mm

Notch angle. 45° ±r

Ligament depth (d2) 8.0 mm, ±0.025 mm

Surface finish 1.6 |im on notched surface and opposite face; 3.2 |im on other surfaces

5.4 Hardness Testing

5.4.1 Process

Two hardness measurements are made on each of the pilot-lot samples, at positions approximately 10 mm from the specimen ends on the face opposite the notch. The two measurements

are averaged to estimate the hardness of the sample.

The hardness criteria for verification specimens relate to three practical aspects of the impact

test: ( 1 ) The minimum hardness requirement for low-energy lots assures an appropriate impulse

load is transferred to the machine frame on impact to verify adequate mounting and overall

stiffness of the machine. (2) The minimum hardness requirement for low-energy lots also

determines the direction in which the 4340 impact specimens exit the machine. (3) The specimento-specimen variation in hardness provides an indication of the variation in energy of the

specimens (particularly for the higher-energy specimens).

The verification specimens are produced so that different

energy ranges leave the machine in different directions.

Specimens with hardness of greater than 44 HRC leave

the machine in a direction opposite to the direction of

the swing, and are needed to evaluate how well the

shrouds on U-type impact machines are functioning.

Specimens with a hardness less than 44 HRC typically

exit the machine in the same direction as the swing of

the pendulum.

In practice we find that when the variation in hardness

exceeds ±0.5 HRC the quality of the lot is questionable

(i.e., the variation in energy is likely unacceptable). As

shown in Figure 10, the correlation between energy and

hardness is much more useful for evaluating variations

-> ^oo - ia

c

lUo

g. 60 - E

0 I 1 1 1 1 30 36 40 4fi 60

Hardness, HRC

Figure 10. Hardness data for low

and high energy verification

specimens.

16
of high-energy (lower hardness) specimens, because at high hardness, the slope of the trend

decreases significantly. So, although hardness evaluations have worked well as a quality-control

procedure in our program, hardness data are principally used to estimate the impact toughness of

the specimens and to assure that the low-energy specimens exit the machine in the required

direction.

5.4.2 Requirements

The average hardness of the pilot lot samples must be within ±1 HRC of the targeted hardness,

unless otherwise agreed.-^ This requirement is most important for the low-energy specimens,

which normally need to have a hardness of 44 HRC or more to exit the impact machine properly.

5.5 Impact Testing

5.5.1 Process

If the dimensional measurements of the specimens and the hardness results are acceptable, the

pilot-lot specimens are divided into three groups of 25 (one group is tested on each of the three

master impact machines) and the extra 25 specimens are held in reserve for any additional testing

that may be required. In dividing the specimens into groups, the furnace locations from which

the specimens were taken are considered and the groups are balanced accordingly.

The certified energy value for a production lot of verification specimens is defined as the grand

average of the 75 specimens tested (25 specimens on each of the 3 master machines). In addition to the grand average impact energy, the standard deviation, sample size, and several

other statistics are calculated for the verification set. These statistics are used to determine the

acceptability of the lot and the performance of the machines. All 75 specimens are included in

these calculations, with the following three exceptions: (1) specimens that are determined to be

outliers as defined in section 6.5.2, (2) specimens having the same lateral expansion but

significantly different energies, and (3) specimens with flaws apparent on their fracture surfaces.

5.5.2 General statistics

The average energy and grand average energy are defined as

X] + X2 + X2 + ...x^ x =

. (1)

n

where n is equal to 25 for calculating the averages of each machine, and n is equal to 75 for

calculating the grand average for the pilot lot.

When using a single impact machine, the same amount of impact energy may be indicated by materials

having different yield strengths. These same materials tested in another machine may indicate different values of

impact energy. The difference is usually greater for the stronger materials, presumably due to the faster rate at which

peak loading occurs. To accentuate these differences, materials of high yield strength are specified for the

verification specimens at each energy level. These requirements are normally monitored by making hardness

assessments rather than tensile testing.

17
The standard deviation is defined as

s -

(2)

n- 1

The pooled standard deviation is defined as

'p = V

p—

where subscripts 1 , 2, and 3 indicate the standard deviation of the 25 samples tested on the three

master machines, and P is equal to three.^

The sample size, which represents the minimum number of specimens from a given production

lot that should be tested in a verification test, is defined as

n =

where E is 1 .4 J or 5 % of the mean energy, whichever is greater. For example, for the lowenergy specimens E is equal to 1.4 J, so the maximum pooled standard deviation allowed for a sample size of 5 is

s„ = E--= 1.4—= 1.047, (5)

P 3 3

which indicates a CV of around 0.07 for specimens with an average energy of 16 J (1.04/16 = 0.07).

For the higher-energy specimens, E is taken as 5 % of the average energy for the lot and the

maximum pooled standard deviation allowed for a sample size of 5 is

Sp = (0.037) (average energy) , (6)

and in this case the CV is 0.037 by definition (Sp /average energy).

3^ '

(4)

The choice of the value of standard deviation depends on whether all the machines used to determine the reference value met the requirements for variability, k (see equation 7). If all the machines met the requirements, the value of s shall be equal to the pooled standard deviation. If all the machines did not meet that requirement, s shall be

equal to the largest of the standard deviations of the machines considered separately.

18
The outlier analysis is performed using box-and-whiskers plots to provide a graphical summary

of the data and identify outliers."^ Outliers are defined as values that are lower than the first quartile or higher than the third quartile by more than 1.5 times the absolute difference between

the first and third quartiles. If a lot has more than 5 % outliers, it may be rejected.

The variation in energy values is calculated for each machine using a ratio of the standard

deviation for the particular machine over the pooled standard deviation. This ratio, k, is expressed algebraically as

P P P

here s^ and Sp are the individual and pooled standard deviations for the three machines

respectively. If the k ratio of any of the three machines exceeds 1 .25 (assuming 25 specimens

tested per machine), the variability in energy values due to that machine is questioned and

appropriate actions are taken (repairs to the machine, testing of additional samples, etc.).^

5.5.3 Uncertainty calculation

The uncertainty of a single specimen in a given lot can be determined by combining three

components of uncertainty: within-machine uncertainty {Sp), uncertainty due to machine bias {s^),

and the uncertainty of specimen homogeneity {Sf^). The total uncertainty is given by

The within-machine uncertainty is the "pooled" standard deviation (see eq (3)) based on 25

verification specimens tested on each of the 3 master machines. The degrees of freedom

associated with Sp is 72 (i.e., 25 + 25 + 25 - 3).

The uncertainty due to machine bias accounts for possible bias in the observed averages

associated with each master machine. The value of can be quantified using a technique called

"BOB" which models the unknown biases with a Type B uncertainty distribution.

The final component of uncertainty, Sfj, can be thought of as a correction for specimen

inhomogeneity and is typically based on engineering judgment. It is common practice to set the

An outlier is defined statistically, but a specimen identified as an outlier is not removed from the analysis unless it shows physical evidence ofjamming, material flaws, or other reasons for atypical behavior.

^ If the k ratio of a machine is greater than 1.25, the results of this machine can be questioned by the contractor who supplied the specimens. This is considered a basis for retesting another group of 25 specimens prior

to determining the acceptability of the lot.

19
number of degrees of freedom associated with a Type B component of uncertainty, such as Sf^ , equal to infinity.

5.5.4 Energy requirements

The most important requirement is the variability in impact energy of the specimens. Our

contracts allow us to reject a lot with a sample size of more than 5. A lot can also be rejected if the average energy is outside the range specified in Table 6. The

certified energy of the specimens must fall within the ranges of 14 to 20 J (10 to 15 ft-lbf) for the

low energy level, 88 to 136 J (65 to 100 fflbf) for the high energy level, and 176 to 244 J (130 to 180 ft-lbf) for the super-high energy level, unless otherwise agreed.

6. Certification and Acceptance of a Pilot Lot

6. 1 Process

Acceptance of a new batch of verification specimens is based on the data obtained from the pilot

lot of 100 specimens, taken from a heat-treatment batch of approximately 1200 specimens.

Although impact energy is the most important criterion, other criteria are also evaluated to determine the consistency and quality of the verification specimens. The pilot lot data (impact

energy, hardness, and dimensional measurements are processed using a computer program to provide standardized output for review in determining the acceptability of the lot.

If the data indicate that the pilot lot is acceptable, the contractor is advised to machine the

remainder of the production lot and submit it for final acceptance. If the random samples

removed by NIST from the production lot are acceptable, a certified energy value is assigned to the lot and it is placed in inventory.

The certified energy of the lot is defined as the grand average energy of the lot. The number of

specimens in a verification set is determined by the sample size calculation. Typically, a set size

of five is used ( for lots with sample sizes of three, four, or five), but occasionally sets having

more or less than five samples are distributed.^

The number of degrees of freedom associated with each of the three components of uncertainty can be combined using the Welch-Satterthwaite formula to obtain the effective degrees of

freedom associated with the total uncertainty, Uj- . The effective degrees of freedom are used to determine the appropriate coverage factor for the confidence intervals.

6.2 Requirements

A pilot lot can be rejected for use as verification specimens if:

1. The verification specimens do not meet the dimensional requirements given in section 6.3.

We routinely reject lots with sample sizes greater than five, but when stocks are very low we have

occasionally accepted lots with larger sample sizes.

20
2. The hardness and energy levels are not within the ranges specified in Table iJ

Table 7: Required ranges for verification specimens.

Energy level Low High Super-high

Absorbed Energy (J) 14 to 20 88 to 136 176 to 244

Hardness (HRC) >44 + 1 of avg + 1 of avg

3. The sample size for the lot exceeds 5.

4. The number of outliers exceeds 5 % of the number of specimens impact tested in the pilot lot (4 is the maximum for a pilot lot of 75).

5. The difference between a machine average and the grand average is greater than the larger of

1.4 J or 5 % of the grand average.^

6. The results from one of the three machines show excessive variability, according to the k

ratio.^

7. The microstructure of the low energy and high energy specimens is not 100 % martensite (no

ferrite, austenite, or bainite should be visible).

6.3 Reports

The pilot-lot data calculations are done by a computer program to provide a consistent

appearance and quality for our records. The evaluation report documents the lot identification,

the reference machine it was tested on, and the energy and hardness of each specimen that was

tested. The calculated values in the report are as follows: (1) the grand average energy and

standard deviation, (2) the average energy and standard deviation for each machine, (3) the

average hardness and standard deviation, (4) the pooled standard deviation in energy for the lot,

(5) the sample size, and (6) the k ratio of each machine. A set of standardized plots in the report

shows outlier data, the distribution in energy for each machine, and the combined distribution in

energy for the three reference machines. The data collected from dimensional measurements on

the specimens are kept separately. The evaluation report and raw data for the pilot lots are filed

for future reference and a copy of the report is sent to the contractor who supplied the pilot lot (with our comments).

The average hardness values of the high and super-high energy specimens are not specified, but we

require a maximum variation of ±1 HRC for the lot.

Q In this case, both the quahty of the specimens and the performance of the machine are questioned.

Appropriate actions are taken that are agreed to between the NIST and the contractor supplying the specimens.

' In this case, both the quality of the specimens and the performance of the machine are questioned.

Appropriate actions are taken that are agreed to between NIST and the contractor supplying the specimens.

21
7. Distribution

7.1 Packaging

Specimens are drawn from the production lot at random to make up the sets of impact

verification specimens. These sets are distributed for verification testing. Each set of

verification specimens is retained henceforth in the sets, as originally drawn.

If a purchaser can demonstrate that one or more specimens of a set are defective, the set is replaced without charge.

7.2 Information

Each set of verification specimens is accompanied by a certificate (Appendix 1) that gives the

following information: name, address, and telephone number of NIST contacts; the test temperature; the identification of steel used; the designation number of the practice or practices

whose specifications for verification specimens are met by the specimens supplied.

8. Customer Certification Procedure

8.1 Process

The results of a verification test are returned to NIST, along with the broken specimens and a

questionnaire that is filled out by the customer. Information from each of these three sources is used in the evaluation of the test. Based on the results of this evaluation a letter is written to the

customer. If the results are acceptable, a verification letter and accompanying verification sticker serve as documentation that the machine meets the requirements of ASTM Standard E23. If the

results are unacceptable, the letter explains why we think the test does not meet the requirements

of E 23 and suggests how the machine might be brought into compliance. '°

8.2 Customer Questionnaire

The information provided by the customer is used to help us understand anomalies in the test data and provide background that allows us to better advise the customer. If test results are uniformly high, for example, the questionnaire might be referenced to determine how the test temperature was measured and the last time the temperature equipment was calibrated, which

might explain the result (test conducted at wrong temperature). Other, nontechnical information

is also provided by the questionnaire that is used to update our database. A copy of the

questionnaire is given in Appendix 2.

8.3 Test Data

The verification test results are calculated and compared with the certified value of the lot, and

with the results of previous verification test results for the machine. A machine is classified as unacceptable if the difference between the average energy of the machine being verified and the

Customers are not required by E 23 to have their machines verified by NIST. They can verify the results of the test themselves, or have a private testing company verify the test results. ASTM E 23 does require that the energy value of the impact verification specimens be "established on the three reference machines owned,

maintained, and operated by NIST in Boulder, CO".

22
certified value is greater than 1 .4 J or 5 % (whichever is larger) of the certified energy of the

verification lot.

8 .4 Examination of Broken Specimens

The specimens are checked to determine the following information: (1) if the anvil marks

indicate that the specimens were centered for the test, (2) if the striker mark indicates that the

striker on the impact machine was centered, (3) if the anvil markings indicate excessive or unusual wear, (4) if the size of the shear lips on the specimens indicate that the test was done at the proper temperature, and (5) if markings on the fracture surfaces of the specimens show

material flaws or unusual textures.

These observations are used either to remove a specimen from the data analysis (in the case of a flaw or off-center strike), or as a basis to fail the test due to worn anvils, etc. More specific

information on how and why the specimen are examined in the NIST procedure are given in

Special Publication 960-4.

8.5 Customer Letter

Based on our judgement and the requirements of ASTM E 23, a pass or fail letter is developed

for the customer. The letters are composed using a word processing program that is integrated

with our database, and with a list of standard paragraphs covering commonly observed problems

with verification test results. The program merges customer data with the selected standard

paragraphs, and then allows final editing for the addition of more specific comments, if applicable. The letter also includes a table that presents the customers' data and the values that were computed by the program to evaluate the data.

If a customer fails the verification test, he/she is typically contacted by fax, phone, or email to discuss the results. If a customer passes the verification test, the letter serves as a file record.

8.6 Verification Sticker

A verification sticker (to put on the impact test machine) is mailed with each pass letter. The

stickers have a NIST logo and give the serial number of the machine, the date of the next

verification, and the range in energy over which the machine is verified.

The stickers are made using a Brady 200M label printer and Codesoft version six software by

Teklynx.

The inclusion of stickers in the customer letters was initiated in September 2001.

9. Program Controls

9.1 Impact Machines

The impact machines are inspected and adjusted by NIST personnel, and experts contracted by

NIST. Critical direct verification measurements were made when the machines were installed,

and are made when a change in the performance of a machine is noted. Example data for the

master machines are given in Appendix 3.

23
The performance of the impact machines is routinely evaluated for each lot of specimens tested.

This evaluation is principally a comparison of the mean and standard deviation of each machine

to the other machines used in the program. The performance of the machines are compared as each pilot lot is tested, and these results are compared with the past performance of the

machines.^' A plot showing the average energy of each machine and the grand average for each

pilot lot is updated for each pilot lot tested, to document and evaluate the relative performance of

the impact machines. Example data are given in Appendix 3.

A log book on the machines is maintained that contains records for the "daily check" procedures

that are conducted on the machines prior to testing a pilot lot: these records allow us to track the

friction and windage, and other factors that affect the performance of impact machines. The log

book also documents maintenance to the machines and the number and types of specimens

tested.

A reserve of impact verification specimens (from past pilot lot tests) are kept and serve as control

specimens. When a change to a machine is suspected, due to its relative performance, a set of

control specimens can be tested and compared to the original performance for this machine with

these specimens. Control specimens are also used to check machines following a repair.

9.2 Measurement Equipment Used in the Verification Program

A Newage Deltronic hardness tester is used to measure hardness. The hardness tester is calibrated annually by Leco Corporation. The hardness tester is checked with calibration blocks

prior to each use. An optical comparator is used to measure the notch angle, notch depth, notch

radius, and L/2 (notch centering in relation to specimen length). The optical comparator is a

Deltronic Model DH 216 and is equipped with an MPC-5 readout. The comparator is calibrated

annually by Precision Gage, Inc. Both the hardness tester and the optical comparator read

directly to a personal computer using NIST developed software.

Mitutoyo, Model CD-6"C, digital calipers are used to measure specimen length, width, and

thickness. The calipers are calibrated annually by Precision Gage, Inc. The calipers are checked

with a one-inch calibration block prior to each use. The caliper data are automatically stored on

a personal computer.

Squareness is measured with a gage manufactured by Laboratory Testing, Inc. The gage was

manufactured using the drawing in ASTM Standard E 23. The gage is calibrated annually by

Laboratory Testing, Inc. The gage is checked with a calibration block furnished by, and annually

calibrated by, Laboratory Testing, Inc. All calibrations by outside companies are traceable to NIST.

The impact machines have characteristic differences from one another in energy level and variation. Changes in these relative differences indicate changes to our program, and are investigated to determine the cause.

24
9.3 Specimens

The quality and consistency of the verification specimens is first controlled by the steel used for

their production. Our contractors are shipped bundles of steel bar that are coded with reference

to ingot location, and production lots are made using steel from a given bundle. This is our best assurance that the steel used for a given production lot is as similar as possible. In the event that

some portion of the bar contains melting or rolling flaws, this procedure would help us to more

quickly identify and remove this material from the stock.

Our second control of specimen quality is careful sampling and pilot lot evaluations. In our

experience we have found that geometric rather than random sampling produces a better estimate

of the mean energy for our pilot lots. Our samples are taken from predetermined positions within

the heat-treating baskets and labeled.

Our final control involves a feedback loop using data from customer verification tests. As

customer data are collected they are stored in a database, and pass/fail ratios can easily be

calculated for a lot of verification specimens that is questioned by either a customer or ourselves.

If these data show normal ratios, this is strong evidence the average energy of the lot was

accurately estimated by our pilot-lot sample. If these data show more machines than normal are

failing using a particular lot of specimens, and the mean energy of the customer data is significantly different from the certified energy value of the lot, this is evidence that the certified

energy value of the lot has changed or that the average energy determined for the lot was not an

accurate estimate.

9.4 Customer Evaluation and Service

In an attempt to control the quality of our customer assessment, we look at both current and past

tests results for the machine. This often helps in understanding a customers' problems and

allows us to better help the customer with comments concerning the performance of the machine.

To provide prompt service for the program, we have two back-up personnel who are capable of

filling in to cover the day-to day operation of the program.

To preserve good documentation of customer verification tests, we save the test specimens for one year. We save customer letters for two years as a hard copy. Digital files of letters are kept

indefinitely, and all database information is saved.

9.5 Database

The software used in the program is managed by one person only. No changes are made without

adequate follow-up. The personnel using the software are always consulted before and after

making software revisions. The database (and software) is backed-up to tape on a regular basis.

In the last 10 years approximately 3 lots have been suspected of having inaccurate certified energies

assigned to them. In all 3 cases, sampling is suspected to have caused the error. The stability of the specimens has

not been suspected as the cause (because the energies increased rather than decreased).

25
10. Support of National and International Impact Standards

The data gathered from customer verification tests and from our pilot lot tests provide a unique

source of information for statistical studies on impact testing. These data allow us to review how

specific designs, capacities, and ages of impact machines perform under the rules of the various

impact standards used in the world. They also allow us to evaluate how well our estimates for

the average energies of pilot lots compare with the average of all the machines in the world that

tested them.

The principal use for these data is to help address issues concerning the indirect verification rules

of ASTM and ISO impact testing standards. For example, currently there are major differences

between the pass/fail range for ASTM and ISO verification tests, and our data can be used to make a strong argument that the ISO tolerance is too large (and the ASTM tolerance may be too

small). Since the data are from actual tests, and include results from many countries, we can more accurately evaluate (and demonstrate) the impact of verification rules to our customers and

the standards community than can any other country in the world. Our database is currently

estimated to have results for more than 14000 tests (sets) and approximately 150 pilot lots.

1 1 . Education and Training

11.1 Customers

Misunderstandings with our customers are minimized once they understand the NIST program,

and how they can best take advantage of it. Tp help educate our customers we have developed a video that provides an overview of the Charpy test and the NIST verification program. We also

have a brochure detailing our specimen evaluation procedures. Special Publication 960-4.

Occasionally we have provided group training to companies that manufacture and repair Charpy

impact machines.

11.2 Staff

General experience is the most valuable education and training for our staff, particularly

experience gained from talking to customers. In addition, we attend ISO and ASTM meetings to gather feedback on the program. Comments at these meetings, and other technical meetings

concerning impact testing, help to keep the program and personnel on track.

12. Safety Considerations

The operation of impact machines requires good safety practices to avoid injury. During our impact tests, the laboratory is locked and signs are posted on the doors to indicate that testing is in progress. Partitions are used to shield the operator and other equipment in the room from

flying specimen halves as they exit the machine after impact. Safety glasses are worn when

impact tests are conducted.

26
Appendix 1. Sample Charpy Verification Certificate

Example of a Certificate that was distributed with Charpy verification samples in 2000.

27
SRMs 2092, 2096, and 2098 Page 1 of 3 National Institute of Standards & Technology

Certificate

Standard Reference Materials ®

2092 - Low-Energy

2096 - High-Energy

2098 - Super High-Energy

Verification Specimens for Charpy V-Notch Impact Machines

Lot No.:

Standard Reference Materials (SRMs) 2092, 2096, and 2098 are intended primarily for the verification of Charpy VNotch machines in accordance with the current ASTM Standard E 23 [1]. Each SRM consists of a set of individual

10 mm X 10 mm x 55 mm specimens needed to perform one verification. These SRMs comply with both ASTM

Standard E 23 and International Organization for Standardization ISO/DIS 12736 dimensional requirements [2].

Material Description: SRMs 2092 and 2096 are made from 4340 alloy steel. SRM 2098 is made from a high

strengthmaraging steel. The bars are finished to length, stamped, heat-treated, and machined in SRM specimen lots ofapproximately 1200. Each specimen has a lot number and an identification number (three or four digits) stamped

on oneend of the specimen. Additional information can be found in References [3-5]. SRM Certification Procedure: Specimens taken at random from each SRM lot are tested by the NIST Materials

Reliability Division on Charpy V-Notch reference machines. The specimen data generated are then statistically

evaluated to assure the homogeneity of the lot, establish the certified value, and determine the number of SRM

specimens required for a user to perform a valid test. See Table 1 for a list of the approximate energy ranges within

which the individual certified values should fall.

If certified values are required immediately after testing, contact the NIST Charpy Program Coordinator as follows: telephone (303) 497-3351; fax (303) 497-5939; or e-mail vigliotti@boulder.nist.gov. The lot number and energy

results of the tested specimens must be provided in order to obtain certified values by telephone or fax.

Expiration of Verification: The verification report issued on an acceptable machine is valid for one year from the date that the SRM was tested. If a user's machine is moved or undergoes any major repairs or adjustments, the current verification will be invalidated and the machine must be retested and reverified. The overall direction and

coordination of the technical measurements leading to verification of test specimens and machines, evaluation of test results, and issuance of the report on machine conformance are under the direction of the NIST Materials Reliability

Division, Boulder, CO.

The support aspects involved in the original preparation, certification, and issuance of these SRMs were coordinated

through the NIST Standard Reference Materials Program by R.J. Gettings. Revision of this certificate was

coordinated through the NIST Standard Reference Materials Program by C.R. Beauchamp.

Fred R. Pickett, Chief

Materials Reliability Division

Gaithersburg, MD 20899 Nancy M. Trahey, ChiefCertificate Issue Date: 14 May 2001 Standard Reference Materials

Program

28
SRMs 2092, 2096, and 2098 Page 2 of 3 NOTE: THESE ARE NOT CERTIFIED VALUES. THESE ARE THE APPROXIMATE RANGES FOR

EACH ENERGY LEVEL.

Table 1 . Approximate Charpy SRM Energy Ranges.

SRM No. (J) (ft-lbf)

2092 13-20 10-15

2096 88-136 65-100

2098 176-244 130-180

Storage: The SRMs are composed of specimens anticipated to have an indefinite shelf life under normal storage

conditions. Each specimen is coated with oil, wrapped in a corrosion inhibiting paper, and sealed in a plastic

envelope.

It is recommended that the specimen be retained in this package to protect them from moisture until used. The

protective oil coating should be wiped from each specimen just prior to testing.

Use: Prior to testing a Charpy V-Notch machine, the machine should be checked to assure compliance with the appropriate sections of the current ASTM Standard E 23 [1]. To comply with the testing procedures specified in the standard, SRM 2092 and SRM 2096 shall be tested at -40 °C ± 1 °C (-40 T ± 2 °F). SRM 2098 shall be tested at 21 °C ± 1 °C (70 °F ± 2 °F). All SRM specimens are to be tested in accordance with the testing procedures of the appropriate sections of the current ASTM Standard E 23. All SRMs shall be tested at the same time. An acceptable

machine will produce an average value widiin 1.4 J (1.0 fflbf) or 5 % of the certified energy value, whichever is greater, providing the specimens appear to have normal markings. Because the source(s) and magnitude of error for energy values at one energy level may not be the same at different energy levels, calibration or correction curves

shall not be used.

Verification of User's Machine: The NIST Charpy Program Coordinator will issue a report of findings to the user's

facility upon receipt of the fracttired specimens and completed questionnaire. If the machine to be verified produces

acceptable values and the specimens appear to have normal markings, this report will verify its conformance. If the machine produces values outside the allowable tolerance of the certified energy values or the specimens have

abnormal markings, the report may suggest repair or replacement of machine parts, changes in testing techniques, or

other appropriate corrective actions. Fractured specimens and completed questionnaires should be returned to the NIST Charpy Program Coordinator, Mail Code 853.07, 325 Broadway, Boulder, CO 80305-3328. A plastic, self- locking bag is provided for the return of broken specimens. The broken specimens shall be taped together as described in the wrapping instructions included with the questionnaire.

Important Information: Shipping charges for the return of broken specimens are the responsibilit>' of the user. The

mailing label provided with each SRM must be used to expedite shipping and, for overseas shipments, clearance by

U.S. Customs.

Note to International Customers: Regular overseas shipments of broken specimens should be sent airmail so that

after they are cleared by U.S. Customs, they can be forwarded directly to NIST-Boulder. If a more rapid shipping

mode is necessary, choose an overnight delivery service that will handle U.S. Customs clearance AND will deli\ er

directly to NIST-Boulder. Unless such delivery is assured, air freight packages may be returned to the customer by

U.S. Customs. SRMs 2092, 2096, and 2098 Page 3 of 3

29
REFERENCES

[1] ASTM E 23, Standard Test Methods for Notched Bar Impact Testing of Metallic Materials, Annual Book of

ASTM Standards, 03.01, ASTM, West Conshohocken, PA.

[2] ISO/DIS 12736, Metallic Materials - Impact Testing - Preparation and Characterization of Charpy V Reference

Test Pieces for Verification of Pendulum Impact Testing Machines, ISO, Geneva, Switzerland.

[3] Siewert, T.A. and Schmieder, A.K., "Pendulum Impact Machines: Procedures and Specimens for Verification,"

ASTM STP 1248, ASTM, West Conshohocken, PA, (1995).

[4] Shepherd, D.A. and Siewert, T.A., "Interlaboratory Test Study for the Determination of Precision and Bias in Charpy V-Notch Impact Testing," ASTM Research Report E 28-1014, ASTM, Philadelphia, PA, (1991).

[5] Holt, J.M., "Charpy Impact Test - Factors and Variables," ASTM STP 1072, ASTM, Philadelphia, PA, (1990). Users of this SRM should ensure that the certificate in their possession is current. This can be accomplished by

contacting the SRM Program at: telephone (301) 975-6776; fax (301) 926-4751; e-mail srminfo@nist.gov; or via the Internet h ttp://www. nist. sov/srm.

Certificate Revision History: 14 May 2001 (updated email address for Boulder contact); 09 August 2000 (updated

mail and zip codes for Boulder facility); 22 March 2000 (editorial revision); 26 July 99 (editorial revision); 20

February 97 (original certificate date).

30
Appendix 2: Sample Customer Questionnaire

Example of a customer questionnaire used for the Charpy program.

31
QUESTIONNAIRE FOR CHARPY IMPACT MACHINE VERIFICATION

IMPORTANT; This questionnaire contains information to help you perform a successful verification

test. Energy results are required for verification. Other specific information is requested to help evaluate

the condition of your machine. The questionnaire and the fractured specimens should be shipped to the

Charpy Program Coordinator, NIST, Division 853, 325 Broadway, Boulder, CO 80305-3328. Phone:

303/497-3351 Fax: 303/497-5939.

Location of Machine

Company

Address

State/

City Province

Zip/

Country Postal Code

Mailing Address for Verification Letter (if different from above)

Company

Address

State/

City Province

Zip/

Country Postal Code.

Test Machine (circle appropriate units where indicated)

1 . Machine Manufacturer and Serial Number

2. What is the maximum energy capacity of the machine?

(J fflbf)

3. If the machine is adjustable, what capacity was used for this test?

(J fflbf)

4. The machine should be securely bolted to a concrete foundation or a steel block having a mass

not less than 40 times that of the pendulum.

(a) What type of bolts are used to mount the machine? (J, lag, etc.)

(b) The machine should be level according to the current ASTM Standard E 23.

32
5. Is your machine equipped with a carbide striker?

6. Is your machine equipped with carbide anvils?

7. Check the appropriate pendulum design below.

cm]

B

Side

Support

If side supports or shrouds are used, what is dimension "d"?

Circle: (mm or in)

C (Other)

Please Sketch

9. Your anvils and striker should conform to the dimensions below:

Spectinea

Anvil

Anvil

Anvil

A:

R:

Anvils

80° approx.

1 ± 0.05 mm

a:

Striker

30° approx.

r: 8 ± 0.25 mm

(0.039 ± 0.002 in) (0.315 ±0.010 in)

33
w 40 ± 0.05 mm w: 4 mm approx.

B:

(1.574 ±0.002 in)

90° ± 10 min b:

(0.157 in)

0.25 mm (0.010")

Pendulum

Shroud

10. If shrouds are used to contain broken specimens, the following requirements should apply:

(A) The shrouds should have a minimum hardness of 45 HRC.

(B) The thickness of the shrouds should be approximately 1.5 mm (0.06 in).

(C) Dimensions a, b, c, and d below should not exceed 1.5 mm (0.06 in).

(D) If dimension "d" in item 8 is more than 13 mm (0.5 in), requirements (B) and (C) above do

11. The striker should pass through the center of the anvils within 0.40 mm (0.016 in).

12. With the pendulum in the free hanging position, engage the energy indicator. The indicator should read

within 0.2% of the maximum energy range being used.

13. What is the friction /windage loss of your machine?

(J fflbf)

(a) Raise the pendulum to the latched position. Without a specimen in the machine, release the

pendulum and permit it to swing 1 1 half cycles; after the pendulum starts its 1 1"* half cycle,

move the pointer to between 5 to 10 % of scale range capacity and record the dial reading.

(b) Divide the value by 1 1, then divide by the maximum scale range of the machine and multiply

by 100. The result, friction and windage loss, should not exceed 0.4 %.

14. With the specimen removed from the machine and the pendulum released from its latched position, what

is the dial reading after one swing?

not apply.

(J fflbf)

34
This reading should be zero. If this reading is not zero and your machine is equipped with a compensated scale, please adjust the dial to read zero. If your machine is equipped with a non-compensated scale, please compensate

the energy values for windage and friction by subtracting the windage and friction value calculated in item 13.

15. When was this machine last verified by the NIST? Date:

16. Is your machine equipped with a direct reading scale or a noncompensated scale?

Important Information

To obtain accurate results the following procedures should be followed closely. For the NIST reference

specimens the test temperature is near the ductile-brittle transition temperature of the steel. Therefore

small variances in temperature and procedure may cause considerable error in energy values.

The cooling bath should be placed directly beside the machine. This enables the operator to remove specimens from the bath and fracture them in the machine quickly.

It is very important that the specimens be removed from the bath and fractured in less than 5 s. Taking longer than five seconds can increase the energy values, which may cause the low energy

specimens to exceed the allowable energy limit.

If your machine is equipped with a centering device, we do not recommend that you use it to center specimens when performing low temperature testing. Instead, we recommend the use of

centering tongs as described in the current ASTM Standard E 23. The centering tongs should be

cooled with the specimens.

Verify temperature-measuring equipment at least twice annually. The measurement equipment

can be checked immediately before the test by checking a medium with a constant temperature

such as dry ice [-78.6 °C (- 109.3° F)] or ice water [0.0 "C (32.0° F)].

When testing super-high energy level specimens or other ductile materials, the anvils should be

checked between each test for material left by the previous test.

When the anvils are replaced it is recommended that practice specimens be broken before NIST

specimens are tested.

TESTING TECHNIQUE

1. Test temperature for SRM 2092 low energy and SRM 2096 high energy level specimens should be

-40 ± r C (-40 ± T F).

2. Test temperature for SRM 2098 super-high energy level specimens should be 21 ± TC (70 ± 2° F).

3. How long were the specimens held at temperature? (NIST recommends a minimum of 10 min)

4. What instrument was used to remove the specimens from the bath and center them in the machine?

35
I

STATE REASON FOR VERMCATION

1 . Compliance with annual ASTM Standard E 23 Indirect Verification

2. Changed striker and/or anvils

3. Moved machine

4. Changed bearings or pendulum

36
WRAPPING INSTRUCTIONS

To expedite the evaluation of your machine, please secure the 5 broken specimens (10 halves) from a particular energy series, as one unit with clear cellophane tape according lO the following instructions. See diagram below.

1. Keep broken halves correctly paired (back to back) with the fracture surfaces facing upward

and notched surfaces facing outward.

2. Coat the FRACTURE SURFACES ONLY with a light coat of oil. DO NOT use grease or coat in plastic.

3. Include this completed questionnaire with the firactured specimens.

4. Be sure that you use the MAILING LABEL, provided with the specimens, and attach the

label so that it is clearly displayed on the OUTSIDE of the package. This will expedite

delivery to the Charpy Coordinator. Customers returning specimens from outside the United

States should include the following statement on the U.S. Customs Declaration:

Contents include U.S. manufactured steel test bars being returned to the U.S. for

evaluation and are valued at less than 10 U.S. dollars.

37
Sample Test Results Report

NOTE: Use ONE questionnaire only to report the NET ENERGY RESULTS of all energy levels

used to test this machine at this time.

INDICATE ENERGY UNITS (circle units used)

Joules ft'lbf

Series Series Series

SRM 2092 SRM 2096 SRM 2098

Specimen

Number

Value Specimen

Number

Value Specimen

Number

Value

Average Value Average Value Average Value

Date of Test

(Month/ Day/ Year)

Test Operator

PRINT Telephone

Test Operator

SIGNATURE^ FAX

Company Representative

PRINT Telephone

Company Representative

SIGNATURE FAX

38
If you require approval of your machine by the Defense Contract Management Command

(DCMC), a DCMC representative should provide his or her signature and the DCMC seal

to indicate that the preceding information was witnessed by a government representative.

Print Name of DCMC Official Seal

Signature of DCMC Official and Seal

DCMC Office Location

39
Appendix 3. Machine Performance Data

c

o

CO0

to

CO

>»O)k-0

c

LJJ CDOi

CO^

CD>

<

34000 35000 36000 37000

Date of Pilot Lot Testing

38000

Figure A4.1.

0) 34000 35000 36000 37000

Date of Pilot Lot Testing

38000

Figure A4.2.

40
Table A4.1.

LEVEL Avg Energy, J Energy_TK, J Energy_TO, J Energy_SI, J STDPOOL, J STDTK, J STDTO, J STDSI, J CV

HIGH 103.89 104.08 104.10 103.47 2.34 2.65 2,48 3,21 0.02 HIGH 102.70 103.25 102.39 102.44 2.76 3.64 2.52 3,32 0.03 HIGH 89.87 90.48 89.03 90.11 2.14 2.04 2.73 2,08 0.02

ni vi n 103.10 103.88 101.59 103.34 2.85 3.02 3.47 3,26 0.03

ni vin 101 .09 1 00.71 101.67 100.90 3.31 3.85 87 w, 1 w nw.wwm

ni on 102.29 102.84 102.31 101.71 2.55 2 67 W.WW W , 1 W nW.Wfc 02 n1 vj n 98 20 100.94 96.99 96.74 2.60 W.WW 2£.ww"^a 2 21 0U.wwO"? nivjn 1 00.97 99.35 99. 1 £ £.ww 2 d1 2 n 09

ni viri 99.76 100.90 99.57 98.81 2.73 3.56 2 &H 3.74 0 0"? W. Uw

niva n 99.84 100.77 100.17 98.55 3.07 3.61 W.W9 3.98 0.03 HIGH ill n 97.55 97.03 101.00 94.63 2.76 3.57 2.60 2.53 0.03 HIGH 97.48 98.42 97.88 96.13 3.04 4.03 2.81 3,58 0.03 HIGH 106.67 1 08.23 106.44 105.35 3.04 3.96 2.85 3,92 0.03 HIGH 104.05 104.77 104.72 102.64 2.60 3.30 2.52 3.01 0.02 HIGH 95.99 95.67 95.67 96.63 2.45 2.54 3.02 2.48 0.03 HIGH 99.34 99.23 99.97 98.70 4.98 5.63 6.03 6.18 0.05 HIGH 102.00 104.32 102.30 99.38 6.02 6,93 7.27 7,86 0.06 HIGH 99.78 100.58 98.70 100.41 2.99 3.61 3.22 3,37 0.03

Hlf^H nivjri 1 09.01 1 1 0.23 108.78 1 08.03 3.41 3.46 4.41 3.45 0.03

ni vjn 91.76 92.50 91.32 91.46 2.28 3.14 1 .54 3,34 0.02

niwiii 96.07 95.21 97.47 95.53 2.78 3,26 3.00 3,53 0.03

ni wn 89 66 90.29 89.69 89.00 2.43 3,49 2£.&w25 W ,*T9 0 0"? W , WW

nivjin 93.14 93.51 94.96 90.96 3.19 3,86 3w.ww50 3,30 0 0'? W . WW HIGH 81.21 81.64 80.45 81.46 1.89 2.21 1 .82 2.46 0.02 HIGH 80.05 78.69 80.02 81.95 3.40 3.83 4.02 3,72 0,04

HIGH 97.21 96.71 98.90 95.91 2.96 2.73 3.97 3.06 0,03

HIGH 99.26 99.98 98.46 99.34 1,89 2,19 1.86 2.43 0.02 HIGH 88.13 88.69 88.20 87.54 2.65 3.53 2.49 2.31 0,03

HIGH 109.52 112.37 108.87 107.38 3.25 4.27 3.12 3,80 0,03

HIGH 1 03.75 105.69 102.52 103.04 3.43 3.39 4.48 3.76 0.03 HIGH 85.34 85.62 84.32 86.09 2.88 2,98 3w.ww63 2.88 0 0"? W . WW

ni van 939W.O611 9W.W& 929£. 171 / 2 22 2 T1 2£.wwfsd £.wO 0 02 HIGH 85 82 85ws.**w 46 87O / . 171 / S40*t. 949*T 2 nfi 11 , 72/ £ 2 7ft / w 9£.wT11 0 02 HIGH 9999.86WW 100 1 WW. 121 £ 9999. 48tw 99.9Q71 2 181 w 2 54 &.WW 2 "^n 2 0 09 HIGH 87.86 87.10 88.61 87.84 2.52 3.29 fc.*fw 2 45 2.30 0 0"? w. Ww HIGH 83.65 83.73 83.94 83.19 1.53 1.68 1 .64 1 ,48 0w. 09L/£ HIGH 98.57 98.34 99.00 98,37 2.17 2.36 2.39 2,87 0.02 HIGH 88.80 88.70 89.59 88.31 2.40 2.55 2,91 2,33 0.03 HIGH 101.96 103.51 101.71 100,75 2.94 2,95 3.62 4.22 0,03

HIGH 102.50 102.79 105.20 99.56 3.05 3,28 3.70 3.37 0.03 HIGH 106.97 106.42 102.93 100,84 3.37 3,69 4.18 3,09 0.03 HIGH 105.89 108.43 104.24 105.20 3.36 4.25 3.58 3.00 0,03

HIGH 104.40 106.03 104.75 102.47 2.71 2,96 3.29 2.46 0.03 HIGH 90.57 90.71 91.07 90.11 2.68 3.52 2.58 2.55 0.03 LOW 16.19 15.19 16.68 16.73 0.85 0.87 0.85 0,70 0.05 LOW 17.56 16.94 17.98 17.76 0.78 0.95 0.61 0.57 0.04 LOW 17.59 17.28 17.91 17.56 0.70 0.83 0.61 0.43 0.04 LOW 16.97 16.29 17.36 17.25 0.75 0,65 0,76 0.68 0,04

LOW 17.26 16.52 18.08 17.16 0.77 0.83 0,77 0,51 0.04 LOW 16.71 15.68 17.49 16.93 0,64 0,58 0,33 0,78 0,04

LOW 16.79 16.01 17.32 17.03 0.71 0,69 0,47 0.80 0.04 LOW 17.46 16.30 18.17 17.93 0.87 0,96 0.67 0.90 0.05 LOW 15.77 14.52 17.22 16,41 0.84 1,08 0.60 0.60 0.05 LOW 18.07 17.08 18.30 18.85 0.93 0,84 0.93 1.03 0.05 LOW 18.92 17.35 19.89 19.52 1.32 1.19 1.59 1.29 0.07 LOW 18.70 17.79 19.03 19.14 0.76 0.56 0.74 0.85 0.04 LOW 18 12 17.39 18.41 18.62 0.63 0.55 0.54 0.59 0.03 LOWW TV 17 24 16.14 17.98 17.59 0.92 0.91 0.97 0.79 0.05 LOW 17 57 16.73 18.17 17.83 0.83 0.72 0.88 0.76 0.05 LOW 15.07 14.19 15.66 15.34 1.01 1.04 1.06 0.83 0.07 LOW 16.49 15.71 17.21 16.54 0.85 0.94 0.76 0.70 0.05 LOW 16.58 15.81 17.42 16.51 0.73 0.58 0.69 0.78 0.04 LOW 15.92 15.15 16.54 15.03 0.57 0.54 0.77 0.47 0.04 LOW 16.09 14.95 16.97 16.35 0.96 0.79 1.20 0.73 0.06 LOW 16.95 16.27 17.26 17.31 0.75 0.53 0.86 0.64 0.04

41
LOW 15.68 14.20 16.32 16.51 0.91 0,.84 0.93 0.91 0..06 LOW 15.81 14.63 16.82 15.97 0.83 0..84 0.81 0.73 0..05 LOW 16.51 15.45 16.92 17.15 0.65 0..60 0.61 0.53 0..04 LOW 18.44 17.55 19.13 18.75 0.95 1,.00 0.75 1.11 0..05 LOW 14.91 13.32 15.90 15.69 0.60 0,.58 0.58 0.42 0..04 LOW 16.68 15.80 17.26 16.96 0.70 0..59 0.68 0.65 0..04 LOW 16.13 15.03 16.71 16.60 1.03 0.91 1.16 0.99 0 .06 LOW 15.34 14.23 15.90 15.90 0.57 0..65 0.46 0.36 0..04 LOW 16.41 15.57 17.12 16.49 0.71 0..68 0.68 0.60 0..04 LOW 15.59 14.68 15.80 16.21 0.66 0..73 0.39 0.60 0..04 LOW 16.13 15.37 16.88 16.29 0.67 0..76 0.46 0.57 0..04 LOW 15.05 15.31 16.75 16.47 0.81 0..72 0.89 0.67 0..05 LOW 15.46 14.29 16.28 15.91 0.59 0.56 0.56 0.42 0 .04 LOW 16.68 15.61 17.64 16.91 0.87 0.89 0.90 0.65 0 .05 LOW 17.35 16.26 18.26 17.75 0.74 0.71 0.70 0.66 0..04 LOW 16.00 15.05 16.50 16.31 0.69 0.57 0.65 0.66 0,.04 LOW 18.63 17.50 19.63 18.76 1.03 1,.12 1.04 0.86 0,.06 LOW 16.27 15.22 17.03 16.75 0.70 0..54 0.67 0.74 0..04 LOW 16.26 15.08 16.83 16.57 0.66 0..59 0.65 0.52 0,.04 LOW 14.10 13.30 14.36 14.75 0.71 0 .48 0.76 0.69 0..05 SUPERHI 222.52 223.38 219.04 225.14 6.98 8,.69 7.93 7.76 0..03 SUPERHI 224.82 225.40 222.86 226.21 4.38 4..65 5.39 6.71 0.,02 SUPERHI 230.78 228.58 229.89 233.86 4.02 5,.22 3.85 6.48 0.,02 SUPERHI 245.98 243.49 245.69 248.65 6.51 8..72 6.43 9.57 0,,03 SUPERHI 227.09 224.65 227.88 228.59 6.45 9,.35 5.46 7.71 0.,03 SUPERHI 237.45 236.42 235.43 240.27 6.52 8,.48 6.95 7.26 0,,03 SUPERHI 245.45 245.30 244.25 246.91 5.98 8..08 5.82 8.21 0.,02 SUPERHI 260.10 261.00 260.09 252.88 5.21 5,.89 6.28 7.32 0,,02 SUPERHI 209.92 203.05 219.72 206.87 6.89 8.,86 7.58 6.42 0.,03 SUPERHI 258.02 260.09 261.00 252.88 5.21 5,.89 6.28 7.32 0,,02 SUPERHI 209.88 203.05 219.72 206.87 6.89 8,,86 7.58 6.42 0.,03 SUPERHI 220.90 222.49 222.78 217.42 6.88 6.,32 9.57 10.56 0.,03 SUPERHI 227.29 226.12 225.68 230.07 7.27 8..64 8.71 8.10 0,,03 SUPERHI 222.76 222.22 224.25 221.54 6.24 7,.62 7.06 8.74 0,,03 SUPERHI 222.76 222.22 224.25 221.54 6.24 7,.62 7.06 8.74 0,,03 SUPERHI 214.08 210.20 210.51 221.83 5.51 7,.39 5.51 6.31 0,,03 SUPERHI 205.95 206.54 200.70 210.49 5.35 6..61 6.10 5.14 0.,03 SUPERHI 238.49 239.66 234.46 241.24 6.04 6,.87 7.41 7.37 0.,03 SUPERHI 200.79 200.19 199.41 202.69 6.23 8..37 6.26 7.21 0.,03 SUPERHI 219.91 224.95 222.24 212.44 5.35 6,.62 6.00 6.18 0,,02 SUPERHI 220.86 221.38 220.89 220.15 6.52 8..70 6.78 5.86 0.,03 SUPERHI 219.54 220.13 224.00 214.54 6.28 7,.71 7.22 6.64 0.,03

42
T. A. Siewert/ M. P. Manahan,^ C. N. McCowan,^ J. M. Holt,^ F. J. Marsh,^ and E. A. Ruth^

The History and Importance of Impact Testing*

Reference: Siewert, T. A., Manahan, M. P., McCowan, C. N., Holt, J. M., Marsh, F. J. and

Ruth, E. A., "The History and Importance of Impact Testing," Pendulum Impact Testing: A

Century ofProgress, STP 1380, T. A. Siewert and M. P. Manahan, Sr., Eds., American Society

for Testing and Materials, West Conshohocken, PA, 2000, pp. 3-16.

Abstract: Charpy impact testing is a low-cost and reliable test method which is commonly

required by the construction codes for fracture-critical structures such as bridges and pressure

vessels. Yet, it took from about 1900 to 1960 for impact-test technology and procedures to reach

levels of accuracy and reproducibility such that the procedures could be broadly applied as standard test methods. This paper recounts the early history of the impact test and reports some

of the improvements in the procedures (standard specimen shape, introduction of a notch,

correlation to structural performance in service, and introduction of shrouds) that led to this

broad acceptance.

Keywords: absorbed energy; Charpy impact testing; impact testing; pendulum impact

Without uniformity of test results from day to day and from laboratory to laboratory, the

impact test has little meaning. Over the years, researchers have learned that the results obtained

from an impact test can depend strongly upon the specimen size and the geometry of the notch,

* Contribution of NIST; not subject to copyright.

^ Supervisory Metallurgist, Materials Reliability Division, National Institute of

Standards and Technology, Boulder, CO.

^ President, MPM Technologies, Inc., 2161 Sandy Drive, State College, PA.

^ Materials Research Engr., Materials Reliability Division, NIST, Boulder, CO.

Alpha Consultants & Engineers, Pittsburgh, PA.

^ Retired (Bethlehem Steel), San Marcos, CA.

^ Tinius Olsen Test Machine Co., Willow Grove, PA.

Further details on the economic impact of Charpy impact testing are included in a

previous version of this report published in Standardization News, February 1999.

43
anvils, and striker. To a lesser degree, impact test results also depend upon other variables such

as impact velocity, energy lost to the test machine, and friction. The goal of those who have

written and modified ASTM Standard Test Methods for Notched Bar Impact Testing of Metallic

Materials (E 23) has over the years been to standardize and control the variables associated with

impact testing. This report looks at the history of impact testing, with emphasis on the key

advances in understanding and application of the impact test, as reflected in the evolution of the

test standard.

Impact Testing: 1824 to 1895

The earliest publication we could find on the effects of impact loading on materials was a

theoretical discussion by Tredgold in 1824 on the ability of cast iron to resist impulsive forces

[1]. In 1849, the British formed a commission to study the use of iron in the railroad industry,

which began by considering practical approaches to impact testing [2]. Apparently, failures of

structures in the field were leading some researchers to speculate that impact loads affected

materials far differently than static loads, so tensile-strength data (from slowly applied loads) was ^ a poor predictor of performance under dynamic loads.

In 1857, Rodman devised a drop-weight machine for characterization of gun steels, and over

the subsequent 30-year period, his machine was widely used to test railroad steels and for

qualification of steel products [2]. Many of the early experiments with impact tests were

performed on final product forms, such as pipes or axles. Thus they served as proof tests for a batch of material, or yielded comparative data for a new product design, or basic reference data on the impact resistance of different construction materials (such as the comparison of wrought

iron to ductile iron). Instrumentation was poor for the early impact tests, so the data is often only

as break or no-break for a mass dropped through a certain distance. These early drop weight tests were conducted using smooth (no notch or crack starter) rectangular bars. While the test worked

well for brittle materials, where crack initiation is easy, specimens of ductile materials often just

bent. LeChatalier introduced the use of notched specimens while conducting drop-weight tests in

1892 [3]. He found that some steels that showed ductile behavior (bending without fracture) in a smooth rectangular bar, would exhibit fragile behavior when the test specimen was notched.

While the addition of a notch was a major improvement in the test method, a test procedure was

needed that would provide a continuous, quantitative measure of the fracture resistance of

materials. Also, substantial work was needed to develop test procedures that produced consistent

data, and to answer the objections of those who doubted the value of impact testing.

1895 to 1922

This period saw the establishment of a number of national and international standards bodies,

which took up the causes of developing robust test procedures and developing consensus

standards for many technologies, including impact testing. One of these standards bodies was The American Society for Testing and Materials, established in 1898. Another was the

International Association for Testing Materials, officially established in 1901, but this association

grew out of the good response to two previous International Congresses that had been held a number of years before. These two standards bodies seem to have had a good working

44
relationship, and the President of ASTM, Prof. H. M. Howe, also served on the Board of lATM

during this time [4].

In 1902, only four years after the founding of ASTM, the ASTM "Committee on the Present

State of Knowledge Concerning Impact Tests" published a bibliography on impact tests and

impact testing machines in the second volume of the Proceedings of ASTM [5]. This

bibliography listed more than 100 contemporary papers on impact testing published in the U.S.,

France, and Germany. Many of these papers contained information that was also known to the

members of lATM. In fact, some of the papers had been presented and discussed at the lATM

Congresses.

Among the references is a report by Russell (published in 1898) that shows remarkable insight

into the needs of the design engineers of the time and introduces quantitative measurement to the

test [6]. He pointed out that none of the machines of the time, typically of a drop-weight design,

had the ability to determine any data beyond whether the specimen broke or remained intact. Therefore, he designed and built a pendulum machine which "would measure the energy actually

absorbed in breaking the test bar". His report shows a test machine that is based on the same

swinging pendulum concept as those in common use today and mentions his careful analysis of

the mechanics of the test, including corrections for friction losses and calculation and comparison

of the centers of gravity and percussion. Since this was before the time of compact, standardized

test specimens, the machine was vary large and massive, and was capable of breaking many full- size products. Besides showing a prototype of the machines used today, this report is valuable in

that it includes data on over 700 tests of typical construction materials, and emphasizes the effect

of the rate of loading in evaluating materials for different service conditions. Russell's pendulum

impact machine finally provided a means for quantifying the energy absorbed in fracturing a test specimen for a wide range of materials and conditions. His paper nicely summarizes the test- machine technology and knowledge for material performance at the end of the past century, and

so served as a benchmark for future research. To the best of our knowledge, Russell was the first to develop and demonstrate the advantages of the pendulum design for impact testing machines.

The members of LATM Commission 22 (On Uniform Methods of Testing Materials)

continued to conduct research that addressed the shortcomings in the impact testing techniques,

until they had developed a knowledge of most of the important factors in the test procedure.

Even though many of these early machines and reports are simplistic by today's standards, they

provided previously unknown data on the impact behavior of materials. France seems to have

been an early adopter of impact testing for infrastructure construction standards, and so French

researchers provided much data on the effects of procedure variables and were the most prolific

contributors to the LATM Proceedings between 1901 and 1912. Incidentally, it was a

representative from France, G. Charpy, who became the chair of the impact testing activity after

the 1906 LATM Congress in Brussels, and presided over some very lively discussions on whether

impact testing procedures would ever be sufficiently reproducible to serve as a standard test method [7]. Charpy' s name seems to have become associated with the test because of his

dynamic efforts to improve and standardize it, both through his role as Chairman of the lATM

Commission and through his personal research [8]. He seems to have had a real skill for

recognizing and combining key advances (both his and those of other researchers) into

45
continually better machine designs and consensus procedures. For example, Charpy

acknowledges the benefits of Russell's pendulum design in his 1901 paper [8] by stating:

"Russell described in a paper presented in 1897 at the American Society of Civil Engineers some

'experiments with a new machine for testing materials by impact.' The machine he is using is designed to determine the work absorbed by the rupture of a bar, for this, the ram used appears in

the form of a pendulum arranged in such a way so that when it is released from its equilibrium

position, it meets the test bar in passing through the vertical position, breaks it and afterward

rises freely under the influence of the acquired speed. The difference between the starting height

and the finishing height of the pendulum allows evaluation of the work absorbed by the rupture

of the bar."

By 1905, Charpy had proposed a machine design that is remarkably similar to present designs

and the literature contains the first references to "the Charpy test" and "the Charpy method". He

continued to guide this work until at least 1914 [7,9-70]. A number of other standard machine

designs and procedures were also under consideration at this time, and in 1907 the German

Association for Testing Materials adopted one developed by Ehrensberger [70]. Because the

pendulum machine had not achieved dominance yet, impact machine designers and

manufacturers offered three major types; Drop Weight (Fremont, Hatt-Tumer, and Olsen),

Pendulum Impact (Amsler, Charpy, Dow, Izod, Olsen, and Russell), and Flywheel (Guillery).

This was a period during which the configuration and size of specimens closely approached

what we use today [7]. Originally, two standard specimen sizes were most popular. The smaller

had a cross section of 10 by 10 mm, a length of about 53 mm (for a distance of 40 mm between

the points of support), a notch 2 to 5 mm deep, and a notch tip radius near 1 mm. The larger and

initially more popular of these specimen sizes was scaled up by a factor of three in all these

dimensions. The group favoring the larger specimen pointed out the advantage of sampling a

larger cross section of the material (for reduced scatter in the data) and the difficulty of producing

the small notch radius on the smaller specimen. However, the group favoring the smaller

specimen eventually won because a more compact and lower-cost machine could be used, and

not all structures were thick enough to produce the larger specimen. Besides specimen

dimensions that are very similar to what we use today, the Commission proposed features for a standard impact procedure that included:

limits for the velocity of the striker,

rigid mounting to minimize vibration losses, a minimum ratio of anvil mass and rigidity to striker size, and

recognition of the artificial increase in energy as ductile specimens deform around the

edges of a wide striker [7].

One report at the 1912 meeting [7] included the testimonial from a steel producer of how the

improved impact test procedures had allowed them to tailor the refining processes to produce less

brittle steel. The report describes a reduction by a factor of 20 in the number of production parts

that were rejected for brittle performance.

46
1922 to 1933: The Beginning of ASTM Method E 23

ASTM Committee E-1 on Methods for Testing sponsored a Symposium in 1922 on Impact

Testing of Materials as a part of the 25th Annual Meeting of the Society, in Atlantic City, New

Jersey. The Symposium included a history of the developments in this area, a review of work

done by the British Engineering Standards Association, several technical presentations, and the

results of a survey sent to 64 U.S. testing laboratories [77]. Twenty-three respondents to the

survey offered detailed information on topics such as the types of machines in use, the specimen

dimensions, and procedures. In addition, many responded positively to a question about their

willingness to develop an ASTM standard for impact testing.

Based on the information in this survey, an ASTM subcommittee began to prepare a standard

test method for pendulum impact testing in 1923. This effort took until 1933, when ASTM

published "Tentative Methods of Impact Testing of Metallic Materials," ASTM designation E

23-33T. (An ASTM specification of "Tentative" indicated that it was subject to annual review

and was a work in progress. The tentative designation is no longer used by ASTM.) (Other

countries also developed their own standards; however, we found it difficult to find their records

and to track their developments.)

ASTM E 23-33T specified that a pendulum-type machine was to be used in testing and

"recognized two methods of holding and striking the specimen", that is, the Charpy test and the

Izod test (where the specimen is held vertically by a clamp at one end). It did not specify the

geometry of the striking edge (also known at the time as the "tup") for either test. It stated that

"the Charpy type test may be made on unnotched specimens if indicated by the characteristics of

the material being tested, but the Izod type test is not suitable for other than notched specimens".

Only a V-notch was shown for the Charpy test. Although the dimensions for both types of

specimens were identical with those currently specified, many tolerances were more restrictive.

The units were shown as English preferred, metric optional. The committee pointed out many

details that influence the test results, but because they did not have the knowledge and database

needed to specify values and/or tolerances for these details, the document was issued as a

tentative. The original document contains an appendix with general discussions of applications,

the relation to service conditions, and comparisons between materials. As our understanding of

the variables in Charpy testing has grown, ASTM E 23 has been revised repeatedly to incorporate

the new knowledge.

1934 to 1940

The first revision of E 23 was issued in 1934 and it added a dimension for the radii of the

anvil and specifically stated that "these specimens (both the Charpy and the Izod) are not considered suitable for tests of cast iron" referencing a report of ASTM Committee A3 on

Cast Iron. The method retained the "tentative" designation.

The geometry of the Charpy striking tup, specifically the radius of the tup that contacted the

specimen, was not specified in the 1934 revision. However, the minutes of the 1939 and 1940

meetings for the Impact Subcommittee of El state that this item was discussed and a survey was

47
made of the geometries used in the United Kingdom and in France. Those countries had been

using radii of 0.57 mm and 2 mm, respectively. For reasons that were not recorded, the members

of the Subcommittee agreed to a radius of 8 mm at the 1940 meeting and ASTM E 23 was

revised and reissued as E 23-4 IT. Two other changes that occurred with this revision were that

metric units became the preferred units, and keyhole and U notches were added for Charpy-test

specimens.

1940 to 1948

Impact testing seems to have been a useful technique for evaluating materials, but was not a common requirement in purchase specifications and construction standards until the recognition

of its ability to detect the ductile-to-brittle transition in steel. Probably the greatest single

impetus toward implementation of impact testing in fabrication standards and material

specifications came as a result of the large number of ship failures that occurred during World

War n. These problems were so severe that the Secretary of the U.S. Navy convened a Board of

Investigation to determine the causes and to make recommendations to correct them. The final

report of this Board stated that of 4694 welded-steel merchant ships studied from February 1942

to March 1946, 970 (over 20%) suffered some fractures that required repairs [12]. The

magnitudes of the fractures ranged from minor fractures that could be repaired during the next

stop in port, to 8 fractures that were sufficiently severe to force abandonment of these ships at sea. Remedies included changes to the design, changes in the fabrication procedures and

retrofits, as well as impact requirements on the materials of construction. The time pressures of

the war effort did not permit thorough documentation of the effect of these remedies in technical

reports at that time; however, assurance that these remedies were successful is documented by

the record of ship fractures that showed a consistent reduction in fracture events from over 1 30

per month in March 1944 to less than five per month in March 1946, even though the total number of these ships in the fleet increased from 2600 to 4400 during this same period [72].

After the war, the National Bureau of Standards released its report on an investigation of

fractured plates removed from some of the ships that exhibited these structural failures and so provided the documentation of the importance of impact testing [13]. The NBS study included

chemical analysis, tensile tests, microscopic examination, Charpy impact tests, and reduction in

thickness at the actual ship fracture plane. A notable conclusion of the report was that the plates

in which the fracture arrested had consistently higher impact energies and lower transition

temperatures than those in which the fractures originated. This was particularly important

because there was no similar correlation with chemical composition, static tensile properties (all

steels met the ABS strength requirements), or microstructure. In addition, the report established

15 ft-lb (often rounded to 20 J for metric requirements) as a minimum toughness requirement,

and recommended that "some criterion of notch sensitivity should be included in the

specification requirements for the procurement of steels for use where structural notches,

restraint, low temperatures, or shock loading might be involved," leading to a much wider

inclusion of Charpy requirements in structural standards.

48
1948 to Present

By 1948, many users thought that the scatter in

the test results between individual machines

could be reduced further, so additional work

was started to more carefully specify the test method and the primary test parameters. By

1964, when the ASTM E 23 standard was

revised to require indirect verification testing,

the primary variables responsible for scatter in

the test were well known. In a 1961 paper,

Fahey [14] summarized the most significant causes of erroneous impact values as follows:

(1) improper installation of the machine, (2)

incorrect dimensions of the anvil supports and

striking edge, (3) excessive friction in moving

parts, (4) looseness of mating parts, (5)

insufficient clearance between the ends of the

test specimen and the side supports, (6) poorly

machined test specimens, and (7) improper

cooling and testing techniques. While the

machine tolerances and test techniques in ASTM

E 23 addressed these variables, it was becoming

apparent that the only sure method

of determining the performance of a Charpy

impact machine was to test it with standardized

specimens (verification specimens).

Much of the work that showed impact tests did not have inherently high scatter, and could

be used for acceptance testing, was done by

Driscoll at the Watertown Arsenal [15].

Driscoll's study set the limits of 1 ft-lb (1.4 J)

and ±5%, shown in Figures 1 and 2. The

data superimposed on these limits in Fig- ures 1 and 2 are the initial verification results

gathered by Driscoll for industrial impact

machines to evaluate his choice of verification

limits.

40 60 80

Energy, J

Figure 1. The deviation and energy values obtained for the first round of tests on industrial machines. The

deviation is calculated as the difference between the

results of the Watertown Arsenal machines and the

industrial machines. These data were originally published

by D.E. Driscoll, Reproducibility of Charpy Impact Test, ASTM STP 176, 1955.

40 60 80

Energy, J 120

Figure 2. The deviation and energy values for the second

and third rounds of tests on industrial machines. The data show that all but two of the machines tested were able to pass the 1.4 J or 5% criteria after appropriate repairs were made. These data were originally published by D. E

Driscoll, ReproducibiUty of Charpy Impact Test, ASTM

STP 176, 1955.

In Figure 1, the verification results for the first attempt on each machine are shown: only one machine fell within the ± 1 ft-lb (1.4 J) limit proposed for the lower energy range. Results for

retests on the same machines after maintenance are shown in Figure 2. Driscoll's work showed

the materials testing community that not all machines in service could perform well enough to meet the indirect verification requirements, but that most impact machines could meet the

proposed requirements if the test was conducted carefully and the machine was in good working

49
condition. With the adoption of verification testing, it could no longer be convincingly argued that the impact

test had too much inherent scatter to be used as an acceptance test.

Early results of verification testing showed that 44%

of the machines tested for the first time failed to meet

the prescribed limits, and it was thought that as many as 50% of all the machines in use might fail [76].

However, the early testing also showed that the failure

rate for impact machines would drop quickly as good

machines were repaired, bad machines were retired,

and more attention was paid to testing procedures. It

600

5 -4 •3 -2 -1 0 1 2 3 4 5 6 Difference, J

Figure 3. Distribution of low-energy

verification data. Data for 1995-1997.

Approximately 2400 tests; each test is an average

400

^ , ^, ^ . , , rvr»/w r.uL L- for five specimens. The vertical lines at ±1.4 J was estimated that approximately 90% of the machines

^^p^.^^^^^^he acceptance criteria, in use could meet the prescribed limits of ± 1 ft-lb (1.4 J) or ± 5%. Recently acquired verification specimen data, shown in Figures 3 through 5, confirm

these predictions. Failure rates for verification tests at low, high, and super-high energy ranges are currently

estimated to be 12, 7, and 10%, respectively [17].

Overall, the incorporation of verification limits in

ASTM E 23 has greatly improved the performance of

impact machines, so that data collected using ASTM

E 23 machines can be compared with confidence.

ASTM E 23 is still the only standard in the world, to our knowledge, that requires very-low-energy impact Figure 4. Distribution of high-energy

specimens (between 15 and 20 J) for verification, and verification data. Data for 1995-1997.

-10 0

Difference, %

as shown by the data in Figure 1, results obtained

using machines in need of maintenance can vary by

more than 100% at this energy level. In effect, the

limits imposed by ASTM E 23 have produced a population of impact machines that are arguably the

best impact machines for acceptance testing in the

world.

While ASTM E 23 is used around the world,

there are other forums for the development of global

standards. One of these, the International Organization

for Standardization, ISO, allows qualified representatives from all over the world to come together as equal

partners in the resolution of global standardization

problems [18]. ISO Committee TC 164 handles the

topic of Mechanical Testing, and its Subcommittee

SC 4 handles toughness testing. While this

Approximately 2400 tests. Each test is an

average for five specimens. The vertical lines at ±5% represent the acceptance criteria

150

100-

- 0.2

•ao

3.

- 0.1 -D

-10 0

Difference, %

0.0

Figure 5. Distribution of the super-high-energy

verification data. Data for 1995-1997.

Approximately 650 tests. Each test is an average

for five specimens. The verfical lines at ±5%

represent the acceptance criteria.

50
subcommittee has developed and maintains ten standards on toughness testing, perhaps the most

pertinent is ISO Standard R 442: 1965 Memllic Materials - Impact Testing - Verification of

Pendulum Impact Machines. This standard covers the Charpy test and is presently undergoing

balloting for revision. An important feature of this document is that it recognizes Charp\ testing

with both the 2-mm and 8-mm radius striker. There are other regional and national standards

that specify" impact testing procedtires, such as the Japanese standard, JIS Z2242, Methodfor

Impact Test for Metallic Materials.

Typical Applications Today

Nuclear

Since it is impractical to measure the fracture toughness of large specimens throughout the life of a nuclear power plant. sup."eillance programs use Charpy and tensile specimens to track the

embrirtlement induced by neutrons. The economic imponance of the Charpy impact test in the

nuclear industry can be estimated b>' noting that most utilities assess the outage cost and loss of revenue for a nuclear plant to be in the range of SSOO.OOO to S5O0.0O0 per da\'. If Charp\- data can be used to extend the life of a plant one year beyond the initial design life, a plant owner

could realize revenues as large as 5150.000,000. Further, the cost avoidance from a vessel

related fracture is expected to be in the billion-dollar range, To date, the XRC has shut down one

U.S. plant as a result of Charpy data trends. It is important to note that this plant's pressure

vessel was constructed from a one-of-a-kind steel and is not representative of the U.S. reactor

fleet.

Nonetheless, with decisions like this based on the Charp)' test, the importance of ASTM E 23 and

the restraints it applies cannot be o\-eremphasized.

Steel

The Charpy \'-notch i CX'N i test specimen and associated test procedure is an effecti\"e cost- saving tool for the steel mdustr.'. The specimen is relativeh' easy to prepare, many specimens

can be prepared at one time, various specimen orientations can be tested, and relati\"ely lowcost equipment is used to test the specimen. In many strucmral steel applications, the CVN test can be used: (1) as a qualir\" control tool to compare different heats of the same type of steel, (1) to check conformance with impact requirements m standards, and ( 3 i to predict ser.'ice

performance of components. .AJso. C\'N test information can be correlated with fracture

toughiness data for a class of steels so that the results of fracture-mechanics analyses can be

compared with the material toughness.

CA'N data have many uses, such as during the design and construction of a bridge or an

offshore oil platform. Before full-scale production of the steel order can begin, the suppKer

needs to demonstrate to the buyer that the steel plate is capable of meeting certain design criteria. The process begins by making the steel grade and then testing a ponion of the plate to determine

if all required criteria are met. Also, steel mill equipment imposes limitations on plate size:

therefore, individual steel plates need to be welded together in the field to produce lengths which

51
can reach deep into ocean waters. Small sections of the sample plate are welded together, and

fracture mechanics tests are conducted to determine the crack tip opening displacement (CTOD)

toughness in the heat affected zone (HAZ) and in areas along the fusion line where the weld

metal meets the base metal. Then, a steel supplier might correlate the CTOD test results with

CVN 50% ductile-brittle transition temperature (DBTT). By agreement between the customer

and supplier, this correlation can allow the steel supplier to use the Charpy test instead of the

more expensive and time-consuming CTOD testing.

Continuing Standardization Efforts

Even after 100 years, the Charpy impact test procedures still have room for improvement.

The ASTM E 23 standard has recently been redrafted to provide better organization and to include new methods such as in-situ heating and cooling of the test specimens. Two new related

standards are also under development through ASTM Task Group E 28.07.08, "Miniature and

Instrumented Notched Bar Testing", which was formed a little more than two years ago. The

first standard covers miniature notched bar impact testing and relies on many of the existing

practices related to test machine requirements and verification as specified in existing standard E

23. The second standard is focused on instrumented testing, where strain gages attached to the

striker provide a force-deflection curve of the fracture process for each specimen. Research is focused on using these data to obtain plane strain fracture toughness as well as other key test parameters. Upon acceptance of the standard by ASTM, both the existing E 23 standard and the

new miniature notched bar standards would reference the instrumented impact standard.

The state of the art in impact testing continues to advance in other parts of the world also.

ISO is balloting a standard (14556) on instrumented impact testing, there is work in Europe on

miniature Charpy specimens, and ESIS is investigating the use of pre-cracked Charpy specimens

for determining fracture toughness. It can be expected that harmonization efforts will bring some

of this work into E 23 in the future.

Conclusion

Several years ago, at the ASTM Symposium on "The Charpy Impact Test: Factors and

Variables" [19], a bystander was overheard to say: "I see that there is a Symposium on the

Charpy Test; what can be new there?" Since then, the document has been updated twice and is currently being revised to reflect new developments and to make it more "user friendly."

Although ASTM E 23 has been a useful standard for many years, it continues to be a "work in

progress," a work used extensively to help evaluate existing and new materials for products and

structures ~ a test to ensure safety as well as to reduce the initial and lifetime costs for structures. The ASTM E 23 standard is a document that continues to improve as our technical knowledge

increases. Knowledge which will help make the test more accurate and reliable is continually

being gained. New technologies such as miniaturization of the test, instrumenting the striker to obtain additional data, and developing mechanics models to enable extraction of plane strain

fracture toughness will be areas of development over the next 100 years. We anticipate that the

benefits from the application of E 23 during the next 100 years will overshadow the benefits

from those in the past 100 years.

52
References

[I] Tredgold, T., Strength of Cast Iron, 1 824, pp. 245-268.

[2] White, A.E. and Clark, C.L., Bibliography ofImpact Testing, Department of Engineering

Research, University of Michigan, 1925.

[3] LeChatalier, A., "On the Fragility After Immersion in a Cold Fluid", French Testing

Commission, Volume 3, 1892.

[4] "Report on the Work of the Council from the Budapest to the Brussels Congress - 1901-

1906," Proceedings of the International Association for Testing Materials, Brussels

Congress, 1906.

[5] Hatt, W.K. and Marburg, E., "Bibliography on Impact Tests and Impact Testing

Machines," Proceedings ASTM, Vol. 2, 1902, p. 283.

[6] Russell, S. B. "Experiments with a New Machine for Testing Materials by Impact,"

Transactions ASCE, Vol. 39, June 1898, p. 237.

[7] Proceedings of the Sixth Congress of the International Association for Testing Materials,

New York, 1912.

[8] Charpy, M.G., "Note sur I'Essai des Metaux a la Flexion par Choc de Barreaux Entailles,

Soc. Ing. Francais, June 1901, p. 848.

[9] "Impact Testing of Notched Bars," The Engineer, Vol. 99, March 10, 1905 pp. 249-250.

[10] Whittemore, H. L., "Resume of Impact Testing of Materials, with Bibliography,"

Proceedings ASTM, Vol. 22, Part 2, 1922, p. 7. [II] Warwick, C. L., "Resume on Notched Bar Tests of Metals," Proceedings ofASTM, Vol.

22, Part 2, 1922, p. 78.

[12] The Design and Methods of Construction of Welded Steel Merchant Vessels: Final

Report of a (U.S. Navy) Board of Investigation, Welding Journal, Vol. 26, No. 7, July

1947, p. 569.

[13] Williams, M. L. and Ellinger, G. A., Investigation ofFractured Steel Plates Removed

from Welded Ships, National Bureau of Standards Report, December 9, 1948.

[14] Fahey, N. H., "Effects of Variables in Charpy Impact Testing," Materials Research

Standards, Vol. 1, No. 11, Nov., 1961.

[15] Driscoll, D. E., "Reproducibility of Charpy Impact Test," Impact Testing, ASTM STP 176,

1955.

[16] Fahey, N. H., "The Charpy Impact Test - Its Accuracy and Factors Affecting Test

Results," Impact Testing ofMetals, ASTM STP 466, ASTM, 1970.

[17] McCowan, C. N., Wang, C. M., and Vigliotti, D. P., "Summary of Charpy Impact

Verification Data: 1994 - 1996," Submitted to the Journal of Testing and Evaluation,

1998.

[18] More information is on the ISO World Wide Web site, at http://www.iso.ch

[19] Charpy Impact Test: Factors and Variables, ASTM STP 1072, J. M. Holt, Ed., ASTM,

1990.

53

M.P. Manahan, C.N. McCowan, T.A. Siewert, J.M. Holt, F.J. Marsh, and E.A. Ruth

For 100 Years, Notched Bar Impact Testing Standards Have

Yielded Widespread Benefits for Industry

ASTM Standardization News, 27(2), February 1999, pp.30-35.

Without standardization, the impact test has little meaning. The test result obtained from

an impact test is dependent upon the specimen size, notch geometry, and the geometry of the

anvils and striker. To a lesser degree, impact test results are also dependent upon other variables

such as impact velocity, energy lost to the test machine, and friction. The goal of those who have

written and modified ASTM E 23, Standard, Test Methods for Notched Bar Impact Testing of

Metallic Materials, has been, and is, to standardize and control the variables associated with

impact testing.

ASTM E 23 describes notched-bar impact testing of metallic materials by both the

Charpy (simple-beam) and the Izod (cantilever-beam) methods. While for some materials the

Izod method is used, the Charpy method is the overwhelming choice for metallic materials.

Because it is the method of choice, ASTM E 23 and this paper give more attention to the Charpy

method.

In the late 1800s and early 1900s, many different specimen and anvil geometries were

being used for Charpy testing. As a result it was difficult, if not impossible, to correlate the

results of various investigators, and the energy absorbed in fracturing a Charpy test specimen was

in question. In 1909, Revillon concluded that standardizing the specimen and anvil geometries was necessary to obtain reproducible results and thus overcome the opposition to impact tests.' The first version of ASTM E 23 accomplished that. Various manufacturers of impact machines

could now make machines that would produce test results similar to tests performed on their

competitors' machines. Further revisions to ASTM E 23 strived to make the impact test result a more accurate measure of the energy required to fracture the specimen. These changes included:

(1) modifications to the striker, (2) standardizing windage and friction losses and coirections,

(3) standardizing requirements for test machine foundations, (4) standardizing specimen supports

and shrouds to reduce "jamming" of the broken specimen, which falsely increases the measured

energy, (5) standardizing the impact velocity, and (6) requiring verification of sources of test errors.

As these changes were made to ASTM E 23, manufacturers of impact test machines and

users of the test methods were forced to change their designs and procedures in order to conform

to the standard. As a result, impact tests performed today, in accordance with ASTM E 23, are

reliable and reproducible throughout the world.

55
The Early Days of Impact Testing

The earliest known publication on the effects of impact loading on materials was a

theoretical discussion by Tredgold in the early 1800s on the ability of cast iron to resist

impulsive forces.^ In 1 849, the British formed a commission to study the use of iron in the

railroad industry, which began its work by considering practical approaches to impact testing.^ In

1857, Captain Rodman ^ devised a drop weight machine for characterization of gun steels, and

over the subsequent 30-year period, this machine was widely used in the testing of railroad steels

and for qualification of steel products. These early drop-weight tests were conducted using

smooth (no notch or crack starter) rectangular bars. While the test worked well for brittle

materials, where initiating a crack is easy, ductile materials would only bend and inducing

fracture was not possible. LeChatalier introduced the use of notched specimens while

conducting drop weight tests in 1892.^* It was discovered that some steels that showed ductile

behavior (bending without fracture) in a smooth rectangular bar, would exhibit fragile behavior

when the test specimen was notched. While the addition of a notch was a major improvement in

the test method, a test procedure was needed that would provide a continuous, quantitative measure of the fracture resistance of materials.

Pendulum Impact Testing

A hundred years ago, in 1898, a report by Russell showed a machine that is based on the

same swinging-pendulum concept as those in common use today. ^ His report included data on many construction materials, and emphasized the effect of the rate of loading in evaluating

materials for different service conditions. The pendulum impact machine of Russell finally

provided a means for measuring the energy absorbed in fracturing a test specimen for a wide

range of materials and conditions, from brittle at low test temperatures to ductile at high test temperatures.

Impact testing was an exciting and active research field near the turn of the century and a 1902 bibliography listed more than 100 contemporary papers on impact testing published in the

U.S., France, and Germany.^ Such studies were compared and discussed at the meetings of the

International Association for Testing Materials (before ASTM took up this topic) during the next

decade.^'^ During this period, the committee members conducted research that overcame the

shortcomings in the impact testing techniques, until they had developed robust and carefully

considered procedures that provided useful information for industrial users. Even though these

early standardized procedures were primitive by today's standards, they proved very satisfactory

in evaluating the impact behavior of materials. For example, these early reports record that the

test procedures were adopted by the French Navy for ship machinery, especially for engine

shafting. Incidentally, it was a representative from France, G. Charpy, who became the chair of

the impact testing activity after the 1906 lATM Congress in Brussels, and presided over some

very lively discussions on whether impact testing procedures would ever be sufficiently

reproducible to serve as a standard test method.^ Although not the inventor of the pendulum

impact test, Charpy' s name is associated with the test because of his efforts to improve and

standardize it.

56
Development of ASTM Method E23

In about 1923, an ASTM subcommittee began to prepare a standard test method for

pendulum impact testing. This effort took until 1933 when ASTM published "Tentative

Methods of Impact Testing of Metallic Materials, " ASTM designation E 23-33T. (An ASTM

specification of "Tentative" indicated that it was subject to annual review and was a work in

progress. The tentative designation is no longer used by ASTM.)

Method E23-33T specified that a pendulum-type machine was to be used in testing and

"recognized two methods of holding and striking the specimen," that is, the Charpy test and the

Izod test. It did not specify the geometry of the striking edge (also known as the "tup") for either

test. It stated that "the Charpy type test may be made on unnotched specimens if indicated by the

characteristics of the material being tested, but the Izod type test is not suitable for other than

notched specimens." Only a V-notch was shown for the Charpy test. Although the dimensions

for both types of specimens were identical with those currently specified, many tolerances were more restrictive. The units were shown as English preferred, metric optional. The authors

pointed out many details that influence the test results, but because they did not have the

knowledge to specify values and/or tolerances for these details, the document was issued as a

tentative one. The original document contains an appendix with general discussions of

applications, the relation to service conditions, and comparisons between materials. As our

understanding of the Charpy-test variables has grown, Method E23 has been revised to incorporate the new knowledge.

The first revision was issued in 1934 and added a dimension for the radii of the anvil and

specifically stated that "these specimens (both the Charpy and the Izod) are not considered

suitable for tests of cast iron" referencing a report of ASTM Committee A3 on Cast Iron. The

method retained the "tentative" designation.

The geometry of the Charpy striking tup, specifically the radius of the tup that contacted

the specimen, was not specified in the 1934 revision, but the minutes of the 1939 and 1940

meetings for the Impact Subcommittee El state that this item was discussed and a survey was

made of the geometries used in the United Kingdom and in France. Those countries used radii

of 0.57 mm and 2 mm, respectively. For reasons that were not recorded, the members of the

Subcommittee agreed to a radius of 8 mm at the 1940 meeting and ASTM E23 was revised and

reissued as E 23-4 IT. Two other changes that occurred with this revision were that metric units

became the preferred units and keyhole and U notches were added for Charpy-test specimens.

Impact testing seems to have been a useful material evaluation technique, but was not a common requirement in purchase specifications and construction standards until the recognition

of its ability to detect the ductile-to-brittle transition in steel. Probably the greatest single

impetus toward implementation of impact testing in fabrication standards and material

specifications came as a result of the large number of ship failures that occurred during World

War n. These problems were so severe that the Secretary of the Navy convened a Board of

Investigation to determine the causes and to make recommendations to correct them. The final

report of this Board summarized the magiiitude of the problems found during this study. *° Of

57
I

4,694 welded-steel merchant ships studied from February 1942 to March 1946, 970 (over 20

percent) suffered some fractures that required repairs. The magnitudes of the fractures ranged

from minor fractures that could be repaired during the next stop in port, to eight fractures that were sufficiently severe to force abandonment of these ships at sea. Also, at least 26 lives were

lost because of these fractures. The total cost of these fractures to our nation (replacement and

repair of ships, loss or delay of critical cargo needed for the war effort, and loss of lives) was

immense.

The problem was complex and remedies included changes to the design, changes in the

fabrication procedures, retrofits, as well as impact requirements on the materials of construction.

Assurance that these remedies were successful is documented by the record of ship fractures that

showed a consistent reduction in fracture events from over 130 per month in March 1944 to less

than 5 per month in March 1946, even though the total number of these ships in the fleet increased from 2,600 to 4,400 during this same period. '°

Benefits from the Introduction of Verification Testing

In 1948, many users felt that the scatter in the test results between individual machines

could be reduced further, so additional work was started to more carefully specify the test method

and the primary test parameters. By 1964, when the ASTM E 23 standard was revised to require

indirect verification testing, the primary variables responsible for scatter in the test were well

known. In a 1961 paper, Fahey" summarized the most significant causes of erroneous impact

values as follows: (1) improper installation of the machine, (2) incorrect dimensions of the anvil

supports and striking edge, (3) excessive friction in moving parts, (4) looseness of mating parts,

(5) insufficient clearance between the ends of the test specimen and the side supports, (6) poorly

machined test specimens, and (7) improper cooling and testing techniques. While the machine

tolerances and test techniques in ASTM E 23 addressed these variables, it was becoming

apparent that the only sure method of determining the performance of a Charpy impact machine

was to test it with standardized specimens (verification specimens).

Much of the work that showed impact tests did not have inherently high scatter, and

could be used for acceptance testing, was done by Driscoll at the Watertown Arsenal.'^

Driscoll's study set the limits 1 ft-lb and ±5 percent, shown in Figures 1 and 2. The data

superimposed on these limits in Figures 1 and 2 are the initial verification results gathered by

Driscoll for industrial impact machines to evaluate his choice of verification limits. In Figure 1, the verification results for the first attempt on each machine are shown: only one machine fell within the ± 1 ft-lb limit proposed for the lower energy range. In Figure 2, results for re-tests on

the same machines are shown, after maintenance. Driscoll's work showed the materials impact

testing community that not all machines in service could perform well enough to meet the

indirect verification requirements, but that most impact machines could meet the proposed

requirements if the test were conducted carefully and the machine were in good working

condition. With the adoption of verification testing, it could no longer by convincingly argued

that the impact test had too much inherent scatter to be used as an acceptance test.

58
Early verification test results showed that 44 percent of the machines tested for the first time

failed to meet the prescribed limits, and it was thought that as many as 50 percent of all the

machines in use might fail.^-' However, the early testing also showed that the failure rate for

impact machines would drop quickly as good machines were repaired, bad machines were

retired, and more attention was paid to testing procedures. It was estimated that approximately

90 percent of the machines in use could meet the prescribed limits of ± 1 ft-lb or ± 5 percent.

Recently acquired verification specimen data, shown in Figures 3 through 5, confirms these

predictions. Failure rates for verification tests at low, high, and super-high energy ranges are

currently estimated to be 12, 7, and 10 percent respectively.^'*

Overall, the impact of incorporating verification limits in ASTM E 23 has greatly

improved the performance of impact machines; data collected using ASTM E 23 machines can

be compared with confidence. ASTM E 23 is still the only standard in the world, to our knowledge, that requires low-energy impact tests for verification, and as shown by the data in

Figure 1, results obtained using machines in need of maintenance can vary by more than 100

percent at the low-energy level. In effect, the limits imposed by ASTM E 23 have produced a population of impact machines that are arguably the best impact machines for acceptance testing

in the world.

Case Study In the Nuclear Industry

Notched-bar impact data do not directly provide engineering data (such as plane strain

fracture-toughness data) which can be used in stmctural integrity analyses. Therefore, it is necessary to conservatively correlate the key test parameters with component performance

objectives. An important illustration of this approach is the nuclear industry's reactor pressurevessel integrity program. It is essential that a nuclear reactor be operated in a manner that ensures that the vessel integrity is maintained under both normal and transient operating

conditions. In particular, the vessel must be protected from both brittle and ductile fracture.

This is accomplished by postulating limiting flaws and using linear elastic fracture mechanics

(LEFM) models to calculate the allowable coolant temperature (T) and pressure (P) during heatup, cool-down, and leak/hydro testing (P-T curves). The P-T limits are revised periodically

throughout the life of a plant to account for neutron damage to the pressure vessel. The Charpy

shift, indexed at 41 J, is used to shift the ASME (American Society of Mechanical Engineers)

reference stress intensity factor (Kjr) curve to account for the effects of neutron bombardment.

Since it is impractical to test large fracture toughness specimens throughout the life of

a nuclear power plant, surveillance programs use Charpy and tensile specimens to track the

neutron-induced embrittlement. As illustrated in Figure 6, the nuclear industry uses the 4 1 J index to define a ductile-brittle transition temperature (DBTT). The effect of neutron irradiation

is to shift the transition region to higher temperatures (AT41) and the Nuclear Regulatory

Commission (NRC) sets screening limits on the maximum shift in the energy-temperature curve

that can occur during the life of the plant. If the screening limits are exceeded, then either the

plant must be shut down or a thermal anneal must be conducted to restore the material properties.

59
The ability of the material to withstand ductile fracture is judged by the upper shelf

energy (USE). In older plants built before fracture toughness testing was widely used, Charpy

testing was used to qualify individual heats of material. The ASME code and the Code of

Federal Regulations prescribe minimum plate properties that must be satisfied prior to service

(e.g., at least 102 J of energy on the upper shelf prior to service). The NRC requires an in-depth

fracture-mechanics assessment if the Charpy USE is expected to drop below 68 J during the

operating life of the plant.

It is difficult to include all the economic benefits realized by using the Charpy impact test in the nuclear industry. Nevertheless, some insight can be gained by noting that most utilities assess the outage cost and loss of revenue for a nuclear plant in the range of $300,000 to $500,000 per day. If Charpy data can be used to extend the life of a plant one year beyond the

initial design life, a plant owner could realize revenues as large as $150,000,000. Further, the

cost avoidance from a vessel-related fracture is expected to be the billion-dollar range. To date,

the NRC has shut down one U.S. plant as a result of Charpy data trends. It is important to note

that this plant's pressure vessel was constructed from a one-of-a-kind steel and is not

representative of the US reactor fleet. Nonetheless, with decisions like this being based on the

Charpy test, the importance of ASTM E 23 and the restraints it applies cannot be

overemphasized

Cost Saving in the Steel Industry

The Charpy V-notch (CVN) test specimen and associated test procedure is an effective

cost-saving tool for the steel industry. The specimen is relatively easy to prepare, many

specimens can be prepared at one time, various specimen orientations can be tested, and

relatively low-cost equipment is needed to test the specimen. In many structural steel

applications, the CVN test can be used ( 1) as a quality-control tool to differentiate heats of the

same type of steel, (2) for quality assurance purposes, and (3) to predict service performance of

components. Also, CVN test information can be correlated with fracture toughness data for a

class of steels so that fracture mechanics analysis can be applied directly. One may question how

all the above factors help a steel producer sell a reliable product

Consider the following case. A steel producer has a contract to supply plate steel for an

off-shore platform. The plate material needs to meet mechanical properties that are quite

rigorous for reasons of safety and end-product reliability. Before full-scale production of the

order can begin, the steel supplier needs to demonstrate to the buyer that the material is capable

of meeting such criteria. To accomplish this, the supplier qualifies the material for the project.

The process begins by making the steel grade and then testing a portion of the plate to determine

whether all required criteria are met. Steel-mill equipment imposes limitations on plate size;

therefore, individual steel plates need to be welded together in the field to produce lengths that can reach deep into ocean waters. Small sections of the sample plate are welded together, and

fracture-mechanics tests are conducted to determine the crack tip's opening displacement

(CTOD) toughness in the heat-affected zone (HAZ) and areas and along the fusion line where the

weld metal meets the base metal. Then, for example, a steel supplier might correlate the CTOD

test results with CVN 50 percent ductile-brittle transition temperature (DBTT). By agreement

60
between the customer and supplier, this correlation can allow the steel supplier to use the Charpy

test instead of the more expensive and time-consuming CTOD testing.

One piece of information that can be used directly in design applications is the critical

Diane-strain stress-intensity. Kj,. value. It is defined as that value occurring ahead of a sharp

crack at the moment of unstable crack propagation. The Kj- value is related to component

geometr\\ applied stress, and flaw size. Barsom and Rolfe proposed a plane-strain fracture

toughness CVN energy correlation that can be used m the transition region:

Ki^, - / E = 2{C^'y^ - (English units),

where K,, is the plane strain fracmre toughness, E is Young's modulus, and is the absorbed

energy \-alue from the Charp}' \'-notch test

If one knows the values of E and CX'N (easily obtainable) for a given material, a quantitative assessment of permissible stress levels and critical flaw size can be calculated. It must be noted that the above equation was developed for a particular grade of steel and therefore

may not be suitable for all grades of steel. However, the development of such correlations for a particular class of steel can be ver}- cost effecdve. As mentioned earlier, the nuclear industry- has

used Charpy parameters qualitatively to indicate the need for in-depth fracmre mechanics

analyses when the Charpy parameter falls below prescribed values. Design criteria for bridge

steels have also been based on such correlative procedures.

Continuing Standardization Efforts

Even after 100 years, the Charpy impact test procedures still have room for improvement.

The ASTM E23 standard has recently been redrafted to pro\"ide better organization and to include new methods such as in-situ heating and cooling of the test specimens. Two new related

standards are also under development through ASTM Task Group E28.07.08 on \Iiniamre and

Instrumented Notched Bar Testing, which was formed a little more than two years ago. The first standard covers mmiamre notched-bar impact testing and relies on many of the existing practices

related to test machine requirements and verification as specified in existing standard E 23. The

second standard is focused on instrumented testing. This involves the use of strain gages that are attached to the striker. In this method, the force-deflection cur\-e can be obtained for each test. Research is focused on using these data to obtain plane-strain fracmre toughness as well as other

key test parameters. Upon acceptance of the standard b}' ASTM. both the existing E23 standard

and the new miniature notched-bar standards would reference the instrumented impact standard.

Conclusion

As can be seen, the ASTM E 23 standard is a document that is improving with increasing

technical knowledge. Several years ago. at the ASTM Symposium on the Charpy Impact Test: Factors and Variables, a bystander was overheard to say, 'T see that there is a S\mposium on the

Charpy Test; what can be new there?""' Since then, the document has been updated twice and is currently being revised to reflect new developments and to make it more "user friendly"'.

61
Although Method E23 is now a standard, and no longer tentative, it continues to be a "work in

progress;" a work used extensively to help evaluate existing and new materials for products and

structures - a test to insure safety as well as reduce the initial and life-of-structure costs. Knowledge is continually being gained that will help make the test more accurate and reliable.

New technologies such as miniaturization of the test, instrumenting the striker to obtain

additional data, and developing mechanics models to enable extraction of plane strain fracture

toughness will be areas of development over the next 100 years. It is anticipated that the

improvements to E 23 obtained over the past 100 years will not overshadow the benefits that will

be realized in the future.

References

1. Louis Revillon, "The definition of Resilience in Impact Tests," Proceedings of the Vth Congress

of the Intemaional Association for testing Materials, Section 1113, Copenhagen 1909.

2. Tredgold T., "Strength of Cast Iron", pp 245 - 268, 1 824.

3. White A.E., Clark C.L., "Bibliography of Impact Testing", Department of Engineering Research,

University of Michigan, 1925.

4. LeChatalier A., "On the Fragility After Immersion in a Cold Fluid", French Testing Commission,

Volume 3, 1892.

5. Experiments with a New Machine for Testing Materials by Impact, S.B. Russell, Transactions

ASCE, Vol 39, p. 237, June 1898.

6. Bibliography on Impact Tests and Impact Testing Machines, W.K. Hatt and E. Marburg,

Proceedings ASTM, Vol. 2, p. 283, 1902.

7. Report on the Work of the Council from the Budapest to the Brussels Congress - 1901-1906,

Proceedings of the Intemational Association for Testing Materials, Brussels Congress, 1906.

8. Proceedings of the Sixth Congress of the Intemational Association for Testing Materials, New

York, 1912.

9. Report on Impact Tests of Metals, G. Charpy, Proceedings of the Vth Congress of the

Intemational Association for Testing Materials, Section HI,., Copenhagen 1909.

10. The Design and Methods of Constmction of Welded Steel Merchant Vessels: Final Report of a

(U.S. Navy) Board of Investigation, Welding Joumal, 26, 7, July 1947, p. 569.

1 1. N. H. Fahey, Effects of Variables in Charpy Impact Testing, Materials Research Standards, Vol

l,No. 11, Nov., 1961.

12. D. E. Driscoll, Reproducibility of Charpy Impact Test, Impact Testing, ASTM STP 176, 1955.

13. N. H. Fahey, The Charpy Impact Test - Its Accuracy and Factors Affecting Test Results, Impact

Testing of Metals, ASTM STP 466, ASTM, 1970.

14. C. N. McCowan, C. M. Wang, and D. P. Vigliotti, Summary of Charpy Impact Verification Data: 1994 - 1996, Submitted to the Joumal of Testing and Evaluation, 1998.

15. J. M. Barsom and S. T. Rolfe, Eng. Frac. Mech., 2(4), 1971, p. 341.

16. ASM Metals Handbook, Ninth Edition, Vol. 8, 1985, pp. 263, 264.

17. Charpy Impact Test: Factors and Variables, ASTM STP 1072, J. M. Holt, editor, ASTM, 1990.

62
40 1 •

1 1 1 1

30

-5 •

Difference,

20 1

10

•

•

• • •

-

_ I 'i • 0 ' . -

-10,

1 1 1 1 1

'

) 20 40 60 80 100 120

Energy, J

Figure 1 : The deviation and energy values obtained

for the first round of tests on industrial machines.

The deviation is calculated as the difference

between the results of the Watertown Arsenal's

machines and the industrial machines. These data were originally published by D. E. Driscoll,

Reproducibility of Charpy Impact Test, ASTM STP

176, 1955.

10

8

1 •

1 1 1 1 .

ince, J

6

4

2

•

0 T • •

• • : a -2

-4

-6

-\

1 1 1 1 1

3 20 40 60 80 100 120

Energy, J

Figure 2: The deviation and energy values for the

second and third rounds of tests on industrial

machines. The data show all but two of the

machines tested were able to pass the 1.4 J or 5%

criteria after appropriate repairs were made. These

data were originally published by D. E. Driscoll.

63
oO

600

500

400

300

200-

100-

0

' I

I

I

I M I M I

I

I

I

I I

' I'

I

'

I

' I ' I ' I

'

1-^ I . I . I . I

0.2

o

T3O

a.o'

T3 0.1 ®

03

0)

-5 -4 -3 -2 -1 0 1 2 3 4 5 6 7 8 9 10 Difference, J

0.0

Figure 3: Distribution of low-energy verification

data. Data for 1995-1997. Approximately 2,400

tests; each test is an average for five specimens.

Figure 4: Distribution of high-energy verification

data. Data for 1995-1997. Approximately 2,400

tests. Each test is an average for five specimens.

64
Figure 5: Distribution of the super-high energy

verification data. Data for 1995-1997.

Approximately 650 tests. Each test is an average

for five specimens.

Figure 6: Some materials, such as ferritic pressure

vessel steels, exhibit a transition in fracture

behavior as the notched-bar impact test temperature

is increased. At low temperatures the fracture is predominantly cleavage; at intermediate

temperatures the fracture is a mixture of both

cleavage and ductile; and above the transition

region the fracture is entirely ductile.

65

T.A. Siewert^

Impact Test Methods: Procedures and Analysis

Abstract

This report discusses recent research in the development and refinement of impact test procedures, as well as some direct and indirect verification procedures to assure that the machine

is operating properly. Most of the recent research can be organized into several broad categories:

the specimen (e.g.. surface finish, tolerances, and miniature sizes), the anvils and striker (e.g..

radii and surface finish), and general test procedures (e.g.. time to reach test temperature and

suitability for cr\'ogenic testing). This report also provides some direct and indirect verification

tests that can be used to evaluate the mounting and condition of the anvils and striker on the

machine. Finally, this report concludes that an open, international discussion of the procedural

details of the test is one of the best ways of promoting harmonization.

Kej-words

impact testing: international intercomparison: machine verification: specimen notching and

conditioning: striker radius: test procedures: test temperatures

Introduction

For over 100 years, researchers have been tr}'ing to understand and to measure the effect of

impact loading on the performance of engineering materials. In fact, the development of

consistent impact procedures was recognized to be of such importance that, even in 1912.

Committee 26 (on impact testing) of the International Association for Testing Materials (L\TM)

summarized its main goals as to "fix the conditions to be fulfilled by two distinct tests in order

that the resuks may be comparable and to correlate these numerically definite results to the

qualities determining the practical values of a material for different uses""[l].

Since then, impact-test procedures and anal\"tical methods have been refined as various

researchers have discovered additional parameters that affect the test results. In some cases, these new results have been widely and uniformly adopted. In other cases, different standards

organizations or machine manufacturers have chosen different approaches. As a result of many

such choices by the different standards organizations over the years, we now find some variation

in impact test procedures around the world. Certainh . worldwide comparison of test data would

be simplified if the procedures could be further harmonized between countries and between the

various standards. The followins section describes recent work directed toward understanding

^Materials Rehabilit}- Division, National Institute of Standards and Technology, Boulder.

Colorado 80305 USA

67
the effect of various procedural details. Publication of such work can persuade the various

standards committees around the world to choose the best procedural details (that produce the

most consistent results) or to determine that some existing differences in procedures have no

effect (so data developed under different procedures are considered equivalent and are mutually

recognized).

Recent research on procedures

The three most common impact test procedures in use around the world are probably ISO R 442

"Metallic Materials - Impact Testing - Verification of Pendulum Impact Machines," ASTM E 23

"Standard Test Methods for Notched Bar Impact Testing of Metallic Materials," and JIS Z2242

"Method for Impact Test for Metallic Materials". While the three have some similarities, they

also differ. Much current research is directed toward both improving these (and other)

standardized procedures, and trying to understand the effect of their differences and move toward

harmonization.

Striker Radius

Perhaps the aspect of the procedure that has been receiving the most study recently is that of

striker radius. At least 5 papers in the past 12 years have investigated the differences between

using strikers of 2 and 8 mm radius.

In 1989, Fink [2] compared the results of a number of variables, including notch preparation and

striker radii, on impact data. He studied a number of types of steel including 4340 (a heattreatable tool steel used for 15 and 100 J reference specimens), ASTM A 537 (a carbonmanganese steel used in pressure vessels), and HY-80 (a quenched and tempered high-strength

steel). For these steels, he reported a nearly 1 : 1 relation for data generated with the two strikers,

where the 2 mm striker produced results about 4 % higher. The precise relationship was:

= (1.042) * Eg + 0.516, (1)

where Ej and Eg are the energies of the 2 mm and 8 mm strikers and where the values are in ftIbf. He reported a coefficient of determination (r^) of 0.9987 and a standard error of 1.36.

Also in 1989, Naniwa et al. [3] compared the results of impact machines with strikers of 2 and 8

mm radius using steels with a range of absorbed energies. Figure 3 in their report showed a 1 :

trend for the two striker radii up to about 200 J, then a gradual increase in the data of the 8 mm

radius striker. When the machine with the 2 mm striker reported 300 J, the machine with the 8

mm striker reported about 400 J. Data for percent shear and lateral expansion showed no trend,

and the shape of the transition curves were the same. Additional impact testing with

instrumented strikers showed that both striker designs produced similar shapes for the first parts

of the record, but the load was substantially higher near the end of the record for the 8 mm radius

striker. This suggested that the difference was occurring near the end of the loading cycle.

Further testing (load measurements combined with imaging of the specimen in the anvils) on a

static bending machine confirmed that the higher energy in the tail region occurred when the

68
sides of the impact specimen made contact with the shoulder of the 8 mm striker. Thus, they

attributed the difference between the two strikers solely to the wider width of the striker with 8

mm radius.

In 1995, Nanstad and Sokolov [4] evaluated the data from machines with the 2 mm and 8 mm

strikers using six different materials. They studied two heats of ASTM A 533 (a pressure vessel

steel), a submerged arc weld with a high upper-shelf energy, a submerged arc weld with a low

upper-shelf energy, a Russian Cr-Mo-V forging steel, and two kinds of reference materials (4340

steel and a maraging steel). Although one plate showed lower values with the 8 mm striker and

the other plate (and the low upper-shelf weld) showed lower values with the 2 mm striker, they

concluded there was no clear trend (within one standard deviation) up to 175 J. Only the highestenergy reference material (near 220 J) showed a clear difference, where the 8 mm striker

produced energies about 1 1 % higher than those for the 2 mm striker.

Also in 1995, Siewert and Vigliotti [5] evaluated the data from two different brands of U-type

pendulum machines, each with both the 2 mm and 8 mm radius striker. They used reference

grade specimens at energies of 18, 45, 100, and 200 J. The small standard deviation produced by

these specimens allowed a very precise measurement of machine or striker effects. They found

very small differences between the two strikers for the three lower energy ranges, and an even

smaller effect between the two brands of machines. At 200 J, they noted that the 8 mm striker

produced energies about 10 J higher than those for the 2 mm striker, and the 8 mm striker

produced standard deviations that were higher by about a factor of three.

These four studies suggest that the striker radius does not seem to be an important variable up to about 150 or 175 J, at least for the steels that were evaluated. However, above 200 J the two

striker radii produce divergent results as the energy increases. Ruth [6] has attempted to produce

a compromise striker, one with the narrower profile of the 2 mm striker, but also with a flatter surface on the very front edge. This was accomplished by grinding the front to an 8 mm radius,

then blending this surface to the edges by the use of a 1.5 mm radius. So far, this approach has

not reached its goal.

Specimen Fabrication

Koester and Barcus [7] compared grinding and broaching of the notch. They found that both

procedures produced data that were equally consistent, but there was a bias between the two

techniques. They attributed this to differences in either the microstructural damage due to the

notching or to slight imperfections in the notch radii.

Fink [2] also looked at notch production by grinding, broaching, and milling (with a fly cutter).

He concluded that the grinding of the notch produces the smoothest and most consistent profile.

Direct Verification (Machine Condition and Mounting)

Schmieder [8] found that direct verification of a machine is not a simple task. He based much of

his work on the concept that the permitted inaccuracy of a metrological measurement must be 10

69
times better than the tolerances specified for the device. In other words, he tried to use

instruments and techniques that were more precise by an order of magnitude than what was

required by the standard, to develop a better estimate of how close the machine was to exceeding

the prescribed tolerance.

He evaluated four C-type pendulum machines and five U-type pendulum machines, spanning

capacities from 3 to 2500 J. He found that the losses due to friction and windage could exceed

the permitted limits on machines of very small capacity or on multi-range machines (where the

bearings are sized for the highest capacity, and so have too much friction for the lower capacity).

He also found that checking the difference between the center of strike and the center of

percussion requires extremely accurate measurement of the period of the pendulum (as the center

of percussion varies as the square of the period of the pendulum). At a 5 degree angle of swing,

the friction would damp the swing before enough cycles would occur. At higher angles, the

nonlinear terms became important, and even the use of elliptic integrals in the theoretical analysis

was unable to correct for these effects.

Schmieder et al. [9] later studied the effect of various machine dimensions, including: tilted

anvils, thinner anvils (striker contacting anvils 5 mm past the normal position), and striker not

contacting the specimen opposite the notch. All these were studied at levels in excess of the

variation permitted by ASTM Standard E 23, and all variations noticeably increased the absorbed

energies. Thus, these data support keeping the machine tolerances that are specified by E 23.

Porro et al. [10] studied the use of compliance to evaluate the quality of the machine mounting,

in terms of such common problems as loose bolts on the base of anvils, paint or other lowfriction materials under the base.

Ruth et al. [11] studied the effect of surface finish of the machine anvils and striker. They found

that surfaces smoother than those required by the standard procedures better simulate the surfaces

of these parts after a period of use. Thus, a better finish will reduce the discontinuity in apparent

energies when these parts are replaced.

Ruth [11] also studied the effect of radius on the comers of the 8 mm striker, because a 0.25 mm

tolerance can rapidly wear beyond the tolerance. He found that increasing this radius to 0.5 mm

had little or no effect, but increasing the radius to 1 mm had a very measurable effect.

Yamaguchi et al. [12] studied the effect of anvil radius and taper. They reported a measurable

reduction in absorbed energy as the taper angle increases from 9 to 12 °, and a 5 % change in

energy as the anvil radius increases from 1 to 1.5 mm.

Effects of Specimen Size and Dimension

Alexander and Klueh [13] compared Charpy specimens of standard size (10 mm by 10 mm) to specimens of half and third size. They found that the smaller specimens allowed more specimens

to be produced from a given amount of material (especially important for irradiated material), but

produced different upper-shelf energies and different transition temperatures. They concluded

70
that the upper-shelf energies could be corrected with a simple volumetric normalization

procedure, but the shift in transition-temperature was more complex. Later, Alexander et al. [14]

revisited this issue and developed sub-size verification specimens that could be used to verify the

performance of machines used to test sub-size specimens.

Manahan et al. [15] also looked at sub-size specimens, and developed a test machine design.

They proposed a minimum cross section of 5 mm by 5 mm, and recommended side grooves to increase the amount of material in these smaller sections that is exposed to plane strain

conditions.

Marsh [16] studied the effect of changing the tolerance on the right angle between the two 10

mm faces of the specimen. He varied the angles outside of the tolerance of 10 minutes of arc and

found that greater variations produced statistically significant changes in the energies, especially

for specimens with absorbed energies near 100 J. He concluded that a tolerance of 10 minutes on

the right angles should be maintained.

Test Temperatures and Specimen Conditioning

Nanstad et al. [17] studied the effect of thermal conditioning, the process of bringing the

specimen to the desired test temperature. They investigated a number of media including water,

oil, acetone, and methanol, at temperatures above and below ambient. They found that water was

a poor choice between 50 and 100 °C because evaporative cooling is so significant that the

specimen may cool below the temperature tolerance even if the specimen is broken within 5 s of

leaving the bath. Also, they found that soaking times used with gaseous media need to be

increased to assure that the specimen has reached thermal equilibrium.

The growing use of cryogenic magnets has promoted the use of impact testing to measure the

ductile-brittle transition of structural materials at temperatures down to 4 K. Tobler et al. [18]

offer several cautions. They found that the very low specific heat of metals below 77 K causes

the specimens to heat rapidly as they are transferred from the bath to the anvils. For this reason

alone, valid tests cannot be performed according to the procedures of E 23. Further, even cooling

the specimen in place in the anvils is unable to provide accurate data, as the work hardening

during the initiation and propagation of the crack raises the temperature substantially. Thus they

concluded that pendulum impact testing is not valid below 77 K, and any attempt to correlate

performance at 4 K from specimens cooled to 4 K is confounded by the variations in workhardening rates in the various materials.

Manahan [19] reported that conditioning of the specimen when on the machine anvil and in

position for testing (by use of a special fixture) reduces the changes in temperature that can occur when a specimen is transferred from a conditioning bath to the anvils. In addition, this procedure

doubles the precision in centering the specimen in relation to the striker, since there is no rush to position the specimen.

71
!

Other Procedure Details

Sundqvist and Chai [20] reported on the production of in-house standard specimens (from a

stable nickel-based alloy) for tracking the performance of an impact machine between the formal

reverifications required by standards. They found that this was an excellent method of tracking

the performance of machines that are used to test specimens that lead to excessive wear of the

striker and anvils.

In spite of the widespread use of notched specimens for evaluating material performance, Galban

et al. [21] report that unnotched specimens can provide standard deviations as small as, or smaller than notched specimens of the same material. Since verification of machine performance

is separate from evaluation of material performance, use of such specimens (with low standard

deviations) could reduce the cost of the verification specimens.

Comparison of Data - Machine-to-machine and country-to-country

Several recent round-robins or comparisons of national reference machines confirm that today's

Charpy test procedures are at least as reproducible as those reported by Driscoll [22] in 1955, and

are consistent between countries. These recent round robins have shown that the certified

energies of verification specimens distributed by national metrological authorities usually agree

within 1 % with the values determined by other national authorities. A 1998 study [23] compared

the four organizations or laboratories that were found to certify the verification specimens for

Charpy impact machines. These organizations were the Institute for Reference Machines and

Measurements (IRMM, in Belgium), Laboratoire National D'Essais (LNE, in France), National

Institute for Standards and Technology (NIST, in the United States), and National Research

Laboratory for Metals (NRLM, in Japan). The study involved a comparison of the 2 and 8 mm

radius strikers, three absorbed energy levels, and a large number of replicate tests for each of

these conditions at each of these organizations. This study concluded that the other organizations

developed average energies very close to those assigned by the laboratory that produced them,

the specimens produced by the four organizations have similar spread in the data (coefficient of

variations between 0.02 and 0.04), and the 2 and 8 mm radius strikers produced similar results

for 4340 steel (absorbed energies below 200 J). Therefore, in spite of the various differences in

procedures between the major standards in use around the world, the basic test procedure is quite

reproducible, so the results developed in different countries and on different designs of (verified,

high-quality) machines can be compared with confidence.

A follow-on three-year study [24] is currently underway. This study is examining the effect of

the differences in the measurement systems (e.g., master machines versus master batches) used

by various National Measurement Institutes (NMIs) that distribute reference specimens. Ten

specimens at each of three energy levels (15 J, 30 J, and 1 10 J) will be tested on seven referencegrade machines every six months. The data are expected to allow comparison of the verification

systems used by the laboratories, as well as provide data on machine variables, offsets, and

uncertainties relative to the harmonization of the respective systems.

72
Machine Installation

The data from a machine are not reliable unless the machine is mounted properly. The following

is the detailed procedure used by NTST to mount its three Master Charpy Reference Machines.

Unless the manufacturer of your machine supplies alternate instructions, you should consider

usmg these guidehnes. These are taken from a NTST Technical Note 150O-8 [25].

A stable foundation for the impact machine is critical to ensure accurate results. Energy losses

through the foundation must be kept to a minimum. The recommended foundation is a highstrength concrete that measures about 1.5 m long by 1 m wide by 0,5 m thick. Usually you wiU

need to cut a hole in the floor to accommodate the new foundation. If other equipment in the area could affect the machine operation, you may want to isolate it from the floor with e.xpansionjoint material.

Hold-down bolts used to secure the machme to the foundation should be of the m\-ened T" or

"J" type. (The next section, on direct verification, describes problems with the use of lag bolts,

which may be tightened up against the base without gripping the concrete. ) The bolts, nuts, and

washers should have a high strength (for example. AISI grade 8 or higher ). NTST recommends

using bolts or rod with a diameter of 22 mm, NTST used 22 mm diameter grade 8 threaded rod. cut into pieces that were about 600 mm long. Then. 150 mm long pieces of the same threaded

rod were welded to the end of the 610 mm (24 in) pieces to make invened T' bolts.

The machine was positioned over the center of the foundation hole. The machine was held

approximately 100 mm above the floor by using spacers suitable to hold the weight of the

machine. The "T" bolts were positioned in the machine-base mounting holes with a nut below

and above the base of the machine. The nuts were tightened to keep the "T" bolts straight while

the concrete was poured. The ends of the T bolts were positioned approximately 25 mm from the

bottom of the hole. The machine was then leveled on the spacers. Leveling did not need to be as accurate as the fmal leveling (mentioned later). Reinforcement bars were attached to the top of

the horizontal rod previously welded to the bonom of the "T" bolts. The reinforcement bars were

attached in the form of a box connecting the four bolts, A second box formation of reinforcement

rods was attached to the "T" bolts 0.25 m above the first box. The concrete was then Doured

under the machine. The concrete was finished as level as possible at this time, Before the

concrete fully hardened, we removed enough concrete, to a depth of approximately 25 mm. from,

around each "T" bolt to be able to thread a nut to below the surface of the concrete, and cleaned

the exposed threads. The machine was left in this position for "2 hours.

After 72 hours, the nuts on top of the base plate were removed and the machine was lifted off the

"T" bolts. The bottom nuts were then threaded down into the cavities that had been created

before the concrete hardened. The nuts were left high enough on the "T" bolts to enable the use

of an open-end wrench to adjust them after the machine was positioned on them. At this point,

the base of the machine was lightly coated with oil to keep the grout from adhering to it. The

machine was then hfted back onto the "T" bolts and w-as positioned on the adjustment nuts. The

machine was now ready to be leveled. A machinist's level was used to ensure meeting the level

tolerance of 3: 1000. The critical leveling procedure was done using the four nuts under the

73
machine. After the machine was leveled, the outsides of the nuts were wrapped with duct-seal

putty to facilitate their removal from the "T" bolts later in the process.

At this point the base of the machine was ready to grout. Heavy cardboard forms were placed

around the base of the machine to keep the grout under the machine. The grout was pushed

under the machine, so that the base of the machine was in total contact with the grout. The

machine was left in this position for 72 hours.

After 72 hours, the machine was lifted off the "T" bolts one last time. The grout was inspected

for cavities and for surface contact with the bottom of the machine. The putty was removed from

around the nuts. Some grout leaked around the putty and had to be chipped away from the nuts

to enable them to turn. The supporting nuts were removed from the "T" bolts. After all debris

was removed from the grout, the machine was repositioned on the "T" bolts and rested on the

grout. Washers and nuts were installed on the "T" bolts and were tightened to pull the machine

tightly against the grout. The level was checked at this point. The "T" bolts were cut off to approximately 12 mm above the nuts. The nuts were torqued to about 500 N-m. The level of the

machine was rechecked at this point.

Direct Verification

This section explains direct verification requirements (based on those in ASTM Standard E 23),

that confirm that a machine is in good operating condition, without the use of verification

specimens. The direct verification tests are physics-based tests, which assure that the machine is functioning as closely as possible to the behavior of a simple pendulum, with only small losses

due to friction and windage. Direct verification is most important when the machine is first installed or when major parts are replaced, but is also important during the periodic

reinspections. While these tests are required for the periodic reinspection in ASTM E 23, NIST

recommends that the free-swing test and windage-and-friction test be performed each day that the

machine is used. The records of these tests then serve as a convenient measure of bearing

performance. The following recommendations also come from a NIST Technical Note [24].

Space limitations prevented including illustrations of these characteristics here. The illustrations are available in the Technical Note.

Since the Charpy test is a dynamic test with vibration and impact loads, the hold-down bolts may

loosen over time. In extreme cases, this may introduce error sufficient to cause a machine to exceed the tolerance limits of the indirect verification test. In marginal cases, the movement may

still be sufficient to add a bias to the results that reduces the likelihood of passing. The tightness

of all bolts should be checked periodically, especially the anvil bolts, the striker bolts, and the

base-plate bolts. The manufacturer can supply the torque values for the anvil and striker bolts.

The base-plate bolts should be torqued to the recommended torque values for the grade and size

of the nuts and bolts. Only "J" or "T" bolts should be used; lag-type bolts can lead to errors. These are made to withstand only static loads. We believe that in some cases, the insert portion

of lag bolts can loosen in the concrete. When lag bolts are retightened, they can pull out of the

concrete and be pulled against the base of the machine, giving the impression of a properly

mounted machine. This condition is very difficult to detect. A machine with this problem will

74
exhibit erroneously high energy values at the low-energy level. The mounting procedure used to eliminate this problem for the NIST Master Reference Machines was described in the previous

section.

Standard E 23 describes a routine check procedure that should be performed weekly. It consists

of a free-swing check and a friction-and-windage check. The free swing is a quick and simple

test to determine whether the dial or readout is performing accurately. A proper zero reading

after one swing from the latched position is required on a machine that is equipped with a compensated dial. Some machines are equipped with a non-compensated dial. Such a dial is one

on which the indicator cannot be adjusted to read zero after one free swing. The user should

understand the procedure for dealing with a non-compensated dial. This information should be

available from the manufacturer.

The friction-and-windage test assesses the condition of the bearings. The pendulum should be

released and allowed to swing 10 half cycles (5 full swings). (The release mechanism should be

held down this whole time to avoid additional friction when the pendulum swings back up to where it may push on the latch.) As the pendulum starts its eleventh half swing, the pointer

should be reset to about 5 % of the scale capacity. Record the actual value and divide by the

eleventh half swings. Divide this number by the machine range capacity, then multiply by 100.

Any loss of more than 0.4 % of the machine capacity is excessive, and the bearings should be

inspected.

Keeping a daily log or shift log with the machine is also recommended. The log can be used to track the zero and friction values. The log can also include information such as number of tests, materials tested, maintenance, and any other useful comments.

The anvil and striker radii should be carefully inspected for damage and for proper dimensions.

Damage (chips or burrs) can be detected easily by visual inspection and by running a finger over

the radii to check for smoothness. Measurement of the dimensions requires more sophisticated

equipment. Radius gages are usually inadequate to measure the cridcal radii. Making molds of

the radii (such as with silicone rubber) or making an indentation in a soft, ductile material (such

as annealed aluminum), then measuring the impressions on an optical comparator is recommended. Occasionally even a new set of anvils and striker may have incorrect radii. Thus, new anvils and strikers should always be inspected before being installed in the machine.

Since the radii will not have local wear before use (the radii are consistent along their length),

they can be measured directly on an optical comparator or other optical measurement system.

Indirect Verification

Indirect verification uses carefully characterized test specimens to stress the test machine

components to levels similar to those experienced during routine usage. Since many machine

problems, such as loose anvils or striker, cannot be detected dunng direct verification, indirect

verification serves as an important supplemental test of the machine pertormance.

75
Centering tongs, such as those described in ASTM Standard E 23, are an excellent way to insert

the specimens at the very center of the anvils. The tongs should be inspected occasionally for wear or damage. A proper set of tongs is critical for the accurate placement of the specimen.

Some machines are equipped with a centering device. The device should also be inspected for wear and proper operation. Centering devices for low-temperature testing should be evaluated

carefully because the centering operation can extend the time between a specimen's removal from

the bath and fracture, and so may exceed the five-second interval allowed for transferring and

fracturing the specimen.

Some reference specimens are designed to be tested at -40 °C (-40 °F). Since the absorbed

energy changes with temperature, accurate temperature control is necessary to obtain valid test data. The temperature indicator should be calibrated immediately before testing. Ice water and

dry ice are very convenient calibration media.

Post-Fracture Examination

Just matching the reference energies is not sufficient to confirm that the machine is fully

satisfactory. For example, worn anvils can combine with high-friction bearings to compensate

for each other and produce an artificially correct value during the verification test. These are called compensating errors. Unfortunately, these errors compensate only over part of the range, so the machine produces generally inaccurate values. The post-fracture examination of

standardized verification specimens is a good way to identify such effects. Therefore, the NIST

verification specimens come with a questionnaire (with critical questions about the machine and

the test procedure) and a mailing label so the specimens can be returned to NIST. All specimens

are examined and compared to the data on the questionnaire before a response is sent to the

customers.

Following are the most common of the problems observed during examination of fractured

specimens. In many cases, suggestions on how to correct or avoid them in the future are included. '

Worn Anvils

Most of the wear of an impact test machine occurs on the anvils and striker. This wear can be

evaluated by examining the gouge marks that are formed on the sides of high-energy specimens

when they are forced through the anvils. Anvils that are within the required tolerance of the

standard will make a thin, even gouge mark all the way across both pieces of the broken

specimen. As the anvils wear, they will make a wider, smeared mark across the specimen halves.

When wide, smeared marks are observed on a customer's specimens, the anvils should be

changed, because the reduction in energy needed to push the specimens through worn anvils

eventually drops the machine below the lower tolerance in the energy range. You can monitor

the wear on your machine by retaining some specimens that are tested with new anvils and

comparing them to specimens of similar composition and hardness that are tested as the anvils wear. For specimens at a similar absorbed energy, the gouge marks will grow wider and

smoother as the anvils wear.

76
Off- Center Specimen

An off-center specimen strike occurs when a specimen is not centered against the anvils, so the

striker contacts the specimen to the side of the notch. The low-energy specimen best indicates

when an off-center strike occurs. This condition can be identified on the specimens by finding

that the gouge marks caused by the anvils are not equidistant from the machined notch edges, and

the striker gouge mark is offset the same amount from the notch. Also, the fracture surface of a

correctly tested low-energy specimen is flat and both halves are even. However, the fracture

surfaces of a specimen that has been tested off-center are on an angle. The more off-center the

strike, the steeper the angle will be. This problem increases the energy needed to fracture a specimen. The most common causes for this slipping are worn or damaged centering tongs, a worn or misaligned machine centering device, careless test procedures, or the use of a cooling

fluid that is too viscous at the test temperature, which causes the specimen to float on the

specimen supports. Most machine manufacturers should be able to provide new centering tongs.

Ethyl alcohol seems to be one of the best cooling medium to prevent floating because it evaporates quickly from the bottom of the specimen.

Off -Center Striker

This differs from the off-center specimen in that the specimen is centered against the anvils so

the anvil gouge marks are equidistant from the machined notch edges. However, the striker does

not contact the specimen precisely opposite the notch. An off-center striker is usually attributed

to the pendulum shaft shifting off center. This shift can be the result of a loose alignment ring on

the shaft or a loose bearing block on the machine. This problem also increases the energy needed

to fracture specimens at all energy levels.

Uneven Anvil Marks

Frequent testing of subsize specimens can cause the anvils to wear unevenly. Since this wear is restricted to only a fraction of the area that the full-size reference specimen contacts, there is usually no effect on the energy required to fracture the specimen. This anvil condition presents

two problems. First, since subsize wear is usually not indicated by a change in the energy

required to break a reference specimen, inspection of the broken specimen is required. This wear

will cause the anvils to be out of tolerance according to the requirements in the standard. This means that the machine does not meet the direct verification requirements of the standard and is therefore, not eligible for the indirect verification process. The second, and more important

problem, is that the subsize specimens are being tested in an area of the anvil that is worn. When

the wear is substantial, this condition will produce artificially low subsize values of energy. The

anvils on a machine with this condition should be replaced.

Chipped Anvils

Sometimes an anvil can be chipped. Lower-energy specimens are affected the least amount

because they are the hardest specimens and therefore have a more brittle fracture. The ductile

high-energy specimens will produce higher than normal energy results, and the very ductile

77
super-high-energy specimens are affected most by a chipped anvil. This condition should be

detected easily by a visual inspection before using the machine. New anvils are required when an

anvil is chipped.

Anvil Relief

Some Charpy machine manufacturers have designed a machined relief at the bottom of the anvil.

This anvil design does not meet the direct verification requirements of ASTM Standard E 23.

The relief increases the measured energy for ductile high- and very-high-energy specimens. It can also cause twisting of the specimens, during fracture, that may also contribute to energy

values higher than normal at all energy levels. However, this feature does not appear to add an

excessive amount of energy to the test. (The results are usually within the allowed tolerances.)

Damaged Anvils

Under some test conditions, usually for elevated-temperature testing, the anvils can wear to a rough finish that creates excessive friction. This damaged condition is detected best on higher

energy specimens. Damaged anvils usually cause the gouge marks to become wider and push the

specimen material to form a ridge that can easily be detected with the fingernail. This damage

usually causes artificially high high-energy results. Damaged anvils must be replaced.

Bent Pendulum

A pendulum bent in the direction of the swing produces gouge marks on a specimen. This gouge

mark is usually deeper on the top edge of the specimen as it sits in the machine. The striker

contacts the top edge of the specimen first, causing excessive tumbling and twisting. This

excessive activity can cause the specimen to interact with the striker or the pendulum after

fracture and create additional energy loss. A bent pendulum can be detected by placing an unbroken reference specimen in the machine and placing a piece of carbon paper on the surface

opposite the notch. At this point, lightly tap the striker against the specimen. This will make a mark on the specimen that can be inspected. If the pendulum is not bent, the mark should appear

the same width across the specimen. If the pendulum is bent, the mark will be wider at one edge

and become thinner or even not visible at the other edge. We recommend that a new pendulum

be installed on a machine with this problem.

Summary ofIndirect Verification

Some aspects of Charpy machine condition and accuracy can be assessed only through the use of

reference specimens. Further, some machine problems cause artificially low results while other

machine problems cause artificially high results. In addition, deviations in procedures can cause

similar results. These machine problems and procedural deviations may go undetected for years

without some sort of physical check. For this reason, examination of the broken specimens is a

critical part of the verification process. Many machine problems can be avoided or corrected by

the use of the information presented in this paper. Also, suggested changes in procedure can help

78
to ensure a successful test. Verification specimens are available from various organizations

around the world, including:

• the Institute for Reference Machines and Measurements (IRMM, in Belgium),

• Laboratoire National D'Essais (LNE, in France),

• National Institute for Standards and Technology (NIST, in the United States), and

• National Research Laboratory for Metals (NRLM, in Japan).

Summary

1 . Recent refinements in the procedures continue to improve the accuracy of the test. Topical

areas include the striker, anvils, specimens, and temperatures.

2. The Charpy scales used by the various NMIs are consistent, and the current round-robin

promises further harmonization of the various procedures.

3. The history of past international interactions shows that a free and open interchange of ideas

between countries is of benefit to all.

4. Direct and indirect verification testing is needed to assure the validity of data developed on a Charpy impact machine.

Acknowledgements

I appreciate the contributions of Dan Vigliotti and Chris McCowan at NIST to the procedures

used to evaluate verification specimens.

References

1. Charpy, G. "Report on Impact Tests and the Work of Committee 26," (1912) In: Proceedings of the Sixth

Congress of the International Association for Testing Materials, New York, IV, p. 1- 8, and the discussion of the

report in Section A, p. 6-1 1.

2. Fink, D.A. (1989) In: Charpy Impact Test: Factors and Variables, ASTM STP 1072, J.M. Holt (Ed.), American

Society for Testing and Materials, Philadelphia, Pennsylvania, p. 94-1 19.

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Impact Test: Factors and Variables, ASTM STP 1072, J.M. Holt (Ed.), American Society for Testing and

Materials, Philadelphia, Pennsylvania, p. 67-80.

4. Nanstad, R.K., and Sokolov, M.A. (1995) In: Pendulum Impact Machines: Procedures and Specimens for Verification, ASTM STP 1248, T.A. Siewert and A.K. Schmieder, (Eds.), American Society for Testing and

Materials, Philadelphia, p. 1 1 1-139.

5. Siewert, T. A., and Vigliotti, D. P. (1995) In: Pendulum Impact Machines: Procedures and Specimens for

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10. Porro, F., Trippodo, R., Bertozzi, R., and Garagnani, R. "Impact Tester Compliance: Significance, Sensitivity,

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ASTM STP 1380, T. A. Siewert and M. P. Manahan, Sr. (Eds.), American Society for Testing and Materials,

Philadelphia, Pennsylvania, p. 164-180.

13. Alexander, D. J., and Klueh, R. L. (1989) In: Charpy Impact Test: Factors and Variables, ASTM STP 1072, J. M. Holt, (Ed). American Society for Testing and Materials, Philadelphia, Pennsylvania, p. 3-18.

14. Alexander, D. J., Corwin, W. R. and Owings, T. D. (1995) In: Pendulum Impact Machines: Procedures and

Specimens for Verification, ASTM STP 1248, T. A. Siewert and A. K. Schmieder, (Ed). American Society for Testing and Materials, Philadelphia, Pennsylvania, p. 32-38.

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Procedures and Specimens for Verification, ASTM STP 1248, T. A. Siewert and A. K. Schmieder, (Eds).

American Society for Testing and Materials, Philadelphia, Pennsylvania, p. 39-69.

16. Marsh, F. J. (1995) In: Pendulum Impact Machines: Procedures and Specimens for Verification, ASTM STP

1248, T. A. Siewert and A. K. Schmieder, (Eds.), American Society for Testing and Materials, Philadelphia,

Pennsylvania, p. 19-30.

17. Nanstad, R. K., Swain, R. L., and Berggren, R. G. (1989) In: Charpy Impact Test: Factors and Variables,

ASTM STP 1072, J. M. Holt (Ed.), American Society for Testing and Materials, Philadelphia, Pennsylvania, p. 195-210.

18. Tobler, R. L., Reed, R. P., Hwang, I. S., Morra, M. M., Ballinger, R. G., Nakajima, H., and Shimamoto, S. (1991) In: J. Test. Eval. 19, 1, p. 34-40.

19. Manahan, Sr., M. P. (2000) In: Pendulum Impact Machines: A Century of Progress, ASTM STP 1380, T. A.

Siewert and M. P. Manahan, Sr. (Eds.), American Society for Testing and Materials, Philadelphia, Pennsylvania,

p. 286-297.

20. Sundqvist, M., and Chai, G. (2000) In: Pendulum Impact Machines: A Century of Progress, ASTM STP 1380,

T. A. Siewert and M. P. Manahan, Sr. (Eds.), American Society for Testing and Materials, Philadelphia,

Pennsylvania, p. 100-108.

80
21. Galban, G., Revise, G., Mougin, D., LaPorte, S. and LeFrancois, S. (2000) In: Pendulum Impact Machines: A

Century of Progress, ASTM STP 1380, T.A. Siewert and M.P. Manahan, Sr. (Eds). American Society for Testing and Materials, Philadelphia, Pennsylvania, p. 109-133.

22. DriscoU, D.E. (1955) In: Symposium on Impact Testing, ASTM Special Publication 176, pp. 70-83.

23. McCowan, C.N., Pauwels, J., Revise, G., and Nakano, H. (2000) In: Pendulum Impact Testing: A Century of

Progress, ASTM STP 1380, T.A. Siewert and M.P. Manahan, Sr., (Eds). American Society for Testing and

Materials, West Conshohocken, Pennsylvania, p. 73-89.

24. McCowan, C.N. (2000) Harmonization of Measurement for Charpy Reference Materials. National Institute of

Standards and Technology, Boulder, Colorado, p. 1-4.

25. Vigliotti, D.P., Siewert, T.A., and McCowan, C.N. (2000) Recommended Practice: Installing, Maintaining, and

Verifying Your Charpy Impact Machine, NIST Technical Note 1500-8, National Institute of Standards and

Technology, Boulder, Colorado, p. 1-17.

81

Part 2: Materials and Heat Treatment

83

C.N. McCowan,' D.P. Vigliotti,' and T.A. Siewert'

Evaluation of ABS Plastic Impact Verification Specimens

Reference: McCowan, C.N., Vigliotti, D.P., and Siewert, T.A., "Evaluation of ABS Plastic

Impact Verification Specimens," Pendulum Impact Testing: A Century ofProgress, ASTM

STP 1380, T. A. Siewert and M. P. Manahan, Sr., Eds., American Society for Testing and

Materials, West Conshohocken, PA, 2000.

Abstract: Valid comparison of impact test energies reported by various laboratories over time

depends on consistent performance of impact test machines. To learn what information

verification specimens might reveal about the impact machines used to test plastics, and to learn

the critical parameters that must be controlled to minimize the scatter in the data, we tested a

batch of acrylonitrile-butadiene-styrene plastic (ABS plastic) specimens. Both Charpy and. Izod

sample configurations were tested. We concentrated on determining the variability in impact

strength, but also studied the effects of room temperature aging, exposure to sunlight, and

clamping pressure (for Izod testing) on the impact strength of the ABS plastic specimens. We

conclude that: (1) the variability in impact strength for the ABS plastic impact specimens is

comparable to that of metal specimens which are now used as verification specimens to test largecapacity impact machines; (2) the ABS plastic specimens did not show significant signs of aging

during our tests; and (3) clamping pressures will have to be specified for verification testing of

ABS plastic Izod specimens.

Keywords: Charpy impact testing, impact testing, Izod impact testing, plastic impact specimens,

plastic impact verification specimens

^ Materials Research Engineer, Engineering Technician, and Supervisory Metallurgist,

respectively. NIST, materials Reliablility Division, 325 Broadway, Boulder, CO 80303.

Introduction

To help ensure valid comparisons of impact data, a requirement for the use of verification

specimens was added to the Standard Test Methods for Notched Bar Impact Testing of Metallic

Materials (ASTM E 23). This occurred because the metals impact testing community discovered

that verification tests of impact machine performance were able to detect certain energy loss

mechanisms that could not otherwise be observed during traditional physics-based measurements

of machine performance (pendulum period, mass, mechanical friction, windage, etc.), and these

mechanisms resulted in significant differences between the results of impact machines. This

study evaluates the applicability of ABS plastic verification specimens (acrylonitrile-butadienestyrene plastic) for testing the low-capacity impact machines used by the plastics industry.

Nationally standardized verification specimens are not available for testing the performance of

this class of impact machine.

Few studies on performance issues for plastics impact machines could be found. Por our

purposes, the most useftil report was the one used to support the precision statement in ASTM (D

256 - 93a), "Standard Test Methods for Determining the Pendulum Impact Resistance of Notched

Specimens." This report describes a round robin that included 6 different plastics and 25

laboratories.^ It indicates that both the materials and the laboratories contribute significantly to the uncertainty in the data. This report also indicates that the impact strength for specimens

produced from ABS plastic has low variability, as shown in Table 1.

The low coefficient of variation (CV) for the ABS plastic, which is the ratio of the standard

deviation to the impact strength, compares well with the values of 0.04 typical of metal impact

85
verification specimens now being produced. So, in terms of variability, the ABS plastic appears

to be a good candidate for use as an impact-verification material.

This report presents Izod and Charpy impact data for ABS plastic specimens to further

confirm the low variability in impact strength. In addition, the effects of room-temperature aging,

exposure to sunlight, and clamping pressure (for Izod testing) on the impact strength of the ABS

plastic specimens are evaluated.

Table 1. Izod impact data summary from a previous study.' Only the laboratories that tested 10

specimens are included here.

Impact Strength Standard Deviation Standard Error Coefficient

Lab of Variation

J/m (ft-lbf/in) l/m j/rri /ft IM/in\ l/m J/m iH IKf/in\ (n-iDT/inj

1 599.97 11.24 24.02 0.45 7.47 0.14 0.04

2 557.80 10.45 9.07 0.17 2.67 0.05 0.02

4 591.96 11.09 10.68 0.20 3.20 0.06 0.02

5 580.22 10.87 13.88 0.26 4.27 0.08 0.02

6 564.74 10.58 13.35 0.25 4.27 0.08 0.02

7 639.47 11.98 17.08 0.32 5.34 0.10 0.03

9 583.96 10.94 13.35 0.25 4.27 0.08 0.02

10 587.69 11.01 28.29 0.53 9.07 0.17 0.05

11 614.38 11.51 14.41 0.27 4.80 0.09 0.02

12 584.49 10.95 19.75 0.37 5.34 0.12 0.03

13 547.13 10.25 20.82 0.39 5.34 0.12 0.04

14 545.52 10.22 19.22 0.36 5.87 0.11 0.04

23 541.25 10.14 8.01 0.15 2.67 0.05 0.01

24 547.12 10.25 8.54 0.16 2.67 0.05 0.02

Avg 577.55 10.82 15.48 0.29 4.80 0.09 0.03

Material and Procedures

All the Izod and Charpy samples used for this study were produced from the same batch

of ABS plastic. They were cast in molds to produce geometries that meet the requirements of

ASTM D 256. The notches were ground, and the dimensions of the specimens were measured

prior to testing. In the case of Izod specimens, two specimens were cast at a time. These two

specimens are distinguished from one another as "gate" and "dam" specimens, which refers to the

end of the mold from which the specimens were taken.

Izod and Charpy specimens were made in thicknesses of 3.2 and 6.4 mm (0. 125 and 0.25

in.) for the study. The measured thicknesses of the individual specimens were used to calculate

the impact strength. All the

notch depths and radii of the

samples were found to be within the specifications of ASTM D 256. The 3.2 and 6.4 mm Izod

samples were clamped in the specimen vise using 5 and 8.5 N-m (45 and 75 Ibf-in.) of force

respectively, applied using a torque wrench on a 12.7 mm diameter bolt (0.5 in. - 14).

All testing was performed at room temperature. The specimens were stored and tested at 21 ±1

°C (70 °F). The specimens were not conditioned at 50 ±5 % relative humidity prior to testing.

The nominal humidity of the laboratory in which the specimens were stored and tested was low

(less than 30 % relative humidity).

86
We used a pendulum impact machine with a maximum capacity of about 20 J. The

machine was designed according to the requirements of D 256.

Results

As-Received hod Specimens - The impact data for the as-received Izod specimens are given in Table 2. For the 6.4 mm (0.25 in.) thick specimens, the samples from the gate end of the

mold had higher impact strength than the specimens from the dam end of the mold. The same

trend was found for the 3.2 mm (0.125 in.) samples: the gate samples had higher impact strength

than the dam samples. The differences due to specimen location were anticipated, and the results

show tiiat considering dam and gate specimens as separate groups helps to reduce unnecessary

scatter for the material. The standard deviations in impact strength for the four groups of as- received specimens are low, confirming that the ABS plastic is a good candidate for use as an

impact-verification material. The gate specimens had higher variation in impact strength than the

dam specimens, but all the as-received specimens have CV values (0.04 or less) comparable to those of the steel specimens used for impact verification specimens.

As-Received Charpy Specimens - The tests results on the as-received Charpy specimens,

given in Table 3, again show low variation in the impact strength for the ABS specimens. The

3.2 mm specimens have more variation in impact strength than the 6.4 mm specimens, but both

thicknesses have fairly low CV values. A practical consideration here is that the th-icker

specimens may seat more firmly against the anvils in the Charpy test, and thus rediice the scatter

in the test.

Table 2. Initial Izod datafor as-received impact specimens.

ID Mean

Impact Strength

J/nfi (ft-lbf/in)

Standard

Deviation

Coefficient of Variation

Cases

J/m (ft-lbf/in)

6.4 mm

6.4 mm

3.2 mm

3.2 mm

gate

dam

gate

dam

25

25

25

24

222.48 (4.17)

216.78 (4.06)

233.21 (4.37)

190.70 (3.57)

7.91 (0.15)

4.43 (0.08)

6.83 (0.13)

4.44 (0.08)

0.04 (O.04)

0.02 (O.02)

0.03 (O.03)

0.02 (O.02)

87
Table 3. Datafor the as-received Charpy impact specimens.

ID Mean Standard Coefficient of Variation

Cases Impact Strength Deviation

J/m (ft-lbf/in) J/m (ft-lbf/in)

3.2 mm Charpy 25 241.54(4.53) 12.26(0.23) 0.05(0.05)

6.4 mm Charpy 25 229.40(4.30) 4.65(0.09) 0.02(0.02)

Room-Temperature Aging Data - The Izod data from the 30-, 90-, and 600-day aging

groups and data from the as-received 3.2 mm gate specimens are given in Table 4. There appears

to be a slight decrease in the impact strength of the specimens aged for 30 and 90 days, compared

to the as-received specimens. However, the standard deviations of the three groups overlap, and

the 30- and 90-day results are almost identical. The statistics for the three groups combined (0-,

30-, and 90-day) show only slightly more variation in impact strength than each group considered

separately: (1) average impact strength of 228.5 J/m, (2) standard deviation of 7.37 J/m, and (3)

coefficient of variation of 0.03.

Table 4. Izod impact datafor aged specimens. The as-received specimens

are estimated to be 1 week old, 0 aging.

ID Aging (Days) Cases Mean Standard Coefficient of Impact Strength Deviation Variation

J/m (ft-lbf/in) J/m (ft-lbf/in)

3.2 mm gate 0 25 233.21 (4.37) 6.83 (0.13) 0.03 (O.03)

3.2 mm gate 30 25 225.67 (4.23) 6.27(0.12) 0.03 (O.03)

3.2 mm gate 90 24 226.60 (4.25) 6.48 (0.12) 0.03 (O.03)

3.2 mm gate 600 25 256.80 (4.81) 6.60 (0.12) 0.03 (O.03)

The impact strength of the specimens aged for 600 days increased by more than 1 0 %

(delta %) and this indicates that aging had a significant effect on the impact strength of these ABS

specimens. This result is somewhat surprising. Aging would generally be expected to degrade

the impact properties of plastics. Unfortunately, since only one machine was used for these tests and the performance of this machine could not be checked with verification specimens, we are not convinced that the results of the 600-day tests are necessarily reliable enough to accept.

These tests will have to be repeated in a more comprehensive study (with humidity control) to convincingly show the effects of aging on the impact strength of the ABS specimens, if any, and

to better document any change in impact properties over time.

Specimens exposed to Sunlight - Izod and Charpy specimens were exposed to sunlight on a window sill and periodically rotated for 30 days prior to testing. The impact results. Table 5,

show the average impact strength of the sunlight-exposed specimens is slightly lower than that of

the as-received specimens. Again, the difference is not significant considering the standard

deviations.

88
Table 5. hod Charpy impact datafor specimens exposed to sunlight.

ID ivicai 1 OUclllOlclll Ul Time Case Impact Strength Deviation Variation

(Days) J/m (ft-lbf/in) J/m (ft-lbf/in)

Charpy (3.2 mm) 0 25 241.54 (4.53) 12.26 (0.23) 0.05 (0.05)

Charpy (3.2 mm) 30 25 238.44 (4.47) 8.54 (0.16) 0.04 (0.04)

Izod (3.2 mm) 0 24 190.70 (3.57) 4.44 (0.08) 0.02 (0.02)

Izod (3.2 mm) 30 24 189.90 (3.56) 5.62 (0.11) 0.03 (0.03)

Table 6. Izod clampingforce.

Clamping Force Mean Standard Deviation

ID N-m (in-lbs) Impact Strength

J/m (ft-lbf/in)

3.2 mm gate 1 Just touching 243.38 (4.56)

3.2 mm gate 1 1.1 (10) 217.09 (4.07)

3.2 mm gate 5 2.3 (20) 247.69 (4.64) - 10.77 (0.20)

3.2 mm gate 1 3.4 (30) 232.16(4.35)

3.2 mm gate 1 4.5 (40) 235.06 (4.40)

3.2 mm gate 4 5.7 (50) 225.44 (4.22) 1.66 (0.03)

3.2 mm gate 1 6.8 (60) 216.96 (4.07)

3.2 mm gate 1 7.9 (70) 215.91 (4.05)

3.2 mm gate 3 9.0 (80) 210.75 (3.95) 3.74 (0.07)

Clamping Force in Izod Tests - The force

with which the ABS specimens were

clamped for Izod testing had a significant

effect on the impact strength of the

specimens, as shown in Figure 1 and Table

6. At low clamping forces (less than 3 N-m),

the scatter in the results and the impact

strength increased. At clamping forces

greater than 6 N-m, it appears that the effect

of clamping on scatter in the impact strength

decreases.

cw^-^ o

CDQ.

E

200 0123456789 10 Clamping Force, Nm

89
Discussion

The average impact strengths for the groups of ABS plastic impact specimens that were

tested ranged between about 1 90 and 240 J/m. Converting to impact energy, the range is from

about 0.6 to 0.8 J. This is a useful energy for a low-energy verification specimen, considering

that the machines used to test plastics have low capacities. The impact machine used for this

study, for example, has a low range of about 0 to 3 J, and weights can be added to the pexidulum

to increase the capacity up to about 20 J.

The variation in impact strength for the ABS specimens was low, and this is important for a material being considered for use as a verification specimen. The variation must be low enough

to allow a small sample size to be used when testing the performance of a machine. Typ ically the

CV values for the ABS material were 0.04 or less, and previous experience with metal impact

verification specimens indicates that this variability would allow a sample size of around 5. The

sample size for a verification test depends on the specific requirements of the test, but clearly the

ABS plastic has low variation.

The age of the ABS plastic specimens did not have a significant effect on the impyact

strength for short aging times, but the impact strength increased for the specimens that v^ere aged

for 600 days. These results imply that the impact strength of the specimens does not remain

stable over time, but further testing is required to confirm these results.

The impact strength of the specimens did not appear to be too sensitive to sunliglit

exposure. The sunlight exposure tests were somewhat simplistic, but they indicate that tlie

specimens could be left out on a sunlit desk for several days prior to testing without adverse

effect.

The impact strength of the ABS Izod specimens decreased significantly as clamping

pressure was increased on the specimen. This is apparently a common effect for some j>lastics, so we included these preliminary tests here to help determine how much the clamping pressure

affected the impact strength of the ABS plastic specimens (D 256, note 7). The results show that

clamping pressure is an important variable and would have to be specified for ABS Izodverification specimens.

Overall, the impact strength of the ABS specimens was consistent and this mak&s the

material a good candidate for the production of verification specimens for low capacity impact

machines. The affect of aging on the ABS plastic will have to be considered and there are other

considerations in choosing a specimen to verify the performance of impact machines. A^

verification specimen should test specific aspects of the machine performance that are difficult or impossible to verify by static measurement alone.

Experience in verifying the performance of large capacity impact machines, used for

metals, has shown that testing hard specimens (HRC 45 or greater) is the best way to verify that a machine is properly mounted. The very rapid loading of the machine associated with the impact

of these hard specimens is needed to reveal a mounting problem for the machine. Softer

specimens, on the other hand, are found to better show the effects of anvil and striker condition

on the machine performance. For the lower-capacity impact machines used to test plastics,

factors that affect the test results and the machine material interactions should be carefally

considered to help ensure that appropriate and useful verification specimens are chosen. It is likely that several different types of specimens will be needed to adequately verify the

performance of the low-capacity impact machines.

90
Conclusions

(1) The variability in impact strength for the ABS plastic impact specimens is comparable to that

of metal specimens that are presently used as verification specimens to test large-capacity impact

machines.

(2) Clamping pressures will have to be specified for verification testing of ABS plastic Izod

specimens.

(3) The effects of aging and relative humidity need more study.

Acknowledgments

We are gratefiil to Dave Scarlett of Bayer Corporation for producing the ABS impact

specimens used in this study.

References

1. Research Report RR: D20-1 134, available from ASTM Headquarters, Conshohocken, PA.

91
Appendix I

The Groups are identified as follows:

- / indicates initial, as-received, test group;

- a indicates aged for either 30 or 90 days (a30 or a90);

- 4 and 8 indicate specimen thickness (4 = 6.4 mm or 1/4 in. and 8 = 3.2 mm or 1/8 in.);

- indicates exposure to sunlight.

92
[able W.\- hod Data

Case Group ID J m ft-lbfm

1 a3028 2101 220.238 4.126

2 a30a8 gl02 228.511 4.281

^ a3028 gl03 223.253 4.182

4 a30g8 2104 217.515 4.075

5 a30s8 2105 225.469 4.224

6 a30g8 2lQ6 219.864 4.119

7 a3028 2107 230.059 4.310

8 a3028 gl08 228.511 4.281

9 a30a8 2109 228.031 4.2"2

10 a30g8 gUO 222.586 4.170

11 a30g8 2111 223.440 4.186

12 a3028 2112 226,002 4.234

13 a30g8 2113 225.469 4.224

14 a3028 2114 215.754 4.042

15 a3028 2115 235.984 4.421

16 a3028 2116 229.952 4.308

17 a3028 gll^ 232.354 4.353

18 a30e8 2118 222.907 4.176

19 a3028 2119 241.322 4.521

20 a3028 gl20 215.807 4.043

21 a3028 2121 224.775 4.211

22 a3028 2122 221.732 4,154

23 a3028 gl23 219.330 4.109

93

24 a30g8 2124 228.938 4,289

25 a3028 gl25 233.849 4.381

26 a90g8 gl26 226.590 4.245

27 a9028 gl27 221.305 4.146

28 a90e8 2128 225.682 4.228

29 a90g8 gl29 234.169 4.387

30 a90g8 gl30 232.515 4.356

31 a9028 gl31 220.131 4.124

32 a9028 2132 232.301 4,352

33 a9028 gl33 230.913 4.326

34 a90g8 2134 224.988 4.215

35 a9028 2135 220.932 4.139

36 a9028 gl36 222.319 4.165

37 a9028 gl37 221.625 4.152

38 a90g8 2138 230.273 4.314

39 a90g8 2139 241.856 4.531

40 a90g8 2140 215.594 4.039

41 a90g8 2141 231.767 4.342

42 a9028 2142 230.593 4.320

43 a9028 2143 229.579 4.301

44 a9028 2144 230.913 4.326

45 a90 2145 220.771 4.136

46 a9028 gl47 229.045 4.291

47 a9028 2148 214.366 4.016
Case Group ID J/m ft-lbCin

48 a90g8 gl49 220.451 4.130

49 a90g8 gl50 229.792 4.305

50 id4 dl 210.630 3.946

51 id4 dlO 215.647 4.040

52 id4 dll 214.206 4.013

53 id4 dl2 213.459 3.999

54 id4 dl3 213.939 4.008

55 id4 dl4 220.558 4.132

56 id4 dl5 216.181 4.050

57 id4 dl6 212.925 3.989

58 id4 dl7 219.811 4.118

59 id4 dl8 216.448 4.055

60 id4 dl9 225.629 4.227

61 id4 d2 214.153 4.012

62 id4 d20 218.636 4.096

63 id4 d21 219.010 4.103

64 id4 d22 226.323 4.240

65 id4 d23 223.974 4.196

66 id4 d24 219.811 4.118

67 id4 d25 212.605 3.983

68 id4 d3 210.896 3.951

69 id4 d4 218.209 4.088

70 id4 d5 216.448 4.055

71 id4 d6 220.398 4.129

72 id4 d7 214.206 4.013

73 id4 d8 215.220 4.032

74 id4 d9 210.096 3.936

75 id8 d26 193.068 3.617^

76 id8 d27 197.605 3.102L

77 id8 d28 183.460 3.437

78 id8 d29 188.424 3.53(*

79 id8 d30 194.776 3.64^

80 id8 d31 193.762 3.63C»

81 id8 d32 183.567 3.435»

82 id8 d33 185.435 3A7m

83 id8 d34 188.584 3.53S

84 id8 d35

85 id8 d36 187.090 3.505

86 id8 d37 198.940 3.727

87 id8 d38 196.751 3.68-6

88 id8 d39 187.677 3.51 ^

89 id8 d40 188.157 3.52 5

90 id8 d41 188.958 3.54 0

91 id8 d42 185.489 3.47 5

92 id8 d43 189.972 3.55 9

93 id8 d44 191.307 3.58 4

94 id8 d45 191.307 3.58- 4

95 id8 d46 191.894 3.5^5

94
Case Group JD J;m ft-lbf m

96 id8 d47 190.613 3.571

97 ids d4S 197.605 3.702

98 id8 d49 18S.424 3.530

99 ids d50 194.563 3.645

100 is4 2l 226.750 4.24S

101 i24 2lO 215,647 4.040

102 ig4 gll 214.046 4.010

103 i24 t£— el2 213.512 4.000

104 i24 2l3 214.046 4.010

105 i24 el4 231.554 4.338

106 i24 2l5 223.761 4.192

107 124 2l6 226.109 4.236

108 i24 el7 233.048 4.366

109 i24 ifZ 2lS 227.871 4.269

110 1^4 el9 232.942 4.364

111 i24 231. S21 4.343

112 124 220 225.415 4.223

113 i24 gll 231.554 4.338

114 232.301 4.352

115 i24 g23 231.340 4.334

116 [ad 224 227,390 4.260

117 i24 217.729 4.079

118 ig4 210.843 3.950

119 i£4 ^4 218.316 4.090

1 20 1 (J A g^ 9 1 6 71 S

1 21 go 910 017 A )7 O

1 22 1 CT.d g/ 2 1 4 OAf, AO] O

1 23 ig4 g« 2 1 1

1 24 gy 2 1 n "^no

1 25 icrS cr2fi 22^S 01 D i 9^ 1

1 26 all Z.J I .OZr 1

1 27 gZ6 24'; '^50

1 28 p2Q 4 5f^8

129 236 625 4 4'S 3

130 229,632 4.302

131 g33 245 058 4.5^1

132 228 61 8 4 2{^3

icrR £; J J 4"T. J3'BM~ -J5

1 34 gJ u

9'?4 •;n8 4 3^0

igo &JCT>7' ZZ. 777/ .7UO 008 •T.Z V

1 ^f, igo gJO 77R 11 4 9*^4

igo gjy 74*; fsAQ T. J J

i J o Igo g-+U aAO ADD 7^f .UvZ 007 4 4(

1 Q Igo g4i 741 744

Igo g42 a42 O'^O 524 4.3H9

141 128 g43 225.373 4.2^2

142 i28 g44 227.086 4.2^4

143 igS 245 228.591 4.^82

95
Case Group ID J/m ft-lbf7in

144 ig8 846 221.780 4.155

145 ig8 g47 222.800 4.174

146 ig8 g48 234.484 4.393

147 ig8 g49 238.802 4.474

148 ig8 g50 237.762 4.454

149 ig8 g60 236.198 4.425

150 uvd8 dlOl 192.641 3.609

151 uvd8 dl02 179.350 3.360

152 uvd8 dl03 183.247 3.433

153 uvd8 dl05 190.506 3.569

154 uvd8 dl06 187.997 3.522

155 uvd8 dl07 192.961 3.615

156 uvd8 dl08 195.150 3.656

157 uvd8 dl09 201.075 3.767

158 uvd8 dllO 190.666 3.572

159 uvd8 dill 191.307 3.584

160 uvd8 dll2 193.655 3.628

161 uvd8 dll3 191.627 3.590

162 uvd8 dll4 185.542 3.476

163 uvd8 dll5 194.616 3.646

164 uvd8 dll6 178.816 3.350

iivdX dll7 194.456 3.643

166 uvd8 dll8 192.001 3.597

167 uvd8 dll9 182.766 3.424

168 uvd8 dl20 185.382 3.473

169 uvd8 dl21 189.812 3.556

170 uvd8 dl22 199.943 3.746

171 uvd8 dl23 186.770 3.499

172 uvd8 dl24 190.026 3.560

173 uvd8 dl25 187.357 3.510

96
Table Al.2: Charpy Data

Case Group ID J/m ft-lbCin

1 i4 76.000 227.123 4.255

2 i4 77.000 227.817 4.268

3 i4 78.000 223.280 4.183

4 i4 79.000 225.522 4.225

5 i4 80.000 226.483 4.243

6 i4 81.000 227.550 4.263

7 i4 82.000 224.134 4.199

8 i4 83.000 226.643 4.246

9 i4 84.000 225.415 4.223

10 i4 85.000 222.373 4.166

11 i4 86.000 241.322 4.521

12 i4 87.000 226.109 4.236

13 i4 88.000 232.568 4.357

14 i4 89.000 236.358 4.428

15 i4 90.000 234.970 4.402

16 i4 91.000 229.152 4.293

17 i4 92.000 227.924 4.270

18 i4 93.000 229.098 4.292

19 i4 94.000 229.899 4.307

20 i4 95.000 237.425 4.448

21 i4 96.000 232.568 4.357

22 i4 97.000 227.390 4.260

23 i4 98.000 230.326 4.315

24 i4 99.000 234.169 4.387'

25 i4 100.000 229.365 4.297

26 i8 51.000 219.864 4.1 15>

27 i8 52.000 239.347 4.48«

28 i8 53.000 250.556 4.69*

29 i8 54.000 234.276 4.38^

30 i8 55.000 251.731 4.71S

31 i8 56.000 248.475 4.653

32 i8 57.000 235.557 4.413

33 i8 58.000 254.880 4.773

34 i8 59.000 244.151 4.57^

35 i8 60.000 207.907 3.893

36 i8 61.000 241.375 4.52S

37 i8 62.000 233.155 4.36S

38 i8 63.000 239.827 4.493

39 i8 64.000 245.165 4.5^3

40 i8 65.000 234.116 4.3S6

41 i8 66.000 224.134 4.ie>9

42 i8 67.000 254.346 4.7^5

43 i8 68.000 238.813 4.4'74

44 i8 69.000 253.118 4.7^=42

45 i8 70.000 258.136 4.8^36

46 i8 71.000 252.958 4.7Z39

47 i8 72.000 229.205 4.2^4

48 18 73.000 249.702 4.6^8

97
Case Group ID J/m ft-lbf/in

49 i8 74.000 251.090 4.704

50 i8 75.000 246.713 4.622

51 uv 251.000 231.447 4.336

52 uv 252.000 242.283 4.539

53 uv 253.000 241.429 4.523

54 uv 254.000 223.440 4.186

55 uv 255.000 241.535 4.525

56 uv 256.000 221.732 4.154

57 uv 257.000 243.777 4.567

58 uv 258.000 249.862 4.681

59 uv 259.000 252.104 4.723

60 uv 260.000 243.991 4.571

61 uv 261.000 241.749 4.529

62 uv 262.000 236.411 4.429

63 uv 263.000 237.585 4.451

64 uv 264.000 247.087 4.629

65 uv 265.000 246.019 4.609

66 uv 266.000 237.799 4.455

67 uv 267.000 239.721 4.491

68 uv 268.000 223.067 4.179

69 uv 269.000 238.706 4.472

70 uv 270.000 229.846 4.306

71 uv 271.000 237.372 4.447

72 uv 272.000 252.425 4.729

73 uv 273.000 238.706 4.472

74 uv 274.000 227.337 4.259

75 uv 275.000 235.450 4.411

98
T.A. Siewert, D.P. Vigliotti, L.B. Dirling, and C.N. McCowan

Performance Verification of Impact Machines for Testing Plastics

NIST Journal of Research, 104(6), Nov.-Dec. 1999, pp. 557-565.

Abstract: Valid comparison of impact test energies reported by various organizations and over

time depends on consistent performance of impact test machines. This paper investigates the

influence of various specimen and test parameters on impact energies in the 1 J to 2 J range for

both Charpy V-notch and Izod procedures, leading toward the identification of a suitable

material for use in a program to verif}' machine performance. We investigated the influences on

the absorbed energy of machine design, test material, specimen cross-sectional area, and

machine energy range. For comparison to published round-robin data on common plastics, this

study used some common metallic alloys, including those used in the international verification

program for metals impact machines and in informal calibration programs of tensile machines.

The alloys that were evaluated include AISI type 4340 steel, and five aluminum alloys: 2014-

T6, 2024-T351, 2219-T87, 6061-T6, and 7075-T6. We found that certain metallic alloys have

coefficients of variation comparable to those of the best plastics that are reported in the

literature. Also, we found that the differences in absorbed energy between two designs of

machines are smaller than the differences that can be attributed to the specimens alone.

Key Words: aluminum; Charpy impact test; Izod impact test; plastics; steel; verification; Vnotch

1. Introduction

About 40 years ago, a requirement for the use of verification specimens was added to the standard for impact testing of metals, American Society for Testing and Materials (ASTM)

Standard E 23 [1]. This occurred because the metals impact testing community discovered that

verification tests of impact-machine performance using reference specimens were able to detect

certain energy-loss mechanisms, mechanisms that could not otherwise be observed during

traditional physics-based measurements of machine performance (pendulum period, mass,

mechanical friction, windage, etc). This present paper evaluates the use of verification

specimens for machines used to test plastics, and suggests what information these specimens

provide about machine performance.

Few studies on performance issues for plastics impact machines could be found. For our

purposes, one of the most useful was the one used to support the precision statement in ASTM

Standard D 256, "Standard Test Methods for Impact Resistance of Plastics and Electrical

Insulating Materials" [2]. That report describes a round robin that included six different plastics

and 25 different laboratories. It indicates that both the materials and the laboratories make

significant contributions to the uncertainty in the data. Another study, also an ASTM research

report, indicates that the effect of notch radius (for plastic materials) is linear over the range of

notch radii of 0.03 mm to 2.5 mm [3].

99
2. Procedure

2.1. Material Selection

The first material to be included in the test plan was AISI type 4340 steel (of a special,

high-purity grade) since this has been used for many years to make verification specimens for

metals impact machines. Therefore, it serves as a good benchmaric against which other materials can be measured. We compared this 4340 steel to several aluminum alloys: 2014-T6, 2024-

T351, 2219-T87, 6061-T6, and 7075-T6. Alloy 6061-T6 was selected because of its reproducible performance in some informal tensile testing programs. The other aluminum alloys were selected because they are readily available, and also because they are known to possess a good blend of strength and ductility in structural applications. All these aluminum alloys have a lower modulus (stiffness), about 70 GPa, than that of the 4340 steel, about 200 GPa [4]. This

means that they deform at a force lower than for steels, yet still at a force much greater than for

plastics, whose moduli usually fall between 2 GPa and 12 GPa [5]. A higher modulus in the

verification test material is not necessarily detrimental, since it can serve to better identify an energy loss mechanism in an impact machine that is due to internal friction in the components

that are loaded during fracture. The larger oscillation during these higher loads with metal

specimens helps us to determine whether this effect is significant during routine testing with

plastic specimens.

Stability of impact energy over time is one of the most desirable features in the

verification specimens used to assess machine performance. The 4340 steel has a shelf life of at least several years, and so is a good benchmark against which other materials can be measured.

Although aluminum alloys such as the 2000, 6000, and 7000 series age harden, these effects were minimized through careful selection of alloy and lots. The 6061 and 7075 alloys were

treated at elevated temperature (although lower than the tempering treatments for the 4340 steel),

and the steep reduction in diffusion rate with lower temperature drastically limits subsequent

aging at room temperature. The 2000 series alloys will age at room temperature, but 80 % to 90

% of the hardening is completed in 4 to 5 days [6]. To minimize the small amount of residual

aging, we took our specimens from bars and plates that had been in inventory for several years.

In summary, we expect very little change over time in the mechanical properties of specimens

made from these metallic materials.

Another desirable feamre for verification specimens is complete fracture of the

specimens during impact. Complete fracture is preferred because we can compare the marks on

both fractured halves. The specimens develop marks during the initial strike (when the

pendulum hits the specimen), during fracture, and during subsequent collisions as the specimens

leave the machines. Assessment of these marks during the post-test evaluation of the specimens

(part of the metals impact test ASTM Standard E 23 procedures) provides guidance to machine

owners about alignment problems and wear. We did not include any plastics in this evaluation, because sufficient reference data exist

in the two reports cited in the introduction. The goal of this study was to compare the data for

these candidate metals to the existing data for the plastics.

100
2.2. Specimen Design and Preparation

We selected a specimen configuration (Tig. 1) that was designed to supplement the

machine verification tests described in International Organization for Standardization (ISO ) Draft International Standard (DIS) 13802 [7]. The dimensions were selected based on the

plastic specimens described in ASTM Standard D256. but were changed where necessar\- to

allow- for the different matenals properties of the metals. We permitted deviations from the

standard configuration, such as side grooves on each side of the notch in some specimens, and

allowing the length to be that specified either for Charpy or for Izod impact testing. One of the

goals of this study was to determine the energies that would be developed by specimens of

various sizes. The approach taken in the metals impact standard is to verif\' the machine

performance, using specimens distributed over the useful range of the machine (up to 80 % of

the machine capacity). For a 22 J impact machine, this means specimens with energies from 1 J or 2 J, up to about 1 5 J.

The standard configuration for the type 4340 steel specimens used in metal impact

machines (as described in ASTM Standard E 23) has a cross section of 10 mm by 10 mm [1].

WTien heat-treated to produce a low energ}-, this specimen configuration absorbs about 15 J of

energy from the pendulum. This 15 J specimen seemed appropriate to evaluate the performance

of our plastics impact machine when configured for its maximum capacit}' of 22 J. However,

our machine was damaged in an attempt to break one of these specimens in some preliminar}'

tests. x\pparenth'. our machine is able to tolerate energies in this range only when the fracture

event is spread over a longer time, such as when the low modulus plastic specimens deform

before fracmre. The impact energy is a single number that is the integral of the incremental

resistance of the specimen to fracture as the pendulum swings through its range. Therefore, by

itself, the impact energy reading is an inaccurate way to compare the responses of low- and highmodulus materials, since it does not reflect the influence of the maximum load on the machinespecimen interactions.

After the impact machine was repaired, we continued our initial evaluations (to estabhsh

the experiment design) using miniature specimens designed to evaluate only the lower end of the

machine range, between 1 J and 2 J. We selected steel specimen cross sections of 5 mm by 5

mm, 4 mm by 5 mm (notched across the 4 mm face), and 4 mm by 4 mm for some preliminar>"

tests. This size range was designed to determine the cross section that would produce energies

within the desired range, as well as a specimen size that would completely fracture upon impact.

Another reason for concentrating on specimens of lower energy in the rest of this paper is that

metal specimens show more ductility m thinner sections. Future tests with larger specimens, for

the higher end of the machine capacity, should exhibit a more brittle fracture.

The impact data from the four specimens tested with each of the three cross sections, 4

mm by 4 mm. 4 mm by 5 mm. and 5 mm by 5 mm, were compared to the cross-sectional area. To obtain the cross-sectional area, we multiplied the two dimensions and subtracted the area of

the notched region (1 mm deep) from the product. We found a linear relationship between crosssectional area and energy (at least for this limited energy range), then used these data to standardize on a specimen dimension of 4 mm by 5 mm for the majorit}' of our tests, since this

size fell near the center of the desired energy range.

101
Unfortunately, the specimens in these preliminary tests did not completely separate into

halves upon impact. Even the 5 mm by 5 mm specimens left the machine with the two halves

still joined by a thin ligament, but bent by the impact to about a 90° angle. The ligament was so

thin that the specimen halves could be bent to a 180° angle with two fingers, at which point the

two halves would separate. Therefore, the specimen absorbed almost all the energy needed to fracture it (and also absorbed the kinetic (toss) energy imparted by the striker), but we did not

gain data on possible jamming between the striker and anvils that might occur as broken halves

left the machine. While the standard 10 mm by 10 mm specimen in a metals impact test shows

almost no ductility and leaves the machine in two halves traveling at high velocity, all specimens

in the range of 4 mm by 4 mm to 5 mm by 5 mm showed substantial ductility. Therefore, an optimal cross section is nearer to 10 mm by 10 mm, but we decided not to increase the section

size since it would increase the required energy for fracture beyond the desired range.

A thorough evaluation of all variables calls for a full factorial experimental program with

a large number of replicate tests, which was beyond the scope, budget, and time available for this

study. Rather, this study is an initial evaluation to determine which variables might be most

important and should provide the basis for selection of the variables to include in a future round

robin. Nevertheless, we have used statistical summaries of the data using common formulas to give some estimates of repeatability.

2.3. Machine design

Our plastics impact machine can evaluate both Charpy V-notch and Izod specimens,

which allowed us to develop data in the two different test configurations. The number of

specimens tested with the Izod technique was much smaller than that tested with the Charpy

technique, and was sufficient only to estimate whether there were some differences in the

coefficients of variation for these specimens between the two configurations. Although many

companies follow the Charpy V-notch and Izod impact test procedures described in ASTM

Standard D 256, we have seen a growing interest in ISO Standard DIS 13802, and decided to follow its procedural requirements. Differences between these two standards include anvil

spacing, included angle and radius of the striker tip, anvil radius, and many other details of the

test procedure. These differences preclude direct comparison of the means between specimens

tested according to the two standards, but we believe that the differences are sufficiently minor

to conclude that the standard deviations and coefficients of variation (CVs, or relative standard

deviations) should be about the same.

While most tests were performed using a single machine set up for a full-scale capacity

of 22 J, we also removed the masses bolted to the pendulum, reducing the machine range for

several tests to 5 J. We also performed some tests using a conventional metals impact machine

with a capacity of 358 J, and using another design of plastics impact machine configured for a

capacity of 22 J.

3. Results and Discussion

All of the data from our tests are included in Table A-1 (Charpy) and Table A-2 (Izod)

in the appendix. This body of data consists of sets of similar specimens, typically sets of 3 to 10

102
specimens each, so we could evaluate the scatter between the specimens tested at the same set of

conditions. These data are summarized in the appendix in Table A-3, which combines the data

in each set.

Tables A-1 and A-2 contain columns that describe all of the parameters that were varied

or measured in the evaluation. Even when summarized in Table A-3. this large body of data

makes comparisons difficult, so we have selected subsets of the data in the following discussion

to highlight the effects of the different parameters. These subsets exclude the columns with data

that were held constant (listed in the notes below each table), but add columns with the

calculated standard deviations and coefficients of variation. For the 4340 steel, comparisons are made only between data from sets that were heat treated in the samie batch (with similar

prefixes).

3.1. Test Material

Table 1 includes summar}' data for the six materials (one steel and five aluminum alloys)

included in this investigation, and is intended primarily to compare the repeatabilities of the data

for different materials. The statistical data should be used with care in comparing the different

materials, since the values are based on only three specimens for each material. Also, standard

deviations cannot be compared fairly when the means are different. Therefore, the next column

hsts the coefficients of variation (CVs), which are the standard deviations divided by the means,

and which are also commonly called relative standard deviations. These permit easier

comparison of repeatability among specimen t}^pes and shapes with different means, but they do

also suffer from the same statistical deficiency that comes from having onh' three specimens.

We can thus make only broad generalizations about the data.

The data for the metals seem to fit into three groups according to the coefficient of

variation: CV up to 0.036, CV near 0.05. and CV near 0. 1 . We compared these to the

interlaborator\' data reported for plastics in Table 1 of ASTM Standard D256-93a [2], These

data fell into two groups: coefficients of variation between 0.042 and 0.058 for the plastics with

lower energN' absorption (phenolic, acetal, reinforced nylon, and polypropylene), and coefficients

of variation between 0.012 and 0.018 for the plastics with higher energy absorption (ABS and

polycarbonate). The summar}' of these data in ASTM Standard D256 does not mention the

material thickness; instead, it uses the usual plastics convention of normalizing the energy to 25

mm of specimen width (notch length). This facilitates comparison of plastics of different sheet

thicknesses (often 3 mm to 12.7 mm), but makes analysis of the data in terms of machine energy

range more difficult. Nevertheless, for a given machine range, the plastics with lower energy

absorption will obviously yield data that are at the lower end of that machine's range.

The ductile plastics have low CVs and so should be suitable for assessing machine

repeatabilit}' at the high end of the machine range. At the low end of the machine range, the

metals with the lower CVs (especially 4340 steel) beat the best of the plastics, by about a factor

of two. Therefore, at the low end of the machine capacity, metals offer the possibility of at least matching the abilit}' of specimens of plastics in resolving machine repeatability or the source of

103
machine uncertainties. In addition, metal specimens put a larger load on the machine striker and

frame, better revealing mounting and other structural problems.

3.2. Side Grooves

Table 2 shows that side grooves reduce the mean energy (by an amount much greater

than that explained by the reduction in cross-sectional area), but the coefficient of variation

either stayed about the same or increased slightly. Also, the specimen halves were still joined by

a similarly sized ligament. These unexpected results do not support more than a few confirming

tests on aluminum alloys in any future evaluation.

3.3. Notch Radius

Table 3 shows the effect of halving the notch radius from 0.5 mm to 0.25 mm. In general, notches are stress concentrators that reduce the ability of a material to sustain a load,

and so notches promote brittle, rather than ductile, failure [8]. In addition, sharper notches

should decrease the scatter, as a sharper notch increases the local stress at the crack, and its variable contribution to the scatter. However, these data show that the consistency was much

worse with the sharper notch, although the absorbed energy indeed decreased by about 10 %.

Once again, these disappointing results seem to minimize the value of including a large number

of tests for this variable in any future evaluations.

3.4. Machine Capacity and Design

Table 4 shows the effect of machine capacity and design. There was as much as 45 %

variation in the energy as the machine capacity was changed from 22 J to 358 J, at least for alloy

7075. This variation is several times greater than the standard deviation and so appears to be

significant. However, the effect does not seem significant for alloy 6061-T6, with the effect

being less than the standard deviation. This lack of significance is also evident for changes in

the machine range from 5 J to 22 J, comparison of plastics impact machines from two different

manufacturers, and comparison between the plastics impact machines (with capacities of 22 J)

and a metals impact machine (with a capacity of 358 J). In retrospect, it would have been better

to have made more specimens of 4340 and repeated this test using specimens having a smaller

standard deviation. However, both these results support the robustness of the pendulum impactmachine concept and indicate that it has been implemented consistently in these different

machine designs.

3.5. Charpy versus Izod

The limited data prevent us from drav/ing strong conclusions about the effect of test orientation (Izod versus Charpy), but an informal comparison of the two types of data

summarized in Table A-3 reveals no clear distinction between the two. It seems as though the

difference in mean energy between Izod and Charpy tests is less than the variability due to other

test parameters and so cannot be resolved. The same statement can be made for the CVs.

104
4. Conclusions

1. At low energies (low end of the machine capacity), certain metallic alloys have CVs that are about half that of the plastics in this range. This seems to support the option of using

metal impact specimens to verify the performance of plastics impact machines at the low

end of their range.

2. Metal specimens would increase the load on the machine striker and frame, permitting

better resolution of problems with machine rigidity and mounting.

3. Verification testing provides valuable performance data. For example, the data for alloy

6061-T6 indicate that two different designs of impact machines for plastics produce

results that agree within the uncertainty that can be attributed to the specimens alone.

5. Acknowledgment

We appreciate the assistance of Jennifer Caragol in revising and formatting the tables.

6. References

[1] ASTM Standard E 23, Standard Test Methods for Notched Bar Impact Testing of

Metallic Materials, Vol 03.01, American Society for Testing and Materials,

Conshohocken, Pennsylvania, (1999).

[2] Research Report RR: D20-1 134, untitled, available from American Society for Testing

and Materials, Conshohocken, Pennsylvania.

[3] Research Report RR: D20-1021, untitled, available from American Society for Testing

and Materials, Conshohocken, Pennsylvania.

[4] Smithell's Metals Reference Book, Sixth Ed., E.A. Brandes, Butterworths, London,

(1983) p. 15-3.

[5] Guide to Selecting Engineered Materials, Advanced Materials and Processing, Vol. 137

(6), ASM International, Materials Park, Ohio, June (1990) p. 23.

[6] Metals Handbook Ninth Edition, (Vol. 2) - Properties and Selection: Nonferrous Alloys

and Pure Metals, ASM International, Materials Park, Ohio, (1979) p. 31.

[7] ISO Standard DIS 13802, Plastics - Pendulum impact-testing machines - Validation of

Charpy, Izod, and tensile impact-testing machines, available from the ISO secretariat

AFNOR, (1995).

[8] Metals Handbook Ninth Edition, (Vol. 1 1) - Failure Analysis and Prevention, ASM

International, Materials Park, Ohio, (1986) p. 85.

105
CHARPY SPECIMENS

45° ±1

f mm

,^r = 0.075 mm

38 mm

5 mm

76 mm- 4 mm

NOTE: When used, sides were grooved with 45° cutter and 0.25 mm radius

Figure 1. Charpy specimen design.

106
Table A-1. Data From Charpy Tests.

SPECIMEN

ID

MATERIAL

ABSORBED

ENERGY

(J)

NOTCH

RADIUS

(mm)

SPECIMEN

DIMENSIONS

(mm)

POTENTL\L

ENERGY

(J) COMMENTS

20 6061 -T6 3.576 0.50 4 by 5 5

Remaining

Ligament

21 6061 -T6 3.648 0.50 4 by 5 5

Remaining

Ligament

22 6061 -T6 3.338 0.50 4 by 5 5

Complete

Breaks

23 6061 -T6 3.301 0.50 4 by 5 22

Remaining

Ligament

24 6061-76 3.571 0.50 4 by 5 22

Remaining

Ligament

25 6061 -T6 3.550 0.50 4 by 5 22

Complete

Breaks

26 6061 -T6 3.036 0.25 4 by 5 22

Complete

Breaks

27 6061 -T6 3.435 0.25 4 by 5 22

Complete

Breaks

28 6061 -T6 2.858 0.25 4 by 5 22

Complete

Breaks

29 7075-T6 1.138 0.25 4 by 5 22

Complete

Laminate

Breaks

30 7075-T6 1.138 0.25 4 by 5 22

Complete

Laminate

Breaks

31 7075-T6 1.238 0.25 4 by 5 22

Complete

Laminate

Breaks

32 2219-T87 1.245 0.25 4 by 5 22

Complete Brittle Fracture

33 2219-T87 1.320 0.25 4 by 5 22

Complete Brittle Fracture

34 2219-T87 1.382 0.25 4 by 5 22

Complete Brittle Fracture

35 2014-T6 2.889 0.25 4 by 5 22

Complete

Laminated

Fracture

36 2014-T6 3.000 0.25 4 by 5 22

Complete

Laminated

Fracture

37 2014-T6 2.791 0.25 4 by 5 22

Complete

Laminated

Fracture

38 2024-T351 1.970 0.25 4 by 5 22

Complete Brittle Fracture

39 2024-T351 1.828 0.25 4bv5 22

Complete Brittle Fracture

107
ABSORBED NOTCH SPECIMEN POTENTIAL

SPECIMEN MATERIAL ENERGY RADIUS DIMENSIONS ENERGY COMMENTS

ID (J) (mm) (mm) (J)

40 2024-T351 1.807 0.25 4 by 5 22

Complete Brittle Fracture

41 6061 -T6 3.084 0.50 4 by 5 358

Compare to 26,

27, 28

42 6061 -T6 3.084 0.50 4 by 5 358

Compare to 26,

27, 28

43 6061 -T6 3.084 0.50 4 by 5 358

Compare to 26,

27, 28

fiOfil-Tfi 3 168 0 25 4 hv 5 358

Compare to A,

R

7075 1 4Qfi 0 25 4 bv 5 358

n 707 11 .\jo '^ftlI 0D.^O?R *T uy o OOD

p I .OO 1 0 7R *T Dy O OOD

p1 0D.^O 4*T hvDy O OOD

A^ 4340 2 Q17 0 25 5 hv 5 35ft 4340 3 168 0W*<LO 25 35ft 47 4340 3 001 0 25 5x5A v-» 35ft

4ft 4340 3 001 0 25 *J5x5A \J 35ft 4Q 4340 11 .OD 5811 0 25 4*T hvDy 4*T 35ft OOD

OD - 1 ^ A'^AD.l 1 RR 0 2R 4H h\/ uy o 92

OD - / \3 4*340-1 *f>jT-D ULOD 1 RR 4 '3^7 OD.^O9R 4f Uyh\/ OR 00

OD - /

«+0'rU /IQ/IO-IL.I-OD 1 RR 4 41

O OR H uy o 00

OD 1 4*340-1 fOHD U-l—1 ODRfi 4 "^ftR 0D.^O?R 4H hvuy o 00 VAKj olUc yiUUVc;

OD ^ 4'^40-LL5fi tOtD I—t—OD 4 484 0D.^O2R 4t hvuy o 00 i>iu oiuc? yiiJvJVt;

4340-LL56 4 573 0 25 4*T hvuy \j5 92 Mn QiH^ nrnr»\/o

OD^iR - 4H- 4340-LL56 4 430 0 25 4"T hvDy o5 29 Mn QiHo nror*\/^

OD O 4 568 0D.^O25 4t hvDy o 00 M/^ ciHa nroriv/o

ODt^R -RD

4*340-1 1 '=iR 4 470 0 2R 4*T h\/ uy oR 00 Mr\ ciHo nro/^\/^

ODRR -/7 4*340-1 HO't-D ULOD 1 RR 4 '3ft9 0 2c; 4H uyh\/ oR 00 Mo c A n iNO olUc yrouvc

ODC^R -OR 4*340-l 1 RR 4 4*^7 O OR uy o oo NO olue yroove

ODRR - Qy 4*340-1 HOtD 1—1L-OD RF, 4 'i4R 0 2R 4^ h\/ uy o 00 M/^ ciHo nr/^/^\/^

4340-LL56 4 298 0 25 4*T hvDy o 00 Mr* ciHo r\Tr\r\\i^ 56 - 31

S

4340-LL56 2.363 0.25 4 bv 5 22ri ft. 56 - 32S 4340-LL56 2.467 0.25 4 by 5 22 SiHp nrnn\/p

56 - 33S 4340-LL56 2.495 0.25 4 by 5 22 Side nronvp

4340-LL56 2.519 0 25 4"T bvuy 5\j 22 WiVJw ViJIUUVw OD OOO 4340-LL56 2 535 0 25 4"T hvuy \J5 22 Oiuo yiuuvw

ODc^R . ODO '5RQ n p^ 4H hvuy oR PP oiuc? yiuuvc?

c;r - ^7^; OD >J / O P 447 0 PR 4*T hvuy o PP oiLic yiuuvc?

56 - 38S 4340-LL56 2.535 or- 0.25 4 by 5 22. Side groove

56 - 39S 4340-LL56 2.447 0.25 4 by 5 22 Side groove

56 - 40S 4340-LL56 2.404 0.25 4 by 5 22 Side groove

Machine #2, no

LL11-1 4340 2.888 0.25 4 by 5 22 side groove

108
SPECIMEN

ID

MATERIAL

ABSORBED

ENERGY

(J)

NOTCH

RADIUS

(mm)

SPECIMEN

DIMENSIONS

(mm)

POTENTIAL

ENERGY

(J) COMMENTS

LL11-2 4340 2.739 0.25 4bv5 22

Machine #2, no

side groove

LL11-3 4340 2.725 0.25 4bv5 22

Macliine #2, no

side groove

LL11-4 4340 2.793 0.25 4 by 5 22

Machine #2, no

side groove

LL11-5 4340 2.725 0.25 4 by 5 22

Machine #2, no

side groove

LL11-6 4340 2.684 0.25 4 by 5 22

Machine #2, no

side groove

LL11-7 4340 2.698 0.25 4 by 5 22

Machine #2, no

side groove

LL11-8 4340 2.671 0.25 4 by 5 22

Machine #2, no

side groove

LL11-9 4340 2.671 0.25 4 by 5 22

Machine #2, no

side groove

LL11-10 4340 2.752 0.25 4 by 5 22

Machine #2, no

side groove

LA46-1 4340 2.210 0.25 4 by 5 22

Machine #2,

side groove

LA46-2 4340 2.115 0.25 4 by 5 22

Machine #2,

side groove

LA46-3 4340 1.424 0.25 4 by 5 22

Machine #2,

side groove

LA46-4 4340 1.356 0.25 4 by 5 22

Machine #2,

side groove

LA46-5 4340 1.315 0.25 4 by 5 22

Machine #2,

side groove

LA46-6 4340 1.302 0.25 4 by 5 22

Machine #2,

side groove

LA46-7 4340 1.342 0.25 4 by 5 22

Machine #2,

side groove

LA46-8 4340 1.410 0.25 4 by 5 22

Machine #2,

side groove

LA46-9 4340 1.356 0.25 4 by 5 22

Machine #2,

side groove

LA46-10 4340 1.396 0.25 4 by 5 22

Machine #2,

side groove

G7 2219-T87 0.759 0.25 4 by 5 22

Machine #2, no

side groove

G8 2219-T87 0.705 0.25 4 by 5 22

Machine #2, no

side groove

G9 2024T-351 1.071 0.25 4 by 5 22

Machine #2, no

side groove

G10 2024T-351 1.071 0.25 4 by 5 22

Machine #2, no

side groove

109
SPECIMEN

ID

MATERIAL

ABSORBED

ENERGY

(J)

NOTCH

RADIUS

(mm)

SPECIMEN

DIMENSIONS

(mm)

POTENTIAL

ENERGY

(J) COMMENTS

G13 2014-T6 2.386 0.25 4 by 5 22

Machine #2, no

side groove

G14 2014-T6 1.302 0.25 4 by 5 22

Machine #2, no

side groove

G3 7075-T6 1.152 0.25 4 by 5 22

Machine IVIW«Wl III 1^^ #2f 1 } no1 side groove

G4 7075-T6 0.936 0.25 4 by 5 22

Machine #2 no

side groove

G1 6061 -T6 2.088 0.50 4 by 5 22

Machine #2, no

side groove

G2 6061 -T6 1.966 0.50 4 by 5 22

Machine #2, no

side groove

G5 6061 -T6 1.790 0.50 4 by 5 22

Machine #2, no

side groove

G6 6061 -T6 2.766 0.50 4 by 5 22

Machine #2, no

side groove

G11 7075-T6 0.664 0.25 4 by 5 22

Machine #2, no

side groove

G12 7075-T6 0.610 0.25 4 by 5 22

Machine #2, no

side groove

110
aDlc /V-Z. J Data From Izod Tests.

iFbLiJVLbJN

ID

MATERIAL

ABSORBhD

ENERGY

fj)

N U i Lri

RADIUS

( mm)

brhLiiMbIN

DIMENSIONS

(mm)

rU i 1 iA_L

EN'ERGY

(]) COMMENTS

1 4*^4.0 1 '^11 1 >w 1 1 0 25 4 by 4 22

Remaining

Lina mpnt

2 11 . 777 / / / 4 hu 4 PP

Remaining

1 inampnt

3

RPiRi DUD .TP, 1 - 1 D 9 743 n fin H uy PP

Remaining

1 in^^mpnt

4 RDfil DUU 1 -TR1 U n fio H u y \j P2

Remaining

1 inampnt

5 n fio 4 bv ^ P2

Remaining

1 insmpnt

6 fiDfil \J\J\J 1 -Tfi 1 VJ 2 817

0.50

4 bv 5 22

Remaining

1^ inampnt 1 y Cl 1 1 1 w 1 1 L

7 UUU 1 1 D P 4fi1 n fin 4 hu fi PP

Remaining

1 inamont

A n Pfi 4 hu fi PP

Remaining

1 inarnont

B yny^-Tfi 1 3Qn n Pfi 4 hu fi PP

Remaining

1L.iycll inampnt 1 Iwl IL

8 4*^40 1 331 n Pfi 4 bv 4 PP

Remaining

u-1 inampnt 1 y Cl 1 1 Id 1 L

9 4.'^4n HO*tU 1 314 n Pfi t uy H 22

Remaining

1.1 yd 1 1 1 CI 1

10 A'^AO 1 31 zl n Pfi 4H h\/ uy 4H PP

Remaining

1 i n a mpnt

11 H-OH\J P 043 n pfi 4H uyh\/ ofi PP

Remaining

1 in a rnont

12 p 1 n Pfi 4 hu fi PP

Remaining

1^ inampnt [ y di 1 1 CI ]

14 4340 2 125 0 Pfi 4 bu fi 22

Remaining

1^1inampnt y cii 1 Iwl [ L

15 cjfi huuy fi\j Remaining

1L.Iinamont y Cll 1 1 CI 1 L

16 4340 2.449 0 Pfi fi hu fi PP

Remaining

1L-iinampnt y u 1 1 Id 1

^7 4340 2.557 0.25 5 by 5 22

Remaining

Ligament

18 4340 2.537 0.25 5 by 5 22

Remaining

Ligament

19 4340 2.731 0.25 5 bv5 22

Remaining

Liqament

111
Table A-3. Combined Izod and Charpy Data.

IZOD

SPECIMEN

ID. MATERIAL

MEAN

ABSORBED

ENERGY (J)

STANDARD

DEVIATION

(J)

COEFFICIENT

OF

VARIATION

NOTCH

RADIUS

(mm)

SPECIMEN

DIMENSION

(mm)

POTENTIAL

ENERGY

(J) COMMENTS

1-2, 8-10 4340 1.314 0.111 0.084 0.25 4 by 4

Remaining

Ligament

34765 6061 -T6 2.698 0.176 0.065 0.50 4 by 5

Remaining

Ligament

A & B 7075-T6 1.573 0.259 0.165 0.50 4 by 5

Remaining

Ligament

35017 4340 2.106 0.558 ' 0.264 0.50 4 by 5

Remaining

Ligament

15 - 19 4340 2.511 0.108 0.042 0.50 5 by 5

Remaining

Ligament

CHARPY

SPECIMEN

ID. MATERIAL

MEAN

ABSORBED

ENERGY (J)

STANDARD

DEVIATION

(J)

COEFHCIENT

OF

VARIATION

NOTCH

RADIUS

(mm)

SPECIMEN

DIMENSION

(mm)

POTENTIAL

ENERGY

(J) COMMENTS

20 - 22 6061 -T6 3.521 0.162 0.046 0.50 4 by 5 5.0

Remaining

Ligament (20 &

21) Complete

Break - 22

23-25 6061 -T6 3.474 0.150 0.043 0.50 4 by 5 22.0

Remaining

Ligament (23 &

24) Complete

Break - 25

26 - 28 6061 -T6 3.110 0.300 0.097 0.25 4 by 5 22.0

Complete

Breaks

29 - 31 7075-T6 1.172 0.058 0.050 0.25 4 by 5 22.0

Complete

Laminate

Breaks

32-34 221 9-T87 1.316 0.069 0.052 0.25 4 by 5 22.0

Complete Brittle Fracture

SPECIMEN

ID. MATERIAL

MEAN

ABSORBED

ENERGY (J)

STANDARD

DEVIATION

(J)

COEFHCIENT

OF

VARIATION

NOTCH

RADIUS

(mm)

SPECIMEN

DIMENSION

(mm)

POTENTIAL

ENERGY

(J) COMMENTS

35-37 2014-T6 2.894 0.105 0.036 0.25 4 by 5 22.0

Complete

Laminated

Fracture

38-40 2024-T35

1

1.868 0.088 0.047 0.25 4 by 5 22.0

Complete Brittle Fracture

41 -43 6061-T6 3.084 0.000 0.000 0.50 4 by 5 358

Compare to 26,

27, 28, (44)

44 6061-T6 3.168 0.000 0.000 0.25 4 by 5 358

112
C -F 7075 1.644 0.142 0.086 0.25 4 by 5 358

45-48 4340 3.022 0.105 0.035 0.25 5x5 358

56-72 - 56-74 4340-LL56 4.441 0.100 0.023 0.25 4 by 5 22

56-1 - 56-10 4340-LL56 4.459 0.089 0.020 0.25 4 by 5 22 No side eroove

56-3 IS

56-40S 4340-LL56 2.476 0.068 0.028 0.25 4 by 5 22 Side eroove

LLll-1 - LLll-10 4340 2.739 0.068 0.025 0.25 4 by 5 22

Machine #2, no

side sroove LA46-1 - LA46-10 4340 1.519 0.339 0.223 0.25 4bv5 22

Machine #2, side groove

G7-G8 2219-T87 0.732 0.041 0.056 0.25 4 by 5 22

Machine #2, no

side groove

G9-G10 202T4-351 1.071 0.000 0.000 0.25 4 by 5 22

Machine #2, no

side groove

G13 -G14 2014-T6 1.844 0.732 0.397 0.25 4bv5 22

Machine #2, no

side groove

G3 -G4 7075-T6 1.044 0.149 0.143 0.50 4 by 5 22

Machine #2, no

side groove

Gl -G2 6061-T6 2.027 0.081 0.040 0.50 4 by 5 22

Machine #2. no

side groove

G5-G6 6061-T6 2.278 0.691 0.304 0.25 4 by 5 22

Machine #2, no

side groove

Gil -G12 7075-T6 0.637 0.041 0.064 0.25 4bv5 22

Machine #2, no

side groove

Table 1 Effect of Alloy [Notes 1 to 3]

NUMBER MEAN STANDARD COEFHCIENT

MATERIAL OF ABSORBED DEVIATION OF

SPECIMENS ENERGY (J) (J) VARIATION

4340 3 4.441 0.100 0.023

2014-T6 3 2.894 0.105 0.036

2024-T351 3 1.868 0.088 0.047

2219-T87 3 1.316 0.069 0.052

6061 -T6 3 3.110 0.300 0.097

7075-T6 3 1.172 0.058 0.050

Notch Radius 0.25 mm

Specimen Dimension 4 mm by 5mm

Potential Energy 22 J

113
Table 2

Effect of Side Grooves [Notes 1 to 3]

MATERIAL

NUMBER

OF

SPECIMENS

MEAN

ABSORBED

ENERGY (J)

STANDARD

DEVIATION

(J)

COEFFICIENT

OF

VARIATION

SIDE

GROOVES

4340 10 4.459 0.089 0.020 N

4340 10 2.476 0.068 0.028 Y

Notel: Notch Radius 0.25 mm

Note 2: Specimen Dimension 4 mm by 5 mm

Note 3: Potential Energy 22 JTable 3

Effect of Notch Radius [Notes 1 and 2]

MATERIAL

NUMBER

OF

SPECIMENS

MEAN

ABSORBED

ENERGY (J)

STANDARD

DEVIATION

(J)

COEFHCIENT

OF

VARIATION

NOTCH

RADIUS

(mm)

6061 -T6 3 3.474 0.150 0.043 0.50

6061 -T6 3 3.110 0.300 0.970 0.25

Note 1 : Specimen Dimension 4 mm by 5 mm

Note 2: Potential Energy 22 J

Table 4

Effect of Machine Capacity [Notes 1 and 2]

MATERIAL

NUMBER

OF

SPECIMENS

MEAN

ABSORBED

ENERGY (J)

STANDARD

DEVIATION

(J)

COEFHCIENT

OF

VARL\TION

NOTCH

RADIUS

(mm)

MACHINE

POTENTIAL

ENERGY (J) 6061 -T6 3 3.521 0.162 0.046 0.50 5

6061 -T6 3 3.474 0.150 0.043 0.50 22

6061 -T6 3 3.110 0.300 0.097 0.25 22

6061 -T6 3 3.084 0.000 0.000 0.50 358

6061 -T6 1 3.168 0.000 0.000 0.25 358

7075-T6 4 1.644 0.142 0.086 0.25 358

7075-T6 3 1.172 0.058 0.050 0.25 22

Note 1 : Specimen Dimension 4mm by 5 mm

Note 2: Potential Energy 22 J

114
C.N. McCowan.^ M.J. Leap." B. Gassner.- D.P. Vigliorti'

Standard and Experimental Heat Treatments for Low-Energy

4340 Steel Charpy V-notch Verification Specimens

Introduction

There is little information available on the production of the Charp},- V-notch verification

specimens used to cemf\" impact machines to ASTM E 23 requirements. This is the first of

several reports that will pro\ide archival information on verification specimens for ASTM

subcommittee E 28.07 (Impact Testing"), w hich presides over the verification testing procedures.

This report documents the results of several heat treatments for 4340 steels, used to produce lowenerg\' impact specimens.

Summary

Five groups of specimens were heat-treated to evaluate the effect of different tempering

conditions on the variabilin^ of absorbed energy. Three of the groups evaluated were determined

to be of a quality suitable for use as verification specimens: groups la. lb. and 3. The

specimens from groups la and lb were heat-treated according to recommendations given in

ASTM E 1271, and the results show that impact specimens with ven.' low standard de\'iations in

absorbed energy can be produced using these recommendations. The group 3 specimens were

heat-treated to promote the formation of fine carbides in the microstructure. and although they

did qualif)' for use as verification specimens, the standard deviation in the absorbed energy for

group 3 was higher than that for the group 1 specimens.

Two groups, 2a and 2b, varied too much in impact energy to be suitable for use as

verification specimens. These specimens were tested onh' at room temperature, not -40 "C ( -40

°F), and low-temperamre testing would be expected to increase the scatter in the test results.

Temper conditions used for groups 2a and 2b were designed to produce verification specimens

for room-temperature testing. So. it appears that this approach is not too promising for use in the

verification program.

NIST. MaKnali Reliabilir. Di\ision. Boulder CO

" The Timken Company. Canion OH

' CNS CO. ISC.. Fullenon CA

115
Procedures

Machining

A total of 130 Charpy V-notch specimens were machined for the test matrix.

Blanks were machined to near-fmal specimen dimensions for heat treatment. The specimens were machined to final size for nondestructive testing, then notched on the face labeled W in

Figure 1 and impact-tested.

Heat Treating

All the specimens were normalized at 900 °C (1650 °F) for 1 hour, air cooled,

austenitized at 870 °C (1600 °F) for 1 hour, and oil-quenched prior to tempering treatments.

The tempering treatments that were used are as follows:

(1) The specimens from group 1 were tempered according to the schedule given in ASTM E

1271 : tempered at 399 °C (750 °F) for 1.5 hours, and quenched in oil. No specifications on the

quench oil were cited. Two different companies produced group 1 specimens, and these

specimens were labeled as group la and group lb.

(2) The specimens from group 2 were tempered at 200 °C (392 °F) for 1 hour. Half of these

specimens, group 2a, were tested in this condition, and the other half, group 2b, were

cryogenically treated prior to testing. The cryogenic treatment consisted of slow cooling to - 184 °C (-300 °F), followed by reheating to 182 °C (360 °F).

(3) The specimens for group 3 were given a two-stage tempering treatment. These specimens were tempered at 200 °C (392 °F) for 1 hour, then

tempered at 399 °C (750 °F) for 1.5 hours.

Elastic Property Measurements

Elastic properties of the specimens were measured by impulse excitation of vibration as described

in ASTM C1259. This is a nondestructive technique

that consists of exciting the appropriate mode of

vibration in the specimen by a single tap and

determining the fundamental resonant frequency.

Figure 1. Specimen orientation. r^^^ ^^^^^^^ frequency of the unnotched impact

specimens was determined in four modes of vibration:

flexure and transverse flexure, and longitudinal and torsional vibration. These frequencies, along

with the weight and dimensions of the specimens, were used to calculate elastic properties.

Values of Young's modulus were calculated for flexural and longitudinal (E^ and E,) vibration, as well as the torsion modulus, G, and Poisson's ratio. The raw frequency data, used to calculate

116
t±ie elastic properties, are identified as RF, RL. and RT. These data \'ar>' with specimen

dimension and weight.

For density' calculation the dimensions of the specimens were individually measured to determine their width, breadth, and length. After the elastic properties were measured, notches were ground and the specimens were impact-tested.

Impact Testing

The five groups of specimens were impact-tested on a U-t\pe impact machine equipped

with an encoder and a digital display. The testing was done in accordance with ASTM E 23

procedures.

Groups la, lb. and 3 were tested at -40 'C 1^-40 '¥). Groups 2a and 2b were tested at room temperature.

Data Analysis

Data from the impact tests and the elastic -propert}' measurements were evaluated with a commercially available statistical software program.

For the impact test results, the principal statistics of interest were the mean absorbed

energ}" and the standard deviation (SD ) in absorbed energy. The coefficient of variation fCV), or

relative SD, which is the SD divided by the mean energ}'. is used as a normalized \'alue to compare the \-ariation in absorbed energies among the five groups of specimens.

For the moduli measurements, data were anal\'zed mainly to determine whether there was

a correlation between the variation in absorbed energies and the variation in elastic

measurements of the fi\"e specimen groups. The raw frequency data (RF, RL, and RT) were

evaluated along with the elastic data to evaluate trends that might be indicative of \'ariations in

sample size.

Results and Discussion

Impact Tests

The CV values of the 5 groups of specimens are shown in Figure 2. Data for the

specimens are given in the Appendix.

Groups la and lb had the lowest CV values of the 5 groups tested. These values

correspond to a mean energ}- of 18.4 J (13.6 ft-lbf) and a SD of 0.77 J (0.57 ft-lbf) for the la

group, and a mean energy of 17.2 J (12.7 ft-lbfj and SD of 0.39 J (0.29 ft-lbf) for the lb group.

117
0.07

O06

0,05 -

I 1

1 +1

+

O04 +

0.03

002

0.01

0.00

_

+

+ ENERGYC RTCV - RLCV

1 RFCV O GTCV « ELCV O EFCV

Gla Gib G2a

ID

G2b G3

Figure 2. The coefficients of variation for

the absorbed energy and the elastic property

variables calculated for the five specimen

groups

the specimens in group 2b, which were cryog(

energy than the specimens in group 2a.

As described in Part I of this report, a SD of 1.02

J (0.75 ft-lbf) is the maximum allowed for lowenergy verification specimens (12 to 20 J). So,

both the la and lb groups have low variations in

absorbed energy. Group lb has CV of 0.024,

which is very low. The CV of group la, 0.04, is typical of CV values for verification specimens

NIST has produced in the past.

Groups 2a and 2b have high CV values, 0.062

and 0.067 respectively. The SD for these 2

groups both exceeds the maximum of 1 .0 J allowed by the NIST verification criteria. These

specimens were tested at room temperature,

which might increase the scatter in absorbed

energy values compared with the other specimen

groups tested here, but we did expect 2a would

differ from 2b in the scatter due to the cryogenic

temper treatment used for the 2b group. Little

difference is apparent between these groups, and

ically tempered, had more scatter in absorbed

The group 3 specimens have lower variation in absorbed energy than the 2a and 2b

groups, but not as low as that for the la and lb groups. The CV for group 3 was 0.055. This

corresponds to a SD of 0.98 J (0.716 ft-lbf) and a mean energy of 17.6 J (13.0 ft-lbf). This group

does meet the criteria for verification specimens. The low-temperature double temper was used

for these specimens to determine whether the first temper would result in seeding fine carbides in

the structure, resulting in a reduced scatter in absorbed energy. Apparently this approach shows

little promise. , , . ,

Elastic Properties

The values of CV for the six elastic constants measured for the sample groups are shown

in Figure 3. If the CV values for the group lb specimens were lower than those of the other

groups, the general trend here would be similar to the trend in scatter for the absorbed energy

(Figure 2,+ symbols). Group lb, however, shows more scatter in elastic properties than do

groups la and 3, and it had the lowest scatter in absorbed energy. So the elastic property

measurements evaluated here provided little guidance in predicting which specimen groups

would have more uniform impact properties.

118
Reviewing the raw frequency data in the

Appendix, we observed that the CV values for the

RF, RL, and RT are highest for the group lb data.

This may indicate that the final dimensions of

these specimens (after notching) were less

uniform than the other groups. These specimens were machined by a different shop than the one making the other specimens. Because the scatter in the frequency data is propagated through the

elastic property calculations and finally to Poisson's ratio, the group lb specimens may have

artificially high scatter in elastic properties: one

outlier was removed from the group lb data, but

several other points on the RF plots in the

Appendix may also be outliers. The data for

group 2a and 2b are interesting because these

groups have more scatter in their elastic property

data than do the other groups, but have a scatter in frequency data similar to that of the other

groups: this may indicate measurement errors in

specimen dimensions.

The final observation apparent in the

scatter plots in the Appendix is that there is not a strong correlation between impact energy of the

specimens and the elastic property

measurements. The range in impact energy of

the group la, lb, and 3 specimens, for example,

probably represents a range in hardness of less

than 1 HRC (from our experience with 4340

steel). Considering the summary data plotted in

Figure 4, EL decreases as the energy increases

(strength decreases).

0,004

0003

0.002

0.001 -

0000

Gla Gib G2a G2D G3

IP

Figure 3. The coefficients of variation for

the elastic properties are shown here on a more appropriate scale than in Figure 2.

Figure 4: Impact energy (J) versus EL for

the specimen groups.

We had hoped that the elastic property

measurements would indicate which specimen

groups have more homogeneous microstructures,

and previous measurements have been shown to correlate well with hardness measurements.

Given the localized notch used for the impact test, the dynamic nature of the test, and the very

small differences in absorbed energies we are considering here, however, it is not too surprising

that a better correlation between the scatter in absorbed energy and elastic properties was not

found.

119
Appendix A: Elastic Property and Absorbed Energy Data

120
Summary Data

Group la (flexure)

EF EL GT RF RL RT Energ

y

N OF CASES 25 25 25 25 25 25 25

MINIMUM 28.060 28.100 10.880 15.660 46.350 26.36 12.369

MAXIMUM 28.290 28.230 10.920 15.760 46.530 26.63 14.668

RANGE 0.230 0.130 0.040 0.100 0.180 0.090 2.299

MEAN 28.188 28.162 10.905 15.722 46.460 25.512 13.548

VARIANCE 0.003 0.001 0.000 0.001 0.002 0.001 0.330

STANDARD DEV 0.058 0.036 0.012 0.031 0.049 0.023 0.574

STD.ERROR 0.012 0.007 0.002 0.006 0.010 0.005 0.115

SKEWNESS (Gl) -0.436 0.045 -0.401 -0.600 -0.613 -0.332 0.099

KURTOSIS (G2) -0.222 -1.027 -0.480 -0.854 -0.562 -0.010 -0.214

C.V. 0.002 0.001 0.001 0.002 0.001 0.001 0.042

MEDIAN 28.190 28.160 10.900 15.730 46.470 25.61 13.517

Group la (torsion)

EF EL GT RF RL RT

N OF CASES 25 25 25 25 25 25

MINIMUM 28.010 28.100 10.880 15.650 46.350 25.38

MAXIMUM 28.300 28.230 10.920 15.760 46.530 25.53

RANGE 0.290 0.130 0.040 0.110 0.180 0.09

MEAN 28.184 28.162 10.905 15.716 46.460 25.51

VARIANCE 0.005 0.001 0.000 0.001 0.002 0.001

121
Summary Data (continued)

Group la (torsion)

EE EL GT RF RL RT

STANDARDDEV 0.067 0.036 0.012 0.031 0.049 0.025

STD.ERROR 0.013 0.007 0.002 0.006 0.010 0.005

SKEWNESS (Gl) -0.793 0.045 -0.401 -0.511 -0.613 -0.332 •

KURTOSIS (G2) 0.4/8 1 r\T7 -1.02/ -U.4o0 -0.54/ -O.JOZ -U.J 1

C.V. 0.002 0.001 0.001 0.002 0.001 0.001

MEDIAN 28.190 28.160 10.900 15.720 46.470 25.51

Group lb (flexure)

EF EL GT RF RL RT Energ

V

N OF CASES 30 30 30 30 30 30 30

MINIMUM 27.820 28.050 10.860 15.640 46.330 26.540 11.987

MAXIMUM 28.370 28.370 10.990 25.670 46.660 26.730 13.325

RANGE 0.550 0.320 0.130 10.030 0.330 0.190 1.338

MEAN 28.236 28.201 10.923 16.038 46.425 26.599 12.692

VARIANCE 0.014 0.003 0.000 3.312 0.006 0.002 0.090

STANDARD DEV 0.082 0.056 0.022 1.820 0.074 0.043 0.293

STD.ERROR 0.022 0.010 0.004 0.332 0.014 0.008 0.055

SKEWNESS (Gl) -1.396 0.159 0.099 5.194 1.162 0.959 0.054

KURTOSIS (G2) 3.032 2.184 2.782 24.999 1.520 1.016 -0.154

C.V. 0.003 0.002 0.002 0.003 0.002 0.002 0.024

MEDIAN 28.255 28.210 10.920 15.705 46.410 26.590 12.687

122
Summary Data (continued)

Group lb (torsion)

EE EL GT RE RL RT

N OF CASES 30 30 30 30 30 30

MINIMUM 28.070 28.050 10.860 15.630 46.330 26.540

MAXIMUM 28.420 28.370 10.990 15.830 46.660 26.730

RANGE 0.350 0.320 0.130 0.200 0.330 0.190

MEAN 28.225 28.201 10.923 15.687 46.425 26.599

VARIANCE 0.005 0.003 0.000 0.002 0.006 0.002

STANDARD DEV 0.073 0.056 0.022 0.045 0.074 0.043

STD.ERROR 0.013 0.010 0.004 0.008 0.014 0.008

SKEWNESS (Gl) 0.035 0.177 0.102 1.283 1.162 0.959

KURTOSIS (G2) 0.535 2.220 3.094 1.740 1.520 1.016

C.V. 0.003 0.002 0.002 0.003 0002 0.002

MEDIAN 28.225 28.210 10.920 15.675 46.410 26.590

Group 2a (flexure)

EE EL GT RF RL RT Energ

y

N OF CASES 25 25 25 25 25 25 25

MINIMUM 27.810 27.790 10.740 15.550 46.100 26.370 21.915

MAXIMUM 28.090 27.990 10.810 15.640 46.240 26.470 26.571

RANGE 0.280 0.200 0.070 0.090 0.140 0.100 4.656

MEAN 27.927 27.874 10.774 15.584 46.155 26.412 23.957

VARIANCE 0.007 0.002 0.000 0.001 0.002 0.001 2.209

STANDARD DEV 0.084 0.044 0.016 0.024 0.039 0.025 1.486

123
Summary Data (continued)

Group 2a (flexure)

EF EL GT RF RL RT Energ

y

STD.ERROR 0.017 0.009 0.003 0.005 0.008 0.005 0.297

SKEWNESS (Gl) 0.144 0.489 0.016 0.681 0.647 0.532 0.713

KURTOSIS (G2) -1.031 0.329 0.092 -0.460 -0.531 -0.148 -0.979

C.V. 0.003 0.002 0.001 0.002 0.001 0.001 0.062

MEDIAN 27.950 27.870 10.770 15.580 46.150 26.410 23.287

Group 2a (torsion)

EF EL GT RF RL RT

N OF CASES 25 25 25 25 25 25

MINIMUM 27.810 27.790 10.740 15.550 46.100 26.370

MAXIMUM 28.170 27.990 10.810 15.640 46.240 26.470

RANGE 0.360 0.200 0.070 0.090 0.140 0.100

MEAN 27.932 27.874 10.774 15.589 46.155 26.412

VARIANCE 0.010 0.002 0.000 0.001 0.002 0.001

STANDAR DDEV 0.098 0.044 0.016 0.026 0.039 0.025

STD.ERROR 0.020 0.009 0.003 0.005 0.008 0.005

SKEWNESS (Gl) 0.544 0.489 0.016 0.657 0.647 0.532

KURTOSIS

(G2)

-0.495 0.329 0.092 -0.380 -0.531 -0.148

SUM 698.310 696.860 269.360 389.720 1153.87 660.310

C.V. 0.004 0.002 0.001 0.002 0.001 0.001

MEDIAN 27.930 27.870 10.770 15.580 46.150 26.410

124
Summary Data (continued)

Group 2b (flexure)

EF EL GT RF RL RT Energ

y

N OF CASES 25 25 25 25 25 25 25

MINIMUM 27.470 27.640 10.680 15.540 46.090 26.370 21.534

MAXIMUM 27.750 27.780 10.730 15.630 46.240 26.460 26.967

RANGE 0.280 0.140 0.050 0.090 0.150 0.090 5.433

MEAN 27.612 27.708 10.703 15.590 46.161 26.410 24.236

VARIANCE 0.007 0.001 0.000 0.001 0.002 0.001 2.601

STANDARDDEV 0.085 0.037 0.014 0.028 0.046 0.028 1.613

STD.ERROR 0.017 0.007 0.003 0.006 0.009 0.006 0.323

SKEWNESS (Gl) -0.010 -0.085 0.417 -0.019 0.254 0.278 0.231

KURTOSIS (G2) -1.047 -0.320 -0.599 -1.288 -1.225 -1.301 -1.226

c.v. 0.003 0.001 0.001 0.002 0.001 0.001 0.067

MEDIAN 27.610 27.710 10.700 15.590 46.150 26.400 23.680

Group 2b (torsion)

EF EL GT RF RL RT

N OF CASES 25 25 25 25 25 25

MINIMUM 27.400 27.640 10.680 15.540 46.090 26.370

MAXIMUM 27.770 27.780 10.730 15.640 46.240 26.460

RANGE 0.370 0.140 0.050 0.100 0.150 0.090

MEAN 27.584 27.709 10.703 15.586 46.162 26.410

VARIANCE 0.007 0.001 0.000 0.001 0.002 0.001

STANDARDDEV 0.081 0.038 0.014 0.029 0.046 0.028

125
Summary Data (continued)

Group 2b (torsion)

EF EL GT RF RL RT

STD.ERROR 0.016 0.008 0.003 0.006 0.009 0.006

SKEWNESS

(Gl)

-0.282 -0.123 0.385 0.472 0.220 0.321

KURTOSIS (G2) 0.940 -0.423 -0.510 -0.777 -1.191 -1.273

C.V. 0.003 0.001 0.001 0.002 0.001 0.001

MEDIAN 27.580 27.710 10.700 15.580 46.150 26.400

Group 3 (flexure)

EF EL GT RF RL RT Energ

y

N OF CASES 25 25 25 25 25 25 25

MEnJMUM 27.890 28.020 10.860 15.670 46.390 26.560 11.162

MAXIMUM 28.150 28.210 10.910 15.760 46.510 26.650 14.284

RANGE 0.260 0.190 0.050 0.090 0.120 0.090 3.122

MEAN 28.045 28.096 10.877 15.716 46.442 26.602 13.004

VARIANCE 0.004 0.002 0.000 0.001 0.001 0.000 0.512

STANDARD DEV 0.064 0.041 0.013 0.025 0.038 0.022 0.716

STD.ERROR 0.013 0.008 0.003 0.005 0.008 0.004 0.143

SKEWNESS (Gl) -0.347 0.560 0.724 -0.176 0.175 -0.017 -0.307

KURTOSIS (G2) -0.148 0.818 0.256 -1.026 -1.164 -0.505 0.396

C.V. 0.002 0.001 0.001 0.002 0.001 0.001 0.055

MEDIAN 28.050 28.090 10.880 15.720 46.440 26.600 13.006

126
Summary Data (continued)

Groups (torsion)

EE EL GT RF RL RT

N OF CASES 25 25 25 25 25 25

MINIMUM 27.970 28.020 10.860 15.680 46.390 26.560

MAXIMUM 28.160 28.210 10.910 15.770 46.510 26.650

RANGE 0.190 0.190 0.050 0.090 0.120 0.090

MEAN 28.049 28.096 10.877 15.720 46.442 26.602

VARIANCE 0.003 0.002 0.000 0.001 0.001 0.000

STANDARDDEV 0.056 0.041 0.013 0.024 0.038 0.022

STD.ERROR 0.011 0.008 0.003 0.005 0.008 0.004

SKEWNESS (Gl) 0.293 0.560 0.724 0.087 0.175 -0.017

KURTOSIS (G2) -0.920 0.818 0.256 -0.786 -1.164 -0.505

C.V. 0.002 0.001 0.001 0.002 0.001 0.001

MEDIAN 28.050 28.090 10.880 15.720 46.440 26.600

127
Raw Frequency Data and Elastic Property Data

Group la

ID$ RF RL RT EF EL GT ENERGY

N7 15.72 46.47 26.61 28.19 28.18 10.90 14.22

N43 15.75 46.51 26.63 28.29 28.23 10.92 14.54

NIO 15.70 46.44 26.59 28.26 28.22 10.92 14.66

N41 15.67 46.39 26.56 28.22 28.18 10.90 12.94

N12 15.74 46.48 26.63 28.18 28.14 10.90 12.36

N71 15.71 46.45 26.61 28.21 28.17 10.91 13.58

N19 15.76 46.53 26.64 28.24 28.20 10.91 14.41

N78 15.75 46.51 26.64 28.23 28.19 10.91 12.81

N20 15.68 46.39 26.59 28.18 28.15 10.92 13.64

N86 15.73 46.47 26.63 28.16 28.13 10.90 13.70

N22 15.72 46.42 26.60 28.06 28.10 10.89 13.38

N44 15.75 46.50 26.64 28.18 28.13 10.90 13.13

N29 15.72 46.47 26.61 28.16 28.16 10.90 12.56

N39 15.72 46.45 26.60 28.20 28.19 10.91 14.28

N30 15.74 46.47 26.63 28.10 28.12 10.90 13.26

N46 15.76 46.51 26.65 28.17 28.12 10.90 13.45

N34 15.73 46.46 26.61 28.09 28.13 10.89 13.32

N52 15.70 46.43 26.59 28.15 28.12 10.88 13.58

N37 15.66 46.37 26.56 28.19 28.16 10.90 13.51

N60 15.76 46.51 26.63 28.26 28.19 10.91 13.90

N61 15.67 46.35 26.56 28.21 28.15 10.91 13.45

N62 15.75 46.48 26.64 28.27 28.18 10.92 13.77

N69 15.68 46.41 26.58 28.20 28.20 10.92 13.32

N45 15.75 46.51 26.64 28.10 28.11 10.88 13.32

128
Raw Frequency Data and Elastic Property Data (continued)

Group la

IDS RF RL RT EF EL GT ENERGY

N67 15.74 46.51 26.64 28.21 28.19 10.92 13.51

T34 15.72 46.46 26.61 28.18 28.13 10.89 13.32

T37 15.66 46.37 26.56 28.15 28.16 10.90 13.51

T29 15.72 46.47 26.61 28.20 28.16 10.90 12.56

T30 15.73 46.47 26.63 28.16 28.12 10.90 13.26

T45 15.75 46.51 26.64 28.01 28.11 10.88 13.32

T39 15.71 46.45 26.60 28.25 28.19 10.91 14.28

T46 15.76 46.51 26.65 28.17 28.12 10.90 13.45

TIO 15.70 46.44 26.59 28.26 28.22 10.92 14.66

T52 15.70 46.43 26.59 28.06 28.12 10.88 13.58

T12 15.74 46.48 26.63 28.18 28.14 10.90 12.39

T60 15.72 46.51 26.63 28.26 28.19 10.91 13.90

T19 15.76 46.53 26.64 28.24 28.20 10.91 14.41

T61 15.65 46.35 26.56 28.17 28.15 10.91 13.45

T20 15.69 46.39 26.59 28.22 28.15 10.92 13.64

T62 15.73 46.48 26.64 28.19 28.18 10,92 13.77

T22 15.69 46.42 26.60 28.07 28.10 10.89 13.38

T67 15.74 46.51 26.64 28.21 28.19 10.92 13.51

T44 15.75 46.50 26.64 28.14 28.13 10.90 13.13

T69 15.68 46.41 26.58 28.30 28.20 10.92 13.32

T41 15.67 46.39 26.56 28.18 28.18 10.90 12.94

T71 15.71 46.45 26.61 28.21 28.17 10, 9i 13.58

T7 15.72 46.47 26.61 28.19 28.18 10.90 14.22

T78 15.75 46.51 26.64 28.23 28.19 10.91 12.81

129
Raw Frequency Data and Elastic Property Data (continued)

Group la

ID$ RF RL RT EF EL GT ENERGY

T43 15.74 46.51 26.63 28.25 28.23 10.92 14.54

T86 15.72 46.47 26.63 28.12 28.13 10.90 13.71

Group lb

ID$ RF RL RT EF EL GT ENERGY

NAl 15.71 46.38 26.60 28.21 28.16 10.93 12.49

NF5 15.64 46.34 26.54 28.16 28.21 10.92 12.30

NA2 15.67 46.35 26.56 28.16 28.15 10.91 12.87

NF7 15.69 46.39 26.59 28.14 28.15 10.91 13.32

NA3 15.68 46.39 26.57 28.25 28.21 10.92 12.36

NF6 15.67 46.36 26.56 28.16 28.14 10.90 12.94

NA4 15.73 46.47 26.61 28.30 28.23 10.92 12.49

NF4 15.67 46.35 26.56 28.14 28.16 10.91 12.94

NA5 15.78 46.52 26.66 28.24 28.18 10.92 12.30

NJ6 15.67 46.36 26.57 28.33 28.23 10.94 13.00

NA6 15.65 46.33 26.54 28.31 28.21 10.93 12.56

NJ7 15.65 46.34 26.55 28.28 28.23 10.94 12.75

NA7 15.69 46.41 26.58 28.37 28.24 10.93 12.56

NJ8 15.72 46.44 26.61 28.36 28.24 10.94 11.98

NA8 15.72 46.46 26.62 28.13 28.18 10.92 13.00

NJ9 15.75 46.49 26.64 28.19 28.19 10.92 12.68

NA9 15.67 46.46 26.62 28.34 28.37 10.99 12.68

NJIO 15.69 46.38 26.58 28.36 28.24 10.94 13.19

NAIO 15.72 46.42 26.61 28.31 28.20 10.94 12.75

NFS 15.71 46.43 26.61 28.12 28.12 10.90 12.62

130
Raw Frequency Data and Elastic Property' Data (continued)

Group lb

IDS RF RL RT EF EL GT ENERGY

NFl 15.73 46.47 26.63 28.09 28.14 10,90 12.68

NJ5 15.71 46.43 26.59 28.27 28.22 10.92 12.36

NF2 15.65 46.37 26.55 28.20 28.18 10.90 12.81

NF9 15.67 46.39 26.56 28.17 28.21 10.91 12.36

NFIO 15.76 46.50 26.65 28.36 28.22 10.94 12.75

NJl 15.86 46.66 26.73 28.37 28,28 10.95 12.56

NJ4 15.69 46.41 26.59 28.26 28,22 10,93 12.56

NJ2 15.75 46.50 26.63 28.34 28.23 10.93 12.62

NJ3 15.70 46.40 26.58 28.33 28.24 10.93 13.19

TF2 15.67 46.37 26.55 28.15 28.18 10.90 12.81

TF3 15.77 46.56 26.67 28.07 28.05 10.86 12.94

TFl 15.71 46.47 26.63 28.14 28.14 10.90 12.68

TA4 15.71 46.47 26.61 28.26 28.23 10.92 12.49

TA5 15.75 46.52 26.66 28.21 28.18 10,92 12.30

TF9 15.66 46.39 26.56 28.26 28.21 10.91 12.36

TA6 15.63 46.33 26.54 28,27 28.21 10,93 12.56

TFIO 15.74 46.50 26.65 28.28 28.22 10.94 12.75

TA7 15.67 46.41 26.58 28.28 28.24 10.93 12.56

TJl 15.83 46.66 26.73 28.42 28.28 10,94 12.56

TA8 15.70 46,46 26.62 28,19 28,18 10.92 13.00

TJ2 15.73 46.50 26.63 28.22 28.23 10.93 12.62

TA9 15.66 46.46 26.62 28.30 28.37 10.99 12.68

TJ3 15.68 46.40 26.58 28.25 28.24 10.93 13.19

TAIO 15.69 46.42 26.61 28.23 28.20 10.94 12.75

131
Raw Frequency Data and Elastic Property Data (continued)

Group lb

1D$ RF RL RT EF EL GT ENERGY

TJ4 15.67 46.41 26.59 28.22 28.22 10.93 12.56

TF8 15.69 46.43 26.61 28.08 28.12 10.90 12.62

TJ5 15.68 46.43 26.59 28.15 28.21 10.92 12.36

TF4 15.65 46.35 26.50 28.19 28.16 10.91 12.94

TJ6 15.65 46.36 26.57 28.28 28.23 10.94 13.00

TAl 15.68 46.38 26.60 28.22 28.16 10.93 12.49

132
Raw Frequency Data and Elastic Property Data (continued)

Group 2a

ID$ RF RL RT EF EL GT ENERGY

Nl 15.56 46.13 26.40 27.84 27.85 10.77 22.96

N58 15.57 46.13 26.41 27.83 27.82 10.76 23.15

N5 15.57 46.13 26.40 28.04 27.94 10.80 23.09

N57 15.57 46.12 26.40 28.01 27.89 10.78 23.74

Nil 15.59 46.16 26.41 27.96 27.87 10.77 23.02

N91 15.60 46.17 26.42 27.99 27.87 10.77 22.76

N14 15.56 46.11 26.40 27.96 27.88 10.79 24.59

N96 15.57 46.15 26.42 27.83 27.85 10.77 26.30

N21 15.56 46.11 26.38 27.81 27.82 10.75 23.28

N97 15.61 46.19 26.44 27.81 27.82 10.76 22.30

N24 15.58 46.14 26.40 28.00 27.87 10.77 25.64

N65 15.56 46.12 26.38 28.07 27.93 10.79 22.69

N27 15.55 46.10 26.38 27.84 27.86 10.77 23.28

N55 15.62 46.22 26.44 27.90 27.88 10.77 26.43

N35 15.56 46.11 26.37 27.86 27.85 10.75 23.41

N73 15.57 46.15 26.40 28.09 27.99 10.81 25.45

N47 15.58 46.15 26.41 27.96 27.91 10.79 22.76

N77 15.64 46.24 26.46 27.97 27.89 10.78 22.69

N51 15.60 46.18 26.44 27.89 27.85 10.77 23.48

N83 15.63 46.23 26.47 27.88 27.83 10.77 26.57

N84 15.60 46.18 26.42 27.97 27.92 10.79 26.57

N88 15.61 46.20 26.44 27.98 27.88 10.78 23.15

N90 15.59 46.14 26.41 27.81 27.79 10.74 23.68

N70 15.59 46.18 26.41 27.92 27.92 10.78 25.91

133
Raw Frequency Data and Elastic Property Data (continued)

Group 2a

ID$ RF RL RT EF EL GT ENERGY

N87 15.57 46.13 26.40 27.95 27.88 10.78 21.91

T47 15.58 46.15 26.41 28.05 27.91 10.79 22.76

T51 15.61 46.18 26.44 27.89 27.85 10.77 23.48

T27 15.55 46.10 26.38 27.97 27.86 10.77 23.28

T35 15.55 46.11 26.37 27.81 27.85 10.75 23.41

T70 15.60 46.18 26.41 28.05 27.92 10.78 25.91

T55 15.63 46.22 26.44 27.94 27.88 10.77 26.43

T73 15.58 46.15 26.40 28.17 27.99 10.81 25.45

T5 15.58 46.13 26.40 28.08 27.94 10.80 23.09

T77 15.64 46.24 26.46 27.97 27.89 10.78 22.69

Til 15.58 46.16 26.41 27.83 27.87 10.77 23.02

T83 15.64 46.23 26.47 27.83 27.83 10.77 26.57

T14 15.57 46.11 26.40 28.00 27.88 10.79 24.69

T84 15.60 46.18 26.42 28.02 27.92 10.79 26.57

T21 15.57 46.11 26.38 27.85 27.82 10.75 23.28

T88 15.63 46.20 26.44 27.93 27.88 10.78 23.15

T24 15.58 46.14 26.40 27.87 27.87 10.77 25.64

T87 15.57 46.13 26.40 27.95 27.88 10.78 21.91

T65 15.56 46.12 26.38 28.03 27.93 10.79 22.69

T90 15.59 46.14 26.41 27.81 27.79 10.74 23.68

T57 15.57 46.12 26.40 27.97 27.89 10.78 23.74

T91 15.59 46.17 26.42 27.82 27.87 10.77 22.76

Tl 15.58 46.13 26.40 27.92 27.85 10.77 22.96

T96 15.58 46.15 26.42 27.87 27.85 10.77 26.30

134
Raw Frequency Data and Elastic Property Data (continued)

Group 2a

IDS RF RL RT EF EL GT ENERGY

T58 15.58 46.13 26.41 27.83 27.82 10.76 23.15

T97 15.61 46.19 26.44 27.85 27.82 10.76 22.30

135
Raw Frequency Data and Elastic Property Data (continued)

Group 2b

IDS RF RL RT EF EL GT ENERGY

N2 15.63 46.22 26.44 27.73 27.78 10.73 26.11

N56 15.58 46.15 26.40 27.61 27.72 10.70 26.24

N3 15.57 46.11 26.38 27.65 27.71 10.70 23.35

N50 15.56 46.10 26.38 27.59 27.68 10.70 22.17

N4 15.59 46.17 26.40 27.63 27.73 10.70 25.05

N93 15.62 46.20 26.44 27.69 27.72 10.71 26.50

N13 15.61 46.19 26.42 27.62 27.71 10.70 25.45

N95 15.59 46.14 26.40 27.49 27.64 10.68 25.25

N15 15.59 46.15 26.40 27.65 27.72 10.71 23.22

N99 15.62 46.18 26.43 27.69 27.72 10.72 23.54

N16 15.59 46.15 26.41 27.63 27.70 10.70 23.28

N59 15.62 46.22 26.46 27.47 27.64 10.69 23.02

N17 15.57 46.13 26.38 27.55 27.70 10.69 26.17

N42 15.62 46.23 26.45 27.60 27.71 10.70 23.61

N25 15.57 46.13 26.39 27.56 27.69 10.70 23.28

N64 15.63 46.24 26.45 27.73 27.76 10.72 23.68

N28 15.56 46.13 26.38 27.56 27.71 10.69 22.63

N66 15.56 46.12 26.39 27.57 27.70 10.70 22.30

N40 15.56 46.13 26.39 27.60 27.72 10.70 26.96

N74 15.58 46.12 26.40 27.49 27.65 10.69 24.07

N81 15.63 46.21 26.45 27.74 27.73 10.72 23.74

N82 15.60 46.18 26.43 27.75 27.77 10.73 26.70

N92 15.55 46.10 26.37 27.50 27.68 10.69 21.52

N63 15.62 46.23 26.44 27.70 27.76 10.72 25.51

136
Raw Frequency Data and Elastic Property Data (continued)

N89 15.54 46.09 26.37 27.50 27.66 10.68 22.43

T28 15.55 46.13 26.39 27.58 27.71 10.70 22.63

T40 15.58 46.15 26.39 27.64 27.74 10.70 26.96

T17 15.56 46.13 26.38 27.51 27.70 10.69 26.17

T25 15.57 46.13 26.39 27.56 27.69 10.70 23.28

T53 15.63 46.23 26.44 27.64 27.76 10.72 25.51

T42 15.63 46.23 26.45 27.62 27.71 10.70 23.61

T64 15.64 46.24 26.45 27.77 27.76 10.72 23.68

T3 15.55 46.11 26.38 27.58 27.71 10.70 23.35

T66 15.57 46.12 26.39 27.60 27.70 10.70 22.30

T4 15.58 46.17 26.40 27.60 27.73 10.70 25.05

T74 15.56 46.12 26.40 27.40 27.65 10.69 24.07

T13 15.60 46.19 26.42 27.58 27.71 10.70 25.45

T81 15.61 46.21 26.45 27.65 27.73 10.72 23.74

T15 15.59 46.15 26.40 27.65 27.72 10.71 23.22

T82 15.59 46.18 26.43 27.71 27.77 10.73 26.70

T16 15.58 46.15 26.40 27.61 27.70 10.70 23.28

T89 15.56 46.09 26.37 27.55 27.66 10.68 22.43

T59 15.64 46.22 26.46 27.55 27.64 10.69 23.02

T92 15.56 46.10 26.37 27.53 27.68 10.69 21.53

T50 15.54 46.10 26.38 27.52 27.68 10.70 22.17

T93 15.60 46.20 26.44 27.56 27.72 10.71 26.50

T2 15.61 46.22 26.44 27.65 27.78 10.73 26.11

T95 15.57 46.14 26.40 27.40 27.64 10.68 25.25

T56 15.57 46.15 26.40 27.57 27.72 10.70 26.24

T99 15.60 46.18 26.43 27.56 27.72 10.72 23.54

137
Raw Frequency Data and Elastic Property Data (continued)

Group 3

ID$ RF RL RT EF EL GT ENERGY

N6 15.72 46.43 26.60 28.04 28.07 10.87 13.58

N49 15.68 46.41 26.58 27.96 28.07 10.86 11.16

N8 15.71 46.42 26.60 27.98 28.04 10.86 13.00

N48 15.68 46.40 26.56 28.05 28.10 10.87 13.07

N9 15.74 46.49 26.62 28.15 28.21 10.91 12.87

N94 15.70 46.40 26.60 27.96 28.04 10.88 12.81

N18 15.74 46.48 26.61 28.08 28.13 10.88 13.58

N98 15.70 46.39 26.59 27.97 28.02 10.86 13.26

N23 15.73 46.47 26.61 28.12 28.15 10.89 13.45

NlOO 15.75 46.49 26.62 28.12 28.12 10.88 13.13

N26 15.68 46.39 26.57 28.06 28.10 10.88 13.13

N53 15.73 46.47 26.62 28.02 28.08 10.87 13.26

N31 15.71 46.43 26.59 28.05 28.09 10.87 12.94

N38 15.73 46.45 26.61 28.15 28.15 10.90 12.36

N32 15.72 46.45 26.61 28.09 28.09 10.88 11.98

N68 15.73 46.46 26.62 28.05 28.10 10.88 13.51

N33 15.67 46.43 26.59 27.89 28.05 10.86 12.36

N72 15.75 46.51 26.63 28.05 28.13 10.88 14.22

N36 15.69 46.39 26.57 28.09 28.10 10.88 12.87

N75 15.70 46.41 26.57 28.04 28.09 10.86 12.36

N76 15.72 46.44 26.61 28.01 28.09 10.88 12.17

N79 15.73 46.47 26.60 28.05 28.11 10.87 12.62

N85 15.74 46.47 26.63 28.12 28.13 10.90 14.15

N54 15.69 46.40 26.58 28.04 28.09 10.88 12.87

138
Raw Frequency Data and Elastic Property Data (continued)

Group 3

IDS RF RL RT EF EL GT ENERG

N80 15.76 46.51 26. 6d 27.99 28.06 10.87 14.28

T33 15.71 46.43 26.59 27.99 28.05 10.86 12.36

T36 15.69 46.39 26.57 28,06 28.10 10.88 12.87

T31 15.71 46.43 26.59 28.05 28.09 10.87 12.94

T32 15.72 46.45 26.61 28.01 28.09 10.88 1 1.98

T54 15.69 46.40 26.58 28.04 28.09 10.88 12.87

T38 15.72 46.45 26.61 28.10 28.15 10.90 12.36

T68 15.74 A ^ AC 46.46 26.62 28.07 28.10 1 A O O 10.88 1 O CI

13. D

T8 15.71 26.60 27.98 28.04 10.86 13.00

T72 15.77 46.51 26.63 28.13 28.13 10.88 14.22

T9 15.73 46.49 26.62 28.15 28.21 10.91 12.87

T75 15.69

A /' At 46.41 26.57 28.00 28.09 10.86 12.36

T18 15.75 A AO 46.48 26.61 28.16 28.13 10,88 13.58

T7d 15.74 A ^ A A 46.44 ^C ^1 26.61 28.10 o r\r\ 28.09 1 A o o 10.88 1 1 ^ 12.17

T23 15.73 A ^ An 46.47 26.61 o r\r\ 28.09 28.15 10.89 1 O A Z

13.4:5

i /y 15. /4 40.47 26.60 o r\c\ 28.09 28.11 10.87 12.62

1 ZD 1 ^ AS AA '^Q ZD. J / OB in 1 n SB ij.ij

T80 15 76 46 51 26 65 27 97 06 1 0 87 141 ". 78w (J

T53 15.73 46.47 26.62 28.00 28.08 10.87 13.26

T85 15.74 46.47 26.63 28.09 28,13 10.90 14.15

T48 15.68 46.40 26.56 28.02 28.10 10,87 13.07

T94 15.70 46.40 26.60 27.99 28.04 10.88 12.81

T6 15.72 46.43 26.60 27.99 28.07 10.87 13.58

T98 15.70 46.39 26.59 27.97 28.02 10.86 13.26

139
Raw Frequency Data and Elastic Property Data (continued)

Group 3

ID$ RF RL RT EF EL GT ENERGY

T49 15.71 46.41 26.58 28.06 28.07 10.86 11.16

TlOO 15.74 46.49 26.62 28.07 28.12 10.88 13.13

140
Summary Statistics and Data for the Initial CV'N Specimens Evaluated

LL49

EE EL GT RE EXER

GY

X OF CASES D.OO D.OO D.OO D.CX) D.OO

25.40 28.05 11.06 14.57 95.79

\L-\XIMUM 25.51 28.15 11.09 14.60 100.98

RASGE 0.11 0.10 0.03 0.03 5.20

25.45 28.09 11.07 14.58 98.64

YAFJASCE 0.00 0.00 0.00 0.00 4.36

STASDARD DEV 0.04 0.04 0.01 0.01 2.09

STD.ERROR 0.02 0.02 0.01 0.01 0.93

SKEA^TsESS iGb 0.11 1.09 0.87 0.27 -0.32

KLUTOSIS iG2i -1.14 -0.17 -0.27 -1.04 -1.31

c.v. 0.00 0.00 0.00 0.00 0.02

MEDL-\N 25.44 28,08 11.07 14.58 99.19

LL55

EF EL GT RF EXE

RGY

N OF CASES 5,00 5,00 5,00 5,00 5,00

^^^^^IU^I 24.91 27.74 10,91 14.43 93.33

MAXIMU^I 25.12 27.84 10.95 14.49 102.1

2

R.ANGE 0.21 0.10 0.04 0.06 8.79

\IE.\N 24.99 27.78 10.92 14.45 96.92

141
Summary Statistics and Data for the Initial CVN Specimens Evaluated (continued)

LL55

EF EL GT RF ENE

RGY

VARIANCE 0.01 0.00 0.00 0.00 13.09

STANDARD DEV 0.08 0.04 0.02 0.02 3.62

STD.ERROR 0.04 0.02 0.01 0.01 1.62

SKEWNESS (Gl) 0.80 0.84 1.29 0.85 0.35

KURTOSIS (G2) -0.82 -0.76 -0.08 -0.73 -1.14

C.V. 0.00 0.00 0.00 0.00 0.04

MEDIAN 24.95 27.76 10.91 14.44 97.58

HH54

EF EL GT RF ENE

RGY

N OF CASES 5.00 5.00 5.00 5.00 5.00

MINIMUM 25.43 28.25 11.13 14.58 17.20

MAXIMUM 25.57 28.38 11.17 14.62 18.76

RANGE 0.14 0.13 0.04 0.04 1.56

MEAN 25.52 28.32 11.15 14.60 17.84

VARIANCE 0.00 0.00 0.00 0.00 0.38

STANDARD DEV 0.05 0.05 0.02 0.02 0.62

STD.ERROR 0.02 0.02 0.01 0.01 0.28

SKEWNESS (Gl) -0.80 -0.35 -0.37 -0.75 0.56

KURTOSIS (G2) -0.67 -0.50 -0.78 -0.64 -1.08

142
Summary Statistics and Data for the Initial CVN Specimens Evaluated (continued)

HH54

EF EL GT RF ENE

RGY

c.v. 0.00 0.00 0.00 0.00 0.04

MEDIAN 25.54 28.32 11.15 14.61 17.63

HH55

FF FT dT RF FNF

RGY

N OF CASES 5.00 5.00 5.00 5.00 5.00

MINIMUM 25.36 28.18 11.10 14.56 18.24

MAXIMUM 25.54 28.27 11.14 14.61 19.97

RANGE 0.18 0.09 0.04 0.05 1.73

MEAN 25.44 28.22 11.12 14.58 19.21

VARIANCE 0.01 0.00 0.00 0.00 0.53

STANDARD DEV 0.08 0.04 0.02 0.02 0.73

STD.ERROR 0.04 0.02 0.01 0.01 0.33

SKEWNESS (Gl) 0.33 0.09 0.35 0.38 -0.15

KURTOSIS (G2) -1.62 -1.41 -1.42 -1.59 -1.39

C.V 0.00 0.00 0.00 0.00 0.04

MEDIAN 25.40 28.22 11.11 14.57 19.11

143
Data for First Pilot-Lot Series

CODES RF RL RT EF EL GT ENERGY

LL49 14.57 45.09 26.06 25.40 28.05 11.06 97.30

LL49 14.59 45.12 26.08 25.48 28.08 11.07 99.19

LL49 14.60 45.17 26.10 25.51 28.15 11.09 99.94

LL49 14.58 45.12 26.07 25.44 28.08 11.07 95.79

LL49 14.58 45.11 26.07 25.44 28.07 11.07 100.98

LL55 14.43 44.84 25.88 24.91 27.74 10.91 93.61

LL55 14.44 44.86 25.89 24.94 27.76 10.91 97.96

LL55 14.46 44.88 25.90 25.02 27.79 10.92 93.33

LL55 14.44 44.85 25.88 24.95 27.75 10.91 102.12

LL55 14.49 44.92 25.93 25.12 27.84 10.95 97.58

HH54 14.61 45.31 26.17 25.54 28.32 11.15 17.46

HH54 14.58 45.25 26.15 25.43 28.25 11.13 17.63

HH54 14.62 45.36 26.19 25.57 28.38 11.17 18.15

HH54 14.61 45.32 26.18 25.54 28.33 11.16 18.76

HH54 14.60 45.31 26.17 25.50 28.32 11.15 17.20

HH55 14.57 45.20 26.12 25.40 28.18 11.11 18.24

HH55 14.60 45.25 26.14 25.51 28.25 11.13 19.97

HH55 14.56 45.21 26.11 25.36 28.20 11.10 19.11

HH55 14.61 45.27 26.16 25.54 28.27 11.14 18.85

HH55 14.57 . 45.23 26.12 25.40 28.22 11.11 19.89

144
Scatter Plots for the Absorbed Energy, Elastic Property, and Frequency Data

Group 2B Scatter Plots

ENERGY

EF

EL ^

im

GT

RF

RL

RT

rHiUn

145
Scatter Plots for Specimen Groups

Group la Scatter Plot (flexure mode)

146
Scatter Plots for Specimen Groups (continued)

Group lb Scatter Plot"^ (torsional mode)

ENERGY

The outlier, apparent in the EF-EL scatter plot was removed from the data, and the

summary statistics. The point was identified as nf3.

147
Scatter Plots for Specimen Groups (continued)

Group 3 Scatter Plot (flexural mode)

148


C.N. McCowan^ and D.P. \'i2liotti^

Homogenization and Subcritical Annealing of 4340 Charpy V-notch

V^erification Specimens

Introduction

When considering the possible contributors to scatter in the impact properties of 4340 quenched

and tempered Chaipy verification specimens, inhomogeneities in the as-received microstmcture

of the material are always a concern. In the Chaipy Impact X'erification Program at NIST, impact

verification specimens are produced from hot-rolled bar stock. The bar is rolled frora a singlemelt ingot of \'TM-VAR 4340 steel. We track both the ingot and the general location within the

ingot from which each bar is made. By not mLxing bar stock from different ingot locations when

making a production lot of verification specimens and b>" tightly controlling the chemistn," of the

\TM-\'AR steel, we minimize these contributions to inhomogeneit\- as much as possible.

However, as the hot-rolled bars are stacked for cooling during the rolling operation, it would be

ejqDected that individual bars are exposed to somewhat different cooling conditions. If so. then

carbide precipitation may var\' from bar to bar, and these carbides affect the trans granular fracture

resistance of the steel. Large carbides are not expected to fully dissoh'e during subsequent heat

treatments, which topically did not exceed 900 'C. Recent work has shown that modification of

these residual carbide net^^'orks during heat treatment between 800 and 900 "C can result in

significant differences in the optimum toughness and the scatter in toughness for these steels. So.

in a continuing effort to minimize the scatter in impact toughness of the Charpy verification

specmiens. the possible benefit of re-homogenizmg our 4340 steel was investigated.

Materials and Procedures

Sample Preparation

The Charpy \'-notch verification specimens were made from \TM-VAR 4340 steel, with lowsulfur and phosphorus contents (0.00 % S & 0.004 % P; residual \' content is approximately 0.04 mass %). We purchased the steel as hot-rolled square bar. The bar was cut to length and ground

to near sample-size dimensions (10 mm x 10 mm >^ 60 mm) pnor to heat treating. Following

heat treatment specimens were machined to final dimensions and the notches were ground. The

specimens used in this study were randomly selected from a production lot that had been prepared

for heat treating. They should contain a representative sample of the bar stock.

^NIST Materials Rehabilit>- Division, Boulder, CO

151
Heat Treatments

Two groups of specimens, Groups A and B, were heat-treated for this study. Each group

contained 30 specimens. The Group A specimens were homogenized and subcritically annealed

prior to hardening and tempering. The Group B specimens, the control group, were annealed

prior to hardening and tempering following our standard practice. The two groups were

combined together in one basket for the hardening and tempering treatments to minimize

differences due to these steps in the heat treatment. Specifics of the heat treatment are given

below.

The Group A specimens were homogenized at 1 100 °C (2012 °F) for 75 min in a vacuum

fiimace and cooled with nitrogen gas at one atmosphere. The rate at which the gas-quench

cooled the samples can be approximated by two linear segments: (1) cooling fi-om 1 100 to

816 °C (2012 to 1500 °F) took 40 min, which represents a cooling rate of near 7 °C/min

(12 °F/min), and (2) cooling from 816 to 38 °C (1500 to 100 °F) took 50 min, which represents a

cooling rate near 16 °C/min (27 °F/min). The homogenized samples were then subcritically

annealed at 650 °C (1200 °F) for 60 min to seed the microstructure with fine carbides (cooled at a

rate of 17.3 °C/min).

The Group B samples were normalized in a vacuum furnace at 893 °C (1640 °F) for 90 min and

gas cooled at a rate of 18.6 °C/min,

Groups A and B were combined for the hardening and tempering, and processed as follows: (1)

held at 870 °C (1600 °F) for 100 min, (2) quenched in oil, (3) tempered at 588 °C (1090 °F) for

120 min in vacuum fiimace and, (4) cooled at rate of 16.7 °C/min.

Mechanical Testing •

The hardness of each specimen was measured by taking the average of two hardness values.

These measurements were made on the face of the specimen that is opposite the notch,

approximately 5 mm from the ends of the samples.

The impact tests were conducted at -40 °C (-40 °F). The impact machine used for the tests is equipped with an optical encoder and digital display that outputs the energy absorbed during the

test. , , .

, ,

Metallographic Samples

Three samples were taken from the groups at various stages during the heat treatment process; (1) a sample from Group A after the homogenization treatment, (2) a sample from Group A after the

subcritical anneal, and (3) a sample from Group B after the normalizing. These samples were

prepared for evaluation by light microscopy. In addition, samples were chosen following impact

testing for evaluations of fracture surface in a scanning electron microscope.

152
Results and Discussion

Mechanical Test Data

The data for absorbed energy of the two groups of

specimens are shown in Figure 1 and listed in

Table 1. Clearly, the different heat treatments

used for the two groups prior to hardening and

tempering treatments affected the distribution and

average values of the absorbed energy data. The

distribution of the Group A data is extremely

skewed and the mean absorbed energy (68.4 J, 50.5 ft-lbf) for the Group A specimens is higher

than that of the Group B specimens (61.6 J, 45.4

ft-lbf).

The range and standard deviation in the absorbed

energy for the Group A data (1 6.2 J and 4.5 J ) are lower than those for the Group B data (19.6 J and

5.7 J), indicating that the homogenization and

subcritical annealing treatments helped to reduce

the variation in absorbed energy in these tests. However, the skewed distribution of the absorbed

energy data for Group A is troubling.

The hardness data in Figure 2 show fairly normal

distributions for both groups of specimens. The

variation for the Group A data (range of 1.7,

standard deviation of 0.40 HRC) is slightly greater

than that for the Group B data (range of 1 .5, standard deviation of 0.36 HRC) , but not

significantly. The mean hardness of Group A

(37.2 HRC) is lower than that of Group B (37.5

HRC), as might be expected from the absorbed

energy results, since hardness generally decreases

as absorbed energy increases. However, within

the small range in hardness for this study, the

relationship between hardness and absorbed

energy (Figure 3) is not too useful for predicting

impact toughness: at a hardness of 37.5 HRC, the

absorbed energy for the Group B specimens has a

range of more than 15 J.

Figure 1. Absorbed energy data for Groups A and B,

notched at the median values and boxed at upper and

lower confidence levels of 95 %.

Figure 2. Hardness data for Groups A & B, notched

at the median values and boxed at upper and lower

confidence levels of 95 %.

80 1 0 1 1 1

o

°

0 —> o ai> X

c

UJ

bed COOx ff' X °X X S 60 <

X X V XXX

X X

=5^ X

50 1 1 1 1 35 36 37 38 39 40

Avg Hardness, HRC

Figure 3. Absorbed energy verstis hardness for the two specimen groups where "o" indicates Group A

and "x" indicates Group B.

153
Figure 4. The decarbxirized layer Figure 5. Lightly etched (picric acid)

(top) at the surface of the microstructure of the homogenized

homogenized sample. Bar equal to 50 sample. Bar equal to 50 |J,m.

|im.

Figure 6. Microstructure of the homogenized and subcritically

annealed sample. Bar equal to 50

\im.

Microstructure

The decarburization of the homogenized samples was approximately twice that of the control

samples (Group B), but the depth of the decarburization should not present any problems. As

shown in Figure 4, the decarburization is approximately 25 |im (0.002 in) deep for the

homogenized sample. Very light machining can remove this surface. The interface between the

decarburized layer and the matrix is smooth and free of stress raisers.

The grain size for the homogenized sample was quite large, as expected (200 to 300 |im). The

microstructure, Figure 5, is presumed from optical-microscopy evaluations to be a combination

of martensite and bainite. The balance of these constituents apparently changes across the

thickness of the sample: near the surface, the darker acicular constituents dominate the

microstructure. Microhardness measurements showed the dark region to have a hardness around

400 to 450 HV, and the matrix hardness to be around 500 to 550 HV. Following the subcritical

annealing, the hardness through the sample was found to be more uniform. The average hardness

of the subcritically annealed sample was around 375 HV. As shown in Figure 6, the

microstructure of the subcritically annealed sample retains many remnants of the homogenized

microstructure, but there appears to be a substantial amount of precipitation and speroidization of

carbides throughout the structure, which results in the darker appearance of the matrix and a

softening of the acicular features.

Fractographv

Six samples were chosen to determine whether the differences in absorbed energy values for the

Group A specimens could be related to fracture-surface characteristics. Three samples from

Group A with low energy and three samples with high energy were used for this comparison.

Even with the unaided eye, the fracture surfaces of the low- and high-energy samples were

observed to differ: the color (shade of grey) of the higher-energy samples was characteristically

darker than that of the lower-energy samples. However, at higher magnifications, no

characteristic differences between the samples were observed. An example of the ductile

154
appearance of the fracture surfaces examined for these

samples is shov,Ti in Figure 7.

Suinmar)'

The comparison of the data for the two groups of

specimens considered here is inconclusive. It is not clear

that the homogenization and subcritical annealing of the

Group A specimens significantly reduced the scatter in

absorbed energ>". However, the Group A data do have a lower standard deviation in absorbed energ>' than that of

the Group B data, and the skewed distribution of the

absorbed energ>' data for Group A contains 18 specimens

that have ver>' low scatter when considered separately

(range of 4 J and standard deviation of 1 . 1 J). This offers some hope that a homogenization treatment can reduce the

scatter in the verification specimens.

The homogenization treatment did not produce stress raisers on the surface of the samples, which

would increase their susceptibilit\' to quench cracking during hardening. So machining the

specimens prior to hardening should not be necessar>'. The depth of decarburization w^as not

excessive, so our machining practices (0.4 mm per side oversize) before heat treatment should be

sufficient

Since &e results of this study show some promise of providing a more robust heat treatment for

the impact verification specimens, the next step should be to conduct a comprehensive study that

v,ill include samples ha\ing a range of precipitated carbide. Most of the small carbides

precipitated during a subcritical annealing treatment would be expected to dissolve during the

hardening treatment, but a homogeneous distribution of carbides might be beneficial to reducing

the scatter in impact energy. The point of the study should be to determine the optimum size and

distribution of carbides required to reduce scatter in absorbed energy.

For any follow-up studies it is recommended that several subcritical annealing conditions be

included in the test matrix to investigate tiie effect of size and distribution of carbides on impact

properties, and tiiat several homogenization treatments be investigated.

Figure 7. The typical appearance of the

fiucture surface near the notch of the Charpy specimens. The length of the bottom of the image is approximately 60

\im.

155
12

3

4

5

6

7

8

9

10

1112

13

14

15

16

17

18

19

20

21

22

23

24

25

26

27

28

29

30

31

32

33

34

: Data for the Group A and B specimens

nROI IP .1 HP Ml

3 75.698 36.200 36 600 36 400

3 79.347 36 300 36.200 36.250

3 72.247 36 900 36.400 36 650

3 65.557 37.100 37.400 37.250

a 70.012 36.700 36.700 36.700

3 67.040 37.200 37.100 37.150

a 74.391 37.000 37.000 37.000

a 65.463 37.500 37.600 37.550

a 66.483 37.600 36 900 37.250

3 64.167 37.300 37.600 37.450

3 65.001 37.600 37.500 37.550

3 63.150 37.000 36 900 36\J\J • 950 \J \J\J

3 64.353 36.700 37.200 36 950

3 64.353 36 800 36 900 36 850

3 72.340 37.600 37.000 37.300

3 64.815 37.700 37.300 37.500

3 66.761 36 900 37.400 37.150

a 76.073 36 900 36 900 36\J\J 900 www

a 72.340 37.500 37.200 37.350

3 65.649 38 000 37.800 37.900

9u 65.463 37.500 37.400 37.450

aCI 66.205 36 900 36 600 36 7*50

CIa 69.082 37 400 37 400 37 400

Cla 66.483 37 400 37 300 37 350

oa 66 297 36 800 37 000 36 QOO

3 66.854 37.900 37.700 37.800

3 64.259 36 900 36 600 36 750

Cia 75.513

b 67.597 37 100 37 200 37 150

b 58.171 37.800 37.800 37.800

b 60.566 38.100 37.800 37.950

b 59.736 37.800 37.300 37.550

b 57.895 38.200 37.900 38.050

b 60.197 37.700 37.600 37.650

156
race rspni ip 1 HR HI HAVn

35 b 69.084 37.300 37.100 37.200

36 b 61.211 38.400 38.100 38.250

37 b 64.076 37.200 36.900 37.050

38 b 68.991 37.100 37.500 37.300

39 b 71.037 36.800 36.700 36.750

40 b 62.689 37.300 36.900 37.100

41 b 53.767 37.500 37.500 37.500

42 b 58.355 37.200 37.100 37.150

43 b 56.425 37.600 37.600 37.600

44 b 64.353 38.300 37.300 37.800

45 b 61.765 37.400 37.600 37.500

46 b 53.309 37.600 37.500 37.550

47 b 57.160 37.100 37.500 37.300

48 b 65.928 37.600 37.200 37.400

49 b 62.412 37.900 37.500 37.700

50 b 55.141 38.300 37.800 38.050

51 b 57.252 37.800 37.200 37.500

52 b 66.299 37.500 37.000 37.250

53 b 69.362 37.400 37.200 37.300

54 b Do. \ 17 Qnn •if .suv lo nnn .you

55 b 55.049 38.100 37.500 37.800

56 b 72.714

157

C. N. McCowan, T. A. Siewert, and D. P. Vigliotti

Heat Treatment of NIST T-200 CVN Specimens

Introduction

A T-type maraging steel was used to produce very-high-energy (near 200 J) verification

specimens that are tested to certify the performance of Charpy impact test machines. The steel is an 18 % nickel alloy in which titanium (rather than cobalt) is used as the primary strengthening

element. In a peak aged condition, these alloys would be expected to have Charpy impact

energies of around 1 10 J (80 ft-lbs) and hardness of about 43 to 47 HRC (Rockwell C scale).

NIST uses the alloy to produce specimens with much higher impact energy, however, and refers

to these specimens as superhigh-energy specimens. Our super high energy verification specimens

have impact energies typically in the range of 175 to 245 J (130 to 180 ft-lbs).

We recently purchased a new heat of T-type maraging steel and planned a study to help optimize

the heat treatment for this new steel and reduce the variation in impact energy of the specimens.

Issues of primary interest to this study include: (1) redistribution of indigenous inclusions by

solution heat treatment, to evaluate the effect on the fracture energy and scatter in impact energy;

(2) grain refmement and beneficial effects of multiple recrystallizations on the variation in

absorbed energy: (3) grain size and morphology effects, and (4) controlled cooling and its effect on the degree of embrittlement and impact toughness.

Literature Review

There has been a significant amount of research done on T-type maraging steels, but there is still disagreement on several of the factors that interest us. It is beyond our scope to present a fiill literature review here, but a number of the pertinent papers are discussed and referenced in the

following review.

Transformation in Maraging Steels

The phase transformations that are of the most interest for the 18 % Ni maraging steels are the

martensite transformation on cooling and the formation of austenite on heating (holding at temperature). As shown in Figure 1, martensite is quite stable during heating, which makes

possible the aging of the martensite. Data cited for T-250 Maraging steels in Table 1, gives A^

and Af temperature bounds for the a+Y region of 661 °C and 730 °C (1223 and 1346 °F),

respectively. ^"^"^ However, substantial amounts of reverted austenite can form in Co-free

maraging steels (and other maraging steels) during aging treatments at temperatures of less than

the As temperature.

159
Table 1: Transformation temperature (reported in Sarma paper, from references)

Steel Af

C(F) C(F) C(F) C(F)

T-250 Co-free 253 (487) 115 (239) 662 (1223) 730(1346)

M-250 7.8 Co 209 (410) 90(194) 630(1166) 720(1328)

There are several ways to introduce austenite into the microstructure of martensitic steels: (1)

isothermal heating in the two phase austenite + ferrite region, where austenite nixcleates and

grows (at Ni-rich precipitates and lath boundaries), and (2) thermal cycling at predetermined

heating and cooling rates between the single phase austenite region and room temperature, which

results in enriched austenite that does not transform to martensite on cooling. In the first case, the

austenite is referred to as reverted austenite, in the second as retained austenite. The nickelenriched austenite has been reported to be stable down to -415 °F (77 K), and is suspected of

having high chemical inhomogeneity. In addition, austenite appears to be hardened by high

dislocation densities that result from phase work hardening (due to an apparent oc-martensite — y

cooperative transformation mechanism that has some similarities to a martensitic transformation).

It is not clear whether reverted or retained austenite adversely affect the scatter in CVN energy.

Some authors report no effect of the stable austenite on the impact toughness, bu-t some others

report beneficial effects. The effects are likely to be unlike those for retained austenite in other

carbon and alloy steels.

Mestastable Equilibrium

1000

hk. Austenite reversion

800 ^^j^ on heating 90% transformation o

s

600

E

1 400

Martensite formation 10%

200 on cooling M, -

0 90%

1 1 L

0 5 10 15 20 25 30

Ni content %

1800 1000

800

1400 Austenite

Temperature

F

Temperature

C Y

Austenite + . Ferrite

600

I Y + a

400 • 200

Farrite a

200

0 ]—1—1 5 10 15 20 25

Ni content %

30

Figure 1. Metastable and equilibrium phase relationship in the Fe -Ni

system. These diagrams are based on those shown for maraging steels in

the Metals Handbook. Desk Edition, (ASM Metals Park, Ohio), p. 4-57.

160
Studies of temperature cycling by Viswanathan show that maraging steels are sensitive to rates of

heating and cooling."^ This study used a steel that was solution-annealed at 950 °C (1 750 °F) for

two hours, air-cooled, then annealed at 820 °C (1500 °F) for 3.5 hours and air-cooled. The steel

was then either conventionally aged at 510 °C (950 °F) for 3 hours or thermally cycled and water

quenched. Thermal cycling was done at 6 °C per min or 9.5 °C per min or 1 1 .5 °C per min.

Cycling was between room temperature and Af (determined to be 1400 °F; at a heating rate of 750

°C). The retained austenite was found to increase with the number of cycles. The faster cycling

rate produced more retained austenite (66 % after five cycles, but at 9.5 °C/min, a single cycle

produced about 40 %. The cycling produces solute-rich austenite that does not transform on

cooling and this results in a less saturated martensitic phase, which reduces the precipitate

strengthening. The Charpy V-notch toughness increased from 12 to 70 J (0 to 60 % Aus) for

specimens in the aged condition as the amount of austenite went from 0 to 60 %.

Grain size and morphology

Work by Sinha on a Co-free (250 grade) maraging steel showed the effect of grain size on

toughness.^ The steel had a composition of 0.008 C, 17.1 Ni, 2.25 Mo, 1.39 Ti, 0.01 Al, 0.01 S,

0.008 P, 0.004 O2 , and 0.003 N2 . In the study, hot-rolled pieces were solution-annealed at 7

different temperatures for 1 h (air-cooled) and evaluated for microstructure, strength, impact

toughness and fracture toughness. Some of the specimens were aged at 477 °C (890 °F) and some

were tested in the unaged condition. Full recrystallization occurred after holding for an hour at 825 °C (1520 °F), and this treatment resulted in the optimum strength/toughness combination

(ftill ductile dimple rupture, no ridges, 25 pm blocky martensite). Grain growth occurred at temperatures above 852 °C (1565 °F), accompanied by a gradual change in the martensite lath

morphology from blocky to stringer type. The transformation corresponded to a grain size of 35-

40 Jim. The transformation to stringer type was complete at 1052 °C (1925 °F). Interestingly,

this transformation was correlated to a reduction in tensile ductility, fracture toughness, and

CVN. There was another decrease in toughness for the 1052 °C (1925 °F) specimens that was

attributed to precipitation at grain boundaries. The CVN energy was the least changed of the

toughness indicators measured, and was most constant for specimens annealed between the

temperatures of 850 and 1000 °C (1565 and 1835 °F). All the CVN data, however, were for aged

specimens. (It was also determined in the Sinha study that grain size has little effect on strength,

because the martensite lath spacing was not changed by change in grain size.)

Another study by Sinha (T-250 Co-free and M-250 7.5 Co) detailed grain-growth behavior for

maraging steel.^ In this study, isothermal annealing temperatures were used: the specimens were

mitially annealed for 1 h at 825 °C (1520 °F) and air cooled, then held for times of 0.25 to 10 h at temperatures ranging from 900 to 1050 °C (1655 to 1925 °F). The results show that only modest

gram growth (less than 50 jim) occurred for the T-250 Co-free steel at 900 °C (1655 °F) for times

up to 3 h. Longer holding times, even at these low temperatures, were shown to sometimes result

in abnormal grain growth.

It seems to be generally recognized that grain refinements can be attained in maraging steels by

cyclic heating and cooling treatments. Specimens with large grain sizes (hundreds of

161
micrometers) can be refined to some minimum size (less than 50 micrometers), after which cycHc

treatments do not result in further refinement. The paper by Saul addresses this issue for the 250

and 300 grade maraging steels.' However, no paper we reviewed presented clear evidence of

how and why the refinement occurs, or why the particular treatment schedule is apparently so dependent on the specific composition (or initial microstructure) of the steel.

Aging

Most research does not include aging data for temperatures as low as those used to produce NIST

T-200 impact verification specimens (315° C ), because aging temperatures this low are not of

commercial interest. There has been some indication, however, that aging at low temperatures

results in the formation precipitates different from those typical of the 480 °C (900 °F) aging

treatment that is commonly used for these alloys.

Studies on an 18 Ni Co-containing 350 grade maraging steel and on T-250 Co-fi-ee maraging steel

showed differences in the precipitates formed above and below 450 °C (845 °F).^ * The studies

indicate that NijTi precipitates are formed in the alloys at high aging temperatures, but at low

aging temperatures (3 1 5 °C, 3 h) actual precipitation probably does not occur. It is more likely

that clusters of Ni and Ti atoms cause the strengthening. The study by Sinha, which include

aging temperatures as low as 468 °C on T-250 Co-free maraging steel, shows that the hardening

due to aging was rapid (increases of 80 to 90 % within the first 15 to 30 min). Interestingly, the

toughness of the under-aged maraging steels in the Sinha study was lower than the toughness for

the peak-aged steels. This is apparently due to the clusters or coherent precipitates that are

present in the imder-aged condition, which restrict cross slip in the matrix. In the peak-aged

condition, precipitates (Ni3Ti) are formed that allow more homogeneous slip in the matrix.

Thermal embrittlement

Maraging steel can become embrittled during high-temperature solution-annealing treatments.

The embrittlement is caused by precipitation of Ti (C, N) at grain boundaries during cooling, and

can be retained even following re-armealing. Quenching from high temperature prevents the

precipitation and subsequent embrittlement.

Sinha studied thermal embrittlement in a T-250 maraging steel and showed that marked

degradation in toughness can result when the steel is cooled from high temperature and held

between 785 and 400 °C (1450 and 1750 °F).^ In the study, two heat treatments were used: (1)

HTl, solution-treated at 1200 °C ( 2192 °F) for 1 h and quenched to intermediate temperature for a hold of 180 min then air-cooled, (2) HT2, solution-treated at 1200 °C (2192 °F) and water

quenched, then reheated to intermediate temperature for 180 min and air cool. The composition

of the T250 steel used in the study was 0.006 C, 0.005 P, 0.001 S, 2.25 Mo, 17.1 Ni, 0.10 Al,

0.003 N, 1.39 Ti, and 0.004 O. The steels were tested in unaged and aged conditions (age at 480

°C for 3 h). The impact energy of the HTl as-quenched steel was 188 J, compared to 25 J for the

HTl specimens that were held at intermediate temperatures and embrittled. No effect of

embrittlement was found at any intermediate temperature for the HT2 treatment (about 190 J for

162
all intermediate treatments). So embrittlement occurred only when the steel was directly cooled

to an intermediate holding temperature.

Other embrittlement studies have shown that the steel must be quenched from re-annealing

treatments to avoid embrittlement, but Sinha reasons that in his study there was uniform

precipitation of Ti (C,N) on the dislocations formed during the transformation to austenite on the

reheating of the solution-annealed and quenched steel (and this kept the Ti out of solution, where

it could not segregate to grain boundaries during cooling from the re-annealing treatment).

Inclusions

One would hope that a solution-treatment could be used, in concert with controlled heating and

cooling to dissolve and redistribute the large Ti(N,C) inclusions in the T-type maraging steels.

This matter is of practical interest because these inclusions can have a significant effect on the

homogeneity of the initiation and propagation of ductile tearing in the steel. The solution

treatment could also help to redistribute chemical inhomogeneity in the material that might reduce

variation in impact properties.

Summary of Past Heat Treatments on the NIST T-200 Bar Stock

A number of heat treatments have been done on the T-200 material at NIST and by NIST

contractors. Results from these heat treatments have contributed to our general understanding of

this particular heat of T-200 steel, and we discuss some specific details below.

Initial heat treatments on the T-200 material provided a general understanding of the energy

levels that might be expected. The mechanical test results for two of the heat treatments are shown in Figures 2-4. In Figures 2 and 3, the specimens were armealed at 955 °C (1 750 °F)

for 1 h and air-cooled, then re-annealed at 760 °C (1400 °F) for 1 h and air cool. These

specimens were then divided into five groups and aged at 260 °C (500 °F), 288 °C (550 °F),

315 °C (600 °F), 343 °C (650 °F), and 370 °C

(700 °F) for 3 h and air-cooled. The data show

the relationship between the impact energy and

the hardness of the specimens, and indicate that

specimens aged at less than 300 °C can reach

toughness levels near 200 J.

The data in Figure 4 are similar to that in Figure

2, but these specimens were annealed at 900 °C

(1650 °F) for 1 h and water-quenched, then

reheated twice to 675 °C (1250 °F) and waterquenched as a grain refinement treatment, and re- annealed at 815 °C (1500 °F) for 1 h and air- cooled prior to aging at 3 1 5 °C (600 °F) and

then at 370 °C (700 °F) for 3 h. Other variations

300 350

Aging Temperature, C

Figure 2: Impact energy versus aging

temperature.

400

163
300 350

Aging Temperature, C

Figure 3: Hardness distributions.

400

of these two heat-treatment schedules produced

similar results. Overall, it appears that this T-200

material can be annealed and aged to produce

Charpy specimens having impact energies of 200

J. The hardness of specimens with impact

energies of 1 15 to 205 J ranged from 40 to 32

HRC.

As shown in Figure 5, quenching from the

annealing temperature clearly results in a softer

(tougher) material, and the difference in hardness

is retained after aging (compared with air-cooled).

These data are from specimens heat-treated at NIST in laboratory furnaces. The specimens were

produced mainly for microstructure evaluations,

but hardness tests were made to give some

indication of the toughness. We found a difference of about 5 HRC, between the quenched

and unquenched specimens, which may be helpfiil

in increasing the toughness of the lots. Based on

the results shown in Figures 2 and 3, an increase

in toughness on the order of 30 or 40 J might be

associated with a difference in hardness of 5 HRC.

Microstructural evaluations on laboratory

specimens suggested limits on annealing

temperatures to control grain growth, treatments

to refme the grain size, and procedures to control

retained austenite levels in the specimens. Examples of the microstructures observed for the

260

100 200 300

Aging Temperature, C

Figure 4: Absorbed energy distributions.

specimens are given in Figure 6-13. 35

GROUP

* WaterQuendi

Air Cooled

The as-received T-200 bar stock (Figure 6) has

a small grain size (likely 10 pm or less), which is desirable. The grain boundaries are decorated with

particles or retained austenite. Based on

microstructural observations of the specimens

heat-treated in our laboratory furnace, grain

refinement is attainable. For example, the

microstructure shown in Figure 7, which has a

grain size of about 25 jam or so, was produced

from a microstructure having an initial grain size

of about 50 pm. In this case, the grain refinement was attained by slowly cooling the specimen through the two-phase region (to room temperature),

then reheating and holding it at the temperature where reverted austenite forms, prior to re- annealing the specimen.

20. -L

1 2 3 4 5 6

Aging time, h Figure 5: Average hardness trends.

164
FigTire 6. MicTOStrucuire of the as- received T-20'0 bar stock. Bar equal

to 10 prrL

ii I

Figure 8. Specimen was annealed at 870 °C and air-cooled, then re- heated to 660 "C for 1 h and waterquenched. Bar equal to 20 jam

Figure 7. Specimen was annealed at 870 'C air-cooled, re-heated to 575

'C and air cooled , then re-annealed

at 840 -C and air-cooled. Bar is equal

to 20 ]jm.

Figure 9. Specmien was annealed at 890 "C and water quenched then re- heated to 590 for 1 h and waterquenched. Bar equal to 50 p.m.

Figure 10. Specimen annealed at 890 "C and water-quenched. Bar

equal to 50 ylul

Figure 11. Specmien was heated to 600 'C for 1 h. water-quenched, then

reheated to 660 "C for 30 min, then

to 840 'C and held 1 h and waterquenched. Bar equal to 50 ]im.

16:
In Figures 8 and 9, results of holding the specimens at temperatures below the Af temperatixre

are shown. In Figure 8, the microstructure of the specimen, which was held in the two-phase a +

Y region, has clearly delineated austenite grain boundaries. The basic morphology of the grains

has changed to an equiaxed austenite-like morphology. However, the grains contain a fine trwophase structure of reverted austenite and a-martensite, and occasional regions of a coarser twophase structure. In Figure 9, the microstructure of the specimen, which was held at a temperature

just below the two-phase region, has significant amounts of reverted austenite at prior austenite

grain boundaries and between laths of martensite within the grains.

We did not determine the or the Af temperature in our experiments, but the estimates given by

Sarma in Table 1 appear to be reasonable for our T-200 material. Our observations also indicate

that specimens containing reverted austenite could not be fully annealed when treated at 760 °C

(1400 °F) with holding times of 1 h. So, we might consider 815 °C (1500 °F) to be a minimxim

temperature for annealing treatments. A maximum annealing temperature of about 870 °C ( 1 600

°F) is suggested by our testing, because we see significant grain growth for annealing treatments

done at 925 °C (1700 °F) for 1 h, and some grain grov^h likely also occurred for the 890 °C

(1650 °F) annealing treatments, as indicated by the microstructure shown in Figure 10.

Overall, the laboratory heat-treatment experiments showed that temperature cycling between

room temperature and the two-phase region (and below AJ yielded some grain refinement in our T-200 material. However, cycles between room temperature to slightly above Af also yielded

some grain refinement, and this cycle avoids the formation of too much reverted austenite. ^We

found that when grain growth occurred due to annealing or solution treatments, it was possible

with additional heat treatments to refine the grain size with addition heat treatments back to a

reasonably small size (25 pm).

The final topic of discussion here is the grain morphology, because there are two characteristic

grain morphologies typical of the specimens. Grain morphologies with little grain-boundary

definition (like shown in Figure 10) and a resolvable stringer-type martensite (not shovm) occur

in specimens annealed at higher temperatures. The literature indicates that the morphology is a

function of grain size, where the morphology changes at grain sizes near 35 or 40 ]im. At smaller

grain sizes, the specimens tend to have grain morphologies more like those shovm in Figure 11.

Here, the grain boundaries were etched to mark and delineate the grain boundaries, and the

martensite morphology is blocky (according to Sinha) which likely results in the different

appearance of the grains. Our T-200 verification specimens that have had the lowest variations in

impact energy have had this small grain morphology with well delineated grain boundaries

166
1200

1000

800

600

400

200

0

Solution

Treatment Grain Annealing Treatments

Refinement Al +A2 + A3 + A4

GRl GR2

.-A

Aging

Figure 12: Heat treatment sequence used with reference to the and Af temperatures.

Materials and Heat Treatments Table 2. Heat treatments for the three groups of 1, Three groups of Charpy V-notch specimens 2, and 3 specimens. The five specimens taken for were heat-treated by a commercial shop for evaluation and plotted in Figure 13 are identified,

this study. There were approximately 70

specimens in each group. As shown in

Figure 12 and Table 2, the initial heat

treatment for the group 1 specimens was as

follows: (1) solution treated at 1204 °C

(2200 °F) for 1 h and cooled using a 10 bar

helium quench, and (2) grain-refinement

treatment consisting of a short anneal (GRl)

at 815 °C (1500 °F) with a 10 min hold and

a 10 bar helium quench, followed by a second anneal (GR2) at 815 °C (1500 °F)

for 30 minutes with a 10 bar helium quench.

The group 2 specimens did not receive the

solution treatment, but did receive the grain

refinement treatments (GRl and GR2). The

group 3 specimens received neither the

solution treatment nor the grain refinement

treatments. So the test matrix has three main

legs: (1) group 1, which received a fiill solution treatment and then grain-refinement steps prior to re-annealing, (2) group 2, which,

received the same grain refinement steps as the Gl specimens but no solution treatment, aa<i (3)

group 3, which was a simple annealing schedule using the as-received material.

GroupGroup2 Group

3

1204° C,l h

10 bar He

Specimen 1

GRl treatment, 815° C,

10 min,

10 bar He

Specimen 2

GRl treatment, 815° C,

10 min,

10 bar He

Specimen 4 GR2 treatment, 815° C,

30 min,

10 bar He

Specimen 3

GR2 treatment, 815° C,

30 min,

10 bar He

Specimen 5

Final anneals

(4 variations)

Final anneals

(4 variations)

Final anneals

(4 variatious)

Aging Aging Aging

Subgroups of groups 1, 2, and 3, containing 15 to 20 specimens each, were annealed togetli_er in

the same basket. The four annealing practices were as follows: (1) 830 °C (1525 °F) for 2 li

167
with a 5 bar helium quench, (2) 830 °C (1525

°F) for 2 h with a 1 bar nitrogen quench, (3)

900 °C (1650 °F) for 2 h with a 5 bar helium

quench, and (4) 900 °C (1650 °F) for 2 h with a

1 bar nitrogen quench.

The effect of aging time and temperature on the

impact toughness of the specimens was not

evaluated. All of the specimens from groups 1

,

2, and 3 were aged together at 600 °F for 3 h

and quenched in nitrogen at 6 bar.

Specimens for mechanical testing and

microstructural evaluations were removed from

the group 1 and 2 specimens prior to annealing

and aging of the specimens. One specimen was removed from the group 1 specimens following

the solution treatment. Another group 1 specimen was removed following the GRl

grain-refinement treatment and still another

follov^ng the GR2 grain-refinement step.

Similarly, two group 2 specimens were removed

following the GRl and GR2 grain-refinement

steps, respectively.

260 \

250-

240-

''l 230-

2 220- G1: Sol UJg 210-

n 1 • G2: GR1

G2: GR1+GRi2

G1: SOI+GR1+GR2

200-

190-

180-

G1: S0I+GRI

1 2 3 4 5

Specimen

Figure 13. Specimens tested at various stages

of the heat treatment are identified by specimen

numbers assigned in Table 2. The group (Gl or G2), solution treatment (sol), and grain

refinement steps (GRl and GR2) for the

specimens are also indicated.

Results and Discussion

Solution Treatments and Grain Refinement

The results for the initial heat treatments of the group 1,2, and 3 specimens indicate that the

solution treatment of the T-200 did not result in significantly increased toughness for the material.

As shown in Tables 1 and 2, and in Figure 13, the solution-treated G 1 specimen, and the

solution-treated and grain-refined Gl specimens had respective impact energies of 224 and 237 J. The G2 specimen, which was not solution treated, had an absorbed energy of 240 J following the

grain-refinement treatment. These results are from single specimens, but comparing the solutiontreated and grain-refined specimen to the as-received and grain-refined specimen, little difference

in the level of toughness is apparent. This implies that the as-received material is a relatively

homogenous bulk material and solution treatments might be expected to have only limited effect on the toughness level of the specimens.

The microstructure of the group 1 specimen (Figure 14) that was evaluated following the

solution treatment at 1204 °C (2200 °F) had a very large grain size, as might be expected. The

microstructure appears blocky rather than a stringer type, however, which may indicate that grain

size does not always dictate the grain morphology. It was also noted that the grain boundaries

168
etch unevenly, and this may indicate that precipitates or slight chemical inhomogeneities are

present at the boimdaries even for the hardest quenched specimens.

The group 1 specimen that was solution treated, then re-heated slowly to 815 °C (1500 °P) for 10

min had a predominantly stringy martensitic structure (Figure 15). The group 1 specimen.

(Figures 16 and 17) that was solution annealed and re-heated twice for grain refinement liad a

mixture of blocky and stringer-type structures. More importantly, the grain size is clearly refined

compared to that for the as-solution-treated structure (Figure 14). This specimen had the highest

toughness.

The group 2 specimens (Figures 18-19), which were not solution treated, show slight differences

in structure fi^om one another. The specimen that received only one grain-refinement step. Figure

18, has reverted austenite at prior austenite grain boundaries and on preferred planes witkin the

grains (as does the as-received T-200 bar stock). This specimen had the highest energy for the

heat-treatment conditions considered here. The specimen that received both grain-refinement

treatments (Figure 19) has a slightly larger grain size, and less reverted austenite is apparent.

Both specimens retained a reasonably small grain size (probably less than 25 pm). The slightly

higher toughness of the specimen in Figure 18, may reflect the smaller grain size and/or the

presence of more reverted austenite in the structure.

Comparing the group 1 and 2 specimens, grain-refinement treatments following the solutiontreatment were effective in reducing the very large grain size of the as-solution-treated specimens.

The large grain in the center of the solution treated specimen shown in Figure 14 is about 500

pm in diameter, compared with grain diameters on the order of 10 to 20 pm in the nonsolxitiontreated group 2 specimens (Figures 18 - 19). It is not clear whether any grain refinement

occurred in the group 2 specimens. These specimens have grain sizes similar to the as-received

T-200 material.

The solution treatment did not result in a change to the large indigenous Ti(N,C) inclusioxi

content or size distribution, as might be expected (the melting point of these inclusions is over

2900 °C) . Measurements on group 1 and group 3 specimens (400 fields per sample) shoAv the

Ti-rich inclusions to favor cube-like morphologies with an average size of about 1 1 pm (cube

edge). The average number of Ti(C,N) inclusions per millimeter squared was estimated to be

about 10. There were differences in the amounts of smaller indigenous inclusions in the samples.

The as-received sample had a significantly higher number of inclusions with diameters in. the

range of about 2 to 30 pm. Detailed evaluations of these smaller inclusions (which may also

include small islands of retained austenite) were not done, however, so no data on these

inclusions are available (the inclusion counts were done at too low a magnification to yield

accurate information on these smaller inclusions).

Since the solution treatment does not result in a beneficial modification to the large Ti-ric^h

inclusions, and the as-received material was not found to be embrittled, it appears that there is

little need to include a solution treatment in the processing of this material, unless it results in

lower scatter to the specimens following the final annealing treatments.

169

Final Annealing treatments (1 through 4)

The microstuctures of the specimens after the final

annealing treatments showed that the group 2 and

group 3 specimens had smaller grain size than the

groiq) 1 specimens. In Figure 20, for example, the

grain size of the group 1 specimen is larger than

that of the group 3 specimens shown in Figures 21

and 22. The group 1 specimens, the only specimen

group that was solution-annealed, had some grains

as large as 100 pm, and many grain diameters were assumed to be between 20 and 40 ]im.

The group 2 and group 3 specimens had similar

grain sizes. However, those specimens that were

annealed at the lower temperature (Figure 21)

generally had a smaller grain size (typical sizes

range between 4 and 20 jam in Figure 21) than

those annealed at higher temperature (typical sizes

range between 15 to 30 pm). So, some grain

growth was associated with the final annealing

treatments, particularly for the higher-temperature

anneals (900 °C, 1650 °F), but grain sizes

remained reasonable for all of the treatments.

The impact-test results are summarized in Figure

23. The results of anneal 1 (1525 °F, 2 h, 5 bar

helium quench) showed that a slightly higher

absorbed energy (157 ft-lbs) was attained for the

group 1 specimens, which were solution-aimealed.

The group 3 specimens, however, had lower

scatter in absorbed energy than the group 1 or group 2 specimens. So the particular solution and

grain refinement treatments used here for the

Group 1 and 2 specimens did not reduce the scatter

in impact energy over that found for the as- received and annealed specimens.

The results of anneal 2 (1525 °F, 2 h, 1 bar

nitrogen quench) again show that the group 1 specimens have slightly higher toughness (153 ft- lbs) than the group 2 or group 3 specimens (147

and 149 ft-lbs). However, the slower quench used

for this annealing step resulted in higher scatter

Figure 20. A group 1 specimen, with a hightemperature anneal, specimen # 391. Bar

equal to 10 |im.

Figure 21. A group 3 specimen, with a lowtemperature aimeal, specimen # 2.

Figure 22. A group 3 specimen with hightemperature anneal (#36)

171
- CODE

G1

G2

G3

Figure 23. Box plot of the data for the final annealmg treatments.

(coefficient of variation) for groups 1 and 2 (0.042 and 0.047), compared with the results for

anneal 1 (0.035 and 0.040). The scatter for the group 3 specimens is the exception here, where

similar low levels of scatter (0.03) were found to be independent of cooling rate.

The aimeal 3 (1 650 °F, 2 h, 5 bar helium quench) produced data of impact energy similar to that

for the anneal 1 and 2 treatments: 154, 149, and 144 ft-lbs for groups 1, 2, and 3, respectively.

The scatter in absorbed energy for the group 1 and 2 specimens (0.039 and 0.039) was similar to previous results, but the scatter for the group 3 specimens increased (0.045).

The data for the anneal 4 specimens (1650 °F, 2 h, 1 bar nitrogen quench) show^ed absorbed

energy levels similar to all the previous data, but had consistently higher scatter in absorbed

energy: The coefficients of variation for group 1, 2, and 3 were 0.049, 0.055, and 0.062,

respectively. Again, the slower quench rate (used to simulate an air cool for this annealing

treatment) resulted in higher scatter for the specimens.

Summary

Results of this study and the initial studies on this T-200 material have provided useful

information for the production and quality control of the super-high-energy specimens. A

summary of our understanding for the new T-200 material is as follows:

• The T-200 material is relatively homogeneous.

• The T-200 material can be used to produce impact verification specimen having energies

ofnear200 J.

• A minimum temperature 815 ° C (1500 °F) is suggested for annealing treatments.

• A maximum annealing temperature of about 870 ° C (1600 °F) is suggested.

• Significant grain growth occurs at temperatures above 900 °C (1700 °F, for 1 h).

172
Grain refinement is possible if grain growth occurs during heat treatment.

A small, more or less equiaxed grain morphology (less than 30 pm) with well defined

grain boundaries is desirable.

Increasing the amount of reverted austenite in the microstmcture appears to increase the

toughness of the material (but the effect on variation in the absorbed energy was not

evaluated)

The variation in the absorbed energy is likely reduced by quenching the material rather

than alloN^ing it to cool more slowly.

The \ ariation in the absorbed energy was not clearly reduced by solution treatments or by

grain refinement treatments.

173
CODE NUM ANNEAL Anneal

code

ENERGY

(ft-m

Energy

fJ)

HARD

fHRC)

Gl 1 SOL 0 165 224 25 Gl 2 SOLGRl 0 139 189.0 30 Gl 3 S0L_GR1_GR2 0 175 237 28 G2 1 GRl_ONLy 0 186 252 28 G2 2 GR1_GR2 0 177 240 27 Gl 4 GR1_5BAR 1 158 214 30 Gl 5 GR1_5BAR 1 158 214 30 Gl 6 GR1_5BAR 1 158 215 30 Gl 7 GRIJBAR 1 147 200 30 Gl 8 GR1_5BAR 1 160 217 30 Gl 9 GR1_5BAR 1 151 205 30 Gl 10 GR1_5BAR 1 155 211 30 Gl 11 GR1_5BAR 1 166 225 30 Gl 12 GR1_5BAR 1 158 214 30 Gl 13 GR1_5BAR 1 152 206 30 Gl 14 GR1_5BAR 1 160 218

Gl 15 GR1_5BAR 1 148 201 Gl 16 GRIJBAR 1 156 212

Gl 17 GR1_5BAR 1 151 204 Gl 18 GR1_5BAR 1 168 228 Gl 19 GR1_5BAR 1 151 205 Gl 20 GR1_5BAR 1 159 215 Gl 21 GR1_5BAR 1 155 210 Gl 22 GR1_5BAR 1 163 221 Gl 23 GR1_5BAR 1 158 214 G2 3 GRI_5BAR 1 152 206 31 G2 4 . GR1_5BAR 1 152 206 31 G2 5 GR1_5BAR 1 146 198 31 G2 6 GRIJBAR 1 133 181 31 G2 7 GR1_5BAR 1 146 198 31 G2 8 GR1_5BAR 1 151 205 31 G2 9 GRIJBAR 1 150 203 31 G2 10 GR1_5BAR 1 156 212 31 G2 11 GR1_5BAR 1 149 202 31 G2 12 GR1_5BAR 1 146 198 31 G2 13 GR1_5BAR 1 153 207 G2 14 GR1_5BAR 1 145 197 G2 15 GR1_5BAR 1 158 214 G2 16 GR1_5BAR 1 144 196 G2 17 GR1_5BAR 1 156 211 G2 18 GR1_5BAR 1 154 208 G2 19 GR1_5BAR 1 142 193 G2 20 GR1_5BAR 1 145 197 G2 21 GR1_5BAR 1 158 214 G2 22 GR1_5BAR 1 150 203 G3 1 GR1_5BAR 1 140 190 31 G3 2 GR1_5BAR 1 143 194 31 G3 3 GR1_5BAJR 1 141 191 31 G3 4 GRIJBAR 142 193 31 G3 5 GR1_5BAR 150 203 31 G3

' 6 GR1_5BAR 146 198 31 G3 7 GR1_5BAR 149 202 31 G3 8 GR1_5BAR 141 191 31

174
CODE NUM ANNEAL Anneal ENERGY Energy HARD

code /ft TkA ( It-IDI) LD mRC)

r^i Cj3n /^D 1 CDAD ijKl_M3AK 1 CA z03 31 Oi lU LrKl_jDAK 1 C>4 1 j4 7AQ zUo 31

f^i Cj3 1 1 1 

CO 1 CDAD 1 >fQ zOz

Cj3 IjK1_3dAK 14U 190 1 'J 1 J

OKI Jt3/VK 1 71

ZlU

riD OKI1 CD

AD 3I5/VK 1 AO I4y 7A7 zUz

13 rio 1 CD

AD OKl_Ji5AK 1 A7 14/ 1 OO lyy

CVl VJJ 1IDA OKI JJj/VK. 13z zUO 1 71 / OK1 jtSAK 140 1 06 iy5 1 &15 npOKI1 jDAK <ia A p 14j 1 07 ly/ 1 01 7 rjp OK11 3x3 ^"RAKA p l*t4 1 o< iy3

l\j riD OKI1 jOAK CD A P 1 A^ 14J 1 07 ly/ en OA rjp OK1 1 R A p 1 1 15AK z 1 AA 1 o< iy3 1A

Vjl npOK1 1 R

Ap 1 1 DAK z 14y 7A7 zUz 1A30

ni\j\ OA npOKl_lI5AK 1 1 R A p z 11 j3 71 1 Zi 1 1A30

(W 07Z / npOK11 1i DAK R A P 7z 133 7AS zUo 1A30 ul 78Zo npOKl_l£>AK 1 1 R A p z 14z 1 Q1 iy3 1A30 m 7Q noOK1_1dAK 1 1 R A D z 13/ 711 zl3 31

Lrl npOK11 tI DRAKA p 7z 1 CO I3y 7 1 < Zl3 1A30

Lrl

J 

npOK11 11 RAp r5AK 7Z loo 77jC zzO 1A30

n.'\ 52. noOKl_l 1 1 RdAKA D Z 133 711 zll 1A30

VjlJJ npOK11 11 R13AKA p 7z 130 71 

Zl 1 1A30

mul 1A npOK1 1 RAp 1 1 IJAK J. 1 <fi 135 71

Zl4 1A30

mul

JJ npOK1 1 R

Ap 1 1 15AK z 1 <c 135 71

zl4 1A30

If, npOK11 11 R15AKA p 7Z 140 1 Ofi lyp 1A30

17 npOK1 1 RAp 1 1 X5AK Z 1 CA 1 34 7AQ zuy 1A30

Ifi JO npOKl_lI5AK 1 1 R A p 7Z 114A7/ 1 oo lyy 1A30

71zj npOK11 11 R15AKA p 7Z 1 30 71 

zl 1 1131

7i4 npOKl_l£5AK 1 1 R A p 7Z 1 CQ 1 3y 71 < zl3 1131 7<Zj npOKl_l 1 1 RtsAK A p 7 1 AA 144 1 OA lyo 1131 7AZO npOKl_lt5AK 1 1 R A p 7Z 1 A1 14j 1 OA iy4 1131 77Z / noOKl_H5AK 1 1 R A P Z 1 A7 14z 1 oi iy3 1131

CVi 7fi Zo npOKl_lX5AK 1 1 R A p 7Z 1140Af 1 07 ly / 1131 Oz 7QzV no 1 1 D A D OKl_ii3AK *>Z 13o 1 CA 154 1131 in no 1 1 R A D OK1_1i3AK Z 1 Af 140 1 OQ ly© 1131 1 

31 noOK1_1dAK 1 1 R A D z 1 AA 14U 1 OA lyu 1131

Lrz 173Z m> 1 1 D

AD OKl_lnAK z 1 Cit 130 717 zlz 1131

m 1133 nD 1 1 RAD OKl_lr5AK z 1 C7 13Z 7AA zUO

1i4 34 npOKl_li3AK 1 1 R A D z 1 CI 131 7AA zU4

Lrz 1<3j noOKl_li5AK 1 1 R A D 'Jz 1 A7 14 / 1 OO lyy

lA3o noOKl_lx>AK 1 1 R A I> z 1 1A 1 30 1 6A 154

173 / npOK1 1 RAD 1 1 15AK *>z 1 CI 131 7A< ZU3 CX Zl71 npOK11 11 RDAKA p 7z 1 Afi 14o 7AA ZW 1131

cvx 77Zz npOK1 1 RAD 1 1 DAK z 1 CI 131 7AA zU4 1131 71Z3 noOK1_1dAK 1 1 R A D z 1 AH 140 1 QQ iy5 1131

cvx vj3 7Hz4 noOK1_1dAK 1 1 R A D z 130 711 zll 1131 7^Z3 no 1 1 R A D OKl_lr5AK z 1 /IT 14 / 1 OQ lyy 1131

cvx 7/; zo nDOKl_lr>AK 1 1 R A D z 1 C7 13z OA7 ZU/ 1131 77Z / npOK1 1 R A p 1 1 DAJv 7z 1 3U 7AA ZU4 31n

G3 28 GR1_1BAR 2 144 195 31 G3 29 GRIJBAR 2 148 201 31 G3 30 GR1_1BAR 2 141 192 31 03 31 GR1_1BAR 2 144 195

175
CODE NUM ANNEAL Anneal ENERGY Energy HARD

code (R-m (J) fHRO

G3 32 GRIJBAR 2 154 209 G3 33 GR1_1BAR 2 149 201 G3 34 GRIJBAR 2 154 208 G3 35 GR1_1BAR 2 146 198 Gl 39 GR2_5BAR 3 160 217 30 Gl 40 GR2_5BAR 3 168 228 30 Gl 41 GR2_5BAR 3 147 199 30 Gl 42 GR2_5BAR 3 150 204 30 Gl 43 GR2_5BAR 3 148 201 29 Gl 44 GR2_5BAR 3 156 212 30 Gl 45 GR2_5BAR . 3 153 207 30 Gl 46 GR2_5BAR 3 151 204 30 Gl 47 GR2_5BAR 3 155 210 30 Gl 48 GR2_5BAR 3 147 199 30 Gl 49 GR2_5BAR 3 , 152 206 Gl 50 GR2_5BAR 3 150 203 Gl 51 GR2_5BAR 3 164 223 Gl 52 GR2_5BAR 3 149 202 Gl 53 GR2_5BAR 3 151 205 Gl 54 GR2_5BAR 3 162 219 Gl 55 GR2_5BAR 3 160 216 Gl 56 GR2_5BAR 3 157 212 Gl 57 GR2_5BAR 3 158 214 Gl 58 GR2_5BAR 3 152 205 G2 38 GR2_5BAR 3 140 189 30 G2 39 GR2_5BAR 3 141 192 31 G2 40 GR2_5BAR 3 151 205 31 G2 41 GR2_5BAR 3 148 201 30 G2 42 GR2_5BAR 3 151 205 31 G2 43 GR2_5BAR 3 151 205 31 G2 44 GR2_5BAR 3 145 197 31 G2 45 GR2_5BAR 3 150 203 31 G2 46 GR2_5BAR 3 149 202 30 G2 47 GR2_5BAR 3 145 197 31 G2 48 GR2_5BAR 3 157 213 G2 49 GR2_5BAR 3 146 198 G2 50 GR2_5BAR 3 152 205 G2 51 GR2_5BAR 3 152 206 G2 52 GR2_5BAR 3 141 191 G2 53 GR2_5BAR 3 163 221 G2 54 GR2_5BAR 3 0 G2 55 GR2_5BAR 3 149 202 G2 56 GR2_5BAR 3 142 193 G2 57 GR2_5BAR 3 150 203 G3 36 GR2_5BAR 3 151 204 31 G3 37 GR2_5BAR 3 138 187 31 G3 38 GR2_5BAR 3 138 187 31 G3 39 GR2_5BAR 3 145 197 31 G3 40 GR2_5BAR 3 136 184 31 G3 41 GR2_5BAR 3 151 204 31 G3 42 GR2_5BAR 3 138 187 31 G3 43 GR2_5BAR 3 144 195 31 G3 44 GR2_5BAR 3 141 192 31

176
CODE NUM ANNEAL Anneal ENERGY Energy HARD

code fft-lbf) amc)

G3 45 GR2_5BAR 3 151 205 31 G3 46 GR2_5BAR 3 134 182 G3 47 GR2_5BAR 3 148 201 G3 48 GR2_5BAR 3 154 209 G3 49 GR2_5BAR 3 148 201 G3 50 GR2_5BAR 3 136 184 G3 51 GR2_5BAR 3 146 198 G3 52 GR2_5BAR 3 143 193 G3 53 GR2_5BAR 3 134 181 G3 54 GR2_5BAR 3 152 205 G3 55 GR2_5BAR 3 148 201 Gl 59 GR2_1BAR 4 149 201 30 Gl 60 GR2_1BAR 4 149 202 30 Gl 61 GR2_1BAR 4 147 199 30 Gl 62 GR2_1BAR 4 152 205 30 Gl 63 GR2_1BAR 4 147 199 30 Gl 64 GR2_1BAR 4 152 206 30 Gl 65 GR2_1BAR 4 150 203 30 Gl 66 GR2_1BAR 4 150 203 30 Gl 67 GR2_1BAR 4 141 192 30 Gl 68 GR2_1BAR 4 154 209 30 Gl 69 GR2_1BAR 4 168 227 Gl 70 GR2_1BAR 4 164 222 Gl 71 GR2_1BAR 4 145 1%

Gl 72 GR2_1BAR 4 164 222 - Gl 73 GR2_1BAR 4 155 210 G2 58 GR2_1BAR 4 143 194 G2 59 GR2_1BAR 4 151 205 G2 60 GR2_1BAR 4 139 189 G2 61 GR2_1BAR 4 139 188 G2 62 GR2_1BAR 4 158 214 G2 63 GR2_1BAR 4 144 195 G2 64 GR2_1BAR 4 150 203 G2 65 GR2_1BAR 4 147 199 G2 66 GR2_1BAR 4 163 221 G2 67 GR2_1BAR 4 134 182 G2 68 GR2_1BAR 4 155 209 G2 69 GR2_1BAR 4 150 203 G2 70 GR2_1BAR 4 146 197 G2 71 GR2_1BAR 4 152 206 G2 72 GR2_1BAR 4 160 217 G3 56 GR2_1BAR 4 153 208 G3 57 GR2_1BAR 4 142 193 G3 58 GR2_1BAR 4 150 203 G3 59 GR2_1BAR 4 136 184 G3 60 GR2_1BAR 4 141 191 G3 61 GR2_1BAR 4 149 202 G3 62 GR2_1BAR 4 142 192 G3 63 GR2_1BAR 4 154 209 G3 64 GR2_1BAR 4 156 211 G3 65 GR2_1BAR 4 146 198 G3 66 GR2_1BAR 4 167 227 G3 67 GR2_1BAR 4 163 221

177
CODE NUM ANNEAL Anneal

code

ENERGY

(ft-lbf)

Energy

(S)

HARD

(HRC)

G3 68 GR2_1BAR 4 139 188 G3 69 GR2_1BAR 4 141 192 G3 70 GR2 I BAR 4 156 212

178
1—TTJ-- STATISTIC EXERGY H_\RI)

code CODE ^4RC'

0 Gl N of cases 1 0 Gl Mean 234 (165) 0 Gl N of cases 1 1 0 Gl Mean 188 (139) 30

0 Gl N of cases 1 1 0 Gl M«n ^^'(175) 28

0 G2 N of cases 1 1 0 G2 Mean 232 (186) 28

0 G2 N of cases 1 1

- Gl N of cases 20 - Gl Vlii i;; ii;i-n 199 (147) 30 Gl Vfye i :

i II 1 1

22s (168) 30 Gl Range 28(21) I Gl 214 (158) 50 Gl Mean 213 (157) 30 Gl Stardard Dev 7(5) 0

G! CA' 0 0 G2 N of cases 20 10 G2 180 (133) 31 G2 MaxiilliUl 214 (158) 31 G2 Range 34(25) 0 G2 Median 203 (150) 31 G2 Mean 202 (149) 31 G2 Staiiisrd De%' S (6) 0 —— G* C A'. 0 G3 N' of cases 20 10 G3 Mini' 111 in 190 (140) 31 G3 NlaXiijJlill 210 (155) 31 G3 Range 20(15) 0 G3 Median 198(146) 31 G3 NIean 198 (146) 31 G3 S^asidard Dc%' 5 (41 0 —— G3 CA'. Q

2 Gl N of cases 15 * 5 2 Gl 'kfii'i, ' ,1 r .1 193 (142) 30

- Gl V(^ii.niiii 225 (166) 31

2 Gl Range 32 (24) 1 2 Gl Median 210(155) 30

2 Gl \lean 207 (153) 30

~ Gl Standard Dev 10 O 0 G! CA', 0 0

2 G2 N of cases 15 10

2 G2 MiiiiiiiiT-r 184 (136) 31 2 G2 MffiQznoni 216 (159) 31

2 G2 Range 32 (23) 2 G2 \tedi2n 198 (146) 31

2 G2 199 (147) 31

- S-3-jij^w D-e''. 10(7) 0 G2 CA'. 0 0

2 G3 N of cases 15 10 G3 191(141) 31 G3 212 (156) 31 G3 Range 21(15) 0
Anneal Group STATISTIC ENERGY HARD

code CODE J (It-lDt) (HRC)

2 G3 Median 201 (148) 31 2 03 Mean 202(149) 31

2 03 Standard Dev 5 (4) 0

? r.7 r V n r\

3 01 N of cases 20 10

3 01 Minimum 199(147) 29

3 01 Maximum 228 (168) 30

3 01 Ran op 20 (77\ ]

3 01 Median 206(152) 30

3 01 Mean 209(154) 30

3 01 Standard Dev 8 (6) 0

3 Ol C.V. 0 0

3 02 N ofcases 19 10

3 02 Minimum 190(140) 30

3 02 Maximum 221 (163) 31

3 Range ]

3 02 Median 202(149) 31

3 02 Mean 202(149) 31

3 02 Standard Dev 8(6) 0

3 02 C.V. 0 0

3 03 N of cases 20 10

3 03 Minimum 182 (134) 31

3 03 Maximum 209(154) 31

i niUj Range 1 1 (20) 1

3 03 Median 197(145) 31

3 03 Mean 195 (144) 31

3 03 Standard Dev 8(6) 0

^ r V n

4 01 N ofcases 15 10 4 01 Minimum 191 (141) 30

4 01 Maximum 228 (168) 30

*T U

4 01 Median 203(150) 30

4 01 Mean 206(152) 30

4 01 Standard Dev 11 (8) 0

4 01 C.V. 0 0

4 02 N of cases 15 0

4 02 Mmimum 182(134)

4 02 Maximum 221 (163) 4 Range

4 02 Median 203 (150) 4 02 Mean 202 (149)

4 02 Standard Dev 11 (8) 4 02 C.V. 0

4 03 N of cases 15 0

4 03 Minimum 184 (136) 4 03 Maximum 226 (167) 4 03 Range 42 (31) 4 03 Median 202 (149) 4 03 Mean 202 (149) 4 03 Standard Dev 12(9)

4 03 C.V. 0
1. Saul, G.; Roberson, J.A.; Adair, A.M.; The Effects of Thermal Treatment on the Grain Size

and Mechanical Properties of 18 Ni Maraging Steels, Met. Trans., Vol. 1, February 1970, pp.

383-387.

2. Viswanathan, U.K.; Kutty, T.R.G.; Ganguly, C; Dilatometric Technique for Evaluation of

the Kinetics of Solid-State Transformation of Maraging Steel, Met. Trans. Vol. 24, December

1993, pp. 2653-2656.

3. Sarma, D.S.; Sinha, P.P.; Structure-Property Correlations in a Co-free Maraging Steel, Trans.

Indian Inst. Met., Vol. 49, No.3, June 1996, pp. 163-169.

4. Viswanathan, U.K.; Kishore, R.; Asundi, M.K.; Effect of Thermal Cycling on the Mechanical

Properties of 350-Grade Maraging Steel, Metallurgical and Material Transactions A, Vol. 27A,

March 1996, pp. 757-761.

5. Sinha, P.P.; Tharian, T.T.; Sivakumar, D.; Sarma, D.S.; Effects of Solution Temperatures on

Microstructure, Tensile, and Fracture Properties of Co-free 18 Ni Maraging Steel, Steel Research,

65, Vol. No. 11, 1994, pp. 494-499.

6. Sinha, P.P.; Sreekumar, K.; Nargarajan, K.V.; Grain Growth in 18 Ni Co-free Maraging

Steel, Trans. Indian Inst. Met, Vol. 52, No. 2-3, April-June 1999, pp. 1 19-124.

7. Tewari, S.; Mazumder, I.S.; Batra, G.K.; Dey, Baneijee, S.; Precipitation in 18 Ni Maraging

Steel of Grade 350, Acta mater, 48, 2000, pp. 1 187-1200.

8. Sinha, K.; Tharian, T.; Sreekumar, K.; Nargarajan, K.V.; Sarma, D.S.; Effect of Aging on

Microstructure and Mechanical Properties of Cobalt Free 18Ni250 Maraging Steel, Materials

Science and Technology, January 1998, Vol 14, pp. 1-9.

9. Sinha, D;. Sivakumar, K.T.; Tharian, K.V.; Nargarajan, and D.S. Sarma, Thermal

Embrittlement in 18Ni Colbalt Free and 18Ni-8Co-5Mo Maraging Steels, Material Science and

Technology, November 1996, Vol 12, pp. 945-954.

181

C.N. McCowan

Evaluation of 11.1 mm Bar Stock for Service in the Charpy V-notch

Program

Introduction

The microstructure of the 1 1.1 mm (7/16 in) bar stock for the Charpy V-notch (CVN) Program

was evaluated to determine whether this stock might be used for making specimens. For

comparison purposes, samples of the 12.7 mm (1/2 in) bar we have been using in the program,

and samples of the 14.3 mm (9/16 in) bar purchased by the Army were also evaluated.

The main purpose of the evaluation was to determine the depths of decarburization, oxidation,

and surface flaws in the bar stock. If the 11.1 mm stock is to be considered for use in the Charpy

Program, superficial and microstructural discontinuities should be limited to depths of less than

0.5 mm.

Procedure

Several metallographic specimens were cut from bars of 1 1.1 mm, 12.7 mm, and 14.3 mm

stocks. Samples were prepared for evaluation by grinding and polishing. Samples evaluated by

optical microscopy were etched with picric acid. Samples evaluated by scanning electron

microscopy (SEM) were not etched. A backscatter detector was used to enhance contrast in the

SEM between oxide scales and the steel. The Rockwell hardness was measured on the outside

surface of the bars and near the center (core) of the cross-sectioned bars. Vickers microhardness

was measured with a 10 g load.

Results

11.1 mm X 11.1 mm Stock: Figure 1 The 11.1 mm cold-rolled bar stock made for NIST has: (1) a zone of internal oxidation typically

less than 0.01 mm deep, (2) a nearly totally decarburized zone with a depth of about 0. 1 mm, (3) a uniform core structure starting at about 0.5 mm, and (4) a surface hardness of about 93 HRB

and a core hardness of about 97 HRB.

The microstructure of the bar near the surface is fine-grain, equiaxed ferrite. The adjacent

partially decarburized zone is a mixture of ferrite and spheroidized pearlite. The microstructure

in the core of the bar is spheroidized pearlite.

12.7 mm x 12.7 mm Stock: Figure 2

The 12.7 mm bar stock made for NIST typically has: (1) internal oxidation to a depth of 0.02

mm with excursions to depths approaching 0.2 mm, (2) a zone of nearly total decarburization

about 0. 1 mm deep at the surface, (3) a uniform core structure that starts at a depth of about 2

mm, and (4) a surface hardness near 89 HRB and a core hardness of 91 HRB.

The microstructure of the decarburized zone near the surface of the bar is equiaxed ferrite. The

microstructure of the partially decarburized zone has pro-eutectoid ferrite at the grain boundaries

with partially spheroidized pearlite within the grains. A band structure containing very fine

uniformly spheroidized carbides separates the decarburized zone from the core structure (zone at

183
approximately 1 to 1 .5 mm from the surface of the bar). The core structure is predominately

pearlite.

14.3 mm x 14.3 mm Stock: Figures 3 and 4

The 14.3 mm bar stock the Army used has: (1) a zone of internal oxidation, (2) an almost totally

decarburized region that is similar to that found in our 12.7 mm bar stock (0.05 to 0.1 mm), (3) a uniform core structure that starts at a depth of about 0.6 to 1 mm, and (4) a surface hardness of

about 104 R(b) and a core hardness of 1 10 R(b).

The microstructure of this bar in the initial decarburized zone has large ferrite grains with what

appears to be bainite (or martensite) at the ferrite grain boundaries. Adjacent to this region, a zone (band) with a morphology similar to that of the initial decarburized zone is found on a finer

scale. The remaining portion of the decarburized zone is a mixture of spheroidized pearlite,

ferrite, and bainite. The core microstructure is spheroidized pearlite and bainite.

The hardness of the grain boundary product is 685 HV (8 1 HRB) compared with 340 and 500

HV (65 and 75 HRB) for the ferrite and pearlite products, respectively.

Discussion

hitemal oxidation often occurs at grain boundaries near the surface of the bars. This results in a network of microcracks that may increase the sensitivity of the bar to quench cracking,

particularly microcracks surrounding laps (Figure 2b). All the bar stocks evaluated have

intemal oxidation, but it is most severe in the 12.7 mm bar stock.

The oxidation in the bars made for NIST (11.1 mm and 12.7 mm) is typically on the order of

several grains in depth ahead of the scale interface, so the very small grain size at the surface of

the 11.1 mm bar stock appears to help limit the depth of microcracking.

The 14.3 mm bar has a larger grain size at the surface than the two other bar sizes evaluated.

The large grain size appears to be due to grain growth during aimealing of the hot-rolled bar.

This coarse-grain region was likely formed by tfie coarsening of the band of microstructure

adjacent to it. The network of microcracks present in the sample indicated that a much smaller

grain size was present when the intemal oxidation occurred.

The specimens from the 12.7 mm bar stock are machined to a final size of 10 mm x 1 0 mm, so

they should be clear of the non-uniform structures found near the surface of the stock. However,

in comparison to the "Army stock," the depth of the nonuniform structure in our 12.7 mm bar is about twice as deep.

Producing CVN specimens from the 11.1 mm cold-rolled bar appears to be feasible: a 10 mm x 10 mm specimen from the core of the bar would be free of surface defects and have a uniform

stmcture.

Recommendations

I recommend that we make 1 00 CVN specimens from our 11.1 mm bar for evaluation. The heat

treatment currently used for the low-energy specimens should be used. The absolute energy

value of the batch is not important; the variance in energy is what needs to be evaluated at this

time.

184
Figure 1. The 11.1 mm bar stock ims: (A) shallow micro cracking near the surface and

surrounding surface defects, (B) a hea^ily decarburized zone and a partially decarburized zone

precending the core structure, and (C) a small equiaxed grain size near the surface of the bar.

185
Figure 2. The 12.7 mm bar stock has: (A-B) microcracking due to internal oxidation near

surface of the bar, and (C) a heavily decarburized zone followed by a partially decarburized

zone, then a band of finely spheroidized carbides preceding the core microstructure.

186
(A)

0.4 mm

(B)

0.1 mm

(C)

0.025 mm

Figure 3. The 14.3 mm bar stock has an elongated-grain morphology near the surface of the bar

in the heavily decarburized zone (A). The depth of the micrographing here is shown by the light

gray lines just ahead of the oxide scale layer. The darker areas at the elongated grain boundaries

are not cracks; these regions are probably a transformation product (bainite or martinsite).

Coarse-grain region and the network of microcracks near the surface (B-C).

187
188
George E. Hicho

Investigation of Cracks in Charpy V-notch Specimens

Introduction

On December 18, 1992, Mr. Paul Limdberg of the SRM group here at the Gaithersburg

NTST site asked us to help the Boulder NIST group determine the cause of the cracking obser.-ed

in a number of prospective standard Charpy V-notch specimens prepared from a new heat of

4340 \TM-VAR steel supphed by Vanadium Alloy Steel Corporation of Latrobe, Pennsylvania.

After phone conversations v,ith our Boulder Labs, we asked them to send us specimens shov-ing

the cracking. Subsequently we received tv,-o cracked specimens, one apparently heat-treated but

not machined for it still had surface scale, and one that was heat-treated and machined to finished size. The hardness on both specimens was determined and found to be HRC 25,

indicative of quenched and tempered 4340 steel. Macroscopic examinations indicated that quite

possibly the cracks could have been present mitially, and grew during the quenching procedure.

These are the so-called "quench cracks." Another suggestion as to the cause of the cracks was

that residual stresses could have been created by the letter-stenciling operation prior to heat

treating, and one or a combination of the subsequent heat treatments could have increased the

residual stresses in the vicinit\' of the letters to the pomt where cracking occurred. And finally

there was supposition that decarburization of the specimen's surface could have led to the

cracking, and surface cracks could have propagated into the specimen after a heat treatment:.

Conversarions \\ith the Boulder scientists who examined the rods using acoustic apparatus

revealed that no deep cracks were present in the as-recei\-ed steel. With this background, it was

decided to metallurgically examine randomly selected stock to determine the as-received

microstructure throughout the specimen and to proceed from there.

Examination Procedure

We requested that Dr. Tom Siewert (Charpy project leader) and Dan Mgliotti send xis

two pieces of the steel rod approximately 15 cm (6 m) m length, from the as-received stock:.

These rods had no cracks in them Upon arrival at Gaithersburg, samples were cut from the two

bars and prepared for metallographic examination. The samples were sectioned so that we could

look into the long axis of the bar that is perpendicular to the rolling direction of the bar. Figure 1 shows a photomicrograph of the "short bar" obser\"ed in this direction. It is e%ident from the

photomicrograph that there was a decarburized la>'er (white region) encompassing the outer edge

of this sample. Decarburization is the remo\"al of elemental carbon from the surface, usually in a heat-treating operation, by exposing the surface to an oxidizing atmosphere. Also e^ident is the

microstructure gradient that started at the decarburized zone and progressed inward toward the

center of the bar. This "grey area" (chemical segregation?) was followed b}' an area that

contained what was apparently the as-received annealed microstrucrure, pearlite plus ferrite.

Figures 2 and 3 show the microstructure at the surface and in the gradient region. Note the

presence of the small surface cracks in the decarburized zone in both of these photos. Even

though the photomicrographs revealed that the decarburized zone was small, hardness

measurements indicated otherwise. Hardness measurements, Table 1, taken on the sample

indicated that the low-hardness zone was about 2 mm (0.080 in) deep. This indicated that in

addition to the presence of a decarburized layer, quite possibly there was some chemical

segregation.

189
Figure 4 shows a photomicrograph of the "long bar." Some decarbiirization is present,

and hardness measurements revealed that the depth of decarbiirization was not as deep as that

found in the "short bar." Figures 5 and 6 also show that the decarburized layer of the long bar

contained small cracks.

On December 22, we received some heat-treated specimens that had cracks in thiem.

The specimen in the center of the photomicrograph, figure 7, is the original specimen which had

a decarburized layer (white ring) around the outside of the specimen. It was decided to section

the cracked specimens to see whether decarburized and "grey" zones, like those found in the

randomly picked bar, were also present in these samples. If so, we could state that the

decarburized layer could be causing the cracking seen in the samples. This was not so, for none

of the samples surrounding the decarburized sample shown in figure 7 showed any exteosive

decarbiirization. It was then concluded that the decarburized layer did not cause the cracks

found in these specimens. The surfaces of the heat-treated and cracked specimens were

examined and small surface cracks that did not grow into primary cracks were found. Figures 8

and 9 show cracks, on these same specimens, and those that did grow after heat treatment.

It was suggested that quite possibly the decarburized and "grey" zones were the result of

some chemical inhomogeneity in the steel. Initially, the centers of the specimens that cracked

were checked for manganese, chromium, nickel, silicon, and molybdenum contents and

compared to the composition of AISI 4340 steel. The composition was determined in each using

EDAX. The results indicated that Mn, Cr, Ni, and Mo were within the composition limits for

4340 steel, but the silicon contents were not. The effects of increased silicon on cracking

susceptibility is not known at this time.

A chemical profile for Cr, Ni, and Mn from the surface layer containing the decarburized

zone through the "grey" zone was performed. The results indicated that there was no significant

change in these elements as the zones were traversed. It appears that these zones were depleted

primarily of carbon.

Conclusions

Metallographic examinations, and chemical and hardness determinations were conducted

on two steel samples that were randomly selected from a batch of rods that were to be made into

standard Charpy V-notch specimens. Microstructure and hardness examinations reveale<i that one bar was severely decarburized. Whether this decarburized layer was the initiator of the

crack in the specimens, as some thought, was determined by metallographically examining these

as-received specimens for decarbiirization. The photos showed that there were small surface

cracks present in the surface of the as-received and heat-treated specimens. Hardness results

showed that the sample's decarburized layer had a lower hardness compared to that of its core

since the decarburized layer is essentially ferrite with the carbon removed, while the core is essentially a mixture of ferrite and pearlite. It is our opinion that it is rather difficult for a preexisting crack to propagate into and through the tough ferrite. There is no fracture meclranics

work, to our knowledge, that would indicate that cracks, even if present in the surface, \v^ould

propagate from the surface into the main body of the specimen. Our initial metallographi.ic

examinations on the two cracked samples showed no deep surface cracks, with the exception of

what we call "scale cracks." We surmise now that quite possibly, during one of the heat treating

processes, one of these small "scale cracks" grew into the crack we saw in the finished specimen.

190
A second set of cracked specimens was received and metallographically examined. The

specimens were cross-sectioned and compared to the specimen that was not cracked, bLit showed

the decarburized and gradient zones (figure 7). The cracked specimens did not show the same

microstructure near the surface; hence it was concluded that the microstructure was not

conducive to promoting the cracking seen in the specimens. The photomicrographs also showed

that there were small surface cracks present.

There should be a caveat here, however, that specimens with this amount of dec^arburized

and gradient zones would appear to be unwanted as standard specimens, since their

microstructures are not homogeneous. The carburized and gradient zones present in the bars

suggests that the bars could have been annealed se\'eral times; or. quite possibly during the

annealing process, the bars were exposed to an oxidizing atmosphere.

It appears that there were two synergistic causes for the cracking in the specimens. One is that the as-received bars contained small surface cracks, and the second is mat stamping the

identification code on these specimens induces a residual stress in the specimen that is severely

increased in the heat-treating operation. The combination of the small surface cracks and the

stresses enhanced during the transformation to untempered martensite appear to cause t±ie

cracking in the specimens.

Recommendations

Prior to recei\"ing the heat-treated cracked samples, two questions were to be ariswered.

One was "Does the excessively deep decarburization layer induce cracking in the specimens?"

The second was "Does stenciling of the numbers on the sample prior to heat treating initiate

cracks?" The first question has been answered: cracks do not initiate in the decarburizejd layer

since we found that none of the heat-treated cracked samples contained an extensi\'e

decarburized layer. To resolve the second question, we recommend that the as-received bars that

do not contain excessive decarburization, as determined by metallographic examination, be

e\-aluated in a rigid heat-treating experiment After metallographic examination, each bar free of

this decarburization should be cut to Charpy-specimen length, but not machined to size. Five of

these ten samples should then be stamped in the usual manner, and the fi\"e remaining lonstamped

samples be sent to the heat treater and included v,ith a batch of samples being heat-trea-ted. In addition to these bars, five specimens should have their surfaces machined and then stamped.

These specimens should be added to the others and heat-treated. The samples should fc>e

normalized in the usual manner, and then examined carefuUy for cracks. If none are \'isible,

perform the next recommended hardening heat treatment that is, austenitizing foUowed by

quenching in oil. After this treatment, remove the samples and examine them carefuUy for

cracks. If none is visible, proceed with the sub-zero treatment, remove the samples, an-d see whether cracking has occurred. Finally, let these 1 0 samples remain as they are, that is- , do not

temper them. .After an hour, obser\-e which group, if an>', are cracked. This experiment should

resolve which heat-treatment step is producing the stresses necessar\' to cause crackmg.

This experiment still does not resolve the problem of the gross decarburization found in

randonily selected specimens. This problem should be addressed since these specimens were

intended to be made into standard Charpy \'-notch irnpact specimens.

191
Table 1. Tukon ha.'dness results. A 1 kg load and 20x objective were xised.

Dlst. from

Surface ( Inch)

Long

Bar Remark

Short

Bar Remark

0.003 157 Decarb

134 Decarb

0.005 160 a 158 m 0.010 169 m 181 w/ferrit(

0.020 187 w/ferrite 198

0.040 194 214 m 0.050 219 hazy

0.060 198 w/ferrlte 213 hazy

0.080 197 core 198 core 0.100 206 II 208 n O.UO 202 202 » 0.180 206 M 205

0,220 204 • 210

Table 2. Chemical composition (vt Z) of 4340 steel. Only selected eleaents

determined.

Element AISI "ghgrt frtf" 'loniy bar"

Manganese 0.60/0.80 0.80 0.81

Chromium 0.70/0.90 0.80 0.87

Nickel 1.65/2.00 1.88 1.95

Silicon 0.20/0.35 0.40 0.58

Molybdenum 0.20/0.30 0.18 0.23

192
Figure 1. Photomicrograph of the "short bar" shov,ing the outside

decarburized layer, the transition region, which could be created

by chemical segregation, and the core, consisting of ferrite plus

pearhte. Mag. 6.5 X; Etch: 2 % Nital

193
Figure 2. Photomicrographs of the "short

bar" showing the decarburized layer and the

adjacent microstructure.

Mag: (A) 50 X; (b) 100 X, Etch: 2 % Nital

194
Figure 3. Photomicrographs of the "short

bar" at higher magnification. Scale and

decarb areas near the surface are shown.

Note variation in microstructure from

surface to center. Mag: (a) 50 lim bar; (b) 50

X; Etch: 2% Nital

195
Figure 4. Photomicrograph of "long bar" showing the absence of

the extensive decarb layer and transition zone as shown in the

"short bar." Mag: 6.5 X; Etch: 2 % Nital

196
Figures. Photomicrographs of "long bar."

Note the lesser amount of decarb layer

compared to the "short bar," and the

presence of the primary microstructure.

Mag: (a) 50 X; (b) 100 X; Etch: 2 % Nital

197
Figure 6. Photomicrographs of the "long

bar" showing another region of the decarb

and primary microstructure.

Mag: (a) 50 X; (b) lOO X; Etch: 2 % Nital

198
Figure 7. Photomicrograph of the specimens with cracks around

the specimen with Xhe decarb layer. Note the absence of decarb

layer in the cracked specimens.

199
Figure 8. Photomicrographs of the specimen after heat treatment,

showing the cracks.

200
Figure 9. Photomicrographs of the specimens after

heat treatment, showing the major crack in the

specimens.

201

Part 3: Specimen, Machine, and Procedure Effects

203

A. K. Schmieder

Report of Measurement of Three Charpy Impact Machines at The National

Institute of Standards and Technology

Introduction

During the period from September 21, 1992, through September 25, 1992, measurements of

force and distance were made by instruments with calibrations traceable to national standards.

In general, the measuring procedures were those prescribed by ASTM - Test Method E23-91a.

Section numbers in this report all refer to this test method.

This study has two primary objectives. The first primary objective was to measure the frictional

losses by the change in elevation. The second primary objective was to evaluate the scale errors

at the absorbed energy levels of the NIST verification specimens. Frictional losses were measured by the change in elevation during a single free swing, as described in Section 5.2.6,

and also by reading the scale after 1 1 half swings, as described in 5.2.6.2. In order to obtain loss

per swing, the accumulated loss is divided by 10 rather than by 1 1, as stated in 5.2.6.2. An

additional test, not described in 5.2.6.2, was made during which the pointer is reset after the first,

third, fifth, seventh, and ninth swings. The difference in scale readings for the two tests divided

by five is an estimate of the loss due to the pointer friction. The results of these measurements

of friction are shown in Table 1.

To evaluate the scale errors at the absorbed energy levels of the NIST verification specimens, the

correction for friction loss was made by each of the two methods recognized by ASTM E23;

namely, loss independent of angle of swing (Section 5.2.7), and loss proportional to the angle of

swing (Section 9.1.1). These results are shown in Table 2.

The Satec machine and the Tinius Olsen machine have scales graduated in foot-pounds of energy

absorbed by the specimen being broken. The indicated reading is simply the reading of the

pointer position along this scale. The Tokio Koki machine has a protractor scale, so the

observed value was the angle of rise of the pendulum after breaking the specimen. This angle was converted to energy in foot-pounds by a conversion table furnished by the manufacturer

of the machine. The difference between the energy values calculated from the calibrating

instruments and the difference calculated from the conversion table was several times as great as the corresponding values for the other two machines. To identify the source of these differences,

the angles measured by the protractor readings were compared to those from the extensometer,

and also the conversion calculations were repeated using various values of the physical

dimensions of the machine. The results of these comparisons are shown in Table 3.

The individual values used to calculate the results in Tables 1, 2, and 3 are listed in Table 4. A discussion of some of the notable results follows each table. The next section of this report

shows an estimate of the accuracy of the measurements made by the calibrating equipment. The

last two sections analyze in more detail observations listed earlier.

205
Table la. Frictional losses in percent of rated energy.

Satec Tinius Olsen Tokin Koki

Serial Number 130005 o 1 ojyjj

Ratp(i Pnprpv ft Ih 240 264 265

Frictional Loss in Pendulum and Pointer

Elf Elevation Measurements, % O.lo 0.34 U.zU

By Successive Swings, % 0.25 0.33 0.15

Friction Loss in Pointer Only

By Repeated Swings Without Reset, % 0.00' 0.04 0.04

Elf Successive Swings With Reset , % 0.01 0.03 0.05

Table lb. Comparison of frictional loss measurements between previous and present

investigations.

This Report

SATEC TINIUS

OLSEN

TOKIO

KOKI

Pendulum Only, Single Swing , % 0.17 0.31 0.15

Pendulum Only, Successive Swings, % 0.24 0.30 0.10

Pointer Only, Successive Swings, % 0.01 0.03 0.05

Earlier Report

Same

Model

Same

Model

Same

Machine^

Pendulum only, Single Swing, % 0.30 0.25 0.10

Pendulum Only, Successive Swings, % 0.38 0.32 0.12

Pointer Only, Successive Swings, % 0.03 0.26 0.01

It was observed that the pointer moved but fell back to its original position.

^ Moved from Watertown, MA, to Boulder, CO, between tests.

206
Table 2. Percent errorfa"^ in absorbed energ}'

For SATEC Machine

Indicated Absorbed Energ}' (, ft-lbf) in

Error, Assuming Fnction Losses are:

Independent of Angle of sv,ing" (%) 0.01 -0.04 0,17 0.27

1O.2O1 (-0.''2'i (0.53 1 (0 43)

Proportional to .Ajigle of Swing-' (%) 0.01 -0.04 0.15 0.23

(0.20) 1 -0.80

(0.4"' 1 r0.3''i

For TTNIUS OLSEX Machine

Indicate Absorbed Energy (ft-lb) 10 ID IJU

Error. .Assuming Friction Losses are:

Independent of .Angle of S\i.ing \ %) -0.00 -0.01 0.03 0.14

(-0.10) I -0.24(0.11

)

(0.25)

Proportional to .Angle of Sv,ing {%) -0.00 -0 .02 -0.00 0.0"

(-0.20) ('-0.40;i (-0.01 : ('0 V)

For TOKIO KOKI Machine

Indicated .Absorbed Energ\~ fft-lb i 10.01 1.^. , 1 , 1 .0 J

1 >n 1 1 ju. 1

Error. .Assuming Friction Losses .Are:

Independent of .Angle of Swing (%) -0.11 -0,15 -0.38 -0.^0

(-2.80) (-3.15' ('-1.41;) (-1.25)

Proportional to .Angle of Swing ; -0.11 - 0,15 -0.40 -0.-6

(-2.90) -0.56

' Error is taken as the indicated value minus the value calculated from the direct verification. Values

without parenthesis are percent of rated energy, with parenthesis, of mdicated energ>'.

'* Friction loss assumed constant at all angles of swing.

^ Friction loss assumed proportional to the sum of the angle of fall plus the angle of rise.

^ Indicated absorbed energ\" is determined by reading the angle of nse on the protractor and then

constilting the table furnished b}' the manufacturer of the machine.

207
Table 3. Percent error for Tokio Koki machine using various assumptions to determine

indicated absorbed energy.

Values Shown in Conversion Table

Scale Reading, Degrees Rise 110° 40' 107° 30' 106° 40' 89° 0' 65° 10'

Absorbed Energy by Specimen, (ft-lbf) — 10.01 12.71 71.85 150.10

Absorbed Energy Determined by Calibration and Error

From Calibration (ft-lb) 0.0 10.29 13.11 72.86 151.5

Error, Percent of Rated Energy (%) -- 0.11 -0.15 -0.33 -0.70

Error Using Indicated Energy from Recalculated Table

Error, Percent of Rated Energy (%) 0.00 0.00 -0.04 -0.10 -0.19

Indicated Energy Measured Force and Length

Error, Percent of Rated Energy (%) 0.00 0.01 -0.02 0.02 0.05

Error, Percent of Indicated Energy (%) 0.00 0.10 -0.05 0.05 0.08

Discussion of Friction Measurements in Table 1

Section 4. 1 .4 requires that for a single swing in the forward direction, the friction loss shall not

exceed 0.75 % of the scale range and that due to the indicating devices, shall not exceed 0.25 %.

The largest values in Table la were less than one half of these maxima.

Section 5.2.6.2 requires that for the successive-swing procedure, the loss per swing shall be less

than 0.4 % of the scale range capacity. This corresponds to the values listed on the second line

times 10/11. This requirement is also met by a generous margin.

A previous investigation^ included machines of the same design as the SATEC and TINIUS

OLSEN and the identical TOKIO KOKI machine. The frictional loss measurements of that

investigation compare to this as follows:

The last two columns show closer agreement between the two reports for the successive-swing

method than for the single-swing method. In the first column, the early report tested an older

machine. If the 0.14 % difference between machines is assumed to be due to dirty bearings, then

the results for the two Satec machines are consistent with each other and with the other two

designs.

The conspicuous anomaly is the high ratio of the two values in the upper right hand comer of

Table 1. This will be analyzed after the individual measurements are presented in Table 4.

^ Schmieder, A.K. , Comparison of Metrological Techniques for Charpy Impact Machine Verification. Charpy Impact Test - Factors and Variables, ASTM STP 1072, 1990, pp. 25

208
The measurement of pointer friction without resets shows 0.04 % values. This is the smallest

value that can be discerned on the scale. It corresponds to a pointer movement of about 0.01

inch. The successive-swing measurements of both pendulum and pointer friction have better

precision because of the large quantity measured. The comparison above also indicates that they

have better reproducibility. Therefore, they were used during the calculation of scale errors.

The pointer fall-back that was observed during operation of the Satec machine (see Table 1,

footnote) indicates that it would be advisable to increase the friction drag on the pointer.

Discussion of Scale Errors Shown in Table 2

Table 2 shows the error as percent of the latched energy (rated capacity) and also as percent of

the indicated reading. Section 4. 1 .3 requires that these errors shall not exceed 0.2 % of the

former or 0.4 % of the latter, whichever is larger. With the exceptions noted, the errors at all levels met these requirements.

1. For the Satec machine, the error was slightly greater than the permitted amount at 150 ft-lbf when the friction loss was assumed to be independent of the angle of swing. When the friction

loss was assumed proportional to the angle of swing, the error was within the required limits.

2. For the Tinius Olsen machine, all the errors were well within the acceptable amount for either

of the two assumptions regarding distribution of function losses.

3. For the Tokio Koki machine, the error at the 72 and 150 ft-lbf levels was 2 to 3 times the

permitted amount. The error was approximately proportional to the level when measured as a percentage of the range. This indicates a systematic error in the method of calculation. This

possibility was analyzed quantitatively. The results are shown in Table 3.

Discussion of Table 3

The manual furnished with the machine gives the formulas used to calculate the conversion table

together with an example of that calculation using hypothetical values for the measured

quantities. The actual measurements for the machine calibrated are not stated. The manual also

describes the method of treating frictional losses. The frictional loss of the pendulum was

determined by a successive-swing method described in Section 5.2.6.2, except that the total loss was divided by 10 instead of 11. The divisor 10 is theoretically correct and is used in this report.

(Note Section 5.2.6.2 does not give a method of determining the frictional loss of the pendulum,

but a method of determining when the bearings are to be cleaned. The divisor 1 1 is probably

appropriate for that purpose.)

The T-K manual apportions the friction loss according to the angle of rise, while the method of

measurement gives the total for both rise and fall. This apportionment is theoretically incorrect.

However, the effect on the calibration error is not nearly sufficient to account for the values

observed.

209
The pointer friction is determined as described in Section 5.2.6.1 and is proportioned according

to the angle of rise. This is theoretically correct. The method of Section 9.1.1, which apportions

pointer friction according to the angle of swing, is theoretically incorrect. However, the small

magnitude of the pointer's frictional loss makes this theoretical error insignificant.

The error shown in Section B of Table 3 is copied from Table 2. The calibration error is determined as for the other machines. The absorbed energy is calculated from the direct

measurement of force and distance. This value of absorbed energy is subtracted from the value

in the conversion table for that angle of rise.

It is not known what friction loss was used to calculate the furnished table. For the recalculated

table, the measured loss of 0.15 % of range was used. The example shown in the manual (not a case from the table furnished) used a friction loss of 0.35 % of range. If this is approximately

the value used for the furnished table, that could cause a significant portion of the calibration

error shown in Section B.

Section C of Table 3 shows that when the conversion tables are recalculated by the method

described in the machine manual but with the measured value of friction loss, the calibration

errors are within the permitted limits. The difference between the furnished table and the

recalculated table appear not to be primarily arithmetical. However, a small arithmetic error is obvious in that the furnished table value for absorbed energy at an angle equal to the angle of

rise for a free swing is not zero, as required by the algebraic formulas. If it is not an arithmetic

error, the difference in the two tables must be due to the input quantities, of pendulum length,

weight, or friction loss.

The pendulum is inscribed with its weight and the distance from the axis of rotation to the center

of gravity. Presumably, these values were used in calculating the furnished table. As shown in

Table 4, the values measured during the direct verification were slightly different. Using the

measured values results in the additional reductions in calibration error shown in Section D of

Table 3. This is not evidence that the measured values are more accurate than those inscribed.

It is simply a mathematical result of the same values being used for the calculated reference

values and for calculation of the conversion table.

It is recommended that:

a. The measurement of friction values be confirmed by several different observers using the

successive-swing method.

b. The weight and length of the pendulum be confirmed using different calibrated instruments of

the highest available precision.

c. Using these confirmed values and both E28 methods, calculate two new conversion tables.

List al^. formulas and input values used.

210
Table 4. Measured quantities and intermediate calculated results.

Machine SATEC TINIUS OLSEN TOKIO

KOKI

Length of Pendulu^l^ inches

Supporting Force, lb. 53.35 59.96 66.64

Measured Latched Angle, degrees 134.91 119.13 120.74

Latched Energy, ft-lbf 238.86 263.37 266.09

Friction Loss, ft-lbf 0.38 0.86 0.53

Angles of Rise at:

Clinomete

rReading

Protract orReading

151 150.10 ft.lb. 68.69° 68.73° 65° 2.5' 65° 10'

75 or / 1 .OJ oU.Z / 0"? ?8 8Q° oy U

12.5 or 12.71 127.83 114.28 106° 32.5' 106° 40'

10 or 10.01 129.20 115.18 107° 23.5' 107° 30'

0 or 0.0 134.69 118.82 110° 34.5' 110° 40'

Scale Reading after 1 1 swings:

Without pointer reset 5.9 ft.lb. 8.6 ft.lb 109° 48'

With 5 pointer resets 6.0 ft.lb. 9.0 ft.lb. 109° 38'

Distance from center of rotation to center of strike.

First value applies to Satec and Tinius Olsen, second to Tokio Koki machine.

211
Discussion of Table 4

The primary purpose of including Table 4 is to facilitate confirmation of the accuracy of the

calculated results and comparison of successive calibrations. In addition, the table reveals an

interesting relationship between the angle as measured by the protractor attached to the T-K

machine and as measured by the clinometer.

The last column shows that the two measurements of angle of rise differ by 5.5 to 7.5 min

throughout the 65° range. Since the protractor is calibrated in 10 min intervals and is readable

to ±1 min, the range of difference is not significant and the value of 6.5 min may be used as a constant bias.

The protractor read zero when the striker rested against a 10 mm diameter pin pressed against

the anvils. The clinometer readings were adjusted to read zero in this pendulum position also.

Sources of this bias include:

a. An inaccuracy in the zero-degree graduation.

b. An error in measuring the adjustment quantity for the clinometer.

c. A calculation error.

Estimates of Accuracy of Calibration Equipment

1. Balance Used to Determine Weight

To calibrate the balance, 0.5 lb. lead weights were hung in place of the pendulum. These were matched to a certified standard weight, using a precision scale reading in 0. 1 gr intervals. The

10-lb poise and a graduated precision level on the beam allowed observation of the angle of

swing of the beam during at least 10 cycles before coming to rest. This resulted in minimizing

friction effects both in the balance and in the bearings of the pendulum.

The scale is graduated in intervals of 0.01 lb. Repeatability is also 0.01 lb. A known source of

inaccuracy is the variability in the amount of adhesive putty used to attach a 10 mm diameter pin

to the striker at the center of strike. Estimated inaccuracy is ±0.03 lb or ±0.05 %. Section 5.2.3.3 requires that the measurement of force be accurate within 0.4 %.

2. Pins and Vernier Caliper Used to Measure Lengths

A calibrated 18 in caliper certified to be accurate to 0.001 in was used to measure the length of

rods bolted together to measure the pendulum length. The caliper is graduated in 0.001 in

divisions. Seven interfaces including the ends occur in the assembled rod system so the

accuracy is estimated to be ±0.007 in, or 0.02 %. The repeatability of these measurements was

±0.002 in.

3. Clinometer Used to Measure Angles

The clinometer is graduated in intervals of one minute of arc. The guaranteed accuracy is 1.5

min. In the range of angles used for these measurements, the instrument was verified by

212
measurement on a typical machine using a precision level to establish a reference plane and a

calibrated ruler, graduated in 1/64 in intervals, to measure height. The two measurements of the

angle of the pendulum agreed within the readability of the ruler, that is, 0.05 %. The accuracy of

the angle measurements is estimated to be 0.04 %.

Both the inaccuracy of the length measurement and the angle measurement contribute to the

inaccuracy of the elevation measurement. Using the variance method of combining, the

estimated overall error of measurement for elevation is

±-^(0.02' + 0.04') = + 0.05%.

Section 5.2.3.4 requires elevation measurements to 0.1 %.

4. Estimated Accuracy of Calibration Values for Absorbed Energy

Similarly, combining the contribution of the force measurement and

the angle measurement gives

±^(0.05' + 0.05') = ± 0.07 %.

Analysis of the Difference Between Protractor and Clinometer Readings

A simple procedure to evaluate the accuracy of the protractor is to measure the chord from 120°

to 60° and compare it to that from 60 to 0°. If a graduation error is the source of the bias, the

latter distance would be shorter than the former by two-thirds of the distance between graduation

marks. The bias could also result from the circular arc of the protractor being eccentric to the

axis of rotation of the pointer. This is easily detected by scribing a fine line on the pointer at the

arc at zero and swinging the pointer through full scale while observing any shift of the line

relative to the arc.

Unlike the other two machines, the pendulum stem of the T-K machine is tapered so the

adjustment factor for the clinometer was several degrees. A test of the accuracy of this factor is to compare the friction loss calculated from a single swing to that calculated from successive

swings. The former depends on only the clinometer readings and the latter is completely

independent of the clinometer readings. The reference cited earlier shows the average ratio of

± ^0.02' + 0.04' = ± 0.05%

these two is 0.83. For the T-K machine, the ratio was 0.83. The ratio of the results in Table 1 is 1.33. If an error in the zero reference of the clinometer gives a smaller angle of rise on the free

swing (as shown in Table 4), a corresponding increase in the latched angle occurs. The apparent

frictional loss and the ratio would both be increased from their correct value, as shown by the

results. However, if the true value of the ratio was 0.83 and the loss was 0.15 %, the latched

± ^0.05' + 0.05' = ± 0.07 %5

213
angle would be 1 10.79° for an angle of rise of 1 10.67°. If a 6.5 min bias is assumed, these angles

would become 1 10.90° and 1 10.56°, respectively, giving an apparent ratio of 2.73 as compared

to the observed value of 1.33. Therefore, it appears that the bias of 6.5 min is not due to an error

in the clinometer correction. If the observed bias is due to an error in the protractor scale near

zero, the value of ratio would not be changed and the unexpectedly high value is left unexplained

Analysis of Unexpectedly High Ratio of Two Friction Measurements for the T-K Machine

The values in Table 1 show that a ratio of the friction measured by a single swing to that

measured by successive swings is 1.33 for the T-K machine. The average of the other machines

in Table 1 is 0.87. The average of eight machines tested earlier is 0.83. The T-K machine tested

earlier also had a value of 0.83. In the interval between the two tests, the machine was moved.

The striker is now magnetized. This occurs when some hardened steels are repeatedly struck by

a blow causing bending. If the machine was originally installed new and not magnetized, the

pendulum would hang vertically and the protractor could be set to a true zero. During the later

installation, the magnetized striker would tend to move the pendulum toward the anvils giving a

false zero. Calculations show that if the true ratio were 0.83, a horizontal force of approximately

2 oz toward the anvils would be sufficient to result in an observed ratio of 1 .33,

Most steel specimens are held in a non-magnetic chuck while grinding. Residual magnetism

might also cause a deviation while the pendulum is being leveled so the scale reads 0 degrees

when the striker is almost touching a steel specimen. During this calibration, a gap of 0.010 in was observed.

214
Daniel P. Vigliotti, ^ Tom A. Siewert, ' and Chris N. McCowan^

Maintaining the Accuracy of Charpy Impact Machine*

Reference: Vigliotti, D. P., Siewert, T. A., and McCowan, C. N., "Maintaining the

Accuracy of Charpy Impact Machines," Pendulum Impact Testing: A Century of

Progress, ASTM STP 1380, T. A. Siewert and M. P. Manahan, Sr., Eds., American

Society for Testing and Materials, West Conshohocken, PA, 1999.

Abstract: The quality of the data developed by impact machines tends to degrade over

time, due to the effects of wear and vibration that are inherent in the test. This is the reason that impact standards specify periodic direct and indirect verification tests. Each

year, we provide reference specimens for indirect verification of over 800 machines

around the world. From evaluation of the absorbed energies and the fractured specimens,

we are able to deduce the origin of energies that are outside the allowed ranges, and

report these observations back to the machine owners. This report summarizes the basis

for these observations and will allow, it is hoped, machines to be maintained at higher

levels of accuracy.

Keywords: absorbed energy, Charpy V-notch, impact test, machine repair,

misalignment, verification testing, worn anvils

Introduction

The low cost and simple configuration of the Charpy impact test have made it a common requirement in codes for critical structures such as pressure vessels and bridges.

However, accurate results can be obtained only from impact machines that remain in

good working condition, such as within the tolerances specified by ASTM Standard Test

Methods for Notched Bar Impact Testing of Metallic Materials (E 23). We find that

many of the critical tolerances can be monitored by post-fracture examination of the

standardized verification specimens that we distribute.

Our examination of over 2000 sets of these specimens each year allows us to identify

problems that are often not recognized during routine measurement of machine dimensions

or routine check procedures. We have learned to recognize what marks on the broken

verification specimens indicate factors that could be affecting the results. We can then

advise our customers to recheck or replace the anvils or the striker, tighten bolts, check

bearings, check machine alignment or level, check cooling bath or thermometer, or review

testing procedures. This paper describes the most common problems that we detect, and

gives advice on how to avoid or correct most of them.

^ Technician, Group Leader, and Metallurgist, respectively. Materials Reliability Division,

National Institute of Standards and Technology, Boulder, Colorado 80303

Contribution of NIST; not subject to copyright.

215
Direct Evaluation ' A routine check consists of a free swing check and a friction and windage check.

The free swing is a quick and simple test to determine if the dial or readout is performing

accurately. A proper zero reading after one swing from the latched position is required on

a machine that is equipped with a compensated dial. Some machines are equipped with a non-compensated dial. Such a dial is one on which the indicator cannot be adjusted to read zero after one free swing. The user should understand the procedure for dealing with

a non-compensated dial. This information should be available from the manufacturer.

The friction and windage test will give the user the condition of the bearings. We

suggest that any deviation of more than 5% is excessive and the bearings should be

inspected.

We suggest that the user develop a daily log or shift log to be kept with the

machine. The log can be used to track the zero and friction values. The log can also

include information such as number of tests, materials tested, and any other useful

comments. A sample log is attached as Appendix 1.

Machine Preparation

The Charpy test is a dynamic test. Therefore, bolts may loosen over time. The

tightness should be checked on the anvil bolts, the striker bolts, and the baseplate bolts.

The manufacturer can supply the torque values for the anvil and striker bolts. The baseplate bolts should be torqued to the recommended torque values for the grade and size of

the nuts and bolts. We recommend the use of "J" or "T" bolts only. (See Appendix 2.)

We do not recommend lag type bolts. These bolts are made to withstand only static

loads. We believe that over time, the insert portion of lag bolts will loosen in the

concrete. As lag bolts are continually tightened, they can pull out of the concrete and be

tightened against the base of the machine, giving the impression of a properly mounted

machine. This condition is very difficult to detect. A machine with this problem will cause high-energy values at the low-energy level. The procedure used to mount the

master reference machines is attached as Appendix 2.

The anvil and striker radii should be carefully inspected for proper dimensions

and for damage. Damage can be detected easily with a visual inspection and a check for

smoothness by running a fmger over the radii. We find that radius gages are usually

inadequate to measure the critical radii. We recommend making molds of the radii and

measuring the molds on an optical comparator. Occasionally even a new set of anvils

and striker may have incorrect radii. We recommend that new anvils and strikers be

inspected before being installed in the machine. Since the radii can be considered

consistent before use, they can be measured directly on an optical comparator or other

optical measurement system.

We recommend centering tongs such as those described in ASTM Standard E 23.

The tongs should be inspected for wear or damage. A proper set of tongs is critical for

the accurate placement of the specimen. Some machines are equipped with a centering

device. The device should be inspected for wear and proper operation. We do not

recommend the use of centering devices for low temperature testing because it can delay

the time between removal from the bath and fracture, and so may exceed the allowed

five-second interval.

216
The temperature indicator should be calibrated immediately before testing. Since ice

water and dry ice have constant temperatures, they make quick and easy calibration

media.

Post-Fracture Examination

Many machine problems can be monitored by post-fracture examination of the

NIST standardized verification specimens. Following are the most common of these

problems. In many cases, suggestions on how to correct or avoid them in the future are included.

Worn Anvils - Most of the wear of an impact test machine occurs on the anvils and

striker. We evaluate this wear by examining the gouge marks that are formed on the sides

of high-energy specimens when they are forced through the anvils. Anvils that are within

the required tolerance of the standard will make a thin, even gouge mark all the way

across both pieces of the broken specimen. As the anvils wear, they will make a wider,

smeared mark across the specimen halves. Figure 1 shows the change in the gouge

marks. When the wider smeared marks are observed on a customer's specimens, we

reconmiend that the anvils be changed, because the reduction in energy needed to push

the specimens through worn anvils eventually drops the machine below the lower

tolerance in the energy range. You can monitor the wear on your machine by retaining

some specimens that are tested with new anvils and comparing them to specimens of

similar composition and hardness that are tested as the anvils wear. For specimens at a

similar absorbed energy, the gouge marks will grow wider and smoother as the anvils wear. Worn Anvils

Thiti gouge mark from Wide (smeared) gouge

an vsl in toleratic© mark from worn msM.

Figure 1

Ojf-Center Specimen - An off-center specimen strike occurs when a specimen is not centered against the anvils, so the striker contacts the specimen to the side of the

notch. The low-energy specimen best indicates when an off-center strike occurs. We

identify this condition on the specimens by finding that the gouge marks caused by the

anvils are not equidistant from the machined notch edges, and the striker gouge mark is offset the same amount from the notch (Figure 2). Also, as seen in Figure 2, the fracture

surface of a correctly tested low energy specimen is flat and both halves are even. However, the fracture surfaces of a specimen that has been tested off-center are on an

217
Off-Center Specimen

Ev«nft«aiff««ort8CM Umiwi fawJuw surtasM Figure 2

angle. The more off-center the strike, the steeper the angle will be. This problem

increases the energy needed to fracture a specimen. The most common causes for this

slipping are worn or damaged centering tongs, a worn or misaligned machine centering

device, careless test procedures, or the use of a cooling fluid that is too viscous at the test temperature, which causes the specimen to float on the specimen supports. Most machine

manufacturers should be able to provide new centering tongs. We have found that ethyl

alcohol is one of the best cooling media because it seems to evaporate quickly from the

bottom of the specimen to prevent specimen floating.

Off-Center Striker - This differs from the off-center specimen in that the

specimen is centered against the anvils so the anvil gouge marks are equidistant from

machined notch edges. However, the striker does not contact the specimen precisely

opposite the notch. Figure 3 shows this appearance. An off-center striker is usually

Off-Center Striker

Figure 3

218
attributed to the pendulum shaft shifting off center. This shift can be the result of a loose

alignment ring on the shaft or a loose bearing block on the machine. This problem also

increases the energy needed to fracture specimens at all energy levels.

Uneven Anvil Marks - Frequent testing of subsize specimens can cause the anvils

to wear unevenly. Figure 4 shows an example of these uneven wear marks at each

energy level of our reference specimens. Since this wear is restricted to a small area that

the foil-size reference specimen contacts, there is usually no effect on the energy required

to fracture the specimen. This anvil condition presents two problems. First, since subsize wear is usually not indicated by a change in the energy required to break a reference

specimen, inspection of the broken specimen is required. This wear will cause the anvils

to be out of tolerance according to the requirements in the standard. This means that the

machine does not meet the direct verification requirements of the standard and is therefore not eligible for the indirect verification process. The second, and more

important problem, is that the subsize specimens are being tested in an area of the anvil

that is worn. When the wear is substantial, this condition will produce artificially low

sub-size energy values. The anvils should be replaced on a machine with this condition.

Uneven Anvils Marks

' ^igure 4

Chipped Anvils - Sometimes an anvil can be chipped. Figure 5 shows that this

condition can be detected easily on all three energy reference specimens. The low-energy

specimen is affected the least amount because it is the hardest specimen and therefore has

a more brittle fracture. The ductile high-energy specimen will produce higher than

normal energy results and the very ductile super-high-energy specimens are affected most

by a chipped anvil. This condition should be detected easily by a visual inspection

before using the machine. New anvils are required when an anvil is chipped.

Chipped Anvils

III

'igure 5

219
Anvil with Machined Relief

Figure 6

Anvil Relief - Some Charpy machine manufacturers have designed a machined

relief at the bottom of the anvil (Figure 6). This anvil design does not meet the direct

verification requirements of ASTM Standard E 23. The relief has caused high-energy

results in our ductile high and super-high-energy specimens. It can also cause twisting of

the specimens during fracture that may also contribute to energy values higher than

normal at all energy levels. Since the relief is designed into the anvils and does not

appear to add an excessive amount of energy to the test, we at NIST continue to verify

these machines.

Damaged Anvils - Under some test conditions, usually for elevated temperature

testing, the anvils can wear to a rough finish that creates excessive friction (Figure 7).

This damaged condition is detected best on the high and super-high specimens. Damaged

anvils usually cause the gouge marks to become wider and push the specimen material to form a ridge that can easily be detected with the fingernail. This damage usually causes

artificially high-energy results at the high and super-high energy levels. Damaged anvils

must be replaced.

Damaged Anvils

Figure 7

220
Bent Pendulum Bent Pendulum - Figure 8 shows the gouge marks created by a pendulum bent in the direction of the swing. This gouge mark is usually deeper on the top edge of the specimen as it sits in the

machine. As shown in Figure 9, the striker contacts the top edge

of the specimen first, causing excessive mmbling and twisting.

This excessive activity can cause the specimen to interact with the

striker or the pendulum after fi^acture to create additional energy

loss. A bent pendulum can be detected by placing an unbroken

reference specimen in the machine and placing a piece of carbon

paper on the surface opposite the notch. At this point, lightly tap

the striker against the specimen. This will make a mark on the

specimen that can be inspected. If the pendulum is not bent, the

mark should appear the same width across the specimen. If the pendulum is bent, the

mark will be wider at one edge and become thinner or even not visible at the other edge

(Figure 8). We recommend that a new pendulum be installed on a machine with this

problem.

Figure 8

Bent Pendulum

Figure 9

Summary

The condition and accuracy of Charpy machines cannot be checked only by

comparing results of NIST reference specimens to the Master Reference Machines

located at NIST, Boulder, CO. Some machine problems cause artificially low results

while other machine problems cause artificially high results. In addition, deviations in

procedures can cause similar results. These machine problems and procedural deviations

may go undetected for years without some sort of physical check. For this reason,

examination of the broken specimens is a critical part of the verification process. Many

machine problems can be avoided or corrected with the information presented in this

paper. Also, suggested changes in procedure can help can help to insure a successful test. To obtain verification specimens or to clarify procedures for verification testing, you may

use the following information:

Questions on verification procedures can be answered by the Charpy

Program Coordinator. Phone: (303) 497-3351, fax: (303) 497-5939, or

email: daniel.vigliotti@nist.gov

221
APPENDIX 1 EXAMPLE LOG

222
APPENDIX 2

MOUNTING PROCEDURE FOR REFERENCE CVN MACHCsES

This is a detailed procedure developed by NIST to mount the three Master Charpy

Reference Machines. This procedure is not intended to be substituted for any installation

procedure provided by the manufacturer of the machme.

The foundation of the impact machine is critical to insure accurate results. Energy losses

through the foundation must be kept to a minimum. We recommend making a foundation of 7000 pound mix concrete that measures 152.4 cm (60 in.) long by 91.4 cm

(36 in.) wide by 45.7 cm (18in. ) thick. Usually you will need to cut a hole in the floor to accommodate the new foundation. If other equipment in the area could affect the machine

operation, you should isolate it from the floor with expansion-joint material.

Hold-down bolts used to secure the machine to the foundation should be of the invened

'T" or "J" t}pe. The bolts, nuts, and washers should have a strength of grade 8 or higher.

We recommend using bolts with a diameter of 22 mm (7/8 in.). At NTST we used 22 mm

(7/8 in.) grade 8 threaded rod, cut into pieces 61 cm (24 in.) long. We then welded 22 mm

(7/8 in.) pieces of the same threaded rod. six inches long, to the end of the 61 cm (24 in.)

pieces to make inverted T bolts.

We then positioned the machine over the center of the foundation hole. The machine was

held approximately 10.2 cm (4 in.) above the floor using spacers suitable to hold the

weight of the machine. The T bolts were positioned in the machine-base mounting holes

with a nut below and above the base of the machine. The nuts were tightened to keep the

T bolts straight while the concrete was poured. The ends of the T bolts were positioned

approximately 2.5 cm (1 in.) from the bottom of the hole. The machine was then leveled on the spacers. Levehng did not need to be as accurate as the final le\'eling.

Reinforcement bars were attached to the top of the horizontal rod previously welded to the

bottom of the T bolts. The reinforcement bars were attached in the form of a box

connecting the four bolts. Another box formation of reinforcement rods was attached to the T bolts 25 cm (10 in.) above these rods. The concrete was then poured under the

machine. The concrete was finished as level as possible at this time. Before the concrete

fully hardened, we removed concrete from around each T bolt to create a cavity of

approximately 2.5 cm (1 in.). This cavir>' would enable a nut to be threaded belo\^' the

surface of the concrete. The machine was left in this position for 72 hours.

After 72 hours, the nuts on top of the base plate were removed and the machine was lifted

off the T bolts. The bottom nuts were then threaded down into the cavities created before

the concrete hardened. The nuts were left high enough on the T bolts to enable the use of an open-end wrench to adjust them after the machine was positioned on them. At this

point, the base of the machine was coated with a light oil to keep grout from adhering to it. The machine was then lifted onto the T bolts and was positioned on the adjustment nuts. The machine was now ready to level. A machinist's level was used to insure meeting the

tolerance of 3: 1000 in. The critical leveling procedure was done using the four nuts under

the machine. After the machine was leveled, we wrapped the outside of the nuts with

duct-seal putty to facilitate their removal from the T bars later in the process.

At this point, the base of the machine was ready to grout. Hea\T cardboard forms were

placed around the base of the machine to keep the grout under the machine. It was

necessar,' for the grout to flow completely under the machine, making sure the base of the

machine was in total contact with :he grout. The grout was installed under the machine.

223
The machine was left in this position for 72 hours.

After 72 hours, the machine was lifted off the T bolts. The grout was inspected for

cavities and for surface contact with the bottom of the machine. The putty was removed

form around the nuts. The nuts were removed from the T bolts. After removing all debris

from the grout, the machine was lifted over the T bolts and rested on the grout. Washers

and nuts were installed and tightened. The level was checked at this point. The T bolts were cut off to approximately 12.7 mm (1/2 in.) above the nuts. The nuts were torqued to 380 ft-lb. The fmal level was checked at this point.

NOTE:

Special non-shrinking grout is recommended. This grout is available at most industrial

hardware stores.

If you have any questions concerning this procedure, please contact Daniel Vigliotti by

phone at (303) 497-3351, by fax at (303) 497-5939, by email at daniel.vigliotti@nist.gov, or by mail at NIST, Division 853, 325 Broadway, Boulder, CO

80303-3328.

224
C.N. McCowan and D.P. Vigilotti

Effect of Reduced Notch Radius on Charpy Impact Energy

Introduction

Impact specimens produced for verification testing need to have the lowest possible variation in

absorbed energy. This reduces the overall variation in the system, and allows for more certain

estimates of the true mean energy for a given group of verification specimens. The system

variation can be defined as:

o^ = a^ + G^ + a^, (D

where is the variation due to inhomogeneities in the test specimens, o^- is the variation due to errors caused by measurement uncertainty and test equipment in general, and Op" represents the

variations due to test procedures (operators, data collection, etc.). Steps have been taken to reduce many of the factors contributing to the system variation, by such means as: (1) a single

operator used to perform all impact tests, (2) optical encoders implemented on each impact

machine to remove operator bias and error from the data-collection process, (3) dimensional

measurements of each verification specimen prior to testing, and (4) careful monitoring of room

temperature and specimen temperature. Further improvements will most likely involve reducing

specimen variation.

Considering the specimens, two types of inhomogeneities can readily be identified, dimensional

and microstructural. As mentioned above, the dimensional variations of the specimens are controlled as closely as possible and the tolerances allowed for the specimens are as near as possible to a practical limit apphcable to standard machining practices. The microstructural

variation of specimens is minimized by controlling the chemistry and processing of the steel, and

then by optimizing the heat-treating process used to produce specimens. But some variation due

to microstructural inhomogeneities must be expected, and accepted. This is not to imply that

there is no room for improved heat treatment procedures and alternate materials that might

reduce variation. For a given material and heat treatment, however, one possible way to reduce

the variation in absorbed energy is to change the specimen geometry in a way that helps to minimize the effects of microstructural variation. This report considers the effect of reducing the

notch radius on the variation in impact energy.

225
Figure 1. Conventional/Standard

notch for Charpy Vnotched specimen.

Materials and Procedure

The impact specimens used for this study are verification specimens

of 4340 steel that were heat-treated to produce a 100 % tempered

martensitic structure with a hardness of approximately 45 HRC and an impact energy of approximately 16 J at room temperature. A control

set of 25 specimens was machined to standard ASTM E23 Charpy

impact specimen dimensions (0.25 mm notch radius, 0.01 in) and tested

at -40 °C. A second group of 25 specimens was also machined to the

standard 10 mm x 10 mm x 55 mm dimensions for an ASTM Charpy

specimen, but the notch in these specimens was cut using an electricaldischarge machining (EDM) process with a wire, 0.25 mm (0.01 in) in

diameter, resulting in a notch radius of 0.125 mm (0.005 in). The

standard notch and the EDM notch respectively are shown in Figures 1

and 2. The straight notch of the EDM sample was grooved near the top

to facilitate the use of centering tongs for impact testing.

Results and Discussion

The data for impact energy of the two groups of specimens are shown in

Figure 3 and listed in Table 1. The smaller notch radius significantly

reduced the mean energy of the specimens. The mean energies for the

modified EDM and standard specimen groups were 10.5 and 15.7 J, respectively. A portion of this decrease in energy might be argued to be a result of changes to the

thin layer of steel at the tip of the modified EDM notch due to melting and resolidification (heataffected zone). We did not attempt to separate this effect from that of the notch radius, because

our purpose was to reduce scatter in the impact energy for the specimen, and not necessarily to determine the impact energy of the steel. Actually, we had hoped for a heat-affected zone at the

EDM notch, which might aid in the crack

initiation and reduce the scatter associated with

the initiation event during fracture. A

comparison between the coefficient of

variation (CV) for the two groups of

specimens, however, indicates that the notch

did not reduce the scatter in the absorbed

energy for the specimens. The CV of the EDM

group, 0.099, is actually higher than the CV of

the standard V-notch group, 0.068.

Figure 2. Modified EDM

notch .

There was some variation in the notch depths

of the modified EDM samples measured prior

to testing, as shown in Figure 4. A number of

the specimens had notch depths well beyond

the 0.025 mm tolerance allowed by ASTM

Standard E 23. The variation in notch depth.

EDM V

Specimen Group

Figure 3. The absorbed energy data for the standard Vnotch and modified EDM notched specimens. The data

are boxed to contain 50 % of the values and notched at the median energy value.

226
however, was not found to significantly affect

the absorbed energy measured for the

specimens. Removing the data for specimens

outside the notch-depth tolerance (outliers)

would decrease the scatter in absorbed energy

for the modified EDM specimens, but not

significantly.

Follow-up Testing

To evaluate the effect of the EDM process on

the absorbed energy of the low-energy

specimens, additional tests were conducted.

In these tests three types of notches were

compared: (1) standard ground notch (45°

angle with 0.25 mm notch radius, 0.01 in),

(2) standard EDM notch (45 ° angle with

0.25 mm radius, 0.01 in), (3) modified EDM

notch (slot with 0.25 mm radius, 0.01 in). The notch radii were keep constant so that differences

between the notch cutting process and notch geometry could be evaluated. In addition to lowenergy samples, two groups of high-energy specimens were also tested.

The results of the testing are given in Figures 5 and 6, and Table 2. At low energy, no

significant difference between the two notch geometries of EDM specimens was found. The

EDM specimens with standard notch geometry had an average absorbed energy of 17.3 ± 0.7 J (12.8 ± 0.5 ft-lbf). The specimens with the modified notch geometry had an average absorbed

energy of 16.8 ± 1.0 J (12.4 ± 0.7 ft-lbf). So, as might be expected, changing the notch from 45 °

to a slot does not influence the energy level of the specimens, while changing notch depth and

radius does. The low-energy EDM specimens, however, were found to have higher energy than

the low-energy specimens that had ground notches. The specimens with ground notches had an

average energy of 14.8 ± 0.6 J (10.9 ± 0.4 ft-lbf). This would indicate that the EDM process

changes the microstructure of the low-energy specimens (very near the notch) and this increases

the absorbed energy of the specimens. At the high-energy level, this difference between EDM

and ground notches was not apparent. The two groups of high-energy specimens tested had

similar absorbed energies. The modified EDM notch specimens had an average energy of 93 ±

2.3 J (68.6 ± 1.7 ft-lbf). The high-energy specimens with ground notches had an average energy

of 92.6 ± 2.4 J (68.3 ± 1 .8 ft-lbf).

2.'

o

1.7-

-T 1 r10 12

Energy, J

Figure 4. The notch depth versus the absorbed energy

of the modified EDM specimens. Data within the band

meet the ±0.025 mm tolerance for notch depth allowed

for standard size Charpy impact specimens by ASTM E

23.

227
CO

05

Q

LUCO

20

isle

14

12

d^^"" 6^^"" 6^^""

6v GR0UP2

Figure 5. A group of standard ground specimens, a group of

modified EDM specimens, and a group of standard EDM notched

specimens. The specimens are low-energy specimens, LL93.

100

0)c

Ltl

GROUND MODIFIED

GROUP

Figure 6. A group of modified EDM notched specimens and a group of standard, ground notched specimens. The specimens are high-energy specimens, HH86.

228
Table 1. Absorbed energies for specimen groups with standard ground V-notches and modified

EDM notches.

Standard Ground V-notch Group Modified EDM Notched Group

Specimen ID Absorbed EnergjJ (Ft-lbf

)

Specimen ED Absorbed Energ>'

J (Ft-lbf)

396 14.79 (10.91y 491 12.93 ('9.54)

40 15.65 (11.54) 9.27 (6.84)

1237 17.46(12.88} 37S 9.74 (7.18)

27 15.80 (11 80) 1101 10.30 (7.60)

417 14.10 (10.40) 602 1 1.10 (8.19)

434 14.45 (10.66) 514 10.36 (7.64}

583 15.05 (11.10) 861 9.27 (6.84)

263 16.77 (12.37) 851 10.41 (7.68)

718 15.48 (11.42) 435 1 1.96 (8.82)

439 14.87 (10.97) 1004 8.15 (6.01)

744 14.20 riO.47) 1284 11.73 (8.65)

676 13.84 (10.21) 986 9.40 C6.93)

613 16.34 (12.05) ^S5 11.50 (8.48)

1013 16.{X) (11.80) 929 10.07 (7.43)

108 15.31 (11.29) 1250 1 1.50 (8.48)

988 15.82 (11.67) -52 10.25 (7.56)

1087 16.60 (12.24) 856 10.64 (7.85)

381 14.87 aO.97) 358 10.93 (8.06)

273 15.31 (11.29) 66 9.40 (5.93)

876 14.87 (10.97) 441 10.59 (7.81)

158 16.08 (11.86) 813 10.59 (7.81)

1213 17.64(13.01) 930 10.18 (7.51)

321 16.08 (11.86) 89 10.93 (8.06)

117 17.64 (13.01) 593 9.50 (7.01)

0 16.26 (11.99) 862 11.39 (8.40)

Mean Absorbed Energy = 15.66 J ( 1 1 .55 ft-lbf)

Standard De\Tation = 1.07 J (0.785 ft-Ibf) Range = 3.80 J (2.80ft-Ibfj

1 Coefficient of Variation = 0.068 (0.068)

Mean Absorbed Energy = 1 0.48 J (7.73 ft-lbf)

Standard Deviation = 1.04 J (0.769 ft-lbf) Range = 4.79 J (3.53 ft-lbf)

Coefficient of Variation = 0.099 (0.099)

229
Table 2. Absorbed Energy (J)

High-Energy Specimens, HH86 Low-Energy Specimens, LL93

Standard npni iND

EDM

IVl \J Uil lA^LJ

Standard GROUND

EDM

MODIFIED

EDM

STANDARD

8^ 17 93.66 13.82 18.68 17.38

80 7 1 95.74 13.92 17.68 18.18

y+.o I 14.02 16.28 17.68

97.93 14.22 17.38 16.38

on 14.32 16.88 17.78

yyj.oj 89.28 14.32 18.48 17.18

7U.O J 14.42 16.88 17.08

01 AO 14 SI 16.08 17.28

01 AO 7 1 .OO 14 SI 17 fix 1 / .uo 1 7 flR 1 / .uo

07 7 1 01 '^'^ 14 fi? Ifi 1 \J. 11 8o Ifi 1 u. 78/ o

07 04 7n 14 72 16 38 17 38

07 S'^ 92 82 14.72 16.88 18 28

07 74 OS n 14 72 17 28 18 98

07 fiS 96 16 14 72 15.38 16 78

0'^ 1(^ 04 78 14 09 1 7 ^iS 171 / . 181 o

93.27 96.06 14.92 17.68 17.88

93.69 94.81 15.02 16.38 16.08

94.51 89.07 15.13 16.58 16.68

94.51 95.33 15.32 16.98 17.28

94.72 95.12 15.32 16.48 17.28

95.98 92.41 15.42 13.89 17.58

96.09 91.78 15.72 17.28 16.38

Absorbed Energy= 92.61 J

Standard Deviation= 2.44 J

Absorbed Energy= 93.56 J

Standard Deviation= 2.31 J

Absorbed Energy=14.78 J

Standard Deviation= 0.62 J

Absorbed Energy=16. 81 J

Standard Deviation= 0.99 J

Absorbed Energy=17.33 J

Standard Deviation= 0.65 J

230
1

Thomas A. Siewert^ and Daniel P. Vigliotti^

The Effect of Charpy V-notch Striker Radii on the Absorbed Energy^

Reference: Siewert, T.A. and Vigliotti, D.P., "The Effect of Charpy V-notch Striker Radii on

the Absorbed Energy," Pendulum Impact Machines: Procedures and Specimens for

Verification. ASTM STP 1248. Thomas A. Siewert and A. Karl Schmieder, Eds., American

Society for Testing and Materials, Philadelphia, 1995.

Abstract: The two most common Charpy V-notch striker designs (8 mm and 2 mm radii on the

striking edge) were compared by using verification (reference-grade) impact specimens. Other

variables in the test matrix included two different brands of U-type pendulum machines and four

different specimen energy ranges (near 18, 45, 100, and 200 J). In this comparative study, we

found a very small difference between the two striker designs and an even smaller difference

between the two brands of machines. At 200 J, the difference between the two striker designs was about 10 J. This difference might not be important in most production testing, but must be

considered in verification testing, where the acceptable range may be no more than 5 %. The

standard deviations of absorbed energy for the two strikers were similar, except at 200 J, where

the 2 mm striker produced standard deviations about 3 times higher than that for the 8 mm

striker.

Keywords: absorbed energy, Charpy V-notch, impact test, striker radius, verification specimens

Two different striker designs are commonly found on Charpy V-notch (CVN) machines.

These two common designs are described in ASTM E23 and ISO Standard R442-1965 (presently

being revised by ISO TC 164/SC 4) and are distinguished primarily by the radii of the leading

edge that strikes the specimen, prompting their common identification as the 8 mm and 2 mm

strikers, respectively [JJ. Figure 1 compares the specified profiles near the nose of the two

striker designs. The 8 mm striker is more common in the U.S. and is required for the CVN

testing procedure described in ASTM Standard E 23 [2]. The 2 mm striker is more common in

Europe and Asia. A few machines with the less common strikers are also found in each country,

since some companies have contracts with organizations in other countries that require impact

data with the other striker design. The difference in the dimensions between the two strikers is many times greater than the tolerances, so there is no possibility of producing a striker that can meet the requirements of both designs.

^Supervisory Metallurgist and Technician, respectively. Materials Reliability Division,

National Institute of Standard and Technology, Boulder, Colorado 80303.

^Contribution of NIST; not subject to copyright.

231
4.0 + 0.1 mm

8-mm Striker

4.5 mm

2-mm Striker

Figure 1. Comparison of the 8 and 2 mm striker

dimensions.

Interchanging strikers (of the two designs) on a single impact machine is not a simple

solution for obtaining data with the two strikers. One reason for not interchanging strikers is that

replacing a striker can be very difficult for certain machine designs. Further, ASTM Standard E

23 requires recertification of the machine performance when the striker is changed [2]. The

generally accepted justification for this requirement is that improper mounting of a striker could

have such a significant effect on the machine's performance that the machine would fall outside

the certification limits. .

Recent comparisons of the two striker designs have reported differences in the energies.

A study by Fink describes the effects of striker radius (8 mm versus 2 mm) and notching

procedure on the energy [3]. He reported a linear correlation between the energies (in ft lb)

produced with the two strikers:

with a coefficient of determination (r^) of 0.9987 and a standard error of estimate of 1.36 ft lb. In metric units, (with the number of significant digits reduced to reflect the standard error), this

equation is

where E is expressed in joules. The coefficient of determination (r^) is 0.9987 and the standard

error of estimate is 1.84 J. The study included three steels (AISI 4340, ASTM A 537, and HY80) covering the range of 15 to 200 J. A study by Naniwa et al. also compares the effect of the two striker designs [4]. They

compared steels over a range in strengths to produce a range in CVN energies (well distributed

within the range of 100 to 400 J when tested with the 8 mm striker). They reported no difference

between the two striker designs for energies below 200 J, but found that the absorbed energy

with the 8 mm striker was greater than that with the 2 mm striker for energies above 200 J. The

difference in absorbed energies was as great as 100 J when the 2 mm striker indicated 300 J. This conclusion is in opposition to the relationship reported by Fink, who indicated that the 2

mm striker developed the higher energies.

(E2_)= 1.0420 (E3J + 0.5160, (1)

(E2_)= 1.042 (E8_) + 0.70, (2)

232
The conflicting conclusions in these two reports indicated a need to further investigate the

effects of striker design. Ideally, a relationship might be developed between the data generated

with the two strikers, so that data for either striker could be calculated from the other. At least,

there is a need to understand the reason for the different conclusions in these two studies. In this

report, we compare the results when high-precision verification specimens were tested on two

different brands of CVN machines using both striker designs.

Experimental Procedure

The primary purpose of our program was to evaluate the effects of striker design (8 mm

versus 2 mm) over a range in absorbed energy, but we broadened the test matrix to include

machine-specific effects. The machine-specific effects were evaluated with two brands of U-type

pendulum machines.

We produced specimens with mean energies near the verification ranges currently used in

the United States (18, 100, and 200 J) and additional specimens near 45 J. We used the same

steel from which we manufacture the verification specimens (NIST Standard Reference Materials

- SRM 2092, 2096, and 2098). We used a heat-treatable low-alloy steel for absorbed energies up

to 100 J and a maraging steel for the 200 J energy. We obtained mean energies from 18 to 100 J in the low-alloy steel by varying the heat treatment (tempering temperature), with the lowest

energies being produced by the treatments with the highest hardnesses. A wide range of energies was considered important because we wanted to span the ranges of the studies by Fink and

Naniwa. We were most interested in the effect at 18 J because this is the energy that is most

commonly the cause of a machine failing the verification test using the NIST specimens. It is also the energy closest to the requirements of many fabrication standards that require a minimum

CVN energy. We do not yet have a reference-grade m.aterial with energies near 400 J. so we

were unable to reevaluate the upper end of the trend noted by Naniwa et al.

Each CVN test is destructive; an individual specimen cannot be evaluated again (without

complex reconstitution techniques). To allow us to compare machines, we produced our specimens in conveniently sized batches (also called series), each identified by one or two letters followed by one or two numbers. The specimens in each batch were kept together through the

machining and heat treating operations to minimize the effect of processing variables. Each

batch was divided into fourths at random, providing between 5 and 1 1 specimens from each

batch for testing on each machine at each combination of energy range and striker. These data were analyzed to obtain an estimate of the standard deviation in the absorbed energy for each

combination.

The use of reference-grade specimens (meeting all the requirements of E 23, but with

stricter controls on their manufacturing procedures) was the most important part of our

procedure. These steels and heat treatments are characterized by a narrow spread in the energy,

which permits very small effects to be resolved. Verification specimens consistently have

standard deviations of 5 % or less of the mean energy, which are smaller than those for most

commercial steels. ASTM Research Report E28-1014 lists 2.4 J as the 95 % Repeatabilit}' Limit

for these 18 J absorbed-energy verification specimens when evaluated by the ASTM E 691

interlaboratory test procedure [5,6]

We purchased two new CVN machines (from different manufacturers), each with both

the 8 mm and 2 mm strikers. We selected new machines because they would reflect the latest in

machine design and construction techniques. Each was of the U-pendulum design (see Ref 2),

and had a maximum capacity of 405 J (300 ft- lb). On both machines the strikers were held in

place by only four bolts and could be removed and replaced easily.

233
Each machine was carefully mounted according to the requirements of ASTM E 23.

Each was evaluated with NIST verification specimens and were certified to the requirements of E

23. We also checked each machine's performance with NIST-certified reference materials for

CVN machines (5 specimens at 18 J nominal energy and 5 specimens at 100 J nominal energy)

after removing and remounting the strikers. We could detect no differences in the mean or standard deviation of the specimen energies after striker replacement. Apparently these two

machines have very tight tolerances for striker mounting and have mounting designs that permit

accurate realignment of the striker.

To further reduce the variation in the data, a single operator performed the tests and

recorded the data.. The strikers were changed between testing each series, for a total of 8

changes on each machine. A machine's performance was not checked after each change in the

strikers. Our initial tests of striker replacement with NIST-certified specimens convinced us that

changing the strikers on these machines had an effect smaller than we could measure. The

specimens with mean energies near 18, 45 and 100 J were tested at -40 °C, and the specimens

with mean energies near 200 J were tested at room temperature.

Results and Discussion

Table 1 lists the mean energies obtained for each combination of energy range, machine,

and striker. Figure 2 shows the data from Table 1 as an X-Y plot with the 8 mm data along the

Y axis and the 2 mm data along the X-axis. The data fall very close to the solid line, which

represents a one-to-one relationship between the two strikers, so close that it is difficult to determine the fine structure on this scale. To reveal the small differences between the two

strikers, we have replotted the data in Figure 3 as the difference in the energy (2 mm minus 8

mm) for the various combinations of the test matrix. Either the 2 mm or 8 mm data could have

been selected for the horizontal scale: we arbitrarily selected the 8 mm data for this axis. Figure

3 shows that the 2 mm striker gives higher energies than the 8 mm striker at 45 J, and vice-versa

at 200 J. It also indicates that the two brands of machines have similar behavior.

Table 2 lists the standard deviation for each combination of energy range, machine, and

striker, together with the number of specimens tested. We have plotted the standard deviation as a vertical bar extending one standard deviation both above and below the mean data from Figure

3. Figure 4 is intended only to show how the standard deviation compares to the bias in the

means as a function of energy. A detailed comparison of the standard deviations for the two

strikers is included near the end of this section. Figure 4 reveals that the standard deviation is roughly proportional to the energy, increasing with increasing energy. The means for the two

different machines differ by less than one standard deviation, according to the data from Table 2,

suggesting that the machine effect is small. However, Figure 4 shows that the difference

between the two strikers is sometimes greater than one standard deviation in the 8 mm data, at least for certain energies. The difference between the two strikers is less than one standard

deviation at 18 J, slightly more than one standard deviation at 45 J, nearly 0 at 100 J, and about

two standard deviations at 200 J. We interpret this to indicate that the radius of the striker nose

can be an important parameter, varying in importance as a function of energy.

I

234
Absorbed Energy for 2-mm Striker (J) Figure 2. Energy means when tested by the two

striker designs. Each point represents data from a

single batch tested by the two strikers.

The data in the report by Naniwa et al were only graphical and the uncertainties in

recovering the correct energies have precluded us from attempting to develop statistical measures

of the scatter. Their report suggests that they performed a comparison similar to ours, splitting

batches for testing by the two striker designs. Figure 5, which we have reproduced from their

report, shows their data as the open circles on a plot with the two energies on the two axes [4].

We have added our data as solid circles, Fink's data as X's, and a line that represents our best

estimate of the mean of the Naniwa data. The apparent scatter in their data suggests a standard

deviation near 40 J. Unfortunately, the scale necessary to contain the Naniwa data is so coarse

that the fme structure in the data below 100 J (the Fink data and our data) cannot be resolved.

To show the small differences between the two strikers, we have replotted in Figure 6 our standard deviation data, the Fink data, and the mean for the Naniwa data in this energy range,

as difference between the two strikers versus the mean. We have drawn smooth lines through

both the upper and lower bounds of our standard-deviation data from Figure 4 and shaded the

band between them. Although we have data gaps with our energy range that could affect the

shape of the band between our data, the band shows the general trend of our data and

emphasizes that the differences between the two striker designs are nonlinear. Figure 6 indicates

that the 2 mm striker (relative to the 8 mm striker) has a small positive bias that grows as the

mean energy increases from 18 to 45 J, but then decreases and goes negative.

The Naniwa data appear to support the negative trend that we observed above 100 J. It is not surprising that steels with a standard deviation this large in absorbed energy would not

resolve the trend noted by Fink between 100 and 200 J.

235
Table 1. Averages (Mean) Energy from CVN Testing of Reference-Grade Specimens

CVN 8-mm 2-mm

Specimen Machine Striker, Striker, Difference,*

Series Brand J J J

LL-39 A 18.3 18.6 0.3

B 18.1 18.4 0.3

LL-40 A 18.5 18.8 0.3

B 18.4 18.2 -0.2

M-6 A 43.3 45.2 1.9

B 44.0 45.8 1.8

HH-37 A 112.5 115.1 2.6

B 116.7 114.0 -2.7

HH-39 A 99.8 102.0 2.2

B 104.7 103.2 - 1.5

TTTT A /\ HH-40 A 100.5 99.8 - 0.7

B 100.9 101.7 0.6

SH-1 A 215.9 204.1 - 11.8

B 216.5 200.9 - 15.6

SS-1 A 225.2 215.6 -9.6

B 223.8 215.3 -8.5

* Derived by subtracting the 8 mm absorbed energy from the 2 mm absorbed energy.

236
Table 2. Standard Deviations of the Energy from CVN Testing of Reference-Grade

Specimens

Specimen

Series

Number of

Specimens in

Series*

CVN

Machine

Brand

8 -mm

Striker, J

2-mm

Striker,

J

LL-39 40 A 0.48 0.36

B 0.60 0.35

LL-40 40 A 0.43 0.58

B 0.86 0.44

M-6 20 A 2.25 2.02

B 1.81 2.21

HH-37 44 A 3.61 3.85

B 3.36 3.48

HH-39 44 A 1.81 2.88

B 3.64 3.32

HH-40 36 A 2.77 2.29

B 3.38 2.35

SH-1 20 A 4.70 11.60

B 2.51 11.67

SS-1 20 A 5.41 16.56

B 2.57 12.97

* Number of specimens in a series which was split between two machines and two strikers, so each standard deviation is based on one-fourth of this number.

237
too 2? 0

E

E

CO

</> Z3C

•10

-15

• •

1 l'' 1 1 1 1 lae 200 300

K

!

X

•

•

^ • mean for machine A

\ X X mean for machine B

•

Absorbed Energy for 8-mm Striker (J)

Figure 3. Graphical representation of the differences in the means for the data in Table 1 (2 mm

minus 8 mm striker data for each series and each machine).

•oo a>£

EI CO

COC

E

E

-5

-10

-15

J"

lCj)J 200 300

• mean for machine A

X mean for machine B

Absorbed energy for 8-mm Striker (J)

Figure 4. Graphical representation of the standard deviations for the 8-mm striker data in Tables

1 and 2.

238
Absorbed Energy for 2-mm Striker (J)

Figure 5. Data from the Naniwa study as open circles (Tigure 3a in their report) to which we

ha\"e added the Fink data, our data, and a line indicating the approximate mean,.

Fink \

Absorb &d Energy for 8-rrni Striker (J)

Figure 6. Comparison of our standard deviation scatter band to the mean of the data reported by

Xaniwa et al and Fink.

239
Our band follows the relationship reported by Fink [Equation (2)] up to about 100 J. In

this range, Fink used materials that were very similar to ours, and this may help to explain the

good fit with our data. Beyond this, our data indicate a deviate significantly negatively from his

prediction. Fink's report included all his data in tabular format, so we were able to calculate

standard deviations for his data [3]. Although his data near 200 J show a positive bias, the

standard deviation (15 J) is sufficiently large that our mean is within one standard deviation of

his mean. However, the HY-80 material that he used for his 200 J specimens has a composition

and microstructure significantly different from those for the martempering steel that we used in

this range, so there might be a material effect, perhaps hardness or strain hardening.

These results indicate that great caution should be used when comparing data developed on two different striker designs. Data generated with the wrong striker (a striker other than the

striker specified in a standard or testing protocol) should be used only to obtain a rough estimate

of material performance. Conversion of data generated with one striker to the other type of

striker is subject to the uncertainty of our scatterband and additional uncertainty if the material

properties are different from those used in our study. While the difference above 200 J is most

dramatic, the bias between the two strikers appears to be about 0.3 J for an energy near 18 J. This bias seems small but can be important when material is very near a specification

requirement. The 1.8 J bias at 45 J makes comparison of data developed with the two different

strikers even more difficult in this range.

We noticed one other difference between the two designs of striker. Figure 7 shows the

standard deviation in our data by striker. Both strikers had similar standard deviations (following

a linear trend) up to 100 J, but at 200 J they differed significantly. For both machines and with

two different batches of specimens, we found the same result: the 2 mm striker produced

standard deviations at 200 J about 3 times as great as those with the 8 mm striker. We have not

yet found the reason for the difference, but it does suggest that the 8 mm striker produces more

reproducible results when testing materials with absorbed energies of 200 J. Perhaps this is due

to the fact that there is a greater tendency for specimens to wrap around the 2 mm striker in this

energy range, rather than completely separate into two pieces.

+

15-

+

B 10

ra>a

•o

w 5

+ 2 mm Striker

o 8 mm Striker

0 100 200 300

Absorbed Energy, J Figure 7. Comparison of the standard deviation in

the energies for the two strikers.

240
Conclusions

1. Up to 100 J. the two striker designs produce ver\' similar data, differing by less than one

standard deviation (2 to 5 9c) of the energies when measured with verification specimens.

The practical effect of this difference is small, based on the qualitative namre of C\^'

impact testing.

2. At 200 J. the differences exceed one standard deviation, about 10 J.

3. Although the differences ber.veen the iwo strikers are small they must be considered in a verification program, where the acceptable range may be near 5 %.

4. The difference between energies measured using 2 mm and 8 mn^ strikers is complex. It is unlikely that a general relationship can be developed that will allow one machine to be

certified for both strikers from a test with only one striker (except perhaps for low

energies, where the difference is least).

5. Near 200 J. the 8 mm striker produces a standard deviation three times smaller than that

for the 2 mm striker, but no explanation for this effect is yet apparent.

References

1. ISO Standard R442. "Verification of Pendulum Impact Testing Machines for Testing

Metallic Materials." International Organization for Standards. 1965.

2. ASTM Standard E 23-92. "Standard Test Method for Notched Bar Impact Testing of

Metallic Materials", Annual Book of ASTM Standards . American Societ}' for Testing and

Materials, Philadelphia. Pennsylvania. 1992.

3. D.A. Fink. "Quantitative Comparison and Evaluation of Various Notch Machining

Methods and how They Affect ASTM E 23 and ISO R442 Testing Equipment Results,"

Charpv Impact Test: Factors and Variables. ASTM STP 1072 . ed. J.M. Holt. American

Societ}- for Testing and Materials. Philadelphia. 1990.

4. T. Naniwa. M. Shibaike. M. Tanaka, H. Tani, K, Shiota. N. Hanawa. and T. Shiraishi.

"Effects of the Striking Edge Radius on the Charpy Impact Test," Charpv Impact Test: Factors and Variables. ASTM STP 1072 . ed. J.M. Holt. American Societv for Testing and

Materials, Philadelphia. 1990.

5. Research Report: E28-1014. "Interlab orator}' Test Smdy for the Determination of

Precision and Bias in Charpy V-notch Impact Testing." D.A. Shepherd and T.A, Siewen,

available from ^American Societ}' for Testing and Materials, Philadelphia. 1991.

6. ASTM Standard E 691-87. "Practice for Conducting an Interlaborator}' Test Program to Determine the Precision of Test Methods." Annual Book of ASTM Standards. American

Society for Testing and Materials, Philadelphia. 1987.

241

Christopher N. McCowan\ James W. Dalh-^. Daniel P. Vigliotti^ and Ouk S. Lee'

Low Cost Lower Bound Toughness Measurements

Reference: McCowan, C.N., Dally. J.W., Vigliotti, D.P.. Lee. O.S., "Low Cost Lower

Bound Toughness Measurements," Pendulum Impact Machines: Procedures and

Specimens for Verification . ASTM STP 1248 . Thomas A. Siewert and A. Karl

Schmieder, Eds., American Societ\- for Testing and Materials, Philadelphia, 1995.

Abstract: A method is proposed for determining the lower bound toughness of

engineering steels based on impact loading of a modified Charpy specimen in a standard

Charpy machine. The modified Charpy specimen employed here is nearly the same as the over-sized specimen smdied previously by Bonenberger et al. The primary

difference is that a sharp notch is employed to initiate the crack instead of precompression. A master cur\'e is developed to relate the stress intensit}' developed at the

tip of the notch to the time of crack initiation. An instrumented mp is then employed to give a force-time trace that can be interpreted to estimate the crack initiation time.

Results are presented for the lower bound toughness of ASTM A 533. Type B. Class 1 steel over the temperature range from 0 to 40''C. These results are higher than the

results due to Bonenberger et al. for the same steel. The differences are attributed to the

different techniques used to develop the starter crack in the modified Charpy specimen.

Keywords: Charpy V-Notch, Instrumented Impact Test, Lower bound toughness

During the past 35 years, significant progress has been made in developing the

theor}' of fracmre mechanics, and in perfecting test methods for measuring crackinitiation toughness. Nevertheless, failures of engineering structures continue to occur

with serious consequences. Many of these failures are due to the fact that the material

employed in the fabrication of the structure was not tested or certified prior to its use in

construction [1]. In many instances, handbook values of fracmre toughness were used

in the fracture-mechanics analysis. This practice is ver}' dangerous because the fracture

toughness varies markedly from heat to heat of steel, and the values quoted in

handbooks for a particular alloy should be considered as only typical.

It is necessar}" to test the material specified in the design to establish the crackinitiation toughness over the temperamre range expected in the ser.'ice of the strucmre.

Test methods used to determine the toughness. ASTM E399 [2] for example, are well

known. What is less well known to the engineer performing the analysis, and to the

designer of the structure, is the huge amount of scatter encountered in measuring the

initiation toughness. Recent results by Link and Joyce [3] from an extensive series of

tests with A 533, T}pe B reactor-grade steel, presented in Fig. 1, illustrate the amount of

scatter for steels commonly employed in pressure vessels.

^ Materials Rehabilit\' Division, NIST, Boulder Laboratories, Boulder, CO 80303

" Mechanical Engineering Department. University of Man'land. College Park. MD

20742

" Mechanical Engineering Department, Universit>' of Inchon. Inchon, Korea

243
Temperature (°F)

700

600

500

E

« 400

-1 20 -80 -40 1 1 1

A533, Gr. B Steel SSY Corrected Data 3.5TSE{B) a/W=0.10

3.5T SE(B) a/W=0.55

A IT SE(B) aAW=0.10

T 1TSE(B) a/W=0.55

• ITSEfT) aW=0.40

68 Specimens

40 80

300

200

100

-100 -50

120 160 200

"1

\—U

50

Temperature (°C)

600

500

400

300 — 200

100

4VJ

100

Figure 1: The scatter in the crack initiation toughness of A 533 Type B reactor

grade steel is very large even near the nil ductility temperature. Data from Link and

Joyce^

The scatter is so large (400% at the nil ductility temperature) that the only

conservative method of design is to determine the lower bound, which is a curve of

toughness as a function of temperature drawn below all of the test data. To establish this

lower bound toughness, it is necessary to conduct many tests at each temperature to ensure that at least one or two "low" values are represented in the data set. It appears that

the difficulty in measuring the lower bound, and the excessive time and expense involved

in testing, are important factors that limit the effectiveness of design against fracture.

This paper describes a low-cost method of measuring the lower-bound toughness

of steels that has potential to be used to certify steels for a wide variety of structural

applications. The method is based on testing an oversized Charpy specimen in a standard

240 J Charpy impact machine. The Charpy specimen, shown in Fig. 2, is oversized in cross section with a height of 19 nmi and a thickness of 12.7 mm, but is of standard

length. Side grooves are cut into the specimen 2.0 mm deep with a sharp tip cutter having an included angle of 45°. The face groove, opposite the impact point, is also 2

mm deep, as in the standard Charpy specimen, but the radius of the tip is approximately

0.06 mm. This approach is similar in many respects to the method described previously

by Bonenberger et al. [4,5]; however, two important modifications have been made to improve the method.

First, in preparing the modified Charpy specimens Bonenberger et al subjected the

specimens to axial pre-compression stresses well above the yield strength of the material.

The purpose of the pre-compression was to deform the material in the local neighborhood

of the notch, thereby sharpening its tip. The practice of pre-compression may be

244
objectionable, because the

material local to the notch is strain-hardened and its toughness may be degraded.

In the method described in

this paper, pre-compression

is not necessar\" because

cleavage ma}" be initiated at temperamres in the lower

transition region, if the

machined notch is sufficiently sharp.

64 trm 22 mm

/ \ 2 rirn S*cbor A-A

notsh

KiG grocvB

Figure 2: Geometn." of the modified

specimen.

oversizedj Charpy

Second. Bonenberger

et al employed four strain

gages on each specimen that were located near the crack

tip to record a strain relative

to time during the impact

period. The strain-time trace was interpreted to give the stress intensit}' factor as a function of time. The time of crack initiation was evident from the strain-time traces because of the rapid decrease in the strain at the gage location due to crack extension.

The value of the stress intensit}' factor Kj, at the initiation time, was taken as Ky, which is an estimate of the lower-bound toughness. The approach described in this paper involved

strain gaging and testing several modified Charpy specimens to develop a master curve showing the stress intensit}' factor K, as a function of time. Strain gaging can then be

discontinued for subsequent evaluations of the lower-bound toughness. An instrumented

mp is employed to establish the time of the initiation of the crack at the notch. Using the

initiation time together with the (Kj-t ) master cun'e, it is possible to measure the lowerbound toughness without strain gaging at test temperatures in the lower transition region.

Eliminating the need for pre-compression removes the question of altering the

material properties prior to testing. Eliminating the strain gages reduces the time and cost

of lower-bound toushness measurements bv at least a factor of ten.

SPECIMEN PREPARATION

The modified Charpy specimens were machined from A 533 B reactor grade steel. This particular lot of material was available from the round-robm test series conducted to

verif}' the arrest toughness testing procedure, ASTM E-1221 [2]. The properties of the

material are well known and are described in reference [6]. It is important to note that the

RTs,-^- was -2°C. because it indicates the temperamre range of interest in defining the

lower-bound toughness in the lower transition region. At room temperature this plate of

A 533B exhibited a yield strength of 480 MPa. and an arrest toughness Kj. of 83.4

Mpa-m*^'-.

The specimens were machined in accordance with the drawing shown in Fig. 2. Of particular concern was the sharpness of the notch, since it was to act as a starter crack.

The notch was machined with a multi-tooth carbide cutter with a 45" included angle. The

tip of the cutter ground with a tip radius of 0.04 mm; however, we expected that tool wear

would increase that radius. We checked the profile of several notches using shadowprojection with high magnification and found some variation in notch quality, with notch

245
radii varying from 0.05 to 0.07 mm. It was

also noted that the notch tip was not a perfect

circular arc. A typical profile of a notch tip,

presented in Fig. 3, shows the quality typical

of the geometry maintained at the notch tip.

Four bonded-foil strain gages, with an

active element 1 .5 mm long, were installed on each specimen 5 mm from the tip of the

notch, as indicated in Fig. 4. This location is identical to that employed by Bonenberger et

al [5] in the initial development of this

approach. During the impact experiments,

the strain gage signals were recorded on a four-channel digital oscilloscope. The

frequency response of the strain measuring

system was controlled by the strain-gage

amplifiers. Whose gains were essentially flat from dc to 100 kHz

Figure 3: Micrograph of a replica of the

notch.

CHARPY MACHINE MODIFICATIONS

The Charpy machine employed in the study had a standard U type pendulum with

a capacity of 240 J, although we made two modifications to the machine. First, to accommodate the added height of the oversize specimen (19 mm versus 10 mm for the

standard specimen), the rails supporting the specimen were reduced in thickness by 4.5

mm. This change maintained the position of the centerline of the specimen relative to the nose piece on the hammer.

The second change involved instrumenting the nose piece, to permit a signal

representing the impact force to be recorded. Shallow slots were ground on both sides of

the nose piece of the hammer to accommodate semiconductor strain gages. Cover plates were bonded to the nose piece to protect the gages from contact with the Charpy

specimens during the fracture process.

The sensitivity of the instrumented nose piece was very dependent on the location of the

applied force along its height. We resolved this problem by an in situ calibration

technique based on absorbed energy. We first calibrated the nose piece in a testing

machine to obtain an approximation of the

calibration constant for

its load sensitivity.

Subsequently, this first estimate was adjusted so

that the absorbed energy as calculated from the forcetime record coincided

with the absorbed energy

measured directly from

the Charpy machine. The

repeatability of this

calibration technique was Figure 4: Location of the strain gages relative to the notch

excellent. tin

246
DE\'ELOPLSG THE >L\STER CUR\X

A upical set of strain-time traces recorded from the four gages is shown in Fig. 5. We note that each gage indicates a slightly different signal depending on the response of

specimen. To accommodate these differences, the signals from the four different gages

are averaged together to give a single strain-time trace as indicated in Fig. 6.

20OO

The strain-time trace in Fig. 6 clearly indicates the d\nainics of the modified Charpy

specimen. The strain increases with time in a nearly linearly manner for the first 50 jis,

and then becomes more nonlinear. The nonlinearit\- is due to tw o effects. First, with

increasing time the Hertzian load at the contact point produces local compressive stresses

that exceed the \ield strength of the A 53 3B steel, and a portion of the hammer

displacement is accommodated by local plastic deformation at the contact point. Second,

with the higher strains (1200 |i€ and abo\'e) the regions near the notch tip have >ielded.

and the specimen deformation is accommodated by plastic deformation near the notch tip.

It is evident that the nonlinearit}' in the strain-time

trace is to be expected, and

that it will be dependent on the yield strength of the

material tested and the

shape of the nose piece on the Charpy machine.

Oscillations are also

evident in the strain-time

record presented in Fig. 6. The first oscillation has a period of 40 jis and the

second has a penod of

about 65 |j,s. The third

oscillation begins, but is damped beyond

recognition and its period

is not evident. Clearly, the

specimen is undergoing

vibration during impact:

however, the magnitude of

the oscillations is small

compared to the mean

strain later after 70 |j,s) in

the impact event. Since all of the specimens tested in

this series of experiments

failed at times longer than

95 [is. it appears that the

effect of specimen

vibration on the strain

recorded at the time of

crack initiation was

relativelv small.

150

Figure 5: T\pical strain-time traces showing gage-to-gage

variation in strain.

250'C

b

X

CO

2O0O

10O

Time t, lis

200

Figure 6: Dynamic characteristics of the strain-time trace.

247
2500

b

2000

1500

9 1000 —

500

The reproducibility

of the strain-time traces is illustrated in Fig. 7, where

the records for four different

specimens tested at temperatures varying from 0 to 40 °C

are presented. Examination

of these results show that the

strain-time traces are almost

identical for times less than

about 75 |is and strains less

than 0.0014. Later in the

impact event larger record-torecord deviations are evident.

It is believed that these deviations are due to the tearing that occurs at the notch tip before

cleavage failure is initiated. In the section on Fractography, we will show evidence of

this tearing and describe the specimen-to-specimen variation of the extent of tearing

observed at the tip of the notch.

100 Time t, |is

150 200

Figure 7: Four stress-time traces showing reproducibility.

The master strain-time curve, shown in Fig. 8, is representative of the individual

traces presented in Fig. 7. It is necessary to convert this master strain-time curve to a corresponding master curve for stress intensity factor versus time.

This conversion was made by employing the relationship derived by Bonenberger

etal. [5]:

K^^ = 38, 880e* ^ (1)

where Kj^ has units of MPa-m'^^, and €* is the strain at the instant of cleavage initiation.

We consider the dynamic initiation toughness Kjj as well as the crack arrest toughness

to be good estimates of the lower-bound toughness. Accordingly the value of is presented on the right hand ordinate of Fig. 8. The smooth nonlinear shape of the

master curve suggested that it would be possible to fit the curve with a relatively simple

relation. We have selected a two-term function of the form shown below to relate

toughness with time.

0 50 100 150 200 Q

Time t, ^is Figure 8: Master curve for e-t and Kj^-t

248
In Eq. (2) the time t is in (is and is in MPa-m' The constants a and b were

determined as 4.88 and 0.0758 by fitting the relation to data taken at t = 100 and 200 fis. K = at^'^ + ht (2)

CRACK INITIATION TEVIE

The dynamic initiation toughness for A 533B reactor grade steel may be

determined from the master cur\'e shown in Fig. 8 or from Eq. (2). if some method is used to measure the time at which the crack initiates during the impact event. There are three different methods for measuring the initiation time. Strain gages can be mounted

near the crack tip, and their signals will sharply decrease within a few microseconds after

the crack initiates [4,5]. A second technique involves the placement of a coil near the

Charpy specimen to sense the magnetic field. When the crack initiates the field suddenly

changes. Recording the voltage produced by the coil with respect to time gives a discontinuitN' in the voltage-time trace that indicates the initiation time [6]. The third

technique involves measuring the impact force as a function of time during the impact

event. The mp signal decreases rapidly with time when the crack initiates (in cleavage)

and the specimen stiffness shows a corresponding decrease, We employed the both the

top and strain gage signals in determining initiation time in this stody.

Oscilloscope records of the average strain-time and tup force-time traces are shown in Fig. 9. We include four records to show the effect of different testing

temperatores. An examination of these records indicates that the tup signal oscillates

with ver>' large amplitude for the first cycle; however, in subsequent cycles the amplitude

of the oscillation decreases markedly but does not vanish. The oscillations in the forcetime records are due to several different harmonics. The fundamental harmonic, at 27

kHz, is probably due to the ringing of the nose piece at its natural frequency. The natural

frequency of the specimen (about 2.5 kHz) is not evident in the top record. For times

larger than about 50 to 60 |is, the top signal appears to track the strain signal with

oscillations producing the pnmar>" deviations between the two signals. When the crack

initiated in the modified Charpy specimen, both signal decreased rapidly with respect to time. The difference in the indicated time of initiation varied from specimen to specimen.

Reference to Fig. 9a shows a ver>' close correspondence, because the oscillation of the

top signal was in phase with the initiation of the crack. That is, the oscillation produced a decrease in the force signal at the same instant that the reduced stiffness of a failing

Charpy specimen produced a decrease in the top signal. On the other hand. Fig. 9b, c, and d show that the oscillation was producing an apparent increase in the force signal, at the instant of crack initiation. In this case, the force-time trace is delayed in its response

to the reduced stiffness of the specimen after crack initiation. It is clear then that

differences occur in the estimate of the crack initiation time as measured from the straintime traces and the force-time traces. The differences are indicated by the graph of

initiation time as measured from the strain-time trace on the ordinate, and initiation time

as measured from the force-time traces on the abscissa in Fig. 10. For ideal

measurements, the initiation times associated with both methods should be equal and the

data points should track the no-delay line shown Fig, 10, However, neither measurement

technique is without error, and we observe a dispersion of data points on both sides of the

no-delay line. Most of the points indicate that the indications from the force-time records

are delaved relative to the time indicated from the strain-time trace.

249
1800-]

1600-

1400-

b 1200-

X 1000-

U;

c" 800-

CO 600-

400-

200-

0--

20 ^0 100 120 140 160 180 200'

Time t, \ls

(a)

20 40 60 80 100 120 140 160 180 200"

Time t, ^is

(b)

Figure 9: Four records showing correspondence between the strain gage signals

(average) and the tup signal: (a) E3-1 1 at 0 °C, (b) E3-3 at 10 °C; (c) & (d), next page.

250
Figure 9 continued: (c) E3-16 at 21 =C. and (d) E3-17 at 30 =C.
The delay in most cases is less than 10 [is. In three cases the force time record

gives an early indication of

initiation, but the differences

in these early signals is usually less than 5 |is. We

note three data points in

Fig. 10 that fall well outside

the ±10 |is band. In two

instances the specimen

yielded, and the crack

experienced a significant

degree of tearing before

cleavage initiated at times

greater than 250 jis. The

effects of tearing are to delay

the fracture event and to produce an apparent

toughness well above the

Figure 10: Comparison of initiation time determined from

strain gage and tup signals.

300

50 100 150 200 250 300

Tup, Time of Maximum Force, \is

lower-bound toughness. In one case, Q3-21, a reasonable toughness was predicted with crack initiation time of 221

|is. In general one might disregard any test with a crack initiation time significantly

greater than about 200 |is.

The effect of small errors in measuring the crack initiation time depends on the

time of crack initiation. If the dynamic crack initiation toughness is relatively low (about

60 MPa-m'^), the specimen will fail early in the impact event, say at 100 [is. A delay

error of 10 |is (10 % in this instance) produces an error in Kj^ of 6.0 %. The mitigation of

the error is due to the nonlinearity of the master curve shown in Fig. 8. At higher

toughness values (say 73 MPa-m which corresponds to an initiation time of 150 |is) a delay error of 10 |is (6.7 % in this case) produces an error in Kj^ of only 4 %. For the

highest toughness, about 85 MPa-m''^, the time to initiation is 200 |is, and the error in the

time measurement will be 5 % and in the toughness measurement only about 3 %. When

compared to the scatter of several hundred per cent in toughness measurements typically

observed in extensive and carefully controlled testing programs for Kj^, these errors are very small.

We conclude that force-time records from instrumented Charpy machine can be

interpreted to accurately estimate the initiation time. The estimate will usually give an

initiation time that is too long by 5 to 10 |is, but this error is small compared to the typical

scatter in the measurement, and the error is mitigated in part by the nonlinearity of the

master K,H-t curve.

RESULTS

A total of 1 6 modified Charpy specimens were tested in this study. Each

specimen was instrumented with strain gages, and simultaneous records of strain and

force were made as a function of time during the impact event. Specimens were tested at temperatures of 0, 10, 21, 30 and 40 °C. The results from the test series include the crack

initiation time measured from the strain gage and instrumented tup records; the dynamic

crack initiation toughness Kj^ and the energy absorbed are shown in Table 1. Due to excessive tearing prior to cleavage initiation, two specimens, E3-8 and -16, were

252
considered to be invalid tests. The results for thie crack-initiation toughness of the

remaining specimens are also presented m Fig. 11.

FR\CTOGR-\Piri'

Tne fracture surfaces from five different specimens were examined m a scanmng

electron microscope to smdy the initiation of fracture from the "sharp ' notch tip. The

results that were obser-'ed varied considerably from specimen to specimen. The smallest

amount of tearmg occurred in specimen E3-1 1. as illustrated m Fig. 12. An overall view

of the notch region at 20X. presented m Fig. 12a. shows a narrow tearing zone at the

notch tip prior to initiation of cleavage over a large region. It is imponant to note that the

extent of tearing is not uniform, but the depth of the tear zone vanes over the height of

the specimen. This fact is better illustrated in Fig. 12 b. where the transition from ductile

tearing to cleavage is presented at a magnification of 700X. We observe that the depth of

the tear zone varies from about 33 jj.m at the bouom of the fractograph to about 47 [im at the top. The cleavage region shows small areas of ductile hole joining at the ridge lines,

which is typical for this t}pe of steel when cleavage occurs at test temperatures close to the RTsDTThe most extensive tearing occurred for specimen E3-8 which was tested at 30'C.

The fractograph shown in Fig. 13a indicates that the depth of the tear zone exceeded 1 mm. The propagation of the crack front by the tearing mechanism is relativeh' slow m

comparison to cleavage propagation. As a consequence, the time required for cleavage

imtiation is very long. In this case the time to imtiation was 26" [is. With this large

amount of tearing the test was considered invalid and the data was not used to determine

the lower-bound toughness. The transition from tearing to cleavage evenmally occurs, as shown in Fig. 13 b. Again the cleavage region shows evidence of late-breaking ligaments

at the ridge lines of the cleavage facets. The ductile hole joining in these regions is t\pical of this steel when tested at 30 to 40 "C above the RTv^-^j.

The area of tearing at the tip of the notch varied from 1.8 mm' to 9.6 mm*. The

time to crack initiation was a function of this area, as expected. The shoner imtiation

times i^less than 150 [is > were associated with tearing areas of less than 2 mm-, and the

longer initiation times (^more than 200 [is ) were associated with areas in excess of

2.5 mm'.

DISCUSSION AND CONCLUSIONS

The concept of determining crack initiation toughness from Charpy specimens is not new. Initial smdies [8.9] coincided with the development of instrumented mps. and

eft'orts were made to relate the toughness directly to the impact force. The method for

determining Kj. from the mp records was described by Radon and Turner [10]. and this

method was employed by Ser»er and Tetelman [11] in their studies of a reactor grade

steel. Both of these investigations used standard Charpy specimens that had been

precracked in fatigue and side-grooved. From these smdies. guidelines [12] were

developed to determine K^. and j.^,

The diftlcult}' in using the standard Charpy specimen to determine toughness is that the small height (10 mm) of the specimen is suftlcient to provide plain strain

constraint for only those materials with extremely low toughness. For more commonly

employed engineering steels, additional constraint is necessar>' to accommodate increased
180

160 -

140 - ^ 120 ^ 100

2

80

60^

40t20

0

This Study

Bonenberger/Dally

B

10 20 30 40 50

Temperature, °C

60 70

(a)

50 60 70 80 90 100

Kid, Estimated from Master Curve

Figure 11: (a) The Kj^, temperature relation for A553 B reactor-grade steel, and (b) the

correlation between K,j values calculated from the data with values estimated from

the master curve.

254
(b)

Figure 12: The tearing zone at the root of the notch in the E3-1 1 sample (0 °C), is approximately 0.1 mm wide. Tearing was followed by initiation of cleavage, which

shown more clearly at higher magnification, b.

255
Quasicleavage Tearing Notch

sSf,

(b)

Figure 13: The tearing zone at the root of the notch in the E3-8 sample (30 °C), is over 1 mm wide in some regions. Tearing was followed by initiation of cleavage (mixed mode),

which is more clearly shown at higher magnification, b.

256
TABLE 1

Results from the Modified Charpv Test Series

Spec.

No.

Temp.

°C

Kid

MPam^^

Strain

Time |is

Force

Time (is

Absorbed

Energy J

E3-11 0 70.4 131 132 32.2

Q3-20 0 75.6 137 132 32.9

E3-3 10 71.2 102 121 44.8

E3-20 10 63.9 98 100 26.6

E3-1 21 60.4 127 139 29.4

E3-6 21 58.2 95 95 23.1

E3-10 21 61.5 111 124 35.0

E3-16 21 71.7 154 161 37.8

E3-18 21 70.4 127 128 28.0

Q3-17 21 95.4 131 132 55.3

E3-7 30 69.0 180 177 35.0

E3-17 30 69.3 122 121 29.4

E3-8 30 100.1 267 296 100.8

Q3-10 40 101.5 218 227 59.5

Q3-21 40 99.0 221 283 79.8

Q3-23 40 90.6 191 56.0

* Instrument malfunction on tup record.

257
toughness. Hoyt [13] had suggested using double-height specimens to increase constraint

as early as 1938, but nothing was done to implement this suggestion until Bonenberger

developed the modified specimen geometry [4] with a height of 19.2 mm. This specimen

offers sufficient constraint to permit testing of engineering steels with Kj^ approaching

about 100 Mpa-m'^l

We have extended the work of Bonenberger et al. by modifying the procedures

followed for specimen preparation and testing. We have employed a sharp notch instead

of a precrack formed with precompression. The results obtained indicate that cleavage can be initiated from the sharp notch, but that the cleavage is preceded by a small amount

of tearing. The depth of the tearing ranged from 30 to 50 ^m for specimens exhibiting

lower toughness (55 to 70 MPa-m to as much as 1 mm for specimens with very high

toughness (in excess of 100 MPa-m ). The tearing transitions into cleavage even in the

high-toughness specimens; however, tearing is a slow process and the initiation time is extended to 200 jis or longer.

We have also shown that an instrumented tup can be employed to estimate the

crack initiation time. There is usually a small delay in the response of the tup, when

compared to the strain-gage response; however, the error produced by the typical delay (5 to 10 |is) is negligible when compared to the scatter observed in carefully controlled and

standardized tests for toughness.

A master curve relating crack initiation time was developed in this investigation.

This curve differs from the master curve developed by Bonenberger et al [5], even though

the material was from the same plate. The curve presented in Fig. 8 is nearly identical to the curve in [5] for small strains, but for the larger strains the curve developed in this

study shows more nonlinearity. We believe that part of the differences may be due to the

different Charpy machines that were employed. Bonenberger employed a U-type

machine with a capacity of 400 J. The machine was old (worn tup), used extensively by

undergraduate students, and not firmly mounted to the floor. The machine used in this

study was a U-type with a capacity of 250 J. It was in excellent condition and grouted

and properly bolted to the floor with a suitable foundation. The nose piece on the

hammer was new. We believe that a significant part of the nonlinearity in the master

curve is due to the plastic deformation at the contact point due to Hertzian stresses that

exceed the yield strength.

A second difference was in the material at the notch. Although all the specimens were from the same plate, Bonenberger pre-compressed the material in the local

neighborhood of the notch. This pre-compression elevated the local yield strength and

the specimen exhibited better resistance to the Hertzian contact stresses and required a

longer time before yielding at the net section occurred. The precompression also

introduced residual tensile stresses that were partially relieved by short cracks at the notch

tip.

The third difference was in the depth of the side grooves. In this study we

employed a depth of 2 mm compared to a depth of 1.9 mm used by Bonenberger et al. This difference would tend to cause yielding of the more deeply side-grooved specimens

at an earlier time and to accentuate the non linearities.

The results that we obtained for Kj^ were higher than those obtained by

Bonenberger at 0 °C. We believe this difference is due largely to the method used to prepare the specimens. Precompression used by Bonenberger probably elevated the yield

strength while suppressing the toughness. Also the precompression produced a residual

tensile stress near the notch that was only partially relieved by the formation of a short

crack at the crack tip. Accordingly Bonenberger measured Kj^ values (40 to 50 MPa-m^^^)

258
that were significantly lower than the average crack arrest toughness (83,4 MPa-m ). In

this investigation the measurements of K^. at room temperature vaned from 58.2 to 95.4

Mpa-m'^' with an average value of 69.6 MPa-m" which better agrees with the arrest toughness of the material.

We expect the arrest toughness of the steel K^^ to be less than the d>Tiarmc crackinitiation toughness Kj., In this senes of measurements, the average value of Kj. was

about 16.5 lower than the average value of K^,. However, m the round-robin testing

[7] of the same lot of A533 T\pe B steel the standard deviation for the crack arrest toughness was 10.6 MPa-m^". If one defines the lower-bound toughness as the mean less

two standard deviations^, the value from the round-robin test program would be 83.4 - 2(10.3) = 62.2 MPa-m' It appears that the results from the sharply notched modified

Charpy specimens are somewhat consen'ative when average values are considered.

However, if one is attempting to determine the lower-bound toughness to be used m a conservative design, then the results using the proposed method appear to be in reasonable correspondence with arrest-toughness measurements when the dispersion of

the test results are taken into account.

It is important to eliminate the need to use strain gages in developing a measurement method for fracture toughness that is based on impact loading with Charpy

machines. The strain gages require additional instrumentation, are e.xpensive to install

and require additional time in data reduction. Master cun'es can be developed for

different allo> s using a limited number of strain gages and then employed in subsequent

certification testing to insure that the toughness of a particular heat of steel exceeds a minimum lower-bound toughness. This approach requires that the initiation time be

established with some degree of confidence, \^'e ha\'e shown that the instrumented tup

gives a reasonably close estimate of the initiation time. However, before we close this

topic it should be noted that initiation is not an instantaneous event. When a crack initiates, even from a fatigue-sharpened crack, the large-scale

initiation event is a series of many small initiation events. Initiation occurs at many

separate, small cleavage initiation sites. Crack-front stretch and e\'en limited amounts of

tearing (ducdie hole-joining) occur before all of the clea\'age initiation sites are triggered.

Sometimes in the tougher steels cleavage will extend across a few grains and then the

fracture mechanism will transition back to ductile hole-joining. The initiation process is a combination of many local initiation events that occur over a small inter\'al, but not at the same instant, '^lien we attempt to measure the initiation time. \^-e should recognize

that it is not a precise number, but an approximation of the time when a critical number of

local cleavage initiation events are occurring over a finite duration.

ACKNO\\XEDGEMENTS

James W. Dalh' appreciates the support of the National Science Foundation

during the later phase of this smdy.

159
REFERENCES

[I] Rosenfield, A.R. and Marschall , C.W., "Fracture Mechanics Based Failure

Analysis," USNRC Report NUREG/CR-5860, June 1992.

[2] Anon, 1993 Annual Book of ASTM Standards, Section 3, Metals Test Methods

and Analytical Procedures, ASTM E-399 and ASTM E-1221 Specification,

ASTM, American Society of Testing Materials, Philadelphia, 1993.

[3] Link, R.E. and Joyce, J.A., "Experimental Investigation of Fracture Toughness

Scaling Models," 2nd Symposium on Constraint Effects in Fracture, ASTM,

November, 1993.

[4] Bonenberger, R.J. and Dally, J.W., "Further Studies of a Modified Charpy

Specimen for Lower Bound Toughness," Fracture Mewchanics: 25 th Volume,

ASTM STP 1220, F. Erdogan and R.J. Hartranft, Eds., American Society of

Testing Materials, Philadelphia, 1993.

[5] Bonenberger, R.J., Dally, J.W., and Irwin, G.R., "Lower Bound Initiation

toughness with a Modified Charpy Specimen," Constraint Effects in Fracture,

ASTM STP 1171, E.M. Hackett, K.-H. Schwalbe, and R.H. Dodds, Eds.,

American Society of Testing Materials, Philadelphia, 1993, pp. 139-157.

[6] Winkler, S. Fraunhofer - Institut fur Werkstoffmechanik 7800 freiburg, Germany,

Private Communication.

[7] Barker, D.B., Chona, R., Foumey, W.L., and Irwin, G.R., A Report on the Round

Robin Program to Evaluate the Proposed ASTM Standard Test Method for

Determining the Plane Strain Crack Arrest Fracture Toughness, Kj^, of Ferritic

Materials, NRC report, NUREG/CR-4996, ORNL/Sub/79-7778/4, January 1987.

[8] Irwin, G.R., Dally, J.W., Zhang, X-J, and Bonenberger, "Lower Bound Initiation

Toughness of A 533B Reactor Grade Steel," Symposium on Rapid Load Fracture

Testing, San Franciso, CA, April 1990, ASTM, Philadelphia, PA, 1990.

[9] Impact Testing of Metals, ASTM STP 466, ASTM, Phildelphia, PA, 1970.

[10] Radon, J.C. and Turner, C.E., "Fracture Toughness Measurement by Instrumented

Impact Test," Engineering Fracture Mechanics, Vol. i, 1969, pp. 41 1-428.

[II] Server, W.L. and Tetleman, A.S., "The Use of Pre-cracked Charpy Specimens to Determine Dynamic Fracture Toughness," Engineering Fracture Mechanics, Vol.

4, 1972, pp. 3673-375.

[12] Server, W.L., "Impact Three-Point Testing for Notched and Precracked

Specimens," Journal of Testing and Evaluation, Vol. 6, No. 1, 1978, pp. 29-34.

[13] Hoyt, S.L., "Notched-Bar Testing and Impact Testing," Proceedings ASTM Part

n, Vol. 38, ASTM, Phildelphia, PA, 1938, pp. 141-156.

260
A. Karl Schmieder.* Patrick T. Punscher.^ and Daniel P. Viglioni"

The Role of Strike Marks on the Reproducibility of Charpy Impact Test

Results

REFERENCE: Schmieder. A.K.. Punscher. P.T.. and Vigliom. D.P.. "The Role of Strike Marks

in the Study of Reproducibilit}' of the Results of Charpy Impact Tests. Pendulum Impact

Machines: Procedures and Specimens for Verification. ASTM STP 1248 . T.A. Siewen and A.K.

Schmieder. Eds.. American Societ\' for Testing and Matenals. Philadelphia. 1995.

ABSTRACT: Charp}' \'-notch specimens from one lot of high-strength steel were tested by use

of three machines to determine reference values for three measures of toughness: absorbed

energ}\ lateral expansion, and height of shear lips. The broken specimens were examined to determine the location and magnimde of changes in specimen features made during testing. The

features of interest were the height and location of the shear lips, the location of the lateral expansion projections ranked by height, and the location, length, width, and angle of the first- and second-strike marks. Changes in these features were compared to the changes in average

absorbed energy for each of the machines in its standard condition. To correlate changes in these

features with intentional machine modifications, ten series of tests were made on a founh

machine. Pattems that could predict the direction of change in absorbed energy for most

modiQcations were obsen'ed. The trends indicated b>' these data are: 1 1 j each mO'dification

resulted in an increase in absorbed energ}-. (2) the distance between second-strike marks is a measure of the compliance of the striking edge and anvil, (3) the offset of the first-strike marks is largely due to hft-off of the specimen at the moment the striker hits the specimen, 1 4j offset and

the angle of second-strike marks are measures of general as>mmetn.' of loading, and (5j lateral expansion and shear lips are N'aluable as means to detect scale errors and excess losses not related

to the work of fracture.

KE\"AVORDS: Absorbed energ}". Charp>- \'-notch. high-strength steel, high-speed photography,

impact machine, lateral expansion, shear lips, strike marks.

Mechanical Testing. Scotia. NT 12302

-NIST. Boulder. Colorado 80303

261
NOMENCLATURE

composite postfracture energy - a measure of the work done to create the fracture surface,

excluding energy expended in shock losses, toss losses, and work to form depressions in the

specimen at points of contact.

lift-off - the momentary loss of contact between the specimen and the anvils, which occurs immediately after first contact.

offset - the horizontal distance from the striking edge to the notch.

reference value - a value obtained from tests made by a machine in standard condition,

specimen locations - the location of the specimen when in the position for testing.

In direction of swing: (N)^ notched surface - the surface parallel to the notch root and nearest it. (S) struck surface - the surface parallel to the notch root and farthest from it.

vertically: (U) upper horizontal surface.

(L) lower horizontal surface.

transversely:

(I) inboard half - portion of specimen originally between the striking edge and the

machine column.

(O) outboard half - the portion originally adjacent to, but not including, the inboard half.

first-strike mark - the mark made on the specimen by the anvils before fracture (see Fig. 1).

second-strike mark - the mark made on the specimen if the broken halves fly away from the

pendulum and strike the anvils (see Fig. 1).

third-strike mark - additional mark made on the specimen after fracture by striking a machine

part or other solid object in the vicinity.

tvpes of second strike marks:

line - the second-strike mark that completely crosses the notched surface (see Fig. 2,

codes A, I and J).

nick - a second-strike mark that impacts both edges, but not the center, of the notched

surface (see Fig. 2, codes B and F).

'Letters are abbreviations used in Tables.

262
others - a nick at one edge only, or a nick on one half and a Ime on the other half, or no

second-strike marks on one half.

INTRODUCTION

Relative to hardness and tension tests of metak impact tests have poor reproducibility.

This is economially important smce it requires more tests to ensure a given degree of precision.

This testing problem has been long recognized [1-4]. Although significant improvements have

been made during the last three decades [5], limited reproducibilit\' remains one of the pnncipal

disadvantages of impact testing. The objective of this smdy is to reduce the variability- of impact

test results by identifying machine deficiencies through inspection of the broken specimens.

This inspection included the usual measurement of absorbed energy, lateral expansion,

and shear lips. In addition, another less well-known measurement was made, the characterization

of subsequent strike marks. The process of forming the marks is shown schematically in Fig. 1. High-speed photographs confirm the transverse flight of the brokenhalves and the strike against

the anvils. Enlarged photographs in Fig. 2 show various types of first- and second-strike marks.

V

1st Strike

Figure 1. Schematic diagram shows how the first- and

second-strike marks are produced.

263
Figure 2. Photographs of the struck surfaces on a broken

half from each of the following series: A, B, F, I, and J. The first-strike marks are on the right-hand side and the

second-strike marks are on the left-hand side.

EXPERIMENTAL PROCEDURES

Specimens

All specimens were drawn at random from a large lot of verification-grade specimens.

The material specified is published in ASTM Standard Practice for Qualifying Charpy

Verification Specimens of Heat Treated Steel, E 1271, Appendix XL The specimen dimensions were those for Type A shown In Fig. 6 of ASTM Standard Methods for Notched Bar Impact

Testing of Metallic Materials E 23, except that the tolerances are smaller.

Machines

The first three machines, that were used to determine the reference values were

manufactured by different companies to meet the specifications of ASTM E 23. All had

capacities of 300 J (220 ft-lbf) or more. All were directly verified within a year of making the

tests reported here. The fourth machine, which was modified during these tests, had less than

half the capacity of the others. The machine designations used in this study, the pendulum types

( as described in ASTM E 23, Fig, 1), and the scale errors in percentage of the reading are:

Designation RIC RIU R2U MIC

Type of Pendulum C U U C

Scale error -3.0% -0.8% -0.3% 0.0%

264
All reported values are corrected for the scale errors that are known. A method to estimate the

scale error for MIC is discussed later. The modifications made to Machine MIC are given letter designations and are listed below. The same letters are used to identify the test series in the

tables of results.

A. As received. History of prior use unknown. Tightness of bolts unknown.

B. Old anvils replaced by new anvils. Bolts tightened to 100 J (75 ft-lbf) at each installation.

C. Old anvils reinstalled.

D. Place specimen on the supports so that they are offset 2 mm toward the machine pedestal.

E. New taller supports installed to raise the specimen 10.6 mm above the standard position.

During this and each subsequent replacement of the supports, the bolts were tightened to 27 J (20 ft-lbf). F. Reinstall the original inboard support only.

G. Remove both supports so the specimen is 10.9 mm below the standard position.

H. Shorten and grind the top surface of the new supports so that the specimen is slanted

upward toward the anvil at one degree with the horizontal; reinstall supports.

I. Reinstall the original standard supports. Remove old anvils and grind the face that bears

against the specimen so that it has an angle of 10: 1000 to the original surface, measured

in a vertical plane when installed. Restore the comer radii and surface finish. Reinstall

modified anvils.

J. Remove anvils and restore contact faces to original condition; also reduce the thickness in

the direction of swing by 5 mm. Reinstall modified anvils.

K. Reinstall new anvils, restoring the machine to standard condition. Photograph specimen

half as it flies transversely and strikes the anvil.

Methods of Testing

All specimens were tested at -40°C (-40°F) in accordance with ASTM E 23. The lateral

expansion of the broken specimens was measured as prescribed in ASTM E 23-93a, Section

12.4.2.

The location, length, and width of the strike marks were measured with an optical

comparator in the reflective mode. Magnifications of lOX and 20X were used for the first- and

second-strike marks, respectively.

Many of the first-strike marks had poorly defined outer boundaries. To be consistent, we

reported the distance from the notch root center-line to the inner boundary of the mark. For a few marks, because both boundaries were poorly defined no dimension was recorded.

The high-speed photographs were taken at 2000 frames per second with a video system.

The lighting system included two banks of incandescent lights arranged around the outboard side

of the impact machine and drawing a total of 1500 watts of power.

265
Methods of Calculation

The statistical calculations were made according to the mathematical definitions of the

average and standard deviation. The lateral expansion was measured as prescribed by ASTM E

23, that is, by the sum of the two highest of the projections at the ends of the pendulum strike

marks on both broken halves. This method is based on these results that correlate lateral expansion and absorbed energy over a wide range, mostly at energies higher than those reported

here. Some information is available in the range of these tests [6].

Since four measurements are taken in any case, they were added to see whether the

correlation with absorbed energy was improved in the range of these tests. To make the results

compatible with the standard value, the sum of the four measurements was divided by two, then

reported.

The composite postfracture energy index reported was calculated by dividing the average

value for a series by the average of all specimens tested during the program. This was repeated

for each of the three types of measurements of postfracture energy, and then the three values were

averaged. The result is a dimensionless number for each series that allows the series to be

compared but does not indicate the magnitude or units of the energy measurement. To allow

comparison to other published results, the composite number was multiplied by the average value

for all tests for the lateral expansion measured according to ASTM E 23. The average values for

all tests are shown on the bottom line of Table 1 and the values for Series RIC on the top line. Using these values, the composite postfracture index for Series RIC is

0.1 33[{ 0.1 2 / 0.1 33) + ( 0.11 / 0.1 1 1) + ( 1.35 / 1.403)] / 3 = 0.1 26 mm

This distance, reported as a measure of position of second-strike marks, is the distance

from the centerline of the notch to the centerline of the mark. The offset is equal to one half of

the difference between first-strike marks, measured on the inboard and the outboard halves.

Unless stated otherwise, the distances at the upper and lower surfaces are averaged before the

difference is calculated.

RESULTS

Table 1 shows averages and standard deviations of energy measurements for some single

series and for combinations of related series. Table 2 shows the energy measurements for all of

the series as a dimensionless ratio to the average values for Machine MIC. This permits

comparison of quantities such as lateral expansion to absorbed energy that have different units

and magnitudes varying by a factor of over one hundred.

Table 3 presents the various second-strike measurements. Table 4 is a tally showing the

number of occurrences of various deformations at specified locations. Its primary use is to

identify asymmetrical conditions. Table 5 presents statistics on first- and third-strike marks and

compares absorbed energy to the composite postfracture energy.

266
Table 1. Weighted average values for quantities which are given as a single number for both

broken ends

Machine

or Class

Absorbed

Energy

J, ft lbf

Lateral Expansion"* Sum of

Shear

E 23, mm Sum/2, mm Lips, mm

RIC 15.9. 1 1 .7 (0.3%)" 0.12 (O.OI)' 0. 1 1 (O.Ol)' 1 .35 (0. 1 I)'" RIU 17.2. 12.7 (0.5) 0. 14 (0.03) 0.12 (O.Ol) 1 .43 (0. 14) R2U 17.5, 12.9 (0.5) O. 14 (0.03) O. 12 (O.Ol) 1 .40 (0. 13) MIC-A 15.2, 1 1.2 (0.6) O. 1 1 (0.10) 0.09 (0.01) 1 .36 (0.09)

A.B.C.K 15.3, 1 1 .3 (0.2) 0. 13 (0.02) 0. 10 (0.01) 1 .34 (0.04)

MlC-Others 15.9, 1 1.7 (0.3) O. 14 (O.Ol) 0. 1 1 (0.01) 1 .45 (0.09)

All RXX 16.9, 12.4 (0.6) 0. 13 (O.Ol) 0. 12 (0.09) 1 .39 (0.04)

All XXU 17.4, 12.8 (0.2) 0. 14 (O.OO) 0. 12 (0.00) 1 .40 (0.04)

All Std.C 15.5. 1 1 .4 (0.2) 0. 13 (O.OO) 0. lO (O.OO) 1 .34 (O.Ol)

^ E 23 is sum of two highest projections. "Sum/2" is one half the sum of the four readings.

*' Values in parenthesis are coefficients of variation.

^ Values are standard deviation in mm.

Values from RIC and MIC: Series A, B, C, K.

Table 2. Deviations^ of energy-related measurements in Table 1 from the corresponding values

for Machine RIC.

ID'' Absorbed

Energy

Lateral Expansion

Height of

Shear Lip

E 23 method Sum of Four

RIC (10) 0.0 (0.0) 0.0 (0.0) 0.0 (0.0) 0.0 (0.0)

RIU (10) 8.5 (0.7) 14.6 (2.7) 12.5 (0.9) 15.9 (0.3)

R2U (10) 10.3 (0.7) 15.2 (1.6) 15.9 (0.6) 2.4 (0.3)

MlC-A (6) -4.5 (1.1) -6.3 (0.7) -14.0 (0.3) 0.7 (-0.2)

B (6) = -5.3 (0.4) 5.7 (5.4) -1.5 (3.5) 0.7 (0.5)

C (5) = -3.0 (0.2) 13.3 (-0.6) 15.1 (-0.4) -4.8 (0.1)

D (5) 2.6 (0.6) 28.5 (0.2) 1.1 (0.4) 14.1 (0.7)

E (5) -1.7 (0.5) 5.3 (1.4) -4.2 (0.7) 3.0 (-0.3)

F (6) -2.7 (0.6) 7.8 (-0.4) -4.5 (0.5) 3.0 (0.1)

G (5) -0.9 (0.6) 12.1 (2.8) 6.8 (1.3) 3.0 (0.9)

H (5) 1.7 (0.0) 18.4 (1.4) 13.9 (0.2) 4.4 (0.9)

I (5) 1.3 (1.1) 20.5 (0.6) 4.6 (0.8) 0.7 (-0.2)

J (5) 3.4 (0.2) 18.4 (3.3) 16.3 (0.2) 10.8 (0.2)

^ Deviation of measured quantities are shown as percentages, deviation of the coefficient of

variation of that quantity as a ratio.

^ Machine identification, series, and number of specimens. ^

" One of the halves not available for measurement.
Table 3a. For the reference machines, average values and coefficients of variation for positions,

width, and direction of second-strike marks.

ID'

Distance from Notch to Mark*" Line

Width

mm

Angle with Upper Edge, deg.

Outboard Ratio O/I Outboard Inboard

Lines Others Lines Other Lines Others Lines Others

RIC 0.94

(0.04)''

0.94

(0.04)

1.03 (0.08)L06

(0.09)

1.22/L12"-- (0 38)

0.3

(0.7)

3.2

(0.4)

0.2

(1.0)

0.5

(0.4)

RIU 0.94

(....)

0.93

(0.04)

0.98

(..-) L03(0.07)

0.28/0.34

(0.31)

1.0 (••)2.0

(0.3)

0.2

(-.)

0.2

(0.5)

R2U 1.06

(0.02)

0.99

(0.04)

LOS

(0.06)

0.99

(0.03)

1.25/1.22

(0.37)

1.6

(0.5)

2.7

(0.8)

0.8

(1-3)

0.2

(1.1)

^ Machine identification and type of pendulum.

Dimensionless ratio of distance to width of specimen, which is 10 mm.

Width of all specimens in a series combined statistically.

^ Value in parenthesis is ratio of standard deviation to average.

^ Upper surface/lower surface.

DISCUSSION

Significance and Limitations of Various Measurements

One of the main objectives of the impact test was to measure the energy required to produce the fracture surface. The loss of potential energy of the pendulum during the swing is reported as absorbed energy, but it also includes:

A. frictional losses due to pendulum motion,

B. shock losses due to vibration and displacement of the machine parts,

C. crushing work to form the depressions on the surfaces struck by the pendulum and the

anvils simultaneously, and

D. the kinetic energy of the broken halves after fracture.

268
Table 3b. For Machine MIC, average values and coefficients of variation for position, width,

and direction of second-strike marks.

ID* Distance from Notch to Mark'' Line Width,"' mm Angle with Upper Edge,

Outboard Ratio O/I Outboard Inboard

A 0 . 94 (0 . 02) 1.01 (0.05) 0.49 (0.16) 1.3 (0.8) 1.2 (1.1)

B 0 . 90 (0 . 06) 1 . 04 (0.06) 0.65 (0.30) 2.5 '0.4> 2.1 (0.7)

C 1J. < on\J \J (

\ n\J t 01)f / 0.57 (0.50) 1.5 (0.9)

D 1 . 09* (0 . 01) ... (...) 0 . 56« (0 . 11) 1 . 6= (0 . 4) ... (...)

E 0.89 (...) 0.98 (...) 0.64 (0.10) 0.9 (0.5) 1.4 (0.8)

F 0.83^0.05) ... (...) 1 .2lM0 . 10) ... (...) 19.0 (0.3)

G 0.86 (0.04) 0 . 95 (...) 0.67 (0.29) 2.6 (0.4) ... (...)

H 0.86 (0.05) 0.99 (0.05) 0.84 (0.28) 2.8 (0.7) 4.7 (0.4)

I 0.89 (0.03) 1.05 (0.06) 0.56 (0.15) 1.3 (0.9) 1.1 (0.6)

J 0.86 (0.06) 1.04 (0.11) 0.49 (0.17) 1 1.8 (0.8) 1.0 ( C . 7

)

^ Series identification.

Dimensionless ratio of distance to width of specimen, which is 10 mm.

" Width of all specimens in a series combined statistically.

^ Value in parentheses is ration of standard deviation to average.

^ Outboard values. No strike marks on inboard halves.

^ No strike marks on upper outboard halves.

269
Table 4. Evaluation of machine asymmetry by number of deformations at various locations.

ID'

Start of Second-Strike Marks*" Shear Lips

U L

Lateral Expansion Projections'

Line U L

Nick

U L

None'' U LUpper Surface A B C D

Lower Surface A B C D

RIC

-0

-I

6/1' 6/4 7/2 7/5

4/2 4/3

3/1 2/1 0 00 1

5/2 4/3 5/3 6/22 3 14

2 4 13

3 2 2 3 4 4 11

RIU

-0

-I

6/6 6/0

3/1 3/2

2/0 4/4

6/2 6/5

2 0

1 1

3/2 6/1 7/4 4/3

2 2 13

13 2 2

4 2 11

13 3 1

R2U

-0

-I

6/6 6/0

6/5 6/1

4/1 4/3 4/3 4/1 0 0

0 0

5/3 5/2

5/3 5/2

5 2 2 1 3 3 2 2

3 3 4 0 12 0 7

MIC

A-0

A-I

2/2 2/1 6/4 6/2

2/1 3/1 0 01 0 0 0

4/2 3/1 3/2 3/0 12 2 1 2 112 3 3 0 0

2 0 13

MIC

B-0

B-I

2/2 2/0

1/1 1/0

2/1 3/1 4/0 5/0

1 0

1 0

2/1 0

3/1 6/3

2 12 0 1112 2 0 12

2 111

MIC

D-0

D-I

5/5 5/0 0 0

0 0 0 00 0 0 01/0 4/1 4/3 1/1 0 112

0 0 13

0 3 10

12 0 1

MIC

H-0

H-1

3/2 3/1

1/0 1/1

2/1 2/1

3/1 4/2

0 0

1 0

3/2 3/2 2/0 2/1

2 12 0

2 0 12

0 3 2 0 112 1

MIC

J-0

J-I

3/3 3/0

5/3 5/2

2/2 2/0 0 0

0 0

0 0

2/1 3/2

3/1 2/1 110 3 10 2 2

3 110

12 2 0

^ Machine identification - Position of half before fracture.

Where first contact between the specimen and the anvil occurs, "U" designates

uppermost surface in positioning for testing, "L" the opposite.

Letters indicate ranking of heights at four locations. A is highest projection. If two are equal, both receive the same rank and the next lowest is omitted.

^ Indicates no second strike mark at that location.

^ Numerator is total number of occurrences at that location. Denominator is the number

of highest values included in the numerator.

270
Table 5. Absorbed energy, index of fracture work, first-strike dimensions, and frequency of

occurrence of third-strike marks.

ID

Absorbed

Energy.

J

Composite

Index of

Fracture, mm

First-Strike Marks Third-Strike Marks'

Fracture End

Lips^ Corner Surface Edge

Offset Angle, deg

mm 0 I

RIC 15.9

(2.5)=

0.126 0.1 (0.5) 0.4' (0.7) 0.2' (1.5) 67 13 7 27

RIU 17.2

(4.0)

0.140 n 4 /I ^\ U.l U-J) LL 1

Z /

R2U 17.5

(3.9)

0.140 U.J no (\ s\ U.Z ^1 .8^ DU

-A 15.2

(5.6)

0.113 U.l (U.y) U.O (U.y) U.Z (I .0)

1 AO luy TO11 43

-B 15.0

(3.6)

0.128 0.1 (0.8) 0.1 (...) 0.9 (1.6) 1 10 20 80 10

- C 15.4

(3.0)

0.135 U.l (U.4) U.J (1.2) 0.3 (1.5) on 11 0/ A A 44

- D 16.3

(3.9)

0.144 l.\ \J.L ( \ A) U.l A

u

A

U

A

U

Au

- E 15.6

(3.7)

0.128 0.0 (0.4) 0.2 (1.4) 0.2 (1.4) 55 18 36 64

- F 15.4

(4.1)

0.127 0.5 (0.5) 12.8 (0.1) 14.3 (0.1) 40 20 60 10

- G 15.7

(4.U)

0.135 0.1 (0.4) 1.1 (0.7) 1 .3 (0.8) 83 8 50 33

-H 16.1

(2.5)

0.141 0.1 (0.3) 0.3 (0.9) 0.5 (1.1) 70 70 20

- I 16.1

(5.2)

0.136 0.1 (0.9) 0.3 (0.9) 0.6 (0.7) 67 44 44

- J 16.4

(2.8)

0.145 0.2 (0.5) 0.2 (1.4) 0.3 (0.9) 22 22 22 22

^ Values shown are number of occurrences as a percent of the number of specimen halves

examined.

Flattening of the tip of the shear lip, thereby reducing the height.

Values shown in parentheses are standard deviations as percentages of the value.

271
Absorbed enerev - ASTM E 23 includes a correction for (a) that adequately removes this amount

of work from the reported absorbed energy [7]. The other losses are included in the reported

value. Measurements of lateral expansion remove all of the above except the crushing work at the struck surface. The shear-lip-height method excludes all of the losses. However, that does

not mean that the machine has no influence on the results from these alternative measures of

energy. These tests indicate that the different machines impose different conditions of force,

displacement, and loading points that influence the work done at or near the fracture; therefore,

the resulting numbers are not solely material properties. Attempts to separate these machinedependent contributions by measurements of the broken specimen halves is discussed further in

other sections.

Lateral expansion - The principal use of lateral expansion in this study is to provide a measure of

the work to produce a fracture without depending on the energy measurement scale of an impact

machine. In order to estimate its discriminatory capability in a single series, a regression analysis

was made with inputs of the average absorbed energy and lateral expansion from each of the

fourteen series unweighted for the number of specimens in each series. The regression of series

averages showed regression coefficients 0.534 for the ASTM method, 0.644 for the sum method,

and 0.552 for the composite postfracture energy.

Shear-lip heights - The shear-lip height has the advantage of being subject to less proportional

reading error than lateral expansion; the shear-lip heights were about 10 times greater than the

lateral expansions. This difference would seem to compensate for the differences in the

reference surfaces for the two methods, a fracture surface for the former and a ground surface for

the latter. Another disadvantage of the shear-lip height is that a maximum is sought along the

crest of the lip by measuring at various distances from the notch. A much more serious source of

error is that most crests were flattened by various amounts due to contact with a hard plane

surface.

A regression analysis of the ten individual specimens vs. the absorbed energy of each

showed that the shear-lip height was superior to the lateral expansion method for a single series.

The regression coefficients were 0.81 for the RIC machine, 0.56 for RIU, and 0.46 for the R2U

machine. Corresponding values for lateral expansion were all less than 0.25.

First-strike marks - The accuracy of the measurements can be judged by comparing the distance

between first-strike marks on the inboard and outboard halves to the specified distance between

anvils. For the three reference machines, the distance between maiks are 41.0 ± 0.2 mm for

RIC, 41.1 ±0.1 mm for RIU, and 41.1 ±0.2 mm for R2U. The specified gap between anvils is 40.0 ± 0.05 mm and the distance between tangent points (gap -i- radius of curvature) is approximately 42.0 mm The average of the values for the three machines differs from the

average of the specified dimensions by 0.2 mm of 0.5 percent, indicating that the distance

measurements have better precision than any other measurements presented in this report. The

angle measurements are derived trigonometrically from the distance measurements. Their

accuracy is estimated at 0.3 deg.

272
For the thirteen series of tests reported in Table 5, the average offset of first-strike marks

was typically less than 0.3 mm; four values are less than 0.1 mm, indicating that the specimens

had been positioned very precisely. One explanation for higher offsets is operator carelessness,

but that would not account for the fact that the angles, which the operator does not control, vary

just as much. Even more puzzling is the fact that the angles on each end of the specimen differ,

in some cases, by more than the estimated inaccuracy. An explanation is that after the first contact with the striker, the specimen bends and loses contact with the anvils momentarily.

Furthermore, when contact is restored, one anvil may touch before the other, allowing the

specimen to continue tilting until the second anvil makes contact. The published records of force

vs. time during the first contact between the specimen and the striking edge of a high-strength

steel specimen show the force decreasing, approaching zero for a short period, thus indicating

that lift-off occurs [8,9].

Second-strike marks - The second-strike marks provide more measurements related to asymmetry that the other reported characteristics of the broken halves. The most conspicuous is whether the marks are lines or nicks. A less obvious but just as useful difference is the width of

the strike mark, the edge showing the widest mark having struck the anvil first.

The feature found most useful for quantitative analysis is the sum of the distance between

second-strike marks, obtained by adding the distance from notch to mark for the outboard half of

the specimen to that for the inboard half This distance seems to be a measure of compliance of

anvils and frame. The test series did not contain a comparison of directly controlled compliance,

but Series I an J have partial contact between the specimen and the anvils during the initial loading, thus simulating the slower rate of increase in force of a less stiff system. The average

distance between marks for the same machine and the same anvils in normal condition (Series A

and C) was 36.9 mm, while that for the less stiff anvils (Series I and J) was 35.8 mm, or a

difference of 3 percent. The corresponding increase in absorbed energy was 15.3 to 16.2 J or an

increase of 3 percent, both changes being larger than those due to other modifications. Bluhm

has demonstrated that the absorbed energy increases with a decrease in machine stiffness [10].

Third-strike marks - For Type-C machines, the specimen halves can leave the machines

transversely without again being near the moving pendulum. For Type-U machines, the

specimens rebound from the shrouds and approach the moving pendulum. With this in mind, we

expected that the Type-U machines would have more third-strike marks. This expectation is not

confirmed by the test results shown in Table 5.

The reported effectiveness of shrouds in reducing jamming was confirmed by these tests

[5]. Only one specimen half showed marks due to jamming. Even in this case, an increase in

absorbed energy was not clearly demonstrated. One specimen of the ten in the series had greater

absorbed energy than the jammed specimen.

273
Evaluating asymmetry of machines - At the beginning of this study, we assumed that a properly

adjusted machine would produce a uniform line as a second-strike mark on the specimen.

Misalignment would result in asymmetrical second-strike marks. The data indicate that the

absorbed energy increases as deliberate asymmetry is introduced, but the rate of increase for a given asymmetry is small. Furthermore, the marks and features of the broken halves are also

asymmetrical. When the specimens show second-strike marks, there are several features that can be used to measure symmetry. Among these are:

A. whether lines or nicks are formed on the two halves,

B. the distance from the notch on each half,

C. the angle of the strike mark with the specimen edge, and

D. whether the upper or lower edge struck the anvils first.

Table 4 shows the tally for these dimensions. Examples of how the second-strike marks are useful will be presented in the discussion of the results that follow.

Components of absorbed energy - The simplest estimate of the effect of a modification is to subtract the absorbed energy of tests with the modification from that without the modification.

This is effective if other variables not obviously related to the controlled variable remain

unchanged. To decrease the proportional effect of the uncontrolled variables, the controlled

variable was changed by an amount five to ten times the specified tolerance on the controlled

variable. An exception is for specimen elevation relative to the striking edge, which could be

changed only to the limits of the specified tolerance. Then the difference in absorbed energy was

divided by the change in the controlled variable to measure the rate of change, assuming a linear

relationship. For example, the effect of offset was calculated to be +0.44 J/mm, that of the angle

of strike to be 0.02 J/deg, and that of compliance as measured by the change in the distance

between second-strike marks to be -0.92 J/mm.

The second approach was to make a regression analysis of individual specimens from a

set which showed an above average range of that variable. That approach showed much larger

values of slope; for example, 4.0 J/mm for offset. The 4.0 J/mm estimated the change between

some modifications better than 0.44 J/mm. However, the higher value is not due to offset alone;

other uncontrolled factors must be present that influence the result that we measure.

Comparison of Pendulum Types

Table 2 shows that the average value for absorbed energy measured by RIU and R2U is 9 percent greater than that measured by RIC. This is greater than the change in absorbed energy

due to deliberate modifications made to exceed the specified tolerance beyond what would be

expected in machines in use. Considering the magnitude of difference due to pendulum type, this

effect will be analyzed in detail.

274
Comparison of different measures of the energy to produce fracture - As shown in Table 5, on

average the composite postfracture energy, after testing is 10 percent higher for tests made using

the Type-U machines than for those using the Type-C machine. Each method of measurement in

Table 1 indicates that the Type C machines produce lower values than for the Type-U machines.

Therefore, the difference in absorbed energy is not due to the means of measurement.

Differences in the striking edge - We know that the stiffness of the loading system affects the

absorbed energy and that the difference can be measured fairly easily [8,10]. The striking edge

of the type-C pendulum is supported along its whole length by the massive disk of the beam.

The stiffness of the latter is no doubt less than the former so the rate of increase of the force on

the specimen is smaller and less uniform from the upper side of the specimen to the lower.

Another factor to consider is the distance between second-strike marks. For a specimen

that is loaded more rapidly, we expect that the halves would have greater velocity after fracture,

and thus the second-strike marks would be closer together. The distance between second-strike

marks for the specimens broken in the Type-C machine average 19.0 mm; those broken by RIU

average 18.6 mm; and those from R2U average 21.8 mm. The average difference of 2.8 mm

between machines RIC and R2U would lead to a predicted increase in energy of about 16

percent for R2U compared to RIC; a 10 percent increase was measured (see Table 2). The

results for RIU do not follow the generally observed trend.

Energy loss due to shrouds - The more complicated flight path for specimens tested in shrouded

machines would be a convenient explanation for an increase in the number of third-strike marks

when compared to the Type-C machine. However, the tallies in Table 5 show that the numbers

are approximately equal. In any case, unless the marked specimen actually makes contact with

the pendulum, it cannot affect the absorbed energy.

Summarv - Of the three uncontrolled variables (energy loss to shroud, difference in striking

edge, and means of measurement), the difference of striking edge appears to be the major factor

that relates to the difference between pendulum type.

Effects of Modifications to Machine MIC

The scale of the modified machine was graduated in ft-lbf, and the readings were recorded

to the nearest 0.25 ft-lbf, which is about 2 percent of the typical value obtained during the tests described in this paper. Modifications producing changes of greater than 2 percent are regarded

as significant and are discussed below.

Offsetting specimen 2 mm - Series D shows that the effect of the offset on the different measures

of toughness (see Tables 2 and 5) was measurable. In Table 4, the effect of the offset on

appearance of the broken halves is consistent. None of the inboard halves (shorter from anvil to notch) showed second-strike marks. All of the outboard halves had second-strike lines. The 2

mm offset shows the most conspicuous relationship between the type of modification and the

appearance of the broken specimens.

275
Relative to the normal condition of the machine is Series C, the effect of the offset is to increase the absorbed energy by 0.9 J or about 6 percent, an amount exceeded by only one other

modification in the series of eleven. The rate of 0.44 J/mm is consistent with previous

experience. Again, the rate of 4.0 J/mm quoted earlier in the discussion is not due to the offset,

but is rather the result of uncontrolled variables.

Tilting the specimen supporting surface - Series H consists of tests made after the new supports were machined to position the specimen at the standard height but with the horizontal surface

sloped upward toward the anvils. The effect on absorbed energy was an increase of 0.7 J or 5

when compared to the machine in its last standard condition. The composite postfracture energy

(Table 5) increases slightly from 5.3 to 5.6 mm, indicating that the change is relatively

insignificant. Comparing the distance between second-strike marks shows values of 18.1 and

17.5 mm, respectively, for the reference Series C and Series H. Multiplying the difference by

-0.92 J/mm (the previously determined slope) predicts a +0.6 J change in absorbed energy due to the reduced stiffness of the modified system. Therefore, the change in absorbed energy can be

predicted from the appearance of second-strike marks in this case.

Remachining the anvils to slant the contact faces - Table 5 shows that Series I averages 0.7 J or 4

percent more than Series C. The misalignment of 10 parts in 1000 is about 0.6 deg, less than that

tested in Series H, where the support was angled 1 deg. The distance between second-strike

marks is 17.8 mm. The 0.3 mm decrease in distance between second-strike marks would account

for a 0.3 J increase in absorbed energy, again consistent with the analysis in the previous section.

Reducing the horizontal thickness of the anvils - Table 2 shows that when the anvils are moved

in the direction of swing by 5 mm (Series J), the absorbed energy is increased by 6 percent

relative to the machine in standard condition (Series C). This is larger than the corresponding

value for any other modification made during these tests. The physical effect of this on fracture

work is that it causes a change in the angle between the specimen and the striking edge, similar to Series H and I. In this case, the angle is about 5 parts in 1000 and the distance between secondstrike marks in 17.5 mm (0.6 mm decrease from that found in series C). The predicted change in

absorbed energy is inconsistent with the predictions in the previous two sections. However, the

change in absorbed energy is consistent with the change in composite postfracture energy

reported in Table 5. CONCLUSIONS

Among the effects of the thirteen variations tested, the largest was a difference between

the absorbed energy as measured on a machine with a Type C pendulum and two machines with

Type U pendulums, the latter indicating values 9 percent higher. The intentional modifications

to MIC indicate that further tightening of machine tolerance would have marginal utility for

normal testing. The alternative energy-related measurements were helpful as additional

information to substantiate our conclusions. These conclusions may be applicable only to tests on high-strength steels with fracture energies below 20 J.

276
For similar investigations in the future, we recommend that a set size of at least ten

specimens be used, and that the energy indicator on the machine should be readable to 1 percent

variations in the absorbed energy.

ACKNOWLEDGMENTS

The authors are grateful to N.V. Gjaja for permission to use the measuring instruments at the Schenectady Materials and Processes Laboratory and to T.P. Quinn and R.B. Madigan

(NIST) for assisting with the high-speed photography.

REFERENCES

L Kahn, N.A., and Imbembo, E. A., "Reproducibility of the Single-Blow Charpy NotchedBar Test," ASTM Bulletin . May, 1947, pp. 66-74.

2. Fry, N.L., "An Inquiry into the Reproducibility of Impact Test Results," ASTM Bulletin. January, 1953, pp. 61-66.

3. Driscoll, D.E., "Reproducibility of Charpy Impact Test," Symposium on Impact Testing. STP 176, American Society for Testing Materials, Philadelphia, 1955.

4. Rickett, R. L., et al., "A Comparison of Impact Testing Machines in the 20 to 30 Ft-Lb

Range," Proceedings ASTM. Vol. 53, pp. 739-754, 1953.

5. Fahey, N.H., "Effect of Variables in Charpy Impact Testing," Materials Research &

Standards. November, 1961, pp. 872-876.

6. Omer, G.M., "Charpy Brittle Fracture Transition by the Lateral Expansion-Energy

Relationship." Welding Journal. May, 1958, pp. 201s-205s.

7. Schmieder, A. K., "Comparison of Metrological Techniques for Charpy Impact Machine

Verification," Charpy Impact Test Factors and Variables . ASTM STP 1072, J. M. Holt,

Ed., American Society for Testing and Materials, Philadelphia, 1990, pp. 20-34.

8. Porro, F., Trippodo, R., Bertozzi, R., Garagnani, G., "Impact Tester Compliance:

Significance, Sensitivity and Evaluation," Charpy Impact Test: Factors and Variables . ASTM STP 1072, J. M. Holt, Ed., American Society for Testing and Materials,

Philadelphia, 1990, pp. 7-19.

9. M.P. Manahan, R.B. Stonesifer, Y. Soong, and J.M. Burger, "Miniamrized Notch Test

Specimen and Test Machine Design," Pendulum Impact Machines. ASTM STP 1248,

T.A. Siewert and A.K. Schmieder, Eds., American Society for Testing and Materials,

Philadelphia, 1995, pp. 39-69.

10. Bluhn, J.I., "The Influence of Pendulum Flexibilities on Impact Energy Measurements,"

Symposium on Impact Testing. ASTM STP 176, American Society for Testing and

Materials, Philadelphia, 1955.

277

Earl A. Ruth,' Daniel P. Vigliotti,^ Thomas A. Siewert^

Effect of Surface Finish of Charpy Anvils and Striking Bits on Absorbed

Energy

Reference: Ruth, E. A., Vigliotti, D. P., and Siewert, T. A., "Effect of Surface Finish of

Charpy Anvils and Striking Bits on Absorbed Energy", Pendulum Impact Machines:

Procedures and Specimens for Verification. ASTM STP 1248 . Thomas A. Siewert and A. Kari

Schmieder, Eds., American Society for Testing and Materials, Philadelphia, 1995.

Abstract: Some new Charpy impact testing machines often report higher energy values than

machines which have been in service for a period of time. An investigation into the cause of this

phenomenon revealed that a major contributmg factor was the surface finish of the Charpy anvils

and the striking bit. Depending on the trajectory of the Charpy specimen halves after impact and

the configuration of the testing machine, a considerable amount of friction may occur between

the specimen and the anvils as the specimen is broken. Friction may also occur between the

specimen and the striking bit as the specimen exits the machine. Polishing the anvils and the

striking bit minimizes the friction between these parts. As a Charpy impact testing machine tests many samples, the striking bit and anvils become burnished and energy absorption due to

friction is reduced. If new parts are highly polished prior to installation, the burnishing, normally

caused by testing many samples, is simulated. In this way energy absorption associated with

friction during the wear-in period is minimized and remains more constant during the life of the

anvils and striking bit. ASTM Standard Test Methods for Notched Bar Impact Testing of

Metalhc Materials (E 23) requires the surface finish of the anvil and striking bit to be better than

4 |im (125 |iin.). Specifying a 0.1 [im (4 |iin.) surface finish as a requirement in E 23 will not

only eliminate the wear-in period, but will minimize shifts in test results when anvils and/or

striking bits are replaced in Charpy impact testing machines.

Keywords: anvils, Charpy impact test, energy loss, friction, impact test, striker, surface finish

' Tinius Olsen Testing Machine Co., Inc., Willow Grove, Pennsylvania 19090

^ Materials Reliability Division, National Institute of Standards and Technology, Boulder, Colorado 80303

279
Background

In the fall of 1991, several new Charpy impact testing machines were verified using

National Institute of Standards and Technology (NIST) Low-Energy Samples for Charpy VNotch Machines (SRM 2092). The test results indicated absorbed energies which exceeded the

average by more than 1.4 J. An extensive investigation into the cause of the discrepancy was

initiated.

All dimensional aspects of the machines were evaluated and were found to be well within

the ranges permitted by ASTM E 23. Measurements were made by three different investigators,

all of whom developed similar results.

The structural integrities of the machines' frames and foundations were investigated.

Experiments were conducted that stiffened the frame. Other experiments that stiffened the

pendulum were conducted. No improvements due to these procedures were noted.

One observation made at the time was that the NIST reference specimens (SRM 2092) were

tending to exit the testing machine in the direction of swing of the pendulum. In the past, it was

typical for these specimens to exit the testing machine in a direction opposite to the direction of

swing of the pendulum. Further, the striking bits and anvils on the NIST reference Charpy

impact testing machines used to develop the reference value for the SRM's, were burnished to a high polish as a result of having been used to test many specimens. We hypothesized that

perhaps the relatively rough surfaces on the new striking bits and anvils were causing additional

friction with the specimens as they were being broken. The friction would add to the energy

absorbed during a test.

,'>;.- In order to test this hypothesis, new anvils and striking bits were polished to a mirror-like

surface and installed on a machine which had previously been unable to pass the NIST

requirements on low-energy specimens. A dramatic decrease in energy absorbed was observed at that time. The average energy obtained on the NIST specimens tested on that machine dropped

approximately 0.7 J. The tests conducted at that time were not well structured or documented,

due to the expediency and shipping schedule demands for the new impact testing machines. As a

result, the data from those tests are not included in this paper. A more structured experiment was

designed and the results are offered here.

Experimental Method

Charpy V-notch tests were performed in accordance with ASTM Standard Test Methods for

Notched Bar Impact Testing of Metallic Materials (E 23). The test temperature in all cases was

-40°C. The tests were performed on a 400 J capacity, pendulum Charpy impact testing machine

having a "U" type pendulum. The standard size Type A specimen was used. Some of the tests were performed with a 2 mm radius striking bit instead of the standard 8 mm radius striking bit. This machine had been evaluated for repeatability when the strikers were interchanged. The

repeatability was found to be better than the uncertainty in the test data.[l] These tests conformed to E 23 in all other ways.

280
Three types of specimens were tested. The three types of specimens were:

LoiLL-l: Nominal 16 J energy specimens which exited the testing machine opposite

the direction of swing of the pendulum.

LotLL-44 & 33: Nominal 16 J energy specimens which exited the testing machine in the

direction of swing of the pendulum (note: the material used for the tests employing the 2 mm striker were not from the same lot of material as the

tests employing the 8 mm striker).

LotHH-1: Nominal 100 J energy specimens which exited the machine in the

direction of swing of the pendulum.

Five different test conditions were used. The five conditions were:

Condition 1 : Rough anvils and a rough 8 mm radius striker.

Condition 2: Smooth anvils and a rough 8 mm radius striker.

Condition 3: Smooth anvils and a smooth 8 mm radius striker.

Condition 4: Smooth anvils and a rough 2 mm radius striker.

Condition 5: Smooth anvils and a smooth 2 mm radius striker.

Under Condition 1, five LL-1 specimens, five LL-44 specimens, and five HH-1 specimens were broken. The anvils had an rms surface finish of approximately 0.25 [im (10 |iin.). The 8

mm striking bit had an rms surface finish of approximately 0.25 [im (10 |iin.) on the nose and

the 30' sides. On the parallel sides the rms surface finish was approximately 0.625 ^im (25 [lin.)

(Fig. 1).

D-25 0.25

0 25/

0.25 V \/ / D . 25

ssxTV; 0 . 25

0 - 525

After the first tests, the anvils were anv i ls

removed and polished to a mirror

finish. The rms surface finish measured

0.05 ^m (2 p,in.) after polishing. The

anvils were reinstalled in the impact

testing machine. Five LL-1 specimens,

five LL-44 specimens, and five HH-1

specimens were then broken. This was

Condition 2.

The striking bit was then removed

and polished to a mirror finish. The

nose, the 30° sides and the parallel sides were all polished to the same rms

finish, approximately 0.05 jim (2 p,in.). Figure 1. Surface finish (|im) of unpolished anvils and striking bit.

625

STR I < I NG BIT

281
Table 1. Results of tests on specimens (lot LL-1) with a nominal energy of 16 J which

exit the testing machine opposite the direction of swing of the pendulum. Test results are

in joules.

Condition 1 2 3 4 5

16.3 16.6 16.9 15.9 16.9

16.9 16.6 16.6 16.6 16.9

16.9 16.9 16.3 16.9 16.6

16.5 15.9 15.6 18.0 16.3

16.0 16.3 16.6 16.6 15.9

Average 16.52 16.46 16.40 16.80 16.52

Standard

Deviation

0.35 0.34 0.44 0.68 0.38

Table lA. Results of tests on specimens (lot LL-1) with a nominal energy of 16 J which

exit the testing machine opposite the direction of swing of the pendulum. Test results are

in joules. Outlier removed.

Condition 1 2 3 4 5

16.3 16.6 16.9 15.9 16.9

16.9 16.6 16.6 16.6 16.9

16.9 16.9 16.3 16.9 16.6

16.5 15.9 15.6 16.3

16.0 16.3 16.6 16.6 15.9

Average 16.52 16.46 16.40 16.50 16.52

Standard

Deviation

0.35 0.34 0.44 0.37 0.38

Five LL-1 specimens, five LL-44 specimens, and five HH-1 specimens were again broken. This

was Condition 3.

The striking bit was again removed and a striking bit with a 2 mm radius nose was installed.

The 2 mm striking bit had an rms surface finish of approximately 0.25 |im (10 |iin.) on the nose and the 30° sides. On the parallel sides the rms surface finish was approximately 0.625 [im (25

[I'm.}. Five LL-1 specimens, five LL-33 specimens, and five HH-1 specimens were broken. This

was Condition 4.

The 2 mm striking bit was then removed and the nose, the 30° sides and the parallel sides were all polished to an rms finish of approximately 0.05 \im (2 jiin.). Five LL-1 specimens, five

LL-33 specimens, and five HH-1 specimens were again broken. This was Condition 5.

282
After a quick examination of

the data, we decided to test five

more LL-44 specimens which had

been resented. We removed the 2

mm striker and reinstalled the

polished 8 mm striker. The five

specimens were then tested under

Condition 3.

Results

Twenty-five specimens were

tested from lot LL-1 (Table 1).

These specimens tended to exit the

testing machine in a direction

opposite the direction of swing of

the pendulum. As seen in Table 1, there was very little difference in

the average of the five different test conditions.

The condition with the highest

variation was Condition 4 (smooth

anvils and a rough 2 mm radius

striking bit). An examination of the

data for that group of tests indicated

that the fourth specimen tested

under Condition 4 was almost two

standard deviations higher than the

average. If this specimen is eUminated as an outlier, Table lA is the result.

An examination of the averages

and standard deviations listed in

Table lA yields very close

agreement between the various

conditions. The differences in the

averages are much less than the

standard deviations.

Table 2. Results of tests with an 8 mm striker on specimens (lot LL44) with a nominal energy of 16 J which exit the testing machine in the

direction of swing of the pendulum. Test results are in joules.

Condition 1 2 3 3

17.5 17.6 17.3 16.3

18.0 18.8 17.6 17.6

17.3 17.4 16.9 17.3

16.9 17.6 16.6 17.3

18.0 17.4 17.3 17.6

Average 17.54 17.76 17.14 17.22

Standard

Deviation

0.42 0.53 0.35 0.48

Average

of 10

17.65 17.18

Standard Deviaron

of 10

0.49 0.42

Table 2A. Results of tests with an 8 mm striker on specimens (lot LL44) with a nominal energy of 16 J which exit the testing machine in the

direction of swing of the pendulum. Test results are in joules. Outliers removed.

Condition 1 2 3 3

17.5 17.6 17.3

18.0 17.6 17.6

17.3 17.4 16.9 17.3

16.9 17.6 16.6 17.3

18.0 17.4 17.3 17.6

Average 17.54 17.50 17.14 17.45

Standard Deviation0.42 0.12 0.35 0.17

Average

of 9

17.52 17.28

Standard

Deviation

of 9

0.32 0.34

Twenty specimens were tested from lot LL-44 with an 8 mm striking bit (Table 2). These

specimens tended to exit the testing machine in the direction of swing of the pendulum.

Originally, five specimens were tested for each of the first three Conditions. After we noticed a

283
substantial drop in the absorbed energy between Conditions 2

and 3, five additional specimens were tested under Condition 3 so a greater population could be analyzed. There is a difference

of approximately a 0.4 J (0.3 ft-lb) between the tests that were

run before and after polishing the striking bit. This difference

amounts to about 1 standard deviation.

There is a difference of approximately a 0.2 J (0.15 ft-lb)

between the tests done before and after polishing the anvils.

The tests done with the polished anvils show a higher average

absorbed energy than the average for tests done with

unpolished anvils.

Table 3. Results of tests with a 2

mm striker on specimens (lot LL-33)

with a nominal energy of 16 J which

exit the testing machine in the direction of swing of the pendulum.

Test results are in joules.

Condition 4 5

17.3** 16.6*

16.9* 17.4"

. 16.9" 16.9"

16.9" 16.9

17.3' 16.6"

Average 17.06 16.88

Standard

Deviation 0.20 0.29 Two specimens in Table 2 are almost two standard

deviations from the average for their condition. Table 2A is a

presentation of the results with these two specimens removed.

Using this data, there is virtually no change between the results for polished and those for

unpolished anvils. The difference between the polished and unpolished striking bit, although not

as notable, is still evident.

Ten specimens were tested from lot LL-33 with a 2 mm striking bit (Table 3). We expected

these specimens to exit the testing machine in the direction of swing of the pendulum. Many of

the specimens exited in the opposite direction or did not exit the area of the anvils at all. Testing

performed with an 8 mm striking bit led us to think that these specimens would exit the testing

machine in the direction of swing of the pendulum. Perhaps it was a higher stress concentration

due to the smaller radius striker used in these tests that caused the specimens in most cases to

exit the machine in the direction opposite that of the pendulum or to not exit at all. Even though

the specimens did not exit the machine in the direction of swing of the pendulum, a reduction in

absorbed energy was observed when the striking bit was polished.

Twenty-five specimens were tested from lot HH-1 (Table 4). These specimens tended to exit

the testing machine in the direction of swing of the pendulum. While the differences in the

average absorbed energies are substantial by comparison with the energies for the low-energy

specimens, with the exception of Condition 1 the difference between the test results is insignificant. The tests performed under Condition 1 with the unpolished anvils may have

significantly higher energies than the other four cases with polished anvils.

Conclusion

For low-energy specimens which exit the testing machine in the direction opposite to the

direction of swing of the pendulum, there seems to be very little difference between polished and

unpolished anvils and strikers. There is only a slight downward trend in the average absorbed

energies as the tooling is polished.

284
I T A

FR I CT I ON

Figure 2. Proposed trajectories of low-energy specimens which cause lost energy due to

friction with the striking bit.

A

Figure 3. Proposed trajectories of high-energy specimens which cause lost energy due to

friction with the anvils.

For low-energy specimens which exit the testing machine in the direction of swing of the

pendulum, the average absorbed energy dropped as much as 0.4 J (0.3 ft-lb) when the striking bit was polished. When a low-energy specimen is broken, the broken halves spin at high velocity

and strike the anvils a second time (Fig. 2). Specimens which exit the machine in the direction of

swing of the pendulum rebound off the anvils and may continue to rotate in such a way as to scrape between the anvil support and the sides of the passing striking bit. We think that when we

polish the sinking bit, the friction is reduced between the striking bit and the specimen after the

specimen breaks.

Table 4. Results of tests on specimens (lot HH-1) with a nominal energy of 100 J which

exit the testing machine in the direction of swing of the pendulum. Test results are in joules.

Condition 1 2 3 4 5

86.8 89.2 96.6 95.6 92.5

91.9 92.9 93.6 90.5 88.1

92.2 85.8 87.1 85.1 91.9

94.2 89.8 89.2 89.5 90.2

92.2 89.8 86.1 94.2 91.2

Average 91.46 89.50 90.52 90.98 90.78

Standard

Deviation

2.47 2.26 3.99 3.71 1.54

285
While the differences reported here may not seem large, 0.4 J can be important when

compared to the ±1.4 J range permitted by ASTM E-23 when verification specimens are tested.

A machine that may pass with an old striker that has been burnished by testing many specimens

may fail when a new striker that is not highly polished is installed.

For high-energy specimens, we found that the absorbed energy dropped as much as 2 J (1.5

ft-lb) when the anvils were polished. Again a 1 or 2 J reduction in energy when attempting to qualify a machine can determine whether a machine passes or fails verification tests.

High-energy specimens are dragged across the anvils as they are broken (Fig. 3). Higher

friction between the anvils and the specimen causes the testing machine to indicate higher

absorbed energy values.

ASTM Standard Test Methods for Notched Bar Impact Testing of Metallic Materials (E 23)

requires a 4 [im (125 |iin.) rms finish on the anvils and the striker. Through use, these parts are burnished to a high polish. In order to reduce the shift in results that may result from a change in

the striker or anvils, ASTM E 23 should require that these parts be highly polished. We suggest

that ASTM E 23 be changed to require that the anvil and striking bit have rms finishes of 0. 1 |im

(4 |iin.) or better.

References

. [1] Siewert, T. A. and Vigliotti, D. P., "The Effect of Charpy V-notch Striker

Radii on the Absorbed Energy," Pendulum Impact Machines: Procedures and

Specimens for Verification. ASTM STP 1248 .

286
C.N. McCowan and D.P. Vigliotti

The Influence of Shear Lip Symmetry on the Absorbed Energy of

Charpy Impact Specimens

Introduction

It has been reported that the symmetry of the shear lip formation on Charpy impact specimens can have a significant influence on the

absorbed energy.^ For high energy level verification specimens,

fracture surfaces having non-symmetric shear lips (Figure 1) have

typically had higher absorbed energies than those having symmetric

shear lips.

Here, data for six high energy pilot lots and two super high energy

pilots lots are reported. The results are plotted in Figures 2 to 25,

and summary data for the pilot lots are given in Table 1

Result and Discussion

The data are grouped by pilot lot and test machine. This is because

each pilot lot has a different absorbed energy, and there are differences between the three machines used for the testing of a given

pilot lot.

The data consistently show a trend of higher energy for specimens

that fractured asymmetrically. In Figure 26, the difference in the

energy between the asymmetric and symmetric specimens ranges

from near 0 to 5 J, for the high-energy specimens, and from about 5

to 12 J for the super-high energy specimens. Assuming the typical

energy levels for the high and super-high energy specimens are about

100 and 200 J respectively, the maximum increase in energy is near

5 % for both types of specimens. This is significant, because the

requirements for the impact verification test only allows ±5 % variation from the certified value

of the specimen.

The effect of specimen squareness on the test results was also evaluated. No correlation between

squareness and energy (or fracture symmetry) was found. However, we are not convinced that

squareness is not an important variable, so more testing is planned.

Currently we have no quantitative method to evaluate the influence of fracture symmetry on the

results of verification tests. However, we do consider the mix of fracture types in our customer

results (and the mix from the original pilot lot data) when we evaluate the data for a given

machine. If, for example, a customer result was slightly high and they failed the verification test for no apparent reason, and all five of the customers specimens had asymmetric fractures, we

might have them retest.

Symmetric

Figure 1: Diagram showing:

(a) symmetric shear lips, which

have both shear lips (up) on the same fracture surface, and (b

)

asymmetric shear lips, which

have one shear lip up on each

fracture surface.

287
115

a s

Asymetrical versus Symetrical Fracture

15

94 -

87 -

sol 1 1 a s Asymetrical versus Symetrical Fracture

115

87 -

60 1 La s

Asymetrical versus Symetrical Fracture

Figure 2: HH84 SI2 Figure 3: HH84 TK Figure 4: HH84 T02

a s Asymetrical versus Symetrical Fracture Asymetrical versus Symetrical Fracture a s

Asymetrical versus Symetrical Fracture

Figure 5: HH86 SI2 Figure 6: HH86 TK Figure?: HH86T02

115

a s

Asymetrica! versus Symetrical Fracture

Figure 8: HH87 SI2

Asymetrical versus Symetrical Fracture

Figure 9: HH87 TK

115

108

t 101 c

lUD

1 94

87 - m —

a s Asymetrical versus Symetrical Fracture

Figure 10: HH87 T02

288
115

Asymetrical versus Symetricai Fractire

Figure 11: HH92 SI2

115

Asymetricai versus Symetricai Fracture Figure 12: HH92 TK

Asymetricai versus Symetricai Fractu-e Figure 13: HH92 T02

115

a s AsymetricaJ versus Symetricai Fracture

Figure 14: HH93 SI2

a s

Asymetricai versus Symetricai Fractis-e Figure 15: HH93 TKAsymetricai versus Symetricai FractLre Figure 16: HH93 T02

a s Asymetricai versus Symetricai Fractire Asymetricai versus Symetricai Fracture

Figure 17: HH94 SI2 Figure 18: HH94 TK

Asymetricai versus Symetricai Fractire Figure 19: HH94 T02

289
190

160

Asymetrical versus Symetrlcal Fracture Asymetrical versus Symetrical Fracture a s Asymetrical versus Symetrlcal Fracture

Figure 20: SH16SI2 Figure 21: SH16TK Figure 22: SH16T02

250

210

Asymetrical versus Symetrical Fracture Asymetrical versus Symetrlcal Fracture Asymetrical versus Symetrlcal Fracture Figure 23: SH24 SI2 Figure 24: SH24 TK Figure 25: SH24 T02

290
I <V ' >w

cnc

UJc

U

c

I

100 200

Mean Energy of Lot, J

300

Figure 26: The difference in the energy of the asymmetric fractures and the

symmetric fracture for high and super-high energy levels.

291
Table 1. Summary statistics for pilot lots.

Pilot Lot

SERIES

MACHINE Asymmetrical Symmetrical

Enerqv, J Energy, J

Difference

J

HH93 T02 89.4 87.1 2.4

HH93 TK 90.4 89.2 1.3

HH93 SI2 90.7 88.0 2.7

HH86 SI2 92.1 89.5 2.6

HH86 T02 92.5 91.0 1.5

HH86 TK 94.9 92.4 2.5

HH92 SI2 99.1 97.7 1.5

HH92 TK 99.3 97.6 1.6

HH92 T02 100.6 95.7 4.9

HH94 T02 100.8 100.4 0.5

HH94 SI2 102.0 99.8 2.3

HH87 SI2 102.8 98.5 4.3

HH87 T02 102.9 100.5 2.4

HH94 TK 103.5 99.1 4.4

HH90 SI2 103.5 100.9 2.6

HH89 T02 103.9 100.8 3.1

HH87 TK 104.9 102.0 2.9

HH88 T02 105.5 102.3 3.3

HH88 SI2 106.8 103.2 3.6

HH84 TK 106.8 103.1 3.7

HH84 T02 108.3 104.4 4.0

HH84 SI2 109.4 106.7 2.7

SH16 T02 184.1 176.2 7.9

SH16 TK 184.3 173.8 10.5

SH16 SI2 187.1 175.2 11.9

SH24 T02 228.5 223.8 4.7

SH24 TK 233.6 223.8 9.8

SH24 SI2 238.4 232.2 6.2

292
Reference

1. C.N. McCowan, J. Pauwels, G. Revice, and H. Nakano, International Comparision of

Impact Verification Programs, in Pendulum Impact Testing: A Century of Progress. ASTM STP1380, p. 73, 1999.

293

Part 4: Statistical Evaluations of Charpy Impact Data

295

C.N. McCowan,^ CM. Wang," and D.P. Vigliom'

Charpy Impact Verification Data (1994-1996): A Summary

Reference: McCowan, C. N., Wang. C. M., and Vigliotti. D. P.. "Charpy Impact Verification

Data (1994-1996): A Summary," Journal of Testing and Evaluation, JTEVA. Vol. 27, No. 2,

March 1999, pp. 89-99.

Abstract: We present a summar}' of Charpy impact verification test data that were evaluated b\-

the National Institute of Standards and Technology from Januar>' 1994 to December 1996. The

Charpy impact machines that met the verification requirements of ASTM Test Methods for

Notched Bar Impact Testing of Metallic Materials (E 23) are broken down by year and by

reference lot. Based on the data, a proposed verification rule that hmits the range of the

verification set has been examined. We also present the results for determining whether two

energies (lower and upper ends of the machine capacit}-) or three (lower, middle, and upper ) are needed to verif,' the performance of the large-capacity impact machines; currendy E 23 requires

three energies to be tested.

Kewords: Charpy V-notch. impact certification program, impact testing, notched-bar testing,

pendulum impact machmes, reference specimens

This report provides a summary of the Charpy impact verification data that were evaluated

by the National Instimte of Standards and Technologv' (NTST) from Januar\' 1994 through

December 1996. An indirect verification program has been used to verif,- the performance of

Charpy impact machines for more than 40 years [1.2], and in 1964 ASTM Standard E 23 [3] was

revised to require verification tesdng. NTST has provided the verification specimens and

administered the program since 1989. In this program, impact machines are verified annually to the requirements in E 23, and the verification data, which are generated by organizations that

own test machines, are returned to NTST for evaluation.

The impact verification program can be divided into three basic parts: production and

distribution of impact verification specimens; verification testing; and evaluation of the

verification test data. Before verification testing, a reference value for the impact toughness of

the verification specimens is determined, and the uncertainty' associated with the reference value

is confirmed to be below a limit that ensures the material homogeneity of the specimens. In

practice, the reference value for the impact specimens is determined by testing a random sample

of 75 specimens from a production lot, which normally contains 1200 heat-treated specimens.

The 75 specimens are divided into groups of 25 and tested on three machines that have been

defined in E 23 as the reference impact machines for the United States. The impact toughness,

defined as the energy absorbed in the test, is the average absorbed energy for the75 tests. If the

INational Institute of Standards and Technology- Materials Reliabilir\- Division. Boulder. CO 80305

National Institute of Standards and Technology, Statistical Engineering Division, Boulder, CO 80305

297
lot meets the statistical requirements of ASTM Practice for Qualifying Charpy Verification

Specimens of Heat-Treated Steel (E 1271), a reference value is assigned to the lot [4]. Once a lot

is accepted, sets of five specimens, or verification sets, are sold to companies that want to verify

the performance of their Charpy impact machines. The specimens are broken using the candidate

machine, and the broken specimens, along with the absorbed energy results, are sent to NIST for

analysis. If the results are within 5 or 1 .4 J of the reference value and the markings left by the

machine on the specimens indicate the machine is in good working condition, the candidate

machine is certified by NIST to meet the requirements of E 23.

A database containing the results of verification tests has been collected that includes the

serial number of the candidate machine, the capacity and the pendulum design of the machine,

the energy obtained for each specimen tested, the reference energy for the specimens tested, and

the date of the test. The principal use of these data is to track the performance of individual

impact machines and to monitor the verification program. The data also provide an opportunity

for cross-validation of the current acceptance criteria and evaluation of new criteria proposed for

the verification of Charpy impact machines.

Data from the verification tests and pilot lot evaluations are presented here to provide a general overview of how the Charpy verification program works in practice. To do this, tables

and graphics are employed to show the proportions of test results that meet the current

verification requirements and to consider the influence of the verification specimen on the test results. The data are also used to examine a newly proposed range rule for verification tests. Finally, the data are used to evaluate the requirement in E 23 to test at three energies (lower,

middle, and upper ends of the machine capacity) when verifying large impact machines over

their full capacity. The three energies available for verification testing are referred to as low,

high, and super-high energy specimens in this paper. There is an interest in determining whether

only two energies (low and super-high) are sufficient to verify the performance of the largecapacity impact machines. i

Materials and Procedures

Charpy impact verification specimens are sold by NIST at three energies: the low-energy

specimens, with energies near 17 J, the high energy specimens, with energies near 100 J, and the

super-high energy specimens with energies near 225 J. The low- and high-energy specimens are made from 4340 steel, which is heat treated to produce specimens at the appropriate absorbed

energies. The super-high-energy specimens are made using a T-200 maraging steel. The data

used here include the customer data from many different lots of low-, high-, and super-highenergy specimens. The total number of verification tests for the low-, high-, and super-highenergy levels are 2401, 2385, and 655, respectively.

If the five absorbed energy measurements from the Charpy machine being tested are denoted by ej, e^, e^,, e^, and E^is the average; of e,, i = 1, 2, • •, 5, then the low-energy

certification criterion is given by:

-1.4 <E,-E,< 1.4

298
where is the reference energy of the lot. To certify a candidate Charpy machine in the high- or super-high-energy ranges, the difference between and E^ must be within 5 % of the reference

value; that is, it requires:

-0.05 < (E, - E,)/E, < 0.05

Results

Low Energy

Figure 1 plots the difference between the customer's average and the reference value, E^ - E^,

for the low-energy verification tests. The dotted lines indicate the 1.4 J pass or fail criteria. In the

right margin, the density estimate for E^ - E^ is also plotted. The distribution of E^ - E^ is centered around 0 and has a longer "tail" on the positive side of E^ - E^. Out of 2401 verification

tests shown in Fig. 1, 283 tests (11.8 %) failed the indirect verification. Figure 1 also shows that

when a machine fails to meet the low-energy verification requirements, it fails more often with

1.4 (9.2 %) than with 1.4 (2.6 %). This is expected because most of the

common factors, such as anvil radius, bearings, and mounting, that wear or loosen over time

increase the energy absorbed by the machine during the test.

S If) -

1994 O 1995 A 1996

Figure 1. The difference, in J, between the customer's

average for low energy verification tests and the reference

value. The horizontal axis is the sequence of the test. The order within each year is not relevant. The dotted

lines indicate the 1 .4 J pass/fail criteria. The density

estimate for the differences is plotted in the right margin.

299
Table 1. Pass/fail data for low energy verification tests (by

year).

Year Status < E, E, > E, Total

1994 Pass 314 (39.3%) 388 (48.5%) 702 (87.87o)

Fall 20 (2.5%) 78 (9.8%) 98 (\2.2%)

i995 Pass 350 (43.6%) 359 (44.77c) 709 (88.37c)

Fail 31 (3.9%) 63 (7.87c) 94(11.7%)

1996 Pass 294 (36.8%) 413 (51.8%) 707 (88.67o)

Fail 11 (1.47c) 80 (10.0%) 91 (1 1.47c) Total Pass 958 (39.9%) 1160 (48.37c) 2118 (88.27c)

Fail 62 (2.6%) 221 (9.27c) 283 (1 1.87c)

In Table 1, the pass/fail data are broken down into test year. Tlie number inside the

parentheses is the percentage of the respective yearly total. The percentages of the row "Total"

are with respect to the total number of the tests. The data show that the failure rate for the lowenergy verification tests remains fairly constant (12.2 %, 1 1.7 %, and 1 1.4 %) from year to year. '

In Table 2, the pass/fail data are broken down by the lot designation of verification specimens

tested. The first column is the lot identification; the second column is the reference value of

absorbed energy (J) of the lot; the third column is the standard deviation (J) of the pilot lot (calculated as the square root of the weighted mean of the variances of the three machines with

weights equal to the number of observations, i.e., the pooled estimate); the fourth column is the

lot size, or the number of machines tested using the verification specimens from that lot; the next

two colunms are, respectively, the number of machines that passed the verification test on the

low and high sides; and the last two columns are the number of machines that failed the test and

the failure rate. Only lots for which at least 100 verification tests were made are listed here. We

present the specimen lot data in this way to evaluate the influence of the specimens on the

outcome of the verification tests. The standard deviation of the pilot lot is one of the primary

subtle trend for increasing failure over the small range in standard deviation present in the data.

Unlike the yearly data, where the same population of machines is compared (since machines are

tested annually), the failure rates of lots are expected to vary because the number of machines

Table 2. Pass/fail data for low energy verification tests (by lot).

No. of Passes Reference Standard No. of Failure Lot Energy Deviation Lot Size Low High Failures Rate

44 17.2 0.62 132 46 70 16 I2.l';f 45 17.4 0.52 167 62 90 15 9.0% 46 17.4 0.58 165 90 69 6 3.6%

47 17.2 0.50 174 90 61 23 1.3.2% 48 17.6 0.72 162 79 67 16 9.9% 49 17.6 0.65 172 59 90 23 13.4%

51 16.9 0.71 148 61 64 23 15.5% 52 17.2 0.71 129 63 54 12 9.3%

53 16,7 0.57 130 49 70 II 8.5%

54 16.8 0.65 114 16 72 26 22.8%

55 17.5 0,84 117 48 54 15 12.8% 56 18.2 0.7.1 108 33 65 10 9.3%

300
tested for each lot is some different fraction of the machine population tested each year. So the

acceptance criteria for the lots, and lower variation should result in less influence by the

specimens on the outcome of the test. The fact that there is not a trend of increasing failures with

increasing standard deviation in Table 2 is not surprising, however, because much of the

variation in the failure rates of the lots is probably due to sampling, which would obscure any

standard deviation cannot be related to the failure rate for these data, and the failure rate alone

should not be considered to have too much significance.

Probably the best indicator of specimen influence on the verification test we have is the

distribution of the pass (low or high) data shown in Table 2. These data, monitored periodically

as they are accumulated on each lot, show how many test results were higher or lower than the

reference value assigned to the verification specimens being tested. A very skewed result here

indicates that the reference energy assigned to the verification specimens could be very different

from the average of the test results for the good machines. We would not expect the two

averages to be identical, but if the good machines tested are a representative sample of the

population of good machines, we typically find good agreement between the two averages. So,

when the pass data are very skewed and the failure rate is high, as for Lot 54, we suspect the lot might be influencing the verification test results.

To more fully evaluate the data

for Lot 54, the distribution in energy

for the customer data is compared to the distribution of the pilot lot data

in Fig. 2. It shows that the

distribution of the customer data for

Lot 54 is approximately Gaussian,

with a slightly higher average energy

than the pilot lot data (the average

energy of the pass data for Lot 54 is 17.2 J). The peak around 18 J in the

customer data is not considered to be

part of the distribution of good

machines. Rather, it defines a population of machines that failed

the test and that differs from the

population of good machines (both

pass and fail data were included in

the plot of the customer data).

Although the apparent shift in

average energy for the customer data

may push some good machines

across the pass/fail limit, most of the

machines that failed appear to be

representative of a population

consisting of bad machines. Note

also the almost bimodal shape near

14 16 18 20

Figure 2. The density plots of the energy absorbed for the

pilot lot data (from 75 individual specimens) associated

with Lot 54. The three vertical dashed lines correspond to the reference energy (16.8 J) and the acceptance region

(16.8 ±1.4 J).

301
the peak of the distribution of the pilot lot data. This example appears somewhat extreme, but is not unexpected, because the pilot lot data are a compilation of data from three machines, each of

which has characteristic differences and biases compared with the two others. For this pilot lot,

two machines had very similar average energies (17.3 and 17.0 J), which differed from the

average of the third machine (16.0 J). The combined data of the three reference machines serve

as a good illustration of how the inclusion of different machines results in a balanced average

reference energy for the specimens, but the distribution is broadened due to the bias between

machines and does not provide a good measure of the inherent scatter of the specimens. To

remove the machine bias and better estimate the scatter of the specimens, the pooled standard

deviation of the three machines is used. For this pilot lot, the pooled standard deviation was 0.65

J, while the (not pooled) standard deviation for the (combined) data shown in Fig. 2 was 0.87 J.

High Energy

Figure 3 plots the relative difference between the customer's average and the reference value,

{E^ - E^IE^ for the high-energy verification tests. The dotted lines indicate the 5 pass or fail criteria. In the right margin, the density estimate for {E^ - E^IE^ is also plotted. The distribution

of {E^ - E^IE^ is centered around 0 and has a slightly longer "tail" on the positive side of {E^ -

oCM

c

(D 3=C

0)o

Q.

O

Figure 3. The relative difference between the customer's

average for high-energy verification tests and the reference

value. The horizontal axis is the sequence of the test. The

order within each year is not relevant. The dotted lines

indicate the 5% pass/fail criteria. The density estimate for

the relative differences is plotted in the right margin.

302
Table 3. Pass/fail data for high-energy verification tests (by year).

Year Status E, < £,. £,. > £,. Total

1994 Pass .363 (46.8%) 357 (46.1%) 720 (92.9%)

Fail 2! (2.7%) 34 (4.4%) 55 (7.1%)

1995 Pass 418 (53.9%) 313(40.3%) 731 (94.2%)

Fail 19 (2.4%) 26 (3.3%) 45 (5.8%) 1996 Pass 397 (47.6%) 379 (45.4%) 776 (93.0%)

Fail 17 (2.0%) 41 (4.9%) 58 (7.0%)

Total Pass 1178 (49.4%) 1049(44.0%) 2227 (93.4%)

Fail 57 (2.4%) 101 (4.2%) 1 JO \\J.\J /O

)

Table 4. Pass/fail data for high energy verification tests (by lot).

No. of Passes Reference Standard No. of Failure Lot Energy Deviation Lot Size Low High Failures Rate

44 98.2 2.48 106 51 54 1 0,9%

45 99.1 2.62 160 76 79 5 3.1% 46 100.7 2.94 167 78 70 19 1 1 .4% 47 108.6 3.42 169 125 19 25 14.8% 48 103.1 3.25 182 94 77 11 6.0%

49 102.6 3.16 183 65 106 12 6.6%

51 101.3.09 191 74 109 8 4.2%

52 102.2 2.96 123 49 70 4 3.3%

53 98.2 2.71 136 28 98 10 7.4%

54 99.8 2.70 136 70 62 4 2.9%

55 97.5 3.47 112 62 39 11 9.8%

56 106.7 3.58 121 65 41 15 12.4%

E^IE^. Out of 2385 verification tests shown in Fig. 3, 158 tests (6.6 %) fail to pass the indirect

verification requirements, with 4.2 % failing on the high side and 2.4 % on the low side.

In Table 3, the pass/fail data are broken down into test year. Like the low energy data, the

failure rate for the high-energy verification tests remains fairly constant (7.1 %, 5.8 %, and 7.0

%) from year to year. The failure rate for the high-energy verification tests is consistently lower

than that for the low-energy verification tests. It has long been recognized by E 23 that the veryhigh-strength, low-energy impact specimens show performance problems with machines that

high-energy specimens do not. The difference in pass/fail percentages highlights this point.

Table 4 displays the pass/fail data by lot of verification specimens. Again, with the small

number of machines tested under each lot, the failure rates vary from lot to lot. Considering the

first three lots (44, 45, and 46), for example, the standard deviations and distributions of the pass

data are similar, but the failure rates vary greatly. We assume that the explanation here is that a

higher percentage of bad machines were tested using the Lot 46 specimens. However, for Lot 47,

which has a very skewed pass data, we cannot necessarily attribute the high failure rate to sampling.

303
o

1S94 o 1695 A 1986

Figure 4. The relative difference between tlie customer's

average for super-high-energy verification tests and the

reference value. The horizontal axis is the sequence of

the test. The order within each year is not relevant. The

dotted lines indicate the 5% pass/fail criteria. The density

estimate for the relative difference is plotted in the right

margin.

Super-High Energy

Figure 4 plots the relative difference between the customer's average and the reference value,

(E^ - E^IE^, for the super-high energy verification tests. The dotted lines indicate the 5 % pass or

fail criteria. In the right margin, the density estimate for {E^ - E^IE^ is also plotted. The

distribution of {E^ - E^IE^ has a longer "tail" on the negafive side oi{E^ - E,)/E^. This implies

when a machine fails the super-high verification test, it tends to have a low E^ value. This differs

from that of the low- and high-energy verification data and is suspected to be a result of the

different specimen-anvil (and striker) interaction for the super-high-energy test. These very

ductile specimens are deeply brinelled by the anvils and wrap around the striker during the test. Overall, 10.1 % of the 652 cases evaluated failed to meet the 5 % verification criteria of E 23.

Table 5 contains the pass/fail data broken down into test year for the super-high verification

tests. The failure rate varies more for these data than it does for the high- or low-energy

verification data, but this is a new test. The number of machines tested from year to year is not as constant as in the low- or high-energy tests, and many machines are being tested for the first

304
Table 5. Pass/fail data for super-high-energy verification tests (by year).

Year Status F < F r > r

1994 Pass 47 (56.0%) 30 (35.7%) 77 (91.7%)

Fail 7 (8.3%) 0 (0.0%) 7 (8.3%)

1995 Pass 117 (47.8%) 97 (39.6%) 214 (87.37c)

Fail 29(1 1.8%) 2 (0.8%) 31 (12.7%)

1996 Pass- 182 (55.8%) 11 6 (35.6%) 298 (91.4%)

Fail 23 (7.1%) 5 (1.5%) 28 (8.6%)

Total Pass 346 (52.8%) 243 (37.1%) 589 (89.9%)

Fail 59 (9.0%) 7 (1.1%) 66 (10.1%)

Table 6. Pass/fail data for super-high energy verification tests (l)y lot).

Lot

Reference EnergN

Standard Deviation Lot Size

No

Lov,'

of Pass-es

High

No. of

Failures

Failure Rate

3 229.9 7.25 135 81 33 21 15.6% 4 226.3 6.78 159 73 74 12 7.6%

5 222.5 8.12 169 81 77 11 6i%

6 224.8 5.59 158 92 52 14 8,9%

time at these energies. For these reasons, we anticipate that the 10. 1 average failure rate for the

super-high energy tests will decrease slightly in the years to come.

Table 6 displays the pass/fail data by lot of verification specimens. Lot 3 has the highest

failure rate among the lots for which we now have data, but we suspect that many of the failures

associated with this first lot were due to not testing the specimens at room temperature (as

required). The rest of the lots ha\"e comparable failure rates.

Range Summanr

The range of the five measurements in a verification test is defined as

i? = max{ej - min{e,}

A new verification rule limiting the range of the absorbed energy measurements for the low- and

high-energ}' tests is being balloted for ASTM E 23. The range test is designed to detect excessive

variation, that is. to identif}' machines that have very high scatter in their measurements and just

happen to have mean energ}' values that agree with the reference energy. Splett and Wang [5]

also proposed an alternate certification procedure that accounts for the lot and machine

variations.

The ranges being considered for limiting the low- and high-energy tests are 5 and 15 J,

respecti\-el}'. So. for the low-energy test, a candidate machine would fail if its range were greater

305
than 5 J even it passed the ±1.4 J criteria, and for the high-energy test, a candidate machine

would fail if its range were greater than 15 J even it passed the ±5% criteria.

Based on the data from 1994 to 1996, these range limits appear reasonable Figs. 5 and 6

display the range of the five measurements for the low- and high-energy verification tests. The

solid markers designate the machines that fail the ± 1.4 J criteria in the low-energy test (Fig. 5)

and ±5% criteria in the high energy test (Fig. 6). There are 16 tests with /? > 5 in Fig. 5 and 17

tests with R> 15 in Fig. 6. If the range rule were in use, nine additional tests would fail in the

low energy test, increasing the failure rate from 1 1.8 % to 12.2 %. Similarly, eight additional

tests would fail in the high-energy test, increasing the failure rate from 6.6 to 7.0 %. In both

cases, the range rules would increase the failure rate by 0.4 %.

An alternate range rule would be to use the normalized range

K = R/E,

and fail the candidate machine if R„ > r, where r is some specified limit. The range rule based on R„ enables us to have the possibility of using one limit for all the three energies. It also

provides a useful interpretation for the rule. It can be shown (e.g., see Ref 6) that for samples of

five observations from a Gaussian distribution

R-^233S

where S is the standard deviation. Thus,

R„^233S/E,

and the rejection criterion R„> ris approximately equivalent to the rejection criterion

S/E, > r/2.33

which can be interpreted to mean that (in additional to the regular "difference" criteria) a machine would fail the test if its noise-to-signal ratio is greater than r/2.33. For example, with r = 25, the threshold would be 10.73 %.

306
Figure 5. The range of the five specimens for the lo\^'-

energ}- verification test. The horizontal axis is the

sequence of the test, The order within each year is not

relevant. The dotted line indicates the 5 J proposed rule. The solid markers designate the machines that fail the ±1.4

J criteria.

Figure 6. The range of the five specimens for the highenerg}- verification test. The horizontal axis is the

sequence of the test. The order within each year is not

relevant. The dotted line indicates the 15 J proposed rule. The solid markers designate the machines that fail the ±5%

criteria. 307
O) o

Low High Super High

Figure 7. The normalized range of the five specimens for

all the three energy verification tests. The solid markers

designate the machines that fail the ±1.4 J or ±5% criteria.

The dotted lines, at 25% and 30 %, are possible values of r

to use in the range value.

Figure 7 plots R„ for all three energy verification tests. Again, the solid markers designate

the machines that fail the ±1.4 J or ±5 % criteria. The two dotted lines indicate the two

possible values of r to use: 25 and 30 %, which correspond to 10.73 and 12.88 % in the

noise-to-signal-ratio scales. A 30 % /?„ criterion would fail eleven additional tests (0.46 %)

for the low energy, one additional test (0.04 %) for the high, and no additional test for the

super-high energy.

308
-1.5 -1.0 -0.5 0.0 0.5 1.0 1.5

Difference, Joules (Low Energy)

Figure 8. The scores of the low and super-high energy

tests for cases that passed both tests. The "o" points are cases that also passes the high energy tests.

Energies Required for Verification Testing

Currently, the testing of the low, high, and super-high energies is required by ASTM

Standard E 23 to certify candidate machines with capacities of greater than 289 J. However,

the responsible ASTM subcommittee and task group has questioned whether the testing of

the high energy is necessary in the certification of high-capacity machines. To address this

question, we examine the verification tests from 1994 to 1996 for which the low-, high-, and

super-high-energy tests were performed. Figure 8 plots the values of - (in the lowenergy test) and {E^ - E,)/E^ (in the super-high test) for the cases that passed the low- and

super-high-energy tests. The 698 "o" points are cases that also passed the high energy test. The 21 "•" points (2.92 %) are cases that failed the high energy test. Among the 21 cases

that failed, 13 tests failed with {E^ - E^IE^ between 5 and 6 %, and five tests failed with {E^ - E^IE^ between 6 and 7 %. Thus, the successful result of the low- and super-high energy

verification tests cannot ensure that a machine will pass the high-energy verification criteria

of ±5 %, but in most cases, the results of the high-energy test are in good agreement with the

reference energy.

Discussion

The failure rates for the low and high-energy verification tests are nearly constant,

averaging 11.8 and 6.6 %, respectively. The failure rate for the super-high-energy test is more variable, in part due to the smaller and unequal number of tests performed in each year

and the recent introduction of the test. Overall, the verification program appears to be

functioning as described by those who originally implemented the program [7]. Data from a

309
1970 report [2] showed that initial failure rates for the impact verification program were 44

% (433 tests); but as machines were repaired and retested the failure rates began to decline

sharply (11.5% was the lowest failure rate reported). Clearly the verification program has

established and maintained a population of impact machines that can be reliably used for

acceptance testing: more than 700 of the 800 machines tested annually in the program are within 1 .4 J, or 5 % of the reference values, indicating that these machines differ from each

other by less than 2.8 J, or 10 %.

The distributions of the data show significant numbers of machines near the limits of the

pass/fail criteria, and we assume that some good machines failed the verification test. If the

pass/fail criteria were widened to include more of these borderline machines, however, we

suspect that the distribution of test results would broaden over time and a similar situation

would develop near the new acceptance limits. In effect, it is the stringent pass/fail criteria

adopted by E 23 many years ago that has resulted in the narrow distribution of impact test results in the program today. Frankly, any less stringent requirements would result in

acceptance tests with little value, particularly when qualifying high-strength steels for severe environments. So accepting the current 1.4 J or 5 % pass/fail requirements as practical and

necessary, we can strive only to decrease the probability of good machines failing the

verification test and of bad machines passing the test.

The current acceptance criteria are based solely on the averages of the verification set and

the pilot lot and do not take the variation of the data into account. The proposed range rule is a step in the right direction to help identify bad machines that would currently pass the

verification test. Based on the low-energy data presented here, though, the probability of

failing good machines with this rule because of one single outlying measurement is too high.

We think it will be necessary to visually examine the specimens for jamming marks and

evaluate the distribution of the five test results to more accurately identify machines with

excessive variation. In addition, the proposed range rule can detect excessive variation only

in the verification set and does not incorporate any of the information available on the pilot

lot variation.

Further consideration of the pass/fail criteria and how they relate to the variations of the

pilot lot data, our best indicator of variability, are needed. It has been proposed [5] that the

candidate machine may be certified if - is in interval (L, U) with

U= -L = d + 0.16S (1)

where S is the pooled standard deviation of the pilot lot and d is a constant. The question is, what value should d be given. For illustration, we will use S = 0.7 J which is a typical

standard deviation for the low-energy verification specimens (Table 2). With S = 0.7 J, U =

1.4 J, d is 0.9 J. Historically, low-energy lots have been accepted for distribution if the

standard deviation of the pilot lot was 1 J or less [4], so when verification tests are conducted

with specimens having a standard deviation of more than 0.7 J, say 1 J, these tests would

have only to meet ±1.66 J criteria according to Eq 1. This is reasonable and would lower the

probability of a good machine failing the verification test, but as we have already stated, to

310
maintain a useful verification program we have accepted a maximum difference of 1 .4 J for

the low-energy range. So we need to reduce the maximum allowable standard deviation for

the acceptance of specimen lots to increase the probability of certifying good machines and

leave the pass/fail criteria at their present values. This is clearly the most direct and best

method by which we can improve the impact verification program.

There is always uncertainty associated with the reference energy assigned to the lot. Overall, the average energies for pilot-lot data are in good agreement with the verification

test results. It is, however, very difficult to evaluate the influence of the specimens on the

verification data. In practice, we mitigate the influence of the specimens on the verification

test by monitoring these data and visually inspecting the five returned specimens. For

example, if a test fails to meet the 1.4 J or 5 % requirement and there are no markings on the

specimens to indicate that the machine is in need of repair, we will retest the machine using

different specimens if we have reasons to question the specimens used in the test. In practice

this approach works well in minimizing the effect of the specimens on test results, but a

better estimate of the mean would help avoid this problem.

To reduce the uncertainty of the reference energy assigned to verification specimens, the

sample size can be increased and/or the sampling method can be improved. We think that

increasing the sample size will improve our estimate only marginally. We are, however,

considering changing from a random sampling method to a systematic sampling related to the positions of specimens in the heat-treating baskets. In addition, the control specimens

placed in the sampling locations would be marked to identify their bar-stock origin (the ingot

location of the bar stock is known, but we do not track the individual bars from which the

samples are made). This type of sampling would allow us to include two variables of our

processing in a consistent manner.

Generally, most machines that perform well at the upper and lower bounds of their

capacities also perform as expected at mid-range energies. But three of machines failed the

mid-range test, and Fig. 8 shows that tests at any one energy alone provide limited

predictability on how the machine will perform at the other energies. For example. Fig. 8

shows the results for a machine that had nearly perfect performance (0 % difference) at both

low and super-high energies, but the machine failed the high-energy test. This is due

primarily to the fact that the specimens used to test at the three energy levels interact quite

differently with the machine. For this reason we believe it is necessary to test at each energy

level at which the machine will be used to have any certainty of the performance of the

machine at that energy level.

Conclusions

Some conclusions based on the Charpy impact verification data that were evaluated by

NIST from January 1994 through December 1996 are as follows:

1. The adoption of the stringent pass/fail criteria (1.4 J or 5 %), originally proposed for

verifying Charpy impact machines in 1955, has produced a large population of impact

machines that are suitable for acceptance testing.

311
2. The range rule now being considered for E 23 can detect excessive variation in

measurements. More studies are needed before the rule is implemented.

3. Impact machines of large capacity should continue to be verified by testing at three

energies.

Acknowledgment

This work is a contribution of the National Institute of Standards and Technology and is not subject to copyright in the United States.

References

[1] Driscoll, D. E., "Reproducibility of Charpy Impact Test," Symposium on Impact

Testing, ASTM STP 176, American Society for Testing and Materials, Philadelphia, 1955,

pp. 70-74.

[2] Fahey, N. H., "The Charpy Impact Test—Its Accuracy and Factors Affecting Test

Results," Impact Testing of Metals, ASTM STP 466, American Society for Testing and

Materials, Philadelphia, 1970, pp. 76-92.

[3] ASTM E 23-88, "Standard Test Methods for Notched Bar Impact Testing of Metallic

Materials," American Society for Testing and Materials, Philadelphia.

[4] ASTM E 1271-88, "Standard Practice for Qualifying Charpy Verification Specimens

of Heat-Treated Steel," American Society for Testing and Materials, Philadelphia.

[5] Splett, J. D. and Wang, J. C.-M., "Proposed Changes to Charpy V-Notch Machine

Certification Requirements," Pendulum Impact Machines: Procedure and Specimens for

Verification, ASTM STP 1248, American Society for Testing and Materials, Philadelphia,

1995, pp. 182-194.

[6] John, P. W. M., Statistical Methods in Engineering and Quality Assurance, John

Wiley & Sons, Inc., New York, 1990.

312
C. N. M'Cowan,^ J. Pauwels,^ G. Revise,^ and H. Nakano^

International Comparison of Impact Verification Programs

Reference: McCowan, C. N., Pauwels, J., Revise, G., and Nakano, H., ^international

Comparison of Impact Verification Programs," Pendulum Impact Testing: A Century of

Progress, ASTM STP 1380, T. Siewert and M. P. Manahan, Sr. Eds., American Society for

Testing and Materials, West Conshohocken, PA, 1999, pp. 210-219.

Abstract: A horizontal comparison is made between the four laboratories that certify Charpy

impact verification specimens. The participants in this study were Japan (NRLM), France (LNE),

the European Commission (IRMM), and the United States (NIST). The exercise was conducted

to show how the impact verification programs, specimens, and test procedures compare with

each other. Results for both 8 mm and 2 mm strikers were compared. The study showed the

following: (1) The certified energies of impact verification specimens distributed by these four

metrological authorities often agreed within 1 % of the average values determined in this study;

(2) the variation in energy for the specimens was low, typically bracketed by a coefficient of

variation of between 0.02 and 0.04; and (3) the energies measured for the tests performed with 2

and 8 mm strikers on the 4340 steel specimens were nearly equivalent, but a trend of slightly

higher energy for the 2 mm tests is indicated.

Keywords: Charpy impact verification, impact testing, verification testing, verification

specimens

Introduction: Charpy impact testing is often specified as an acceptance test for structural

materials, and companies performing acceptance tests are typically required to verify the

performance of their impact machine using certified verification specimens. To our knowledge

there are only four laboratories in the world that certify and distribute reference materials for the

verification of Charpy impact machines: (1) The Institute for Reference Materials and

Measurements (IRMM, Belgium), (2) Laboratoire National D'Essais (LNE, France), (3) The

National Institute of Standards and Technology (NIST, USA), and (4) The National Research

Laboratory of Metrology (NRLM, Japan). These four laboratories supply impact verification

specimens to verify the performance of an estimated 1800 impact machines annually.

' Materials Reliability Division, NIST, 325 Broadway, Boulder, CO, U.S.A., 80303

^ Reference Materials Unit, EC-JRC-IRMM, Retieseweg, B-2440, Geel, Belgium

^ Mechanical and Equipment Testing, LNE, 5 Avenue Enrico Fermi, 78197, Trappes

Cedex,France

Materials Measurement Section, NRLM, 1-4, Umezono 1 -Chrome, Tsukuba, Ibarki, 305, Japan

313
This study provides the first horizontal comparison between these four laboratories. We

compare both the impact verification specimens and the machines (or systems) used to certify the

absorbed energies of the specimens. Our goals are to use this comparison to better understand

the details of each other's verification programs, and to consider how any differences between

our verification systems and specimens might affect the users of our respective programs. It is only through these types of horizontal comparisons that we can assess the equivalency of the

results for impact acceptance tests made around the world. These initial results help confirm that

acceptance tests performed under one system are equivalent to the others, making the verification

systems and specimens transparent to the user.

Background Information on Verification Programs

The four verification programs represented in this report have similar goals and much in

common, but each program is unique. To start with, there are two fundamentally different

approaches used to stabilize the certification procedures for impact verification specimens. The

European philosophy for stabilizing their certification procedures (EN Standards) is based on

traceability to "master specimens." This system is represented here by IRMM, which currently

certifies and distributes impact verification specimens for the Community Bureau of Reference

(BCR). The stability of the national impact verification programs in the United States, Japan,

and France is based on traceability to designated impact reference machines that are maintained

by the respective metrological authorities (NIST, NRLM and LNE). This difference and details

of the certification procedures discussed below (and given in Tables 1 and 2) make direct

comparisons of our systems difficult. However, keeping these differences in mind, we can make

useful comparisons of our results.

IRMM Program - In the IRMM program, "master batches" of impact verification specimens

are tested in a round robin for the BCR. From the results of the round robin a certified energy for

the specimens is determined, and these "master specimens" are then tested over time to track and

normalize the results of the impact machine that is used to determine certified values for BCR

verification specimens. The certified values for BCR impact verification specimens are determined as follows: (1) New batches of BCR impact verification specimens are tested (30

samples per batch) on a single impact machine, together with sets of "master specimens"

(generally 25 or 35 specimens) of similar nominal energy; (2) if the samples have acceptable

variation and energy, the difference in the average energies for the "master specimens" and the

batch being evaluated is determined for the machine; (3) this difference is added or subtracted

from the average energy for the batch of BCR verification specimens to determine a certified

energy value[l]. These procedures meet the EN 10045 and ISO 148-3 requirements for

verification specimens [2,3],

For this study, the results of the impact machine used by IRMM were reduced by 4 % to represent the BCR procedure, so these results represent the certification system, not a single

impact machine. The IRMM specimens can be used to verify impact machines in accordance

with EN 10045 and ISO 148-2 [4].

314
Table 1 - Details on the specimens and impact machines used in the study.

Under machine information, the penduhim type iC or U). the striker radius w hich

was tested, the method of reading the absorbed energy (dial or encoder), and the

maximum capacity of the machine are given.

Laboratory Specimen Information Certilied Values

Machines

Test Details

Temperauire

(-C)

Energy'

Level

Steel T>peStnker

Radius

(mm)

Value

(J)

20 1 4340

-) 2".0 - C- t>-pe

- 2 and 8 mm

-Dial

- 300 J

26.8 20

4340

113.6

g 112.6 20 XM32 15".

-> 20 1 E,40

NCD"

- 26.- - U-t>pe

- 2 and 8 mm

- Enccxier

- 350 J

20 2 6".

20 -1 111,8

-4-0 1 4340

8 16.5 - U- t>pe

- 2 and 8 mm

- Encoder

- 350 J

-40 2 8 99.2

20 T-20O 8 258.0

4 0

4340

- C-r)pe

- 2 and S mm

- Dial

- 500 J

0

0

-1

The certified values were not a\-ailable because the lot size for the specimens was

too small to both certif>- the specimens and provide the number of specimens required

for this study.

XRLM Program - In Japan, there are two C-t}pe impact machines m the program, but a single impact machine is used to determine the cenified values for the verification specimens.

The second machine is used as a back-up machine and for comparison to the machine used for

certifications. Because a single machine is used to determine the cenified value for the

verification specimens, the results for the NRLNI impact machine used in diis smdy represent

those that would be attained for the Japanese certification system. The certified value for the

NRLNI specimens is determined by testing 25 specimens. The results are e\-aluated and if all

statistical criteria are met. die average energ>- of the 25 specimens can be used as a certified

value. Tbis procedure meets the JIS B 7740 and the ISO 148-3 requirements for verification

specimens ( the requirements for the JIS B 7740 and ISO 148-3 standards are identicalj [5].

The impact specimens cenified b>' the NRLM program are primarily used to verif\- the

performance of impact machines according to JIS B 7722. which is the national standard for

impact testing m Japan [6]. The NRLM specimens can also be used for the ISO 148-2 machine

315
verification (again, the requirements for the JIS and

ISO standards are identical).

Table 2. Summary of the verification

requirements for national and

international standards: difference

allowedfor the verification result and

the certified energy value (EJ.

Designation Requirements

ISO 148-3 ±2 J for E, < 40 J ±5% for Ec>40 J ISO 148-2 ±4 J for E, ^ 40 J ±10% for E,>40J

EN 10045 ±2 J for E, < 40 J ±7.5 % for E, > 40 J ASTM E23

ASTM 1236

±1.4 J for E, < 28 J ±5% for E, > 28 J JIS B 7722 ±4 J for E, < 40 J ±10% for E,>40 J JIS B 7740 ±2 J for E, < 40 J ±5% for Ec > 40 J

NIST Program - In the United States, the certified values for impact verification specimens are determined using three impact machines (2 U-types

and 1 C-type). The certified value is determined

as follows: (1) 25 specimens are tested on three

different impact machines, (2) the results are evaluated to determine whether the differences in

the variation for the specimens and the average

energies of the three machines meet established

criteria, (3) if the specimen variation is acceptable

and the comparison of results for the three machines

are within normal bounds, the results are combined

and the certified value is defined as the average value

for the 75 specimens. The certified specimens meet

the ASTM E 1236 Standard for Qualifying Charpy

Impact Machines as Reference Machines and ISO

148-3 requirements for verification specimens. In

this study, only one impact machine was used, so the

NIST results represent that machine, not the

certification system. However, at energies above 40 J, the results for the NIST machine are expected to closely represent the average value of the three NIST machines. For the low-energy

(<40 J) specimens, the results for this single machine are often higher than the average of the

three machines used to determine a certified value for NIST specimens.

The NIST specimens are used primarily to verify the performance of impact machines

according to ASTM E 23, the Standard Test Method for Notched Bar Impact Testing of Metallic

Materials. The specimens can also be used for ISO 148-2 verification tests.

LNE Program - In France, two impact machines are used to certify verification specimens

(a U-type and a C-type). The machines are located at two different laboratories in France (LNE

and CTA), which have an agreement from the French Community of Accreditation (COF^AC)

for the certification of impact verification specimens. The procedure used to develop a certified

value is in accordance with EN 10045-2 and is similar to the NIST procedure: 25 specimens are

tested on each machine, and the grand average (50 specimens) is used as the certified value for

the specimens if statistical evaluations show acceptable performance for the machines and the

specimens. In this study only the U-type impact machine was used, so the LNE results represent

that machine, which typically has a slightly higher absorbed energy than the C-type used by LNE.

The LNE specimens can be used to verify impact machines according to EN 10045 and ISO

148-2.

316
Materials and Procedures

Each of the participating laboratories provided 100 verification specimens at three different

levels of absorbed energy, referred to as levels 1, 2, and 3 in Table 2. The laboratories are coded

using numbers (1-4). The specimens provided by each laboratory are also coded (1 -4) to identify the laboratory that supplied the specimens.

Information on the impact machines used for the testing is given in Table 2, along with the

test temperature specified for each group of specimens. The certified absorbed energies for the

specimen groups, which each laboratory had previously determined for the specimens, are also

given in Table 2. (These values were not made available to the participants until the testing was

completed.)

All four laboratories used a similar type of steel to produce verification specimens for

energy levels 1 and 2 (Table 2). The steel is a medium-carbon, low alloy, high-strength steel,

designated in the U.S. as AISI/SAE 4340. Although these steels are ver>' similar in composition,

the heat treatments used by the various laboratories are assumed to vary significantly.

The types of steel used to produce verification specimens for energy level 3 differ between

the various laboratories. So at this energy level, differences in impact properties due to alloy

content must be considered. Basically, three types of alloys were used for energy level 3: (1) a XM32 steel, which is a Cr- Mo type alloy, (2) a T-200 steel, which is an 18 Ni-0.7 Ti maraging

steel, and (3) the AISI/SAE 4340 type steel, which is also used for energy levels 1 and 2.

The impact testing was conducted using the procedures normally followed by each

laboratory, and any special instructions that were provided with the specimens. From each

specimen group of 25 specimens, 15 were tested using a 2 mm striker, and 10 were tested using

an 8 mm striker. Some outlier data were removed during testing, but most of the statistical

outliers were left in the data for our analysis.^

Specimen Variation

The variation in the absorbed energy for the specimen groups is given in Table 3. The

results are presented in terms of the coefficient of variation (CV), which is the ratio of the

standard deviation in the absorbed energy to the average absorbed energy of the specimen group

being evaluated. This relative measure provides a convenient means of normalizing the various

energy levels being considered here. A CV of about 0.04 (or less) represents a low level of the

variation in absorbed energy for impact verification specimens. This CV value is related to sample size calculations (for a sample size of 5), and would generally ensure a variation small

enough to conduct a verification test within the statistical considerations upon which our

^ Statistical outhers are defined here as those data that were identified as outliers in box-andwhisker analysis. For the most part, data were removed during testing only due to a testing

problem (specimen placement, etc.).

317
verification programs are designed.

The ISO 148-3 requirements for

verification specimens allow a CV

of 0.05 or less.

The pooled CV is considered

the best indicator of specimen

variation, due to its inherently larger

sample size. It is defined as the rms

value: the square root of the sum

of the squares of the CVs (for

laboratories 1-4) divided by 4.

At the lowest energy level

evaluated, level 1, the variation for

the 8 and 2 mm tests are similar.

Considering the pooled CVs, no

clear effect of striker radius on the

variation is apparent. Occasionally one laboratory had a high or low

CV with respect to the other three

laboratories. But typically the high

CVs reported for the level 1 specimen groups were due to one or two statistical outliers in the data.

Generally, the variation for the level

1 specimens was low, and all of the

pooled CVs are 0.04 or less.

At energy level 2, the

interlaboratory CVs for the various

specimen groups are even more

consistent than the results for level

1 . Typically, three laboratories had

the same CV for a specimen group,

and one would differ slightly. All

of the pooled CVs are 0.04 or less; many are 0.02. Again, there was no

apparent effect of striker radius on

the variation.

Table 3 - The coefficient of variation is given for the four

laboratories , organized by energy level, striker (tup),

specimen group. The pooled CV ( CVP) of the

laboratories is also given. The CV is defined as the ratio

of the standard deviation and the average absorbed

energyfor the group.

Energy

T 1 Level

TUP

(mm)

Specimen

Group

CVs for Labs 1-4, and the

pooled CV (CVP)

CVl CV2 CV3 CV4 CVP

2 1 0.02 0.04 0.04 0.04 0.04

2 2 0.02 0.03 0.02 0.05 0.03

2 3 0.02 0.07 0.04 0.02 0.04

2 4 0.02 0.02 0.02 0.03 0.02

8 1 0.02 0.02 0.02 0.04 0.03

8 2 0.03 0.04 0.03 0.06 0.04

8 3 0.05 0.06 0.03 0.02 0.04

8 4 0.02 0.03 0.04 0.01 0.03

2 2 1 0.04 f\ f\ A 0.04 0.05 0.03 0.04

2 2 2 0.02 0.02 0.02 0.01 0.02

2 2 3 0.02 0.03 0.03 0.02 0.02

2 2 4 0.02 0.02 0.03

r\ r\r\ 0.02 0.02

2 8 1 U.U4 U.UJ U.Uj U.U3 U.UJ 2 8 2 u.uz U.Ul n CO u.UZ U.UZ C\ CO

2 8 3 0.02 0.02 0.03 0.02 0.02

2

o8 4 0.02 0.03 0.03 0.03 0.03

3 2 1 0.02 0.02 0.04 0.04 0.03

3 2 2 0.03 0.04 0.03 0.04 0.04

3 2 3 0.12 0.15 0.03 0.11 0.11

3 2 4 0.03 0.03 0.03 0.04 0.03

3 8 1 0.03 0.03 0.03 0.04 0.03

3 8 2 0.04 0.04 0.03 0.02 0.03

3 8 3 0.03 0.05 0.03 0.03 0.04

3 8 4 0.05 0.05 0.04 0.05 0.05

At energy level 3, the interlaboratory CV results are slightly less consistent than the level 2

results, and differentiate the results for several specimen groups. The 2 mm tests for specimen

group 3 have a significantly higher CV than the other level 3 groups (pooled CV equal to 0.1 1), and the 8 mm tests for specimen group 4 had interlaboratory CVs slightly higher than 0.04

(pooled CV equal to 0.05). Both of these specimen groups, however, had acceptable CVs for

318
tests made using the striker radius for which ihey were de\-eloped; The CVs for the 8 mm tests on

the group 3 specimens and the 2 mm tests on the group 4 specimens were 0.04 and 0.03

respectively. So. for the more ductile high-energy specimens, striker radius apparent!)- had an

effect on the variation in absorbed energ}'.

In general, the interlaborator.' C\' for the \'arious specimens groups \».ere in good agreement

and appear to reflect differences in the specimens, not the impact machines. However, laboraton.-

1 had the lowest variation most often, and this likely indicates some influence of the impact

machine on the variation assigned to the specimens.

Absorbed Energy

The absorbed energy results for the specimen groups are sho\^ n m Figures 1- 6 and the data

are given in the Appendix. The plots show the average absorbed energ\- for each specimen

group, with error bars representing ± 1 standard de\'iation. The results for each energy level are shown separately for the 2 and 8 mm tests. For comparison purposes, a fifth average and error bar was added to the data for each specimen group. This bar shows the average absorbed energ}'

of the four laboratones for each group (the grand average ) and the ASTM E23 and EN 10045

limits for verif>"ing the performance of reference qualit}' impact machines (see Table 1 1. These

error bars offer convenient scales for comparisons of these data, but the limits have no real

significance for the honzontal comparisons \vq make here. The data for each laborator.-. within

each specimen group, are plotted from left to right as laboratory' 1, 2, 3. and 4.

Energ}- Level 1- As shown in Figures 1 and 2. laboraton.' 4 alway s had the lowest absorbed

energy for the specimen groups tested at this energy le\'el. The highest \"alues wtit always from

laboratory' 2 or 3. and laboratory 1 always had the intermediate \'alues.

The relati\'e relationships between these results were found consistent for both the 2 and 8

TTiTn test results, but the magnimdes of the differences between the results were dependent on the

particular specimen group that was tested. The apparent differences in the results for the

laboratories were greatest for the tests made with specimen groups 1 and 3. slightly smaller for

specimen group 2, and smallest for specimen group 4 (^±0.8 Jj.

The average results for the tests \'.'ere all within ± 2 J of the grand a\'erage determined for

the energy level 1 specimen groups, and most were within ± 1.4 J of the grand as'erages. So

these results are within the limits deemed acceptable for reference-quality impact machines. The

range in the results, however, pushes these limits. This is partly due to differences in the designs

of the C and U-t>pe impact machines represented here. The impact machine used by laboratory

4. for e.xample. may be the most rigid machine in this study (large capacity C-type machine) and

the lower values produced by this machine may be \'ery good values (less energy absorbed by the

machine in the test).

It must also be kept m mind here that the a\'erage values for two of the laboratories would

likely be lower if the C-type machines that are also used in their \-erification programs were

included in their averages. This would decrease the range in the results.

319
The 2 and 8 mm test results were equivalent, considering the standard deviations associated

with them. But there is a trend for the 2 mm results to have slightly higher absorbed energies.

The average differences between the 8 and 2 mm results for each specimen group were as follows: (Gl) -0.01 J, (G2) +0.13 J, (G3) +0.44 J, and (G4) +0.06 J. The pooled standard

deviations associated with the average energies for these specimen groups were in the range of

0.5 to 1.0 J.

Energy Level 2 - The results of the laboratories for the level 2 specimen groups

(Figures 3 and 4) are well within ± 5 % of the grand averages determined for the groups (none

exceed ± 3 %). The results do not show significant differences between the laboratories, and the

effect of striker radius is small. In general the 2 mm test results have slightly higher energy than

the 8 mm results: the average differences between the 2 and 8 mm tests for the laboratories were

1.6, 1.1, 1.1, and 1.5 J for the specimen groups 1-4 respectively. In most cases, the differences

are less than the standard deviations associated with the average energies of the specimen groups

being compared.

Energy Level 3 - The tests at energy level 3, Figures 5 and 6, show a significant

difference in the results for the 2 and 8 mm striker radius. The differences between the 2 and 8

mm test results are apparently related to the type of steel used for the specimens as much as they

are to the energy level of the specimens. For the specimens made from 4340-type steel (groups 2

and 4), the differences in the 2 and 8 mm tests were the smallest. The group 2 results were mixed with respect to the influence of the striker radius and the differences (average difference of

-0.7 J) were not statistically significant. The results for the group 4 specimens consistently show

a higher energy for the 8 mm tests, and the differences (average difference of -4.8 J) are on the

order of one standard deviation. For the specimens made from the XM32 steel (group 1), the 2

mm test results were consistently higher than those for the 8 mm tests. The average difference

for the group 1 specimens was +10.0 J, which is statistically significant (standard deviations of

the average energies for group 1 were around 5 J). The tests with the specimens made from the

maraging steel (group 3) had the largest difference in 2 and 8 mm results (average difference of

-44.7 J), and this shows the results for the 8 mm tests to be higher than those for the 2 mm tests. So the results for two steels (XM32 and maraging) show significant differences for 2 and 8 mm

test results, but the effect of the striker radius was quite different (opposite).

The results also show that the group 3 maraging-steel specimens are better verification

specimens for the 8 mm test, for which they were developed. The variation in the absorbed

energies for the 2 mm tests were much higher than those for the 8 mm tests, and the differences

in the interlaboratory average energies were also greater for the 2 mm tests. So it appears that the

group 3 maraging-steel specimens perform adequately as 8 mm verification specimens, but are a poor choice for 2 mm verification tests. The group 1, 2, and 4 specimens all performed well as

verification specimens for level 3, for both 2 and 8 mm tests ( all within the ±7.5 % limit, and

most within the ±5% limits).

320
12 3 4

Specimen Code

Figure 1. Energy level 1, 2 mm striker.

12 3 4

Specimen Code

Figure 2. Energy level 1, 8 mm striker.

130

120 ^ 110

B 100

m 90 h

80

70 h

60 12 3 4

Specimen Code

Figure 3. Energy level 2, 2 mm striker.

300

12 3 4

Specimen Code

Figure 5. Energy level 3, 2 mm striker.

12 3 4

Specimen Code

Figure 4. Energy level 2, 8 mm striker.

300

12 3 4

Specimen Code

Figure 6. Energy level 3, 8 mm striker.

321
Lateral Expansion

The lateral expansion for the specimens, Figure

7, increased with increasing absorbed energies, as expected. The correlation is linear up to about 150 J. The 2 mm tests resulted in slightly increased lateral

expansion (and absorbed energy) for most of the

specimen groups. The exception was specimen

group 3 (level 3), where the 8 mm tests had higher

lateral expansion. This result is consistent with the

higher absorbed energies for the 8 mm tests on these

specimens.

Hardness

0.0 0.5 1.0 1.5 2.0 2.5

Lateral Expansion, mm

Figure 7- The grand averages for the

lateral expansion and absorbed energy

for each specimen group are plotted

here (averages of the 4 laboratories for

each group). Results for the 2 and 8 mm

tests are shown.

As shown in Figure 8, the absorbed energy

decreases with increasing hardness. The relationship

is approximately linear, if only the type 4340 steel specimens are considered. The XM32 steel fits the

trend, but the T200 maraging alloy does not. This

result is not surprising, but the plot does provide a good example of the relationship between hardness and absorbed energy for the impact

verification specimens. The variation in hardness measured for the specimen groups was

generally low. Reviewing these data it appears that a standard deviation in hardness of between

0. 1 and 0.2 HRC is routinely attained for samples at all energy levels. This corresponds to a range in the hardness of 1 HRC for a specimen group (25 specimens). The specimens having the

most consistent hardness, however, did not

necessarily have the lowest variation in absorbed

energy.

There was a consistent offset in the hardness

values reported by the various laboratories. Since

hardness measurements were not the focus of this

study (not mandatory), this result is not of direct concern here. However, it is of general interest that

the average hardnesses for the specimen groups

measured by the four laboratories were almost never

within 1 HRC, and sometimes differed by more than

2 HRC.

Discussion

The specimens used for verifying Charpy

impact machines around the world have similar

variation in absorbed energy, which is the principal

p

0}c

in

300

200

100

1

o T200

1

. T200

• V XM32

"TUPMM ^\

; o 8

^

. • 2

1 1 20 30 40

Hardness, HRC

50

Figure 8 - Hardness and absorbed

energy of the steels usedfor vertification

testing.

322
factor used to evaluate the quality of these specimens. The coefficient of variation for the

specimens from all laboratories and energy levels typically ranged from 0.02 to 0.04. This result

is interpreted as a benchmark for the quality of impact verification specimens for the 1990s.

Machine/specimen interactions (and other variables) affect the variation assigned to a particular

specimen group by a single machine, but in general the differences in specimen variation were

resolvable and appear to reflect the homogeneity in the impact properties, rather than effects of

machines, operators, and sampling.

The relative outcome of the absorbed energy results for the laboratories was sometimes

dependent on the specimen group that was tested. This was particularly true for the tests made

using the lowest energy specimens (group 1), where the changes in interlaboratory results due to the specimens altered the general interpretation of the results. For example, considering only the

tests made with specimen groups 1 and 2, one might conclude that the results from laboratory 4

were different and lower than those of the other laboratories, but considering only the results for

specimen group 3, the results from laboratory 2 appear to be different and higher than those of

the other laboratories. If only the results for the group 4 specimen were considered, we would

conclude that the laboratories all produce similar results for low-energy impact tests. These

specimen effects are secondary in many cases, but raise questions concerning what these apparent

differences in the results show us about the performance of the impact machines tested. Because

these specimen effects are not well understood for the most part and are not clarified by these

results, we refrain from speculating here. The data do, however, provide a useful side-by-side

comparison of these differences for our verification specimens and will hopefully lead to discussions and testing that improves our understanding of specimen/machine interactions.

The summar}^ plot of the 2 and 8 mm tests results in Figure 9 shows that the effect of

striker radius is small (statistically insignificant)

for the specimens made from 4340 steel. The

small differences in energy for the 2 strikers are not consistent over the 10 to 150 J range: they

show the tests results for the 2 mm striker to be

shghtly higher for the specimens at energy levels

1 and 2 (average difference of +0.16 and +1.33 J

respectively), but lower at energy level 3

(average difference of -2.75 J). The lateral expansion results indicate a more consistent

trend of increased ductility (higher toughness)

for the 2 mm tests over the range of 10 to 150 J

for the 4340 specimens. This finding is consistent with several previous studies that

evaluated these effects for 4340 steels [7-11].

But, for certifications even small differences in

the test results must be considered. So, we

conclude that separate certifications for 2 and 8

mm tests are needed for verification specimens

made from 4340 steel.

300 I 1 y

d3 200 4340 X 1-E

E

//XM32

00 100

0 / 1 1 100 200

2 mm Tup, J

300

Figure 9 - The 2 and 8 mm striker results

( grand averages) are shown. The grand

average of the 2 and 8 mm strikers include

about 60 and 40 tests respectively. The

error bars represent ±1 standard deviation

for the 8 mm averages.

323
The results for the XM32 and T200 steels were strongly affected by the striker radius used

for the test. For the very ductile T200 material, this greater sensitivity has been explained by the

fact that the larger radius 8 mm striker (and striker design) requires more bending of the

specimen during fracture and this requires more energy [7,8,10]. These same mechanical

differences may also explain the better performance of the T200 specimens with the 8 mm

striker: the increased bending results in more complete tearing during fracture, which results in a more consistent fracture energy for the specimens. The higher energy for the 2 mm tests on the

XM32 material, however, has not yet been convincingly explained. Studies with other materials,

showing similar effects of striker radius, have speculated that this increased energy for 2 mm

tests may be due to deeper penetration of the smaller striker combined with mechanical

properties specific to the material (strain hardening, strength, hardness, etc.) [10]. Here, the

lateral expansion for the 2 mm tests on the XM32 samples was lower than those for the 8 mm

tests (and similar to the 2 and 8 mm results for the 4340 steel), so considering the ductility

associated with the fracture does not provide an explanation.

Overall, the comparisons of our results are reassuring. Even though the different

specimen groups clearly had an effect on some test results, the trends for the test results were

generally independent of the specimens tested. The average absorbed energies determined by the

four laboratories for the various specimen groups, and the variation in absorbed energies, are in

good agreement. The laboratories differed mostly on results for verification specimens of the

lowest and highest energy (energy levels land 3). But the results never differed from the average

by more than 2 J (or 7.5 %). In most cases the ASTM 1236 requirement for qualifying reference

machines of 1 .4 J (or 5 %) was met, and if the verification systems could have been more directly

comparable (by including C-type machine results in the averages for laboratories 2 and 3), the

range in absorbed energies between the verification laboratories would likely decrease.

Our laboratories have long recognized that differences in the designs of impact machines

have an effect on impact test results, and our verification systems include this factor (with the

exception of Japan) when determining a certified energy for impact verification specimens.

Japan uses a single C-type machine for certifying verification specimens, but the vast majority of

the companies verifying impact machines in Japan also have a C-type machine (so Japan does

not necessarily have to consider this variable). In Europe and the United States, however, many

different types of impact machines are used by our industries, so we try to include the differences

due to machine design in our certification procedures. As explained in the background section,

the certified energy for ASTM verification specimens is determined by using the average

energies from three different impact machines (2 U-types from different manufactures and 1 Ctype). The French system uses one C-type and one U-type machine to determine certified values

for verification specimens. The verification system used by the European Commission (BCR)

uses the results from round-robin testing on master specimens (which includes many different

impact machine designs) to adjust the result of reference machines used to assign certified values

to verification specimens. This practical approach of averaging the machine design effects has

worked reasonably well, but the limits specified for verification tests are quite stringent in some

cases, and the machine design effects can be significant, particularly when testing low-energy

specimens (as shown by our results). However, considering the ISO 148-2 requirements for

verifying the performance of impact machines (which allow a ±4 J error for test at energies of

324
40 J or less, and a 10 % error for tests at energies over 40 J), the agreement found for our systems

appears to be adequate for purposes of international commerce. As impact machines of newer

design and larger capacity are produced we will have to reevaluate this approach (and our

respective limits on verification of test results).

As a final point for comparison, we consider how well the certified energies for the

specimen groups tested in this study agree with the average energies determined by our

laboratories. As shown in Figure 10, the agreement is good. Again, it was difficult to make a

true comparison here, because the certified values for the Japanese specimens were not available.

However, the results for the Japanese impact machine in this study were substituted for their

certified values, and these values are expected to closely estimate the certified values. For the 17

cases compared here, the average energy determined by the laboratories in 12 cases differed from

the certified value by less than 1.0 %, 4 differed by 1.0 to 2.2 %, and 1 differed by 3.2 %.

This is a comforting result, which indicates that the certified values being determined by the

laboratories independently, are virtually identical to the grand averages determined by the four

laboratories in this study. So, we believe that industries using our specimens to verify the

performance of Charpy impact machines to the requirements of ISO 148-2 (which is the machine

verification document we all have in common) can be assessed in a fair and meaningful manner.

Conclusions

1) The certified energies assigned to verification specimens by the laboratories which

supplied them for this study agree closely with the average energies determined by the four

participating laboratories.

2) The impact verification specimens

distributed by all of the laboratories are similar

in quality, based on the similar variations in

energy found for the specimens tested in this

study. The variation in the energy of impact

verification specimens is currently controlled

within a CV range of 0.02 to 0.04.

3) The specimens used to verify the

performance of a Charpy impact machine can

affect the apparent performance of the machine.

4) The absorbed energies measured with

2 and 8 mm strikers are very similar for 4340

steel, but can differ significantly for other

materials used to produce verification

specimens.

Certified Reference Value, J

Figure 10. Average energy of the four laboratories versus the certified energy of the laboratory that

provided the specimens.

325
Acknowledgments

C. N. McCowan would like to thank D. P. Vigliotti and T. A. Siewert from the NIST

Materials Reliability Division for their efforts in measuring and testing the specimens, and in the

preparation of this manuscript. J. Pauwels wants to thank Mr. Gyppaz, responsible of the Quality

Department of Cogne Acciai Speciali srl, Aosta (IT) for carrying out the IRMM/BCR

measurements and Dr. C. Ingelbrecht, responsible for physical reference materials within the

IRMM Reference Materials unit for his valuable assistance in the evaluation of the results.

References

[I] Pauwels, J., Gyppaz, D., Varma, R., and Ingelbrecht, C, European Certification of

Charpy Specimens: Reasoning and Observations, Pendulum Impact Testing: A Century of

Progress, ASTM STP 1380, T. A. Siewert and M. P. Manahan, Sr., Eds., American Society for

Testing Materials, West Conshohocken, PA, 1999.

[2] EN 10045-2; Metallic Materials - Charpy Impact Test - Part 2: Verification of

Pendulum Impact Testing Machines.

[3] ISO 148-3, Metallic Materials - Charpy Pendulum Impact Test - Part 3: Preparation and

Characterization of Charpy V Reference Test Pieces for Verification of Test Machines,

International Organization for Standards, Geneva, Switzerland.

[4] ISO 148-2, Metallic Materials - Charpy Pendulum Impact Test - Part 2: Verification of

Test Machines, International Organization for Standards, Geneva, Switzerland.

[5] JIS B 7740, Charpy Pendulum Impact Test - Reference Test Pieces for Machines,

Japanese Institute of Standards, Japan.

[6] JIS B 7722, Charpy Pendulum Impact Test - Verification of testing machines, Japanese

Institute of Standards, Japan.

[7] Ruth, E. A., Striker Geometry and its Effect on Absorbed Energy, Pendulum Impact

Machines: Procedures and Specimens for Verification, ASTM STP 1248, T. A. Siewert and A.

K. Schmider, Eds., American Society for Testing Materials, Philadelphia, 1995, p 101-1 10.

[8] Nanstad, R. K, and Sokolov, M. A., Charpy Impact Test Results on Five Materials and

NIST Verification Specimens Using Instrumented 2 and 8 mm Strikers, Pendulum Impact

Machines: Procedures and Specimens for Verification, ASTM STP 1248, T. A. Siewert and A.

K. Schmider, Eds., American Society for Testing Materials, Philadelphia, 1995, p 1 1 1-139.

[9] Revise, G., Influence of Dimensional Parameter of an Impact Test Machine on the

Results of a Test, Charpy Impact Test: Factors and Variables, ASTM STP 1072, J. M. Holt, Eds.,

American Society for Testing Materials, Philadelphia, 1995, pp. 35- 53.

[10] Siewert, T. A. and Vigliotti, D. P., The effect of Charpy V-Notch Striker Radii on the

Absorbed Energy, Pendulum Impact Machines: Procedures and Specimens for Verification,

ASTM STP 1248, T. A. Siewert and A. K. Schmider, Eds., American Society for Testing

Materials, Philadelphia, 1995, pp. 140-152.

[II] Yanaka, M., et. al.. Effects of the Striking Edge Radius and Asymmetrical Strikes on Charpy Impact Test Results, Pendulum Impact Machines: Procedures and Specimens for

Verification, ASTM STP 1248, T. A. Siewert and A. K. Schmider, Eds., American Society for

Testing Materials, Philadelphia, 1995, pp. 153-170.

326
Appendix

Data Summary

T 'C'VrCT orliCliVlIllN T ATI TT TP 1 Ur

EMEAN

(J)

ESTD

(J)

LEMEAN

(mm)

LESTD

(mm)

HMEAN

(HRC)

HSTD

(HRC)

1 1 2 26.60 0.56 0.16 0.02 46.32 0.18

1 2 2 27.71 1.03 0.32 0.04 45.92 0.20

1 3 2 27.88 1.13 0.22 0.02 45.69 0.14

J 4 2 25.35 1.14 0.27 0.04 45.78 0.10

1 8 26.66 0.55 0.12 0.03 46.35 0.13

2 8 27.62 0.63 0.22 0.02 45.00 0.00

3 8 28.31 1.12 0.19 0.03 45.69 0.10

J 4 8 24.99 0.34 0.22 0.02 45.90 0.11

J 2 1 2 25.95 0.61 0.16 0.01 44.03 0.09

J 2 2 2 26.65 0.90 0.26 0.02 42.82 0.39

2 3 2 26.51 0.58 0.22 0.03 43.64 0.18

J 2 4 2 24.89 1.13 0.20 0.03 43.58 0.15

2 1 8 25.79 0.71 0.12 0.02 44.20 0.20

2 2 8 26.55 0.94 0.22 0.02 42.15 0.34

2 3 8 26.58 0.72 0.20 0.02 43.68 0.23

2 4 8 24.58 1.37 43.48 0.17

3 1 2 16.52 0.25 0.04 0.01 47.75 0.21

3 2 2 18.75 1.26 0.19 0.05 46.33 0.99

3 3 2 16.83 0.72 0.08 0.02 46.57 0.36

3 4 2 15.83 0.38 0.11 0.01 47.10 0.34

3 1 8 15.87 0.78 0.02 0.01 47.70 0.20

3 2 8 17.81 0.50 0.10 0.02 46.60 1.70

3 3 8 16.85 0.58 0.07 0.02 46.54 0.25

3 4 8 15.64 0.33 0.09 0.01 47.09 0.33

4 1 2 28.25 0.67 0.11 0.01 43.90 0.16

4 2 2 29.43 0.64 0.21 0.03 43.00 0.00

4 3 2 29.19 0.68 0.13 0.01 42.91 0.10

4 4 2 28.11 0.74 0.18 0.02 43.29 0.12

4 1 8 28.41 0.66 0.09 0.01 43.85 0.17

1 4 2 8 29.08 0.98 0.18 0.02 41.89 0.22

4 3 8 29.00 1.03 0.11 0.02 42.92 0.10

4 4 8 28.25 0.37 0.17 0.02 43.22 0.15

2 1 2 112.13 4.59 1.31 0.05 34.88 0.16

2 2 2 113.88 4.00 1.47 0.04 33.88 0.30

2 3 2 114.92 5.69 1.46 0.06 34.08 0.21

2 4 2 111.23 3.50 1.41 0.05 34.03 0.18

2 1 8 112.66 4.66 1.28 0.06 34.92 0.18

2 2 8 111.12 2.86 1.39 0.04 32.65 0.67
LEVEL SPECIMEN LAB TUP

EMEAN ESTD LEMEAN LESTD HMEAN

\ns\.K^ )

HSTD

i^nrs.^^) Z 1 oo

1 1 9 84 Z.o'+ 9 84 1 41 0 OS 11 08 O 1 7 U. 1 / L

11 oo

1 no 1

Q

41 1 41 Ow.wjOS 11 80 Jj.oy O 9S U.Zj

Z

9Z

11 9Z 84 1 SI O 71 O 04 18jo.y07/ Ou.uy OQ

Z

9Z9Z9Z 7n 9'5 /U.Zj 1 fSi 1 .06 ou.ywoo Ou.wy OO 17 /;i O IS

z 9Z 9z ^8 4Q 1 08 0W. 74/t- 0W.WD06 18 49 O 1 7 U. 1 / 9z 9z

/I 9z D / .70 1 00 0W. 78/ o 0 04 18 1 S O 1 1 W. 1 1 z 9z

1X co f\'\ Q1 11 .Z911 0 6S 0W.Wj01 18 08 OW. 14IH- 9z 9z 9z

Qo 0 06 0 7'^ w. / •? 0 04 17J / . 101 w 0U.Z9 11 9z z9

-3 Qo 8Q 11 .wz 09 0 71 0w.w^04 18 4S OU. 10lU

9Z

9Z

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J

9Z

11 9Z OAyo.zo98 11 78 . /o 11 . 11 ow O 04 1A 1 7 oo. 1 / OU.Z'f 94

9Z9Z9Z QQ 19 SO Z.JW 11 .zo 9A O OS 11jj.yQ71 O A1 U.Ol

9Z J9Z Q8 87 Z.O^ 9 A4 1 91 1 .Z

O 04 IS 1 1 JJ. 1 1 OU.ZU90

9Z

'J /I H- 9Z QQ 1 ^ 9 OS 11 .ZJ 9S O 04 14 09 OU. 10jU

9z

11 Qo Q^ 1 1 9Z. 14 11.111 1 0 04 16 01 O 14

9z 9z

oo Q^\ pO.79/z 9z.zw 90 11 . 10ly 0W.WD06 11 6S OU.JSIJ

9z

'1 co 07 16. 9 6S 11 . 14It- 0W.WH04 14 OS OU.J17/ 9z J

QO 08yo.'\471 9 91 Z.Z 1

11 .ZZ 99 O 01 IS 01 o 1 a U. lo

9z

/I 11 9z 1 04 '^O 9Z. jUSO 11 .W07/ O 04 14 08 OU.UO OA

9Z

/I 9Z 9Z 1lU07/ . 79/Z 9 04 11 .Zw 90 O 04 14 oo O 11 U.J J

9z 9z 1 04 04 'I 1 1 1 1 S 1. 1

J

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9z

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J

OW.WJOS 14 94 OU. 1I'f4

9z /I 11 oo

10"^ 10 9 '^9 11 .Wt 04 O 01 14 OS O 1 A U. lO

9 4 9 Qo 10^ SO %J.WZ 09 11 . 161

0W.WJOS J1 11 .yD OS 0U. 1i 6o

9 4 J

co \(W 00 J.Z9SJ 11 .wo 08 0W.WJOS jj11 .yD OS 0u.uy OQ

9 4 A co 104 77 99 1 1 S 0 04 14 90 0 1

U. 1

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-2 11 9z 74 11 . 70/y O 1 1 U. 1 1 90zy.'f47/ Ou.uyOQ

J 9Z 9Z 1 A1 4A "5 09 11 .y^f 04 O 1 9 W. IZ 98 f^n Zo.O / O 1A U.jO

9Z1 S7 SA 1 J /.DO S SQ 1 09 1 .yz o oe U.Uo 9Q 9Q zo.zy O 91 U.ZJ

J 4 9Z

11 S8 97 JO.Z / A 1 Q 0. ly 1 81 1 .6 J Ou.woOA 98 1 Q Zo. 1 o O 1 0 U. lo

•?

11 Qo 147 74 j.y '\ 07/

11 . 79/z OU.Uo08 zy. 9Q 1luO O 1 7 U. 1 /

'JJ 9 co 140 08 4 '^8 11 .OJ 8S 0w.wy00 96ZO.O8SJ OU. J'+ 14

Qo 146 08 4 96 11 .o^ 89 0w.w07/ 40 07 40HW. JISJ

-5 4 Qo 146 09 6u.wo08 1 77 0w.^u96 98 41 0W. 181 O

J 9z

11 9z 1 1 0'^ 81 11 .H-41

0w.wo06 10 78 OU. 11 6o

J 9Z 9z Z9 1 14 08 4^.yyOQ 11 .JSSJ 0U.UJOS 98Zo.Uj01 11 .y^ 04

-3J 9Z

a 9Z

1 1 S 89 9 QQ z.yy 1 SI 1 .J 1 O OS U.Uj zy.y 9Q Q7/ OU.ZO9A

9Z9Z4 4S 1 SI 1 .J J O OS U.UD 9Q 89 zy.oz O 1 S U. 1

J

'3J

9Z

11 Oo

II1I J.Jo S S8 4't. JZS9 1V.J171 OU.UjOS jU.o 10 8SJ O 1 7 U. 1 / 3 2 2 8 120.16 5.01 1.54 0.09 27.45 0.86

3 2 3 8 115.07 3.53 1.44 0.04 30.13 0.40

3 2 4 8 113.55 2.25 * 29.88 0.16

3 3 1 2 213.29 24.61 2.06 0.09 30.33 0.18

328
LEVEL SPECIMEN LAB TUP

EMEAN

(J)

ESTD

(J)

LFMEAN

(mm)

T FSTD

(mm) (HRC)

HSTD

(HRC)

3 3 2 2 208.17 18.80 * 29.00 0.00

3 3 3 2 198.04 6.82 2.17 0.07 29.25 0.30

3 3 4 2 223.34 25.00 2.21 0.16 29.12 0.18

3 3 1 8 255.46 7.56 2.25 0.12 30.30 0.20

3 3 2 8 259.91 13.14 * 29.11 0.22

3 3 3 8 254.99 7.13 2.27 0.04 29.28 0.16

3 3 4 8 257.93 7.53 2.47 0.04 29.33 0.16

3 4 1 2 162.60 4.15 1.81 0.07 24.55 0.33

3 4 2 2 175.47 4.81 1.94 0.04 22.10 0.47

3 4 3 2 157.88 4.35 1.84 0.04 23.27 0.14

3 4 4 2 167.61 6.69 1.97 0.17 23.30 0.17

3 4 1 8 170.24 9.23 1.79 0.06 24.70 0.26

3 4 2 8 178.40 8.80 1.94 0.06 22.03 0.61

3 4 3 8 162.31 6.37 1.79 0.03 23.22 0.15

3 4 4 8 171.69 8.20 1.89 0.04 23.28 0.16

* Not fractured completely

329

C. Mi-g Wang- arc Jolene D, Splett^

CONSENSUS ^'ALUES AND REFERENCE \"ALUES

ILLUSTRATED BY THE CHARPY MACHINE CERTIFICATIONPROGRAM

REFERENCE: Warg. C. M.. and Splett, J. D., '-Consensus Values and Ref- erence \-alues Illustrated by the Charpy Machine Certification Program,"

Journal of lesung and Evaluazion. JTE\-A. \'ol. 25. Xo. 3, May, 1997. op. 308-314.

ABSTRACT: We present an overvievr of consensus values and reference values

for various situations and apply the computational methods to data from the Charpy

machine certification program administered by the National Institute of Standards and

Technology'. Candidate Charpy machines are certif.ed by comparing a reference %'alue to the average of a set of verincation specimens measired by the candidate machine.

CTirrently, the reference value is the average of three averages observ-ed for measure- ments taken using three Charpy reference machines. Hovrever. the simple average,

which does not account for dLEerences among reference machines, may not be the optimal method of computing the reference value for every situation. We describe four

difierent methods based on the simple average, the average vreighted by sample size, the

weighted average that takes into account machine diuerences. and the weighted average

when machine biases are known, for computing the consensus value and its imcertainty.

We also -oresen: two methods for computing reference values using weighted and un- weighted averages. The techniques and recommendations described in the paper are aonlicable to consens'us value and reference ^"alue problems in general.

KE"\'ll\"ORDS: consensus value, reference value, weighted averages, variance com- nonems. notched-bar testing, reference specimens. Charpy machine certification program, pendul^um impact machines, impact :es::ng

-Mathematical Statistician., Statistical E-gineerlng Division, Information Tecimology Laboratory, Natio-ai Institute of St-andards and Tecimolog;.-, Boulce:. CO 80303.

331
Sometimes it is necessary to combine data from independent testing methods to

determine a single quantity that represents all the data collected. Such is the case in

the Charpy Machine Certification Program administered by the National Institute of

Standards and Technology (NIST). The implementation of the program depends on the

computation of a reference value which represents data collected using three different

Charpy reference machines. The reference value quantifies the breaking strength of a sample of Charpy specimens when no particular reference machine is known to give

the true value.

Basically, the Charpy Machine Certification Program works as follows. NIST obtains a pilot lot of about 100 Charpy specimens from a supplier and measures the

impact toughness of the specimens using one of three Charpy reference machines so that roughly 33 specimens are tested on each machine. Impact toughness is measured /

as the energy absorbed by the specimen (in joules, J) during the test. If a pilot lot

is acceptable, according to contract specifications set forth by NIST, the remainder of

the lot (about 1100 specimxens) is machined and delivered to NIST. NIST then sells the lot of 1100 reference specimens in sets of five to companies who wish to have their

Charpy machines certified. The NIST procedure for qualifying Charpy specimens as reference specimens is similar to ASTM Standard Practice E 1271-88 [1]. To certify a candidate machine, five reference specimens obtained from NIST are broken using the machine under test and the results are sent back to NIST for analysis

along with the broken specimens. A candidate machine is certified, according to limits

specified in ASTM Standard Practice E 23-88 [2], if the average energy absorbed by the

five reference specimens broken on the machine under test is within 1.4 J, or 5% of the

reference value, whichever is greater. The reference value for a single lot is computed

as the average of the individual reference machine averages based on the associated

pilot lot data. NIST currently certifies Charpy machines for "Low" and "High" energy

ranges; the reference value for a typical Low energy lot is between 15.4 and 21.0 J, while High energy reference values are usually between 92.4 and 109.2 J. The certification process can be improved through careful study of the statistical

procedures currently in use. For example, the certification limits described in the

previous paragraph (the average absorbed energy of the candidate machine is within

1.4 J or 5% of the reference value, whichever is greater) are arbitrary, and alternate

limits that take into account pilot lot variability have been proposed [3]. However, the

alternate certification limits are applicable only if we can determine an appropriate

reference value and its uncertainty.

The purpose of this paper is to describe different ways of combining data from

332
independent testing methods for various situations. We first present four methods for

computing a consensus value and its uncertainty when there are significant differences

among the reference machine averages, standard deviations, and sample sizes. The four consensus values are the simple average, the weighted average based on sample size,

the weighted average that takes into account machine differences, and the weighted

average based on known machine biases. Next, we discuss two reference values based on weighted and imweighted averages of individual machine averages. Reference value

and consensus value problems are similar; however, there is an important distinction

between them which will be discussed in detail in the Model section of this paper. The

consensus value and reference value computations are applied to data from the Charpy

machine certification program, and guidelines for use are given.

Although we have used the Charpy Machine Certification Program to illustrate the

methods discussed in this paper, consensus value and reference value computation is common in many fields and the techniques demonstrated here will apply to similar

problems in general.

The Model

The concept of combining data from independent testing methods is intuitively

appealing; however, it is important to formally define the problem in a statistical

framework if we are to examine various procedures for computing a consensus or refer- ence value. The basic statistical model we will use to describe pilot lot measurements

from Charpy reference machines can be written as

y;,- = /i -f &i + €,j (1)

where Yij represents the jth measurement observed for the ith machine. In addition,

i = 1, 2, • • • ,m, where m is the number of reference machines on which measurements

are taken (3 in our case), and ; - 1,2, • • ,ni, where n,- is the number of specimens

broken by the zth machine. For example, a typical pilot lot of 100 specimens might

have the following number of specimens broken on m = 3 reference machines.

Machine (i) Number of Specimens (n.)

1 33

2 34

3 33

The quantity fi is the true average energy absorption inherent in the whole lot of

specimens, bi is the amount by which the true response of the zth machine deviates

333
from the lot average fi, that is, the effect or bias of the ith machine, and e,j is the error associated with a single value. In other words, each observation can be thought of as an overall average plus a machine bias plus random error. We will assume that the Sij are all independent with average 0 and variance af. Sometimes, it is assumed that all are equal, but we will allow the reference machines to have different error variances.

It is critical to understand the various model assumptions and their impact on the

analysis. Two basic assumptions can be made about the reference machines used in the

study: 1) the machines are a random sample from a population of Charpy machines

(random-bias assumption), and 2) the machines are not a sample at all but comprise a

"fixed" (finite) population of interest (fixed-bias assumption). These two assumptions

define the scope of the study. For example, if we assume our reference machines are a random sample from a population of Charpy machines, then the value of fi we are / estimating pertains to the entire population of Charpy machines. In contrast, if our population of reference machines is fixed, the value of fi in eq. (1) is the mean of the

three reference machines. Thus the scope of the study, estimating the mean of the

population versus estimating the mean of a fixed set of machines determines whether

the estimated mean is a consensus value or a reference value. If estimating the mean

of the population is required, then our estimator is called the consensus value, while

reference values are computed when the reference machines are fixed.

Most authors [4, 5, 6, 7, 8] study consensus values based on the random-bias as- sumption, so the bi are normally distributed with average 0 and variance a^. The

quantity cr^ is a measure of the variation among the population of Charpy machines. A

third assumption, that the reference machines are a nonrandom sample from the population of Charpy machines, is a combination of the two previous assumptions since

the reference machines are fixed but the goal of the study is to estimate the mean for

the population of Charpy machines. Eberhardt, Reeve, and Spiegelman [9] derive a consensus value based on the third assumption that the biases bi are fixed and their

bounds are known.

One feature of the random-bias assumption is that when the number of machines m

is very small (say, 2 or 3), the number of degrees of freedom (usually m — 1) associated

with the estimate of is also very small, resulting in an unstable estimate of a^.

Since an estimate of is used in the calculation of the standard error of the estimated

fj,, or in the estimation of fj. itself, the random-bias approach is most useful when m

is relatively large. However, the random-bias assumption can be inappropriate even when m is large if the m machines are not representative of the population of Charpy

machines. Although the fixed-bias assumption is not affected by small values of m, it

334
may not be practical in some situations to completely specify the bias bounds required

to compute the consensus value.

In the Charpy machine certification program, the three reference machines used to evaluate a pilot lot are not randomly selected from the population of Charpy machines,

so the fixed—bias assumption applies. In addition, our estimate of /i in eq. (1), defined

as the true average energy absorption inherent in the whole lot of specimens, appHes

to the three reference machines rather than the entire population of Charpy machines

as in the random-bias case. In other vrords. the purpose of the Charpy program is to compare candidate machines to the three reference machines, so vre are not interested

in estimating the mean of the entire population of Charpy machines; vre are estimating

the mean of the three reference machines so that candidate machines can be adjusted

to match the values produced by the reference machines. Thus o-jx estimate of /x in

the Charpy program is a reference value.

Although estimating fj, in the Chairpy program is clearly a reference value problem,

vre vrill compute the various consensus values using the Charp}' data for illustration.

Consensus \'alues and Weighted Averages

There are many possible estimators of the true absorbed energy p. based on the

model in eq, (1). In this section vre will describe fo-^r different consens^os values (estimators of y.) when data has been collected using diferent Charpy reference machines.

Specifically, a discussion of three random-bias estimators will be followed by the presentation of the fixed-bias estimator of Eberhardt et al. We wiU use ii to denote the true, unknown value of the population parameter (the

true average absorbed energj' of the lot), while /x represents our parameter estimate,

or consensus value, calculated from, observed data. AH proposed estimators of pL have

the form of a weighted average of the individual machine averages Yi,

rr. ^

aiX^ = a-iY-^ — a2Y2 — — arrXm

with the weights summing to 1, that is,

rr. ^Ci = G: — 02 -i r = 1 i=l

where Y. = XI^i Yij/rii is the average of Ui measurements from the ith machine.

It is common to estim^ate p by weighting each machine average based on the sample

335
size Hi •

so that the weights are proportional to the number of measurements used to calculate

the machine average F,. Alternatively, the unweighted estimate of /x

= (3) m

assigns equal weights to each machine average Yi. When all rii are equal, /io and fii are identical.

The third estimator of uses weights that are proportional to the inverse of the

variance of Yi. The variance of Yi is obtained by utilizing the assumption that machine biases are random. Based on this assumption, we know that individual machine

averages Yi are normally distributed with average /x and variance al + erf jrii. In other

words, the variance of Yi is the sum of the variance due to machine differences and

random error variance. This particular estimator of /i m

A = E^«^« (4)

where

\i = Wil^^Wj^ and = 1/var (Yi^ — (^al + cr^/ni^

j=i

has some desirable statistical properties [7]. However, since and are usually

unknown in practice, we have to estimate them based on our sample data before we

can calculate the weights ty,. The sample variance of the ith. machine is

Ui — 1

While computing Sf is straightforward, it is not quite as easy to estimate the betweenmachine variance <j^. Paule and Mandel [4, 10, 11] provide an iterative algorithm for

estimating and we denote their estimate by S^^ . It is also possible to derive estimates

of cr^ analytically based on statistical theory [12]. In particular,

and

1 ^ / \ 2 1 9?

s4 = -r^E (6)

336
correspond to the previously discussed weighted and unweighted consensus value esti- mators fiQ and /ii, respectively. Since the parameter al is always positive, 5^. and

are replaced by 0 whenever they turn out to be negative. (Negative variance estimates can occur for a nimiber of reasons. See [131 for more information.) The estimator based on weights proportional to the mverse of the variance of Yi is

/i2 = f;A,F, (7)

i=l

where

when sample-based estimates are used in place of al and af in eq. (4). The quantity Sl

used to determine the weights is replaced by either 5^. , 5^, , or 5^ . In our application,

the choice of Sl has ver}' Httle effect on the calculated consensus value fi2 because

individual machine sample sizes, variances, and averages are fairly similar. One subtle

difference among the three random-bias estimators of jj. is that the weights used in /ig

are random \'ariables. that is. they depend on actual data, while the weights in jiQ and

jii are nonrandom since they depend only on individual machine sample sizes. The problem, of estimating /i using these three weighted averages has been studied

by many authors. For example, when all are equal (the \'ariances associated with

each, machine are the same) Cochran [6] recommends using /iq if the differences among

machines is negligible [al is small), jii if al is large relative to al, and in intermediate

cases.In a simulation study, Weiler and Culpin 1. compare the variances of jjiQ, fix, and ^.2

for two special cases: 1) the machine variances are equal, a} = u^, and 2) machines are

identical, o"^ = 0. For the first case [a^ = a^), the Weiler and Culpin study indicates

that fj.2 lia^ a- smaller variance than jlo and /or fj.i, except when all are equal. For

the second case {al = 0), Weiler and Culpin provide a rule for deciding among fiQ, and jx2 based on prior information about a^ when m = 2. Thus, the guidelines for

choosing among /io, Aii to estimate the true overall average y. are well defined

when all aj are equal; there are, however, no specific recommendations for the general

case when the machine variances are not equal.

The three estimators [jiQ, /ii, and /ia) of jj. considered thus far can be used when

machine biases are thought to be random. We now consider an estimator of /x that

is based on the fixed-bias assumption. The minimax estimator of proposed by

337
Eberhardt, Reeve, and Spiegelmaji [9] is given by

m

(8)

where the q weights are chosen to minimize the maximum average squared error of fi^

t=i .1=1

subject to the constraints

Ci > 0 and

^Ci = 1

i=l The quantities Li and Ui are the lower and upper bounds of bias bi. The bias bounds Li

and Ui are not estimated from the data, but are known quantities. If the bias bounds

are identical for all machines, fis is simply a weighted average of Yi with weights

Ci -I

Thus As = fi2 for situations in which machines are the same (5^ = 0) and bias bounds

axe equal.

Now that we have defined our four consensus values, we should also present their

associated standard errors. Formulas for computing the standard errors of the four

weighted averages are shown below. Details involving the derivation of the standard

error formulas can be found in [7, 9, 10, 12].

s.e (Ao)

5.e.(/ii)

s.e.[fi2)

y/ET=^nyWi

(9)

(10)

(11)

(12)

338
Consensus Values for Charpy Data

Since there axe known strategies (under the random-bias assumption) for choosing a

specific weighted average when all af are equal, we should determine whether our data

have this property before computing the consensus value. We can use Levene's test [14]

of homogeneity of variances to determine whether the equality of the machine variances

is an acceptable assumption. The hypothesis of interest, called the null hypotheses,

is that the machine variances are equal. (In statistics, a null hypotheses can only be

rejected; we can never prove that the null hypothesis is true.) To perform Levene's

test, we must first compute the test statistic F, which is given by

^ ^

T.7L,n.{Z.-Z) l{m-l)

^^^^

where

rn Zi = ^Zijjni

j=i

m rii m

Next, we need to define the values of test statistic F for which we will reject the null

hypotheses. If the null hypothesis is true and all the machine variances a"^ are equal,

our test statistic F of eq. (13) is distributed as an F distribution with dx =m-\ and

d2 = YJiLii'^i ~ 1) degrees of freedom. Thus, the decision rule for a size a test is to

rtject the null hypothesis and conclude that machine variances are not equal, if

F > Fi-a:di42

where Fi-ardi.dj is the 100(1 - a)th percentile of the F distribution with di and d2

degrees of freedom. In statistical terms, the quantity a usually a small number such as

0.1 or 0.15, represents the probability of incorrectly rejecting the null hypothesis and

concluding that the machine variances are not equal. Also, if we do not reject the null

hypothesis (concluding there is insufficient evidence to indicate that machine variances

are not equal), the common variance cr^ can be estimated by the "pooled" estimator

Y:T=ini-m

339
The pooled variance estimator will be used in place of the individual machine variances

Sf when it is assumed that machine variances are equal. (It is common practice to assume variances are equal if we do not reject the null hypothesis in Levene's test; however, this does not prove that the variances are equal.)

Example 1. This example illustrates the application of Levene's test and the computation of the consensus value and its standard error for a particular pilot lot in

the Charpy machine certification program. The data are from High energy pilot lot HH30. Table 1 contains the summary statistics for each of the three reference Charpy

machines.

TABLE 1-Summary statistics for lot HH30.

Machine 71, (J) Sf (J2)

1 32 93.9963 4.5544

2 33 92.7503 6.2459

3 32 94.5620 4.0058

Levene's test is performed first to determine whether we can assume that machine

variances are equal. The test statistic F for testing af = is 0.9916, and the quantity

-^0.9:2,94 from our F table is 2.3599. Thus, we do not reject the hypothesis that all

are equal because the test statistic is less than Fo.9:2,94- When the machine variances

are equal and the differ only slightly, Weiler and Culpin [7] show that the unweighted

average fli is the "best" random-bias estimator of fi. The consensus value is then given

by

/ii = (93.9963 + 92.7503 + 94.5620)/3 = 93.7695 J. To obtain the standard error of the consensus value, we first obtain the pooled estimate

of a2 = (31 X 4.5544 + 32 x 6.2459 + 31 x 4.0058)/94 = 4.9493

and use this value in eq. (5) to obtain 5"^^ = 0.7132 (the iterative procedure of Paule

and Mandel yields = 0.7073 J^). The standard error of the consensus value is

= -^(0.7132 + 4.9493/32) + (0.7132 + 4.9493/33) + (0.7132 + 4.9493/32)

3 = 0.5374 J.

If the fixed-bias assumption is more appropriate and if we have no prior knowledge

about the bias for each machine, we assign identical lower and upper bias bounds (the

340
actual mirnbers are no: important) for each machine. The '.veishts c, are obtained as

(c-., C2, C3) -- (0.3299. C.3^:2, 0.3299}

and U3 - 93.7590 J with the standard error 0.2259 J. If the following bias bounds

{In, L2. Ls) = (-1.2. -0.9, -0.9;

iU^, U2. l\] = (1.2, 0.9, 0.9)

are specined. the weights are

(ci, C2. C3) = (0, 0.5077, 0.4923)

and /i3 = 93.6422 J Trith the standard error 0.2759 J. The actual bias botmds for

individual machines used at XIST to determine the consensus value of a lot of specimens

are unknown. The bounds used in this example are arbitrary and were specined for

illustrative purposes onlv.

This example demonstrates an interestmg feature of the f_xed-bias problem formulation: it is possible to obtain zero weights. In other words, the zero weight of c-. means

that the data from this machine are not used to determhne the consensus value. From

a statistical standpoint, it is never wise to throw away data -unless there is a physical

or engineering justification, so the possibihty of obtaining zero weights is disfurbing.

However, the developers of the technique do not consider zero weights to be a prcblem.

but rather an indicator that the partic^ilar machine associated with the zero weight is

"relatively poor for deterrnining the true value" 9 . Ultimately, the interpretation of zero weights is application dependent.

Example 2. The Low energy pilot data, summarized in Table 2 for each reference

machine, demonstrates the performance of the four u estimators when the data is unbalanced and machine variances are •mecual.

TABLE 2-Summary statistics for lot LL34.

Machine n, (J)

1 42 19.1751 0.88O7

2 25 17.6799 0.3416

3 40 17.80S~ 0.3870

Table 2 shows that the data are net nearly as balanced as in lot HH30 since is much smaller than n-. and ts. Levene's test statistic for testing af = cr^ is 5.9080. while

341
the value of Fo.9:2,io4 is 2.3544. Therefore, we reject the null hypothesis and conclude

that the machine variances are not equal since the test statistic is larger than Fo.9:2,io4-

Under these circumstances, there is no rule for choosing among the possible consensus

values when machine biases are random. However, by examining the weights used to compute the various estimators, we can decide which random-bias estimator is best for

this data. Table 3 contains the values of /i^, 5.e.(/ii), and the weights used to calculate

each fii. For completeness, noninformative, or equal, bias bounds were specified to calculate fiz- TABLE 3-Weighted averages and weights.

i k (J) s.e.{fi,) (J) weights

0 18.3149 0.5070 0.3925 0.2336 0.3738

1 18.2212 0.4957 0.3333 0.3333 0.3333

2 18.2170 0.4773 0.3303 0.3339 0.3358

3 18.0573 0.0668 0.2127 0.3264 0.4609

The weights assigned to each Yi for computing (lq are proportional to Ui, so the

weights shown in Table 3 for fiQ are quite similar for Machines 1 and 3 since their sample sizes are nearly equal, and the weight assigned to Machine 2 is small relative to the

other weights because the group sample size is small. Although unequal sample sizes are accounted for in the computation of /io, information on within-machine variation is ignored. The unweighted average fii also ignores information on within-machine variation S,^, since each Yi is assigned equal weight, 0.3333. The Paule-Mandel estimator

fL2 assigns weights to machine averages based on values of Si and S'f /n^ obtained from

the data, so both the between- and within-machine variation, as well as the group

sample size, are accounted for in the computation of jl2. The weights associated with

(l2 in Table 3 are similar for all machines because the small variance of Machine 2

compensates for the small sample size. Since the hypothesis of equal within-machine variance was rejected by Levene's

test, it seems appropriate to use methods, such as the random-bias estimator /i2, which take machine variability into account to calculate the consensus values. The

fixed-bias estimator /is determines the weights based only on values of Sf/rii. Thus

the weight for Machine 3 is the largest because it has small variance while Machine 1, which has relatively high variability, is assigned the smallest weight.

We can compare the performance of the random-bias estimators fio, /ti, and fl2 by simulating data that has the same and similar magnitudes of the within- and

between-machine variances as our LL34 pilot lot data. The simulation study shows

342
that fi2 has smaller variance than /to, and that the variance of jjL\ is nearly as small

as the variance of jx2- Since the variance of (12 was the smallest, the simulation study

supports our conjecture that machine variability should be considered when machine

variances are not equal. While the simulation results are useful in examining a particular data configuration, they cannot be interpreted in a general sense. The estimator

/i3 was not included in the simulation study because it was derived based on different

assumptions than the other estimators. Therefore, the standard error of the fixed-bias

estimator /is is not comparable to the standard errors of the three random-bias esti- mators.

Reference Values for Charpy Data

Although we have demonstrated the computation of consensus values using data

from the Charpy machine certification program, estimating /i for the Charpy program

is actually a reference value problem since a fixed set of reference machines constitutes

the population of interest and the goal of the program is to estimate the mean of the

three reference machines to compare to candidate machine means. We will discuss two

methods of computing the reference value based on weighted and unweighted averages.

The fixed-bias consensus value developed by Eberhardt et al. is a weighted average

that can be used to compute a reference value

771

i-1

by specifying identical bias bounds so that the weights are

If the individual machine variances are assumed to be equal, they can be replaced with

the pooled variance 5^ so the weights would only be dependent on the group sample

sizes Tii. In the Charpy machine certification program, the reference value is currently computed as the unweighted average of the three reference machine averages

As = ^^^^ (14) m

If individual sample sizes are equal and machine variances are assumed to be equal,

then the weighted and unweighted averages will result in the same reference value.

343
While the formulas for computing unweighted average in the reference value problem

in eq. (14) and in the consensus value problem in eq. (3) are identical, the standard

errors of the two estimates are different since the scope of the consensus value problem

is different from the scope of the reference value problem. The standard errors of fb^ and /is

s.e.{fl^) ^ I ^ =

(15)

s.e.{fi5) = 16

m

are the same as for s.e.{fi2) and 5.e.(/ii), respectively, when the machines are identical

(5^ = 0). The individual machine variances in eqs. (15) and (16) can be replaced with

the pooled variance S^, if we can assume that machine variances are equal.

Example 1. Computing the reference value for pilot lot HH30 is straightforward.

Based on the results of Levene's test performed in the previous section, we will assume

that the individual machine variances are equal, so the pooled variance, — 4.9493

J, is used to determine the weights

(ci, C2, C3) = (0.3299, 0.3402, 0.3299)

and the reference value is given by

/i4 = (0.3299 X 93.9963) + (0.3402 x 92.7503) + (0.3299 x 94.5620) = 93.8946 J,

with standard error ^r^. ^

1

1 ^(4.9493/32)-! +

(4.9493/33)-i +(4.9493/32)-i = 0.2258 J.

The reference value computed as the unweighted average of the three reference

machine averages is fir, = 93.7695 J which corresponds to fii computed in the previous

section. The standard error of /is is

sJ{Slln,)^{Slln,)-\-{Slln,)

s.e.[fis) - m

^(4.9493/32) + (4.9493/33) + (4.9493/32)

m

0.2259 J.

3A4
Weighting the machine averages does not appear to affect the reference value significantly for this pilot lot probably because the pooled variance was used and the sample

sizes were similar for the three machines. The standard errors of the two reference

values are nearly identical as well.

Example 2. The Low energy pilot lot, LL22, was found to have significantly different

variances among reference machines, so we will use individual machine variances rather

than the pooled variance in the calculations. Since the individual machine variances

and sample sizes are different, we expect the weighted average to be slightly more

precise (smaller standard error) than the unweighted average.

The weights used to compute fi^

(ci, C2, C3) = (0.2127, 0.3264, 0.4609)

are identical to the weights used to determine the fixed-bias consensus value fi^ in the

previous section, so the weighted reference value, fi^ = 18.0573 J and its standard error,

s.e.[fii) = 0.0668 J also correspond to the previously computed values. The unweighted

reference value fis — 18.2212 J, has a larger standard error, 5.e.(/i5) = 0.0702 J, than

the weighted reference value as expected.

In the Charpy program, most pilot lots have similar individual machine sample

sizes and variances, so the unweighted and weighted reference values would produce

virtually the same result.

Discussion

We have defined consensus values and reference values with respect to their model

assumptions. Four methods for estimating the consensus value and two methods for

computing the reference value were outlined, and their application was illustrated for

Charpy machine certification program data.

A general recommendation regarding the use of any particular consensus value is not possible because one estimator is not uniformly better than the rest. Thus we

can only examine the relative performajice of the estimators and choose one that is adequate for the problem at hand. The unweighted consensus value fii is probably

satisfactory for many applications in which group sample sizes and variances are similar. However, it is also beneficial to compute the other types of estimators because the

imweighted average may be less efficient in cases where the data is very unbalanced

and/or the group variances are not equal. For example, if /ii, ft2 and their associated

345
weights are similax, we have additional assurance that the unweighted average /ii is performing adequately. In situations where some information is known regarding the

physics of the testing methods such that bias bounds can be derived, the fixed-bias consensus value (I3 may be optimal, especially when the number of machines is small.

Acknowledgments

The authors thank Dom Vecchia and Chris McCowan of the National Institute of

Standards and Technology, and Hari Iyer of Colorado State University for their input

during the preparation of this document. This work is a contribution of the National

Institute of Standards and Technology and is not subject to copyright in the United

States.

References

[1] ASTM E 1271-88, "Standard Practice for Qualifying Charpy Verification Specimens of Heat-Treated Steel, " pp. 1-7.

[2] ASTM E 23-88, "Standard Test Methods for Notched Bar Impact Testing of

Metallic Materials," pp. 198-213.

[3] Splett, J. D. and Wang, J. C.-M., "Proposed Changes to Charpy V-Notch Machine Certification Requirements," Pendulum Impact Machines: Procedures

and Specimens for Verification, ASTM STP 1248, T. A. Siewert and A. K.

Schmieder, Eds., American Society for Testing and Materials, Philadelphia,

1995, pp. 182-194.

[4] Mandel, J. and Paule, R. C, "Interlaboratory Evaluation of a Material with Unequal Numbers of Replicates," Analytical Chemistry, 42(11), 1970, pp. 1194-

1197.

[5] Schiller, S. B. and Eberhardt, K. R. "Combining Data from Independent Chemical

Analysis Methods," Spectrochimica Acta, 46B, 1991, pp. 1607-1613.

[6] Cochran, W. C, "The Combination of Estimates From Different Experiments,"

Biometrics, 10, 1954, pp. 101-129.

[7] Weiler, H. and Culpin, D., "On the Variance of Weighted Means," Technometrics,

12(4), 1970, pp. 757-773.

[8] Birkes, D., Seely, J. and Azzam, A.-M., "An Efficient Estimator of the Mean in a Two-Stage Nested Model," Technometrics, 23(2), 1981, pp. 143-148.

346
[9] Eberhardt, K. R., Reeve, C. P. and Spiegelman, C. H., "A Minimax Approach to Combining Means, With Practical Examples," Chemometrics and Intelligent

Laboratory Systems, 5, 1989, pp. 129-148.

[10] Paula, R. C. and Mandel, J. "Consensus Values and Weighting Factors," Journal

of Research of the National Bureau of Standards, 87(5), 1982, pp. 377-385.

[11] Paule, R. C. and Mandel, J., "Consensus Values, Regressions, and Weighting

Factors," Journal of Research of the National Institute of Standards and Technology, 94(3), 1989, pp. 197-203.

[12] Rao, P. S. R. S. and Sylvestre, E. A., "ANOVA and MINQUE Type of Estimators for the One-Way Random Effects Model," Communication in Statistics - Theory and Methods, 13(14), 1984, pp. 1667-1673.

[13] Searle, S. R., Casella, G. and McCulloch, C. E., Variance Components, John Wiley

& Sons, Inc., New York, 1992.

[14] Levene, H., "Robust Tests for Equality of Variances," Contributions to Probability and Statistics: Essays in Honor of H. Hotelling, I. Olldn, Ed., Stanford

University Press, 1960.

347

Jolene D. Splett^ and C. Ming Wang^

PROPOSED CHANGES TO CHARPY V-NOTCH MACHINE

CERTIFICATION REQUIREMENTS

REFERENCE: Splett, J. D., and Wang, C. M., "Proposed Changes to Charpy

V—Notch Machine Certification Requirements," Pendulum Impact

Machines: Procedures and Specimens for Verification, ASTM STP 1248 , Thomas A.

Siewert and A. Karl Schmieder, Eds., p. 182, American Society for Testing and Materials, Philadelphia, 1995.

ABSTRACT: In 1989 the administration of the Charpy V-Notch Certification

Program was assumed by the National Institute of Standards and Technology. The

United States Army originated the program to insure the measurement integrity of

Charpy V-notch machines across the country. The program has been operated for many years using candidate machine acceptance limits which can possibly be traced

to a 1955 paper by DriscoU, however, the original statistical justification for using

these acceptance criteria has been lost or never existed. A statistical analysis of recent

certification program data indicates that the existing candidate machine acceptance

limits should be modified. In this paper, we will discuss and justify potential changes

to candidate machine acceptance limits.

KEYWORDS: notched-bar testing, reference specimens, Charpy V-notch machine

certification program, pendulum impact machines, impact testing

The Charpy V-Notch Machine Certification Program has been operating for many

years to ensure the measurement quality of Charpy machines across the country. Basically, the program works as follows. The National Institute of Standards and Technology (NIST) obtains a pilot lot of 100 specimens from a supplier and measures the

•'Mathematical Statistician, Statistical Engineering Division, Information Technology Laboratory, National Institute of Standards and Technology, Boulder, CO 80303.

349
impact toughness of the specimens using three reference machines. Impact toughness is measured as absorbed energy in the test. If the 100 measurements meet certain criteria,

then the remainder of the lot is machined and sent to NIST where an additional 30

specimens, called the confirm lot, are randomly selected and broken. If the absorbed

energy of the confirm lot is in agreement with the absorbed energy of the pilot lot,

the lot is certified as a reference material by NIST. Sets of five reference specimens

are then sold to companies that want to certify their candidate Charpy machine. The

specimens are broken using the candidate machine and the broken specimens, along

with their observed absorbed energies, are sent to NIST for analysis. If the specimens

and measurements are satisfactory, then the candidate Charpy machine can be certified.

The origins of the current Charpy V-Notch Machine Certification Program can be

traced to a 1955 paper by Driscoll [1]. Evidently, the reliability of Charpy measurements

was in question and people were reluctant to use Charpy machines for acceptance

testing, so Driscoll conducted a large-scale study to prove that Charpy machines were

reliable. Driscoll realized that the key to demonstrating reliability and reproducibility

for this type of destructive testing was in the homogeneity of the material used in the

study, so he was able to show that the Charpy machines were in fact reliable by carefully

selecting homogeneous specimens. Driscoll's contribution to the certification program

is observed in the general limits which encompass the majority of absorbed energy

averages from samples of size five for the Low and High energy ranges. Specifically,

most of the Low energy samples had an average absorbed energy withm 1.4 Joules (J)

of the lot average, while the average absorbed energy of five High energy specimens was usually within 5% of the lot average. These limits are identical to those used in

the current certification program to pass or fail a company's Charpy machine.

Various aspects of the current program merit careful study if we are to understand

and improve the performance of the certification process. For instance, the number

of specimens in both the pilot lot and the candidate machine's verification set, and

the definition of a single reference value from measurements on two or more "stan- dard" machines, ultimately affect the error rates associated with classifying machines

as conforming or nonconforming. In this article, however, we limit our attention to the

properties of the current criteria by which machines are judged acceptable.

The main concern regarding the present certification criteria is the arbitrary nature

of the acceptance limits. The justification for using these limits is unknown and does

not appear to be linked to any statistical reasoning or evaluation of their performance.

We intend to demonstrate the statistical properties of the existing limits as well as il- lustrate potential new candidate machine certification limits which are more objective

and statistically defensible.

ASSUMPTIONS AND DEFINITIONS

Several assumptions regarding the certification program in general are necessary

to simplify the discussion in this paper. While somewhat artificial, these assumptions nevertheless serve to illustrate cind explain the potential shortcomings in current

evaluation procedures.

For demonstration purposes, we will assume that the three reference machines used

350
to determine the reference value of the lot are basically identical. Under this assiimption, the process of determining a reference value can be simplified, hypothetically, by

assuming that all 100 pilot lot specimens are measured on one "standard" machine.

Furthermore, we assume that the pilot lot is a representative sample of the machined

lot of specimens. Thus the unknown average absorbed energy of ail specimens in a lot can be estimated by breaking the random sample of 100 specimens known as the Dilot

lot. The computed average absorbed energy of the pilot lot serves as o-.ir estimate of

the true absorbed energy of the lot. Because of the destructive nature of the Charpy test, we are ^unable to separate

the sources of variation inherent in testing. Specifically, the inherent variation among

specimens in a lot cannot be distinguished from measurement error variation associated

with operators, ambient conditions, and other noise sources. We will define the sum of

these two sources of variation as the system variation, a^, which can be written as

where is the true specimen variation and is the true variation associated with,

measurement errors. Since specimen in h omogeneity is a critical component of the

system variation, it is clear that one way to minimize cr^ is to reduce the specimen

variation.

Under our simplifying assumptions, the NIST system variance would be estimated

by computing the sample variance of the pilot lot, S^, while the candidate machine's

system variation would be estimated by the sample variance of five reference specimens,

5|, measured on the machine under test. Since the pilot lot and the five reference

specimens, or verification set, come from the same lot, the specimen variance, a^, is the same for every machine tested on a single lot. In addition, we will assume that both

the absorbed energy and measurement error distributions can be well-approximated by

Gaussian distributions, and that the variance of the candidate machine's measurement

errors is the same as that of the NIST measurement system.

Finally, we make no provision for the occurrence of outHers in the calculations and

results presented below. (In actual practice, a value is considered an outher if there

is physical evidence to indicate that a test result is '"bad".) While outliers are an important issue, and their detection and treatment deserves more consideration, the only

effect of generating outHers in our hypothetical situation would be to reduce the size

of the verification set. EXISTING LOW ENERGY CERTIFICATION LIMITS

Since the existing certification acceptance criterion are cifierent for the Low and

High energy ranges, we will discuss the two cases separately, starting with the Low

energy specifications. A candidate Charpy machine is certified in the Low energy

range if the measured values of five reference specimens meet two conditions: (i) the

average of the five measurements is within 1.4J of the reference value, and (ii) the

range of the five measurements is not greater than 2.8 J. {The range of a sample is the

difference between the largest and smallest measurements.) If the five measiirements

351
from the Charpy machine under test are denoted by Ci, C2, C3, C4, C5, then the Low

energy certification criteria can be expressed as: -lAJ <P -C <1AJ and max {CJ - min {CJ < 2.8J (2)

where P denotes the average of the 100 pilot lot specimens, and C is the average of

the five reference specimens measured on the Charpy machine under test. To understand the implications of the current Low energy certification criteria,

we computed the probability of certifying a candidate machine based on the limits

in (2) under the assumptions outlined in the previous section. The probability of

certification depends on three parameters: the unknown average absorbed energy of

the lot as defined by the NIST system, fip, the unknown average absorbed energy of

the lot according to the candidate machine, fj,c, and the system standard deviation,

cr, which is assumed to be the same for both NIST and the candidate machine. In other words, we calculated the probability that a machine will be certified (that is, the

probability of observing \P — C\ < 1.4 J and range{Ci} < 2. 8 J) for specified values

of the parameters fj,p, ^c, and a. By specifying the parameters, we know a priori if the "simulated" candidate machine is behaving properly or not, but random variation

makes it impossible to correctly classify a candidate machine as "good" or "bad" all the time. The probability of certification is given by

Pr [-1.4J < P - C < 1.4J, range{CJ < 2.8 j] -

where 6 = fip — fMc, <f>{x) — (27r)~-^/^e~^^'^^ is the standard Gaussian probability distribution, and $(x) = <j){z) dz. The probability of certification, calculated using numerical integration routines in IMSL [2], is shown in Fig. 1 as a function of \S\ = \fXp — fid

and a. (The probability is a symmetric function of S.)

The horizontal axis in Fig. 1 represents the magnitude of the difference between the

true average absorbed energy of specimens measured on a single candidate machine and

specimens measured on the NIST reference machine. The vertical line at 1.4 J in Fig.

1 is added as a point of reference; it denotes the point at which the difference between

NIST and candidate machine sample average absorbed energies separates "good" and

"bad" candidate machines according to the current certification criteria. Because of

system variation, a "good" machine may fail, or a "bad" machine may pass purely

by chance, and the probability of either type of error can be read from Fig. 1. For

example, if \fj,p — /xd = 0 and the system standard deviation is high (cr = l.OJ), then

the chance of correctly certifying a candidate machine is only about 72%, even though

the candidate machine is in agreement with the NIST system.

If the current criterion \P — C\ < 1.4J is interpreted to mean that we would like to correctly certify candidate machines 100% of the time if |//p — /id < 1.4J while

correctly failing candidate machines 100% of the time if |/ip — fJ-d > 1-4J, then the

ideal probability curve for certification limits would be a step function. Of course,

perfect certification is possible only in the unrealistic circumstance that a — 0. The

352
curves displayed in Fig. 1 show the chance of making an error, either by certifying a "bad" machine or not certifying a "good" machine, for three reaHstic values of the

system standard deviation a. The increasing frequency of misclassification as the

system standard deviation increases from the smallest value, a = 0.6J, to the largest

standard deviation, cr = 1.0 J, demonstrates that the performance of the certification

criteria is very sensitive to random variation. We will return to this point shortly.

c

.o

COo

0) o**-o

COO

COd

CDd

'St d

\

\

. \ \ \ \ \ \ \ \ \ \ \ \ \ \ \\

\ \

\ w

\ \

a = 0.6J — a = 0.8J

- - o = 1.0J

d

'•.\\

o

d

0 1 2

FIG. l-Probability of certifying a candidate machine tmder the existing Low energy

certification criteria.

EXISTING HIGH ENERGY CERTIFICATION LIMITS

To certify a candidate Charpy machine in the High energy range, the average of the

five reference specimens measured on the candidate machine must be within 5% of the

pilot lot average, or reference value; that is, we require 0.95P < C < l.OoP. In other

words, there is only one certification limit which we may write as:

0.95 < ^

< 105 (3)

353
where P and C are defined as in the Low energy case.

C30 d

c

q< o

CD

•c d 0) O

o

d CDO

CMd

o

d

1.00 1.02 1.04 1.06 1.08 1.10 1.12

FIG. 2-Probability of certifying a candidate machine based on the existing High energy

certification criterion.

To illustrate the properties of the current High energy certification criterion (3),

we calculated the probability of certification for given values of the parameters ^p, /^c, and (J. The parameters are defined as in the Low energy case. The probability of

certification for the High energy criterion is given by

Pr [0.95P <C < l.OSP] =

1.05\/5x iicl^ip - 1.05\ ^ / 0.95\/5x /ij/ip - 0.95', , , - * —77^ —7^Tmr~ r <^(^)

10 CV/\/5 I V 10 CV/n/5

where $(•) and 4){-) were defined in the previous section. While there are three parameters, the certification probability only depends on the ratio, /Xc//ip, and the coefficient

of variation, CV = cr//ip, of the NIST reference machine. CV is a unitless measure of

relative variation which may as well be called the noise-to-signal ratio. Fig. 2 displays

35A
the probability of certification for the High energy range as a function of /ic//^p for

various values of CV that are acceptable according to the specimen manufacturer's

contract. Since the probability of certification is symmetric about the value /ic//ip = 1,

it is sufficient to show the curves for ratios greater than or equal to 1. The horizontal axis in Fig. 2 denotes the ratio of true averages, so machines having

a ratio smaller than 1.05 could be called "good" while a "bad" machine would have

a ratio larger than 1.05. From the figure, we see that the probability of certifying a candidate machine is roughly 0.5 when /Xc//Xp = 1.05, regardless of the value of CV.

In other words, if the average absorbed energy of the machine under test exceeds the

NIST system average by 5%, the candidate machine essentially has a 50% chance of

being certified.

As in the Low energy case, the probability of making an error deteriorates rapidly

with increasing system variation. Fig. 2 shows how the probability of making an error, either by certifying a "bad" machine or not certifying a "good" machine, increases with

CV. For example, if f^dl^p = 1-06 the probability of incorrectly certifying a candidate

machine is roughly 0.1 when CV = 0.017; however, if CV = 0.037 (i.e., a greater

noise-to-signal ratio), the probability of incorrect certification rises to about 0.3. ALTERNATE CERTIFICATION LIMITS

The results illustrated in Figs. 1 and 2 show that system variation, which comprises

both inherent specimen-to-specimen differences as well as laboratory measurement er- rors, is the critical determinant of misclassification rates in the Charpy testing program.

Yet, estimates of system variation (both for NIST and the candidate machine) are not

assigned a prominent role in the existing certification procedure. Currently, the only

part of the procedure which addresses system variation explicitly is the Low energy

bound on the range of the candidate machine's verification set measurements. The

range-test is designed to detect excessive candidate machine system variance, but does

not actually incorporate any of the information from the pilot lot. Similarly, neither

the Low or High energy criteria for comparing the average of the verification set to the reference value presently depend on the estimated NIST system variance, our best

indicator of variability in the current lot of specimens.

An alternative to the current practice is to replace the existing Low and High en- ergy certification procedures by a single protocol that incorporates system variation

into the test procedure in two ways. First, the candidate machine's system variation

would be compared to the NIST system variation by conducting a standard statistical test based on the sample variance of the verification set and the sample variance of

the pilot lot. Because specimens come from the same lot, candidate machine variance

which exceeds the lot variance can only be attributed to excessive measurement error

variability in the candidate machine. The second way of accounting for system variation in the test criteria is to evaluate the difference between the reference value and

the candidate machine's verification set average based on an acceptance limit which

implicitly depends on the actual specimen-to-specimen variation of the cturrent lot, so the computed limit would fluctuate with an estimated system variation based on the

pilot lot and candidate machine data. A test for excessive candidate machine system

355
variability and alternate limits on the average of the verification set measurements are discussed below.

The standard test to compare NIST and candidate machine system variation is the

ratio of the sample variance of the five reference specimens to the sample variance of

the pilot lot. In this application we will only be concerned if the variance of the five

reference specimens is large compared to the variance of the pilot lot. Regardless of

the energy level, the sample variance of the five reference specimens will be considered

too large if the ratio

02 ^ = 4 (4)

is greater than the 100(1 — a)th percentile of the F distribution, denoted by i^i -0:4,99.

Here 5f and denote, respectively, the sample variance of the five reference specimens

broken on the candidate machine and the sample variance of the pilot lot. The values

4 and 99 appearing in (4) are the respective degrees of freedom for and S^, and a

is the probability of erroneously concluding that the system variation of the candidate

machine exceeds that of the NIST system when it does not. Usually a is small; for

example, if a = 0.05 we would claim that the system variation associated with a candidate machine is excessive if the F-ratio is greater than the the threshold value

-^^0.95:4,99 = 2.46. See [3] for more information about statistical tests for comparing

variances.

If the candidate machine passes the variability test, we would assume the system

variation of the candidate machine does not exceed that of NIST, and the second test would be applied. The procedure we have developed is similar to the current Low

energy criterion, but is based on an explicit statistical test of the hypothesis that

IfJ-p — f^c\ ^ d. By the new test, we will conclude that the candidate machine may be

certified \i P — C is in some interval, say [L, U). The acceptable deviation d, which

represents the amount by which the conceptual lot average of the candidate machine is permitted to differ from the NIST lot average, can be adjusted according to program

requirements. The acceptance limits L and U, derived in the Appendix, are given by

U = -L^{d-V 0.4583 • S U-c.:ioz)J (5)

where = {99 + AS^)/103, ti-a:io3 is the 100(1 - a)th percentile of Student's t distribution with 103 degrees of freedom, and a is the probability of failing a "good"

candidate machine.

The particular value of the allowable deviation, d, between the true reference av- erage and that of the machine under test at a given energy level is a choice that is based on engineering judgement. For illustration, we will choose d = 1.4 J to coincide

with the current Low energy acceptance criterion. (No value of d is defined under the

current program for High energy.) The particular value d = 1.4 J is not necessarily the

best Low energy limit for actual use in the certification program. Driscoll [1] implies

that system variation is already accounted for by the value 1.4J. Having specified d, the certification limits in (5) are quite easy to calculate; all that

is required is a t-table. Under the current program, for example, the Low energy limit

356
is d — 1.4J. Supposing that a machine under test has passed the variability test in

(4), and that the pooled standard deviationjs 5_= 0.5J, then the final certification

criterion when a = 0.05 would require that \P -C\ < 1.7803J. The acceptance limit was calculated by substituting d, S and the tabled value to.95:io3 = 1.6598 in (5).

00d

c

o

roo

CD c d CD O•4—o

1q d (0 Si

o

d

o

d

0

G = 0.6J a = 0.8J -- - G= 1.0J

\\\

. \ \

••. \

X

\ \ \

\ \ \

\ \ \

\ \ \

\ \ \

\ \ \

'. \

^

\

•. \ V

\ \ \

\ \ \

\ \

^

\ \ \ \

\ \

\ \

\

1 2

l^ip - ^ic|, Joules

FIG. 3-Probability of certifying a candidate machine at the Low energy level versus the theoretical difference between the reference value and the true candidate machine

average absorbed energy when a = 0.05.

By contrast to the fixed Hmits on P — C in (2), the new criterion based on (5)

takes the variation of the specimens into consideration when computing the acceptance

limits; the more variable the specimens, the wider the acceptance interval. The current

Low energy acceptance interval ( — 1.4J, 1.4J) is believed to be too stringent, resulting

in unsatisfactorily low probabilities of certifying a "good" machine (see Fig. 1). The

probability of certifying a candidate machine using the interval {L,U) based on d =

1.4J is displayed in Fig. 3. In Fig. 3, the probability of certification is quite close to 1.0 regardless of variation

for values of |;Xp — /id less than 1.4J; that is, a "good" machine will almost always

357
be certified. While the probabiHty of certification is still dependent on cr, the main

influence of increasing variability occurs when |/ip — /^d > 1.4J, where the effect of

increasing a is to make it more likely that we will certify a "bad" machine. Again, a step function would be ideal, but progress toward the ideal could only be achieved by

accepting lots with very small specimen variation.

COd

c

o

COo

CD

-c d 0) O*^o

1q d COp

CMd

o

d

C5 = 2.8J a = 3.5J •---o = 4.2J

\\\

•• \ V \\\

\\\

\ \ \

\ \ \

'\ \

\ \ \

•. \ X

\ \ \

\ \ \

\ \ \

' \

^

\ \ \

\ \ \

- , \ \ \

'••\

'\

5 10

||ip - Joules

15

FIG. 4-Probability of certifying a candidate machine at the High energy level versus the theoretical difference between the reference value and the true candidate machine

average absorbed energy when a = 0.05.

The procedure described above can be applied to the High energy case as well.

Assuming that the F-test has been passed, the acceptance limits for the High energy

case would be calculated exactly as they were for the Low energy test, but substituting

any appropriate value of d in (5). For illustrative purposes, we have used d = 5.6J in

(5) and computed the probability of certifying a candidate machine for realistic values

of the variance for High-energy specimens. Fig. 4 displays the probability of certifying

? candidate machine for the High energy case, assuming that the system variation for

the candidate machine is the same as for NIST.

358
The probabiHties shown in Fig. 4 are similar in structure to those observed for the

Low energy case. The probability of certification is nearly 1.0 for "good" machines,

\fJ-p — f^c\ < 5.6J, while the probability of certifying "bad" machines, \fj.p — fid > o.6J,

drops off gradually depending on a. CONCLUSIONS

We have shown that the current limits for certifying candidate Charpy machines

need to be reconsidered, since they are not adjusted for lot-to-lot system variation.

This paper introduced one alternate certification procedure that accounts for system

variation and is more rigorous in its statistical validity. The procedure we have developed could be applied within the existing constraints of the program. There are many

other potential procedures that could be considered for this problem.

Other aspects of the certification program that merit further study include: verifi- cation set sample size, pilot lot sample size, and a method for calculating appropriate

estimates of the lot mean and NIST system standard deviation when there are differ- ences among the three NIST reference machines.

ACKNOWLEDGEMENT

The authors thank Dom Vecchia, Chris McCowan, Dan Vigliotti and Tom Siewert of the National Institute of Standards and Technology for their input during the

preparation of this document. This work is a contribution of the U. S. government,

not subject to copyright in the United States.

APPENDIX

For the Lower energy, the hypothesis we want to test is Hq : \fj,p — fic\ < 1.4J. The

decision rule is to reject Hq \i P — C is not in (L, f/), where L and U are chosen so that

the test has size a. The size of the test is defined as the probability of rejecting Hq

when Hq is true, or the probability of not certifying a "good" machine. Notationally,

we write

Vv\P -C ^ {L, U) 1 - 1.4J < /ip - /ic < 1-4J] = a. Common values of a used in practice are 0.05 and 0.01.

With a pre-determined a, acceptance limits L and U can then be determined from

1-a = Pt[l <P - C <U \ fip- fMc = -1-4:J] = FT[L<P-C<U\fip-fic = lAj] (6)

359
(e.g., see [4], p. 427). Equation (6) can be depicted in Fig. 5 if we assume that P — C

is normally distributed with average //p — //c and variance cr^( 1/100 + 1/5) = 0.21<j^.

-1.4 0 1.4 U

FIG. 5-Acceptance limits L and U with respect to two normal distributions.

From Fig. 5, it is easily seen that U = —L. Furthermore, by making use of the

result that

P - C - (/xp - /.,) \/0^5

is distributed as a Student's t with 103 degrees of freedom, the value of U satisfies

J-IOZ \ ..r.r. r. ) " ^103 I - 1=1-0

0.4583 5 0.4583 5

(7)

where — (995^ + 45^)/103 is the pooled estimate for cr^, and T„(-) is the distribution

function of Student's t with n degrees of freedom. If S is not too large, say 5 < 4,

which should be the case in our problem, a simple solution for (7) is U = 1.4 + 0.4583 5 ii_a:io3 (8)

where ii_a:io3 is the 1 — a quantile of Student's t with 103 degrees of freedom. The

solution can be verified as follows. With the solution in (8), the left-hand side of (7)

reduces to(oj^ + ^'-''^ ~ Txo3(-tl-a:103) = T,0Z (^^^ + t^-<^:lO^ '

With 5 < 4 and commonly used values of a,

( 2.8

^^°Ho.4583 5

+ ^l-a:103^

is very close to 1, and (7) is satisfied.

360
REFERENCES

[1] Driscoll, D. E., "Reproducibility of Charpy Impact Test," Symposium on Impact

Testing, ASTM STP 176, American Society for Testing and Materials, 1955,

pp. 70-74.

[2] IMSL Inc., The IMSL Math/Library, Houston, Texas, 1987.

[3] Walpole, R. E. Introduction to Statistics, Third Edition, Macmillan Publishing

Company, New York, 1982.

[4] Mood, A. M., Graybill, F. A, and Boes, D. C. Introduction to the theory of Statistics, Third Edition, McGraw-Hill Inc., New York, 1974.

[5] Fahey, N. H., "Effects of Variables in Charpy Impact Testing," Materials Research

and Standards, Vol. 1, No. 11, November 1961, pp. 872-876.

[6] ASTM E 1236-91, "Standard Practice for Qualifying Charpy Impact Machines as Reference Machines," pp. 1-6.

[7] ASTM E 23-88, "Standard Test Methods for Notched Bar Impact Testing of

Metallic Materials," pp. 198-213.

361
J. D. Splet^ and C. M. Wan^

Uncertainty in Reference Values for the Charpy V-notch

Verification Program

REFERENCE: Splett, J. D. and Wang, C. M., "Uncertainty in Reference Values for the Charpy Vnotch Verification Program," Journal of Testing and Evaluation, JTEVA, Vol. xx. No. x, December, 2002,

pp. xx-xx.

ABSTRACT: We present a method for computing the combined standard uncertainty for reference values used in the Charpy machine verification program administered by the National Institute of Standards and

Technology. The technique is compliant with the ISO GUM and models the between-machine bias using a Type B distribution. We demonstrate the method using actual data from the Charpy machine verification program. KEYWORDS: Charpy V-notch, impact certification program, impact testing, ISO GUM, notched-bar

testing, reference specimens, uncertainty

For the past 1 3 years, the National Institute of Standards and Technology (NIST) has

administered a program to ensure the measurement integrity of Charpy V-notch machines across

the nation [1]. A brief description of the program follows. NIST obtains a verification set of 75

impact specimens from a manufacturer and measures the impact toughness of each specimen on

one of three "master" Charpy machines. Impact toughness is measured as energy in joules

absorbed by the specimen during the test. If the verification set meets certain criteria, then the

remaining specimens in the production lot will be machined. A sample of 1 5 specimens from the

production lot is then tested on a single master machine to determine if the production lot is in

agreement with the verification set. Once the production lot has been accepted, NIST assigns a

reference value to the lot and sells sets of five specimens to companies who wish to certify their

own Charpy machine. The program is administered within the guidelines of ASTM 1271-88 [2]

and ASTM E23-88 [3].

Several other Charpy machine verification programs exist throughout the world, however they

differ widely from the NIST program [4]. Since there are no intemational standard practices for

verifying Charpy machines, it is important to develop some common ground for comparison.

There is some interest in conducting a long-term interlaboratory comparison of Charpy machines

using a master batch of specimens. To facilitate this comparison, a measure of the uncertainty in

the computed reference value is needed. While other Charpy programs already utilize the

uncertainty of the reference value, ASTM E23 does not provide guidelines for computing this

quantity.

We propose a method for estimating the combined standard uncertainty in the computed

reference value for the NIST Charpy machine verification program and demonstrate the method

* Statistical Engineering Division, National histitute of Standards and Technology, Boulder, CO 80305.

362
using actual data from the verification program. The method provides an uncertainty estimate

that is compliant with NIST [5] and ISO GUM [6] guidelines.

Reference Value and Uncertainty

The reference value is based on verification set data and is defined as ^K^=\h,, (1)

where represents the average absorbed energy observed for the 25 specimens tested on the

master machine.

The combined standard uncertainty of the reference value can be determined by combining

three components of standard uncertainty: within-machine standard uncertainty ), standard

uncertainty due to machine bias ( u{b) ), and the standard uncertainty of specimen homogeneity

( u{h) ). The combined standard uncertainty ( ) is

u^=^u\w)^u\b) + u\h) . (2)

The degrees of freedom associated with each of the three components of uncertainty

{v^,Vi,,v^) can be combined to obtain the effective degrees of freedom using the WelchSatterthwaite formula [6],

^eff ~ 47 \ 4/7 \ 477V

u jw) (b) ^ u jh)

K

The effective degrees of freedom v^^ associated with the total uncertainty are used to determine the appropriate coverage factor for confidence intervals.

Within-Machine Standard Uncertainty

The within-machine standard uncertainty u{w) is based on the "pooled" standard deviation

S

= («l - 1)^1 + («2 - 1)^2 + (»3 - IK ^ (4) ^

\ «j + «2 + «3 - 3

where n^, «2 , «3 are the number of verification specimens tested on each of the three master

machines and , are the associated standard deviations. Typically 25 verification

specimens are tested on each machine, so the within-machine standard uncertainty is

363
The degrees of freedom associated with u{w) are (25 + 25 + 25 - 3) = 72. Since Charpy

testing is destructive, within-machine uncertainty and specimen inhomogeneity cannot be

separated, so u{w) contains both within-machine and specimen inhomogeneity.

Standard Uncertainty Due to Machine Bias : The standard uncertainty due to machine bias accounts for possible bias in the observed

averages associated with each master machine. The value of u(b) can be quantified using a

technique that models the unknown biases with a Type B uncertainty distribution. (See Levenson

et al. [7] for details regarding the technique.) Using observed data for 75 verification specimens

(25 specimens tested on each of the three master machines), a rectangular distribution bounded by

the extremes of the averages of the three master machines is used to model the machine biases so

that w(^) is

u(b) = ' ^"""^^""^ ' , (6)

with degrees of freedom

1 (xV mm -Xmax V/ (^mm) + « (^max)

The quantity 3c^„ corresponds to the smallest average among the three master machines and

"(^min) is the associated uncertainty of x^^ . The largest average among the three master

machines and the associated uncertainty are denoted by x^ and u(x^), respectively. (Note

that u{x^) and w(3c^) do not correspond to the smallest and largest uncertainties among the

three machines. Rather, they are the uncertainties that correspond to the minimum and maximum

averages.)

Standard Uncertainty Due to Specimen Inhomogeneity

The final component of standard uncertainty u{h) can be thought of as a correction for

specimen inhomogeneity and is based on test results for 25 verification specimens broken on a

single master machine and the results for 1 5 production lot specimens tested on the same master

machine. Let //q , cTq be the unknown true mean and standard deviation of absorbed energy (by a master machine) for the specimens in the verification lot, and //, , cr, the corresponding

parameters in the production lot. Let Xq, Sq, X^, and 5", be the sample estimates for ju^, (Tq,

fj^ , and <7^ , respectively. We want to make inferences about //, of the production lot based on

the sair^le estimates Xq , Sq of the verification lot. That is, we want to fmd a standard

364
uncertainty S such that

or

or

?[Xo-2S<jU, <Xq+2S]«0.95,

?[ju, -2S<X,<M, +25']«0.95,

Ml -Mo IS ^ ^Q-Mo ^ Mi-Mo , 2S

_aJ425 ajSs <7o/V25 aJ^lS aJ^fE

= 0.95

.

X — u

Assuming the distribution of —°. is approximately standard nonnal, then

<7o/V25

_2<^0—&<2

<To/V25 = 0.95.

If we know the true values of //q , //, , and (Jq , one can show that an appropriate choice for S is

I

, \Mi-Mo

2aj5

Thus, the uncertainty GqI5 is inflated by a factor of 1 +

^——^

.

2gJ5

Substituting the sample estimates Xq, Xy, and u{Xq) from the verification lot data for the

Once the inflation factor is estimated, we

\X —X

true values, the inflation factor becomes 1 + ' ° 2u{X,)

can use the uncertainty information from all the verification lot data to obtain uQi) . That is.

4u\w) + u\b) 1+ 1^1-^0

= ^lu\w) + u\b) + u\h).

2u{X,)

Thus, if the production lot is accepted, u{h) is calculated using

uih)=ku\w) + u\b)) 0

2u(Xo)

-1

0^ )

(8)

where X^ is the average absorbed energy of the 1 5 production lot specimens tested using a master machine, and X^ is the average absorbed energy of the 25 verification specimens tested on the same master machine. A conservative estimate of the degrees of freedom associated with

5"^ is t>^ = 1 5 - 1 = 14 . If the distributions of results for the 25 verification specimens and the 1

5

365
production lot specimens are in good agreement, then the standard uncertainty due to specimen

inhomogeneity will be small.

Example

The quantitative measurement results for an actual verification set are shown in Table 1 . Fig.

1 displays box plots of the verification set measurements for each master machine. Each box plot

shows the minimum, 25* percentile, median, 75* percentile, and maximum impact energy

observed for each machine.

Table 1 - Verification set measurement results. Machine #1 Machine #2 Machine #3

n, = 25 «2 = 25 «3 = 25

J, =219.782 J =226.761 J J3 =226.408 J

s, = 7.488 J 52 =6.077 J 53 = 6.669 J

w(jci) = 1.498 J w(jc2) = 1.215 J M(jc3) = 1.334 J

250 i —

2A0 T T

-3i 230- [1, i

^ n B 0

1 220 - I T

200J , ^ ,

1 2 3

Master Machine Number

FIG. 1 - Distribution ofverification set measurementsfor each master machine.

The reference value (see equation (1)) associated with the verification set shown in Table 1 is 224.317 J. To compute the combined standard uncertainty associated with the reference value,

366
we must first compute Sp using equation (4) and u{w) from equation (5) (with = 72 degrees

of freedom).

_ j s^+sl+sl _ 1 (7.488)^ +(6.077)^ +(6.669)^ , _ - ^ ^

= 6.769 J

, . Sp 6.769 ^^^^ ^ u(w) = -f£= = .— = 0.782 J

V75 V75

Next, equations (6) and (7) are used to compute u(b) and , respectively, where

=219.782 J, =226.761 J, m(x^„) = 1.498 J, and «(J^) = 1.215 J.

. .

1219.782-226.7611 ^ ^ ^ 2V3 2V3

^^6 =

^ ^ (-^min -^max

)

^ n

(219.782-226.761)' _

^ {2]u\x^) + u\x^) [2) (1.498)' +(1.215)' " ' '

and Vg is rounded down to 6.

If 15 production lot specimens are tested on Machine #3 with Xp^oduction = 223.738 J, then from

equation (8), u{h) (with = 14) is

u(h)=l(u\w) + u\b))

^ \x -X

2 I

I -^Production "^3 2u{x-^)

-1

J

((0.782)' + (2.015)')

r I 223.738-226.408 h

1 + —

V 2(1.334)

-1

= 3.745 J. The combined standard uncertainty (see equation (2)) associated with the reference value is

=Vw'(>v) + w'(^) + m'(/2) =V(0.782)' +(2.015)' +(3.745)' =4.324 J.

The degrees of freedom, calculated using equation (3), are

(4.324)'

u\w) u\b) u\h) (0.782)' (2.015)' (3.745)'

1 1 1 1-. = 20.805,

K 72 14

which roimds down to 20.

Thus, the expanded uncertainty, corresponding to a 95 % confidence interval on the true 367
reference value, is /o.o25,2o(4-324) = 2.086(4.324) = 9.020 J

Conclusions

We have presented a method for computing the combined standard uncertainty of a reference

value for the Charpy machine verification program and have demonstrated the application of the

method for actual data. The method is corrpliant with ISO GUM and NIST uncertainty

guidelines.

By developing a procedure for computing the standard uncertainty of a reference value, we

hope to provide a means for improving the limits used to certify customer Charpy machines. The

certification limits currently in use are somewhat arbitrary and do not account for uncertainty in

the reference value. The development of an uncertainty for a reference value has no practical

effect on the verification program at this time.

Acknowledgements

The authors thank Chris McCowan of the National Institute of Standards and Technology for

sharing his expertise regarding the Charpy program, and Hari Iyer of Colorado State University

for his technical suggestions. This work is a contribution of the National Institute of Standards

and Technology and is not subject to copyright in the United States.

References '

; ;

[1] McCowan, C. N., Siewert, T. A., Vigliotti, D. P., The NIST Charpy V-notch Verification

Program: Overview and Operating Procedures, NIST IR 6618, NIST, Gaithersburg, MD, in

press.

[2] ASTM E 1271-88, "Standard Practice for Qualifying Charpy Verification Specimens of HeatTreated Steel," American Society for Testing and Materials, Philadelphia.

[3] ASTM E 23-88, "Standard Test Methods for Notched Bar Impact Testing of Metallic

Materials," American Society for Testing and Materials, Philadelphia.

[4] EN 10045-2: 1993, "Charpy Impact Test on Metallic Materials - Part 2: Method for the

Verification of Impact Testing Machines," European Committee for Standardization,

Bmssels.

[5] Taylor, B. N., Kuyatt, C. E., Guidelines for Evaluating and Expressing Uncertainty in NIST

Measurement Results, NIST TN 1297, NIST, Gaithersburg, MD, 1994.

[6] International Organization for Standardization (ISO), Guide to the Expression of Uncertainty

in Measurement, International Organization for Standardization (ISO), Geneva, Switzerland,

1993 (corrected and reprinted 1995).

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369

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