NAT L INST. OF STAND & TECH R IX r

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MIST

PUBLICATIONS

10.960-1

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Installing

Maintaining,

Verifying Your

Charpy Impact

Machine

STTR

D.P. Vigliotti

T.A. Siewert

C.N. McCowan

Nisr

National Institute of Standards and Technology

Technology Administration

U.S. Department of Commerce

Special

Publication

960-4

NIST Recommended Practice Guide

Special Publication 960-4

Installing,

Maintaining, and

Verifying Your

Charpy Impact

Machine

D.P. Vigliotti

T.A. Siewert

C.N. McCowan

Materials Science and

Engineering Laboratory

October 2000

Previously issued as NIST

Technical Note 1500-8

U.S. Department of Commerce

Norman Y. Mineta, Secretary

Technology Administration

Dr. Cheryl L. Shavers, Under Secretary of Commerce for Technology

National Institute of Standards and Technology

Raymond G. Kammer, Director
Certain commercial entities, equipment, or materials may be identified in this document in order to describe an experimental procedure or concept adequately.

Such identification is not intended to imply recommendation or endorsement

by the National Institute of Standards and Technology, nor is it intended to imply that the entities, materials, or equipment are necessarily the best avail- able for the purpose.

National Institute of Standards and Technology

Special Publication 960-4

Natl. Inst. Stand. Technol.

Spec. Publ. 960-4

22 pages (October 2000)

CODEN: NSPUE2

U.S. GOVERNMENT PRINTING OFFICE

WASHINGTON: 2000

For sale by the Superintendent of Documents

U.S. Government Printing Office

Washington, DC 20402-9325
Foreword

This Special Publication is a reprint of NIST Technical Note 1500-8, a TN

series started by the Materials Reliability Division. This TN series describes

their division's significant research accomplishments in measurement technol- ogy, reported so that producers and users of materials can improve the quality

and reliability of their products.

This particular Practice Guide provides practical advice on how to correct problems discovered during the testing of Standard Reference Materials 2092,

2096, and 2098 on Charpy impact machines. Although only a small percentage of machines fail to meet the requirements during their annual performance

tests, the failure of a machine can have large economic implications to the

machine's owners, and we try to assist in correcting the problems. From the

study of the fractured specimens and test data that are returned to NIST for

evaluation, we have learned how to identify many of the common problems.

Also, over the years, we have had discussions with thousands of engineers and

technicians at these companies, and have learned the most efficient sequences

for identifying the sources of the problem and for correcting them. Now, we

have collected and organized the various problems and solutions in this Special Publication. We hope that you will find it useful for installing a new

machine correctly, and then for preparing for the annual verification tests. More information on the SP 960 series can be found on the Internet at http://www.nist.gov/practiceguides. This web site includes a complete

list of NIST Practice Guides and ordering information.
Abstract:

The quality of the data developed by pendulum impact machines depends on how well the machines are installed, maintained, and verified. This is the rea- son that ASTM Standard E 23 Standard Test Methods for Notched Bar Impact

Testing of Metallic Materials specifies annual direct and indirect verification

tests. Each year, NIST provides reference specimens for indirect verification of over 1000 machines around the world. From evaluation of the absorbed energies and the fractured specimens, we attempt to deduce the origin of energies

that are outside the ranges permitted by Standard E 23, and report these observations back to the machine owners. This recommended practice summarizes

the bases for these observations, and hopefully will allow machines to be

maintained at higher levels of accuracy. In addition, we provide details of the NIST verification program procedures and the production of the specimens.

Key words:

absorbed energy, Charpy V-notch, impact test, machine repair, misalignment,

mounting, pendulum impact test, verification testing, worn anvils
1. Introduction .1

2. Overviev/ of the NIST Program ............ .2

2.1 Materials .2

2.2 Acceptance Criteria .3

2.2.1 Dimensional Tolerances .3

2.2.2 Impact Energy Requirements .3 2.2.3 The Direction in Which the Specimens

Leave the Machine During Impact Testing ....... A

3. Machine Installation .5

4. Direct Verification .7

5. Indirect Verification . . ,9

5.1 Post-Fracture Examination 9

5.1.1 Worn Anvils 10

5.1.2 Off-Center Specimen .11

5.1.3 Off-Center Striker .12

5.1.4 Uneven Anvil Marks ...................... .12

5.1.5 Chipped Anvils .13

5.1.6 Anvil Relief ..13

5.1.7 Damaged Anvils ........ .14

5.1.8 Bent Pendulum .15

6. Summary ................................ .16

7. Bibliography .17

Introduction

1. Introduction

The low cost and simple configuration of the Charpy impact test have made it a common requirement in codes and standards for critical structures such as pressure vessels and bridges. For many years, engineers and designers have

recognized that materials behave differently when loaded statically than when

loaded dynamically, and that a number of materials have a brittle-to-ductile transition temperature. The Charpy impact test is a very cost-effective method

of evaluating the behavior of materials for applications where these attributes are important.

The history of the pendulum impact test extends back about 100 years. Over

the years, procedure improvements, such as the addition of shrouds to prevent

specimen jamming and the addition of indirect verification to the verification

requirements, have resulted in a simple yet robust test method. The attached

bibliography shows how NIST has contributed to the understanding of the test method, and for those interested, points to a brief history of the test method.

Accurate impact results can be obtained only from machines that are installed

correctly, then remain in good working condition, such as within the tolerances

specified by Standard E 23. Our indirect verification program is referenced in Standard E 23, and supplements the direct verification requirements found

there. Our examination of over 2300 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 (such as the free-swing test). We have learned to recognize which marks on the broken verification speci- mens 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 ther- mometer, or review testing procedures. This recommended practice describes

the most common problems that we detect, and gives advice on how to avoid

or correct most of them. We have divided the description of impact test problems into four major sections: Overview of the NIST Program, Machine

Installation, Direct Verification (evaluation of the machine alone), and Indirect

Verification (evaluation of the machine by the testing of specimens).

While the following sections give suggestions to improve the accuracy of your

impact machine, the machine manufacturer and Standard E 23 also are important sources of information.

1
Overview

2. Overview of the NIST Program

The NIST program focuses on evaluating the performance of pendulum impact

test machines through indirect verification (the production and evaluation of

standardized verification specimens that we distribute to customers of our veri- fication service), as described in ASTM Standard E 23. In chronological order,

the NIST program involves obtaining steel that can be made into verification

specimens, heat treatment and machining of batches of verification specimens,

inspection of representative specimens from each batch to check quality,

assignment of a reference value to each batch, packaging of sets of specimens

for shipment to customers, evaluation of the fractured specimens and customer

test data, and preparation of a certificate of compliance for the customers or suggestions on how to correct any problems. Further details on these tasks are found in the following sections.

The basic materials and procedures currently used by NIST have remained

unchanged for the past 10 years, and date back to the procedures maintained

by the U.S. Army at their arsenal in Watertown, Massachusetts (AMMRC).

2.1 Materials

Two materials are currently used to make the specimens for indirect verification of Charpy impact machines to E 23 specifications. A 4340 steel is used to make specimens for the low- and high-energy levels. A type T-200 maraging

steel is used to make specimens for the super-high-energy level. The steels are purchased as square bar. The bar stock is supplied to subcontractors that machine and heat-treat the impact specimens to meet the NIST

specification. In these steels, the hardness, impact energy, and strength are interrelated. Since

hardness correlates to impact energy and is a more convenient property to measure during processing, it is used as the initial process control. The lowenergy specimens are typically heat-treated to attain a room-temperature hard- ness (HRC) of 45, which corresponds to a Charpy impact energy near 16 J (12

ft-lbf) at -40 °C (-40 °F). The high-energy specimens are typically heat-treated

to attain a room temperature HRC of 32, which corresponds to a Charpy

impact energy near 100 J (65 ft-lbf) at -40 °C (-40 °F). The super-high-energy

specimens are typically heat-treated to attain a room-temperature HRC of 30,

which corresponds to a Charpy impact energy near 220 J (163 ft-lbf) at room

temperature. Note that the two different steels have different responses to heat

treatment, and are tested at different temperatures.

2
Materials

2.2 Acceptance Criteria

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

from a pilot lot of 75 specimens, taken from a heat-treatment batch of 1000 to 1200 specimens. Although impact energy is the desired output, three criteria are combined to determine the acceptability of verification specimens: (1)

dimensional tolerances of specimens, (2) mean and standard deviation of

impact energy, and (3) the direction in which the specimens leave the machine

during impact testing.

2.2.1 Dimensional Tolerances

The dimensional requirements for NIST specimens exceed some of the E 23

specifications for verification specimens. This minimizes variations in impact

energy due to physical variations in the specimens. Also, the notch centering

and the length tolerance for NIST specimens is equivalent to the ISO 164

standard, which permits the specimens to be used in impact machines with

end-centering devices. The NIST requirement for surface finish is equivalent

to the ISO requirement. All of these dimensional requirements can be met with

standard machining practices.

2.2.2 Impact Energy Requirements

NIST lots are currently produced to have average energy values that fall within the ranges of 13 J to 20 J (10 to 15 ft-lbf) for low-energy, 88 J to 136 J (65 to 100 ft-lbf) for high-energy, and 176 J to 244 J (130 to 180 ft-lbf) for

super-high-energy levels. NIST has never produced a low-energy lot having a reference energy of less than 15 J (11 ft-lbf). The upper limit of the super-highenergy range has been limited by NIST because such energies significantly

reduce the final swing velocity for machines with capacities of less than 300 J. (The standard states that machines should not be evaluated with reference

specimens that exceed 80 % of the machine capacity).

The reference energy for a lot of verification specimens is determined from the

pooled average of 75 specimens (25 specimens tested on each of 3 different

impact machines) . In addition to the average impact energy, the standard deviation, and a sample size for the verification set are determined (the number of

specimens required to estimate the performance of a machine with a specified

measurement uncertainty). All 75 specimens are included in these calculations,

except for specimens that are determined to be outliers. An outlier is defined

statistically, but a specimen identified as an outlier is not removed from the

3
Overview

analysis unless it shows physical evidence of jamming, material flaws, or other reason for atypical behavior. At present, any lot with more than 5% outliers is rejected. The E 1271 standard sets a minimum number of specimens for a veri- fication set at five, and the maximum number of specimens in a set is deter- mined by a sample size calculation. Occasionally, a batch has such a small

standard deviation that the equation indicates a minimum set size of four. The average hardness value for a pilot lot is determined for 75 specimens. The

variation in hardness for the lot is also determined, and a lot can be rejected on hardness data alone if the variation in hardness exceeds 1 HRC of the lot average. A variation in hardness of 1 HRC results in a very large variation in absorbed energy. In practice, we find that when the variation in hardness

exceeds 0.5 HRC the quality of the lot is questionable. The correlation

between energy and hardness is much more useful for evaluating variations

of high-energy (lower hardness) specimens, because at high hardness, the slope of the trend decreases significantly.

2.2.3 The Direction in Which the Specimens Leave the Machine

During Impact Testing

The average hardness of the low-energy specimens also can be used to esti- mate whether the specimens will have the desired behavior (leaving the

machine in the forward or backward direction). When the average hardness

value falls below 44 HRC, low-energy specimens begin to exit in a forward

direction, which is undesirable.

Low-energy specimens are designed to exit the machine in the direction opposite to that of the pendulum swing to assure that the shrouds around the anvils

of U-type machines are functioning properly. If the shrouds are not adjusted

properly or are of inadequate hardness, the specimens will jam between the

pendulum and anvils, causing artificially high energy values.

4
Machine Installation

3. Machine Installation

This is the detailed procedure developed by NIST to mount the three Master

Charpy Reference Machines. The manufacturer of your machine should be

able to supply procedures that are designed specifically for your machine.

A stable foundation for the impact machine is critical to ensure accurate

results. Energy losses through the foundation must be kept to a minimum. We

recommend using a foundation of high-strength concrete (if your concrete

supplier uses a commercial-grade description to characterize the quality of

the concrete, you should ask for a 7000-pound mix) that measures 1525 mm

(60 in) long by 910 mm (36 in) wide by 450 mm (18 in) 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 may want

to isolate it from the floor with expansion-joint material.

Hold-down bolts used to secure the machine to the foundation should be of the

inverted "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

strength of grade 8 or higher. We recommend using bolts or rod with a diameter of 22 mm (7/8 in). At NIST we used 22 mm (7/8 in) grade 8 threaded rod,

cut into pieces that are 610 mm (24 in) long. We then welded 150 mm (6 in long) pieces of the same threaded rod to the end of the 610 mm (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 100 mm (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

25 mm (1 in) from the bottom of the hole. The machine was then leveled on

the spacers. Leveling did not need to be as accurate as the final leveling.

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. A second box formation of

reinforcement rods was attached to the "T" bolts 25 cm (10 in) above the first box. The concrete was then poured under the machine. The concrete was fin- ished as level as possible at this time. Before the concrete fully hardened, we

removed enough concrete, to a depth of approximately 25 mm (1 in), 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 72 h.

5
Machine Installation

After 72 h, 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 were 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 the grout from adhering to

it. The machine was then lifted back onto the "T" bolts and was 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 machine. After the machine was leveled, we wrapped the outside of the nuts 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 h. After 72 h, 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 removing all debris from the grout, the machine was repositioned on the "T" bolts and rest- ed 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 12.7 mm (1/2 in)

above the nuts. The nuts were torqued to 380 ft-lb. The final 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 vigliotti@boulder.nist.gov, or by mail at NIST, Division 853, 325 Broadway,

Boulder, CO 80305-3328.

6
Direct Verification

4. Direct Verification

This section of the practice explains the direct verification requirements of

Standard E 23. confirmation that the 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 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 reinspections, we recommend 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.

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 indi- rect verification test. In marginal cases, the movement may still be sufficient

to add a bias to the results that reduces the likelihood of passing. Check the tightness of all bolts, especially the anvil bolts, the striker bolts, and the baseplate 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. We recommend the use of "J" or "T" bolts only. We do not recommend lag-type bolts. These are made to withstand only static loads. We believe that over time, 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 exhibit erroneously high

energy values at the low-energy level. The mounting procedure used to elimi- nate this problem for our 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.

7
W Direct Verification

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).

(We recommend holding the release mechanism 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 11th half swing, the pointer should be

reset to about 5 % of the scale capacity. Record this value and divide by the 1

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.

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, mainte- nance, 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. We

find that radius gages are usually inadequate to measure the critical radii. We

recommend 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. Occasionally, even a new

set of anvils and striker may have incorrect radii. We recommend that new

anvils and strikers 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.

8
Indirect Verification

5. 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 during direct verification, indirect verification serves as an important supplemental test of the machine performance.

We recommend using centering tongs, such as those described in ASTM

Standard E 23. to insert the specimens at the center of the anvils. 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 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 of the 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 quick and

easy calibration media.

5.1 Post-Fracture Examination

Just matching the reference energies is not sufficient to confirm that the machine is fully satisfactor}'. 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 the NIST standardized verification specimens is a good way to identify such

effects. Therefore, the NIST specimens come with a questionnaire (with criti- cal 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 formal response is sent to the customers

.

Following are the most common of these problems. In many cases, suggestions on how to correct or avoid them in the future are included.

9
Indirect Verification

5.1.1 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 wide, smeared

marks are observed on a customer's specimens, we recommend 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 speci- mens 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.

Figure 1. Worn Anvils.

LO
Post-Fracture Examination

5.1.2 Off-Center Specimen

An off-center specimen strike occurs when the notch is not centered between

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 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.

Even fracture surfaces Uneven fracture surfaces Figure 2. Off-Center Specimen.

il
Indirect Verification

5.1.3 Off-Center Striker

This differs from the off-center specimen in that the notch 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. Figure 3 shows this appearance. 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.

Figure 3. Off-Center Striker.

5.1.4 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 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

12
Post-Fracture Examination

that is worn. When the wear is substantial, this condition will produce artifi- cially low subsized energy values. The anvils should be replaced on a machine

with this condition.

LowIll energy specimen showing

subsize wear

High energy specimen showing

subsize wear

Super high energy specimen showing

subsize wear Figure 4. Uneven Anvil Marks.

5.1.5 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 there- fore 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.

Figure 5. Chipped Anvils.

5.1.6 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 veri- fication requirements of ASTM Standard E 23 . The relief has caused highenergy 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.

13
Indirect Verification

^Relief Mark

Figure 6. Anvil with Machined Relief.

5.1.7 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. Rough

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.

Figure 7. Damaged Anvils.

14
Post-Fracture Examination

5.1.8 Bent Pendulum

Figure 8 shows how gouge marks are 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 8, 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 (Figure 9). We recommend that a new pendulum be installed on a machine with this problem.

Figure 8. Bent Pendulum.

Figure 9. Bent Pendulum.

15
6. 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 artifi- cially 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, sug- gested changes in procedure can help to ensure a successful test. To obtain

verification specimens or to clarify procedures for verification testing, you

may use the following information:

Verification specimens can be ordered from the NIST Standard Reference

Materials Program. Phone: (301) 975-6776, fax: (301) 948-3730, or email:

SRMINFO@nist.gov

Questions on verification procedures can be answered by the Charpy Program

Coordinator. Phone: (303) 497-3351, fax: (303) 497-5939, or email:

vigliotti @boulder.nist .gov

16
Bibliography

7. Bibliography

1 ."Absorbed Energy Differences between 8-mm and 2-mm Charpy V-notch

Striker Radii when using Verification Specimens," T.A. Siewert and D.P.

Vigliotti, Pendulum Impact Machines: Procedures and Specimens for

Verification, ASTM STP 1248, American Society for Testing and Materials,

1995, 140-152.

2."Effect of Surface Finish of Charpy Anvils and Striking Bits on Absorbed

Energy," E.A. Ruth, D.P. Vigliotti, and T.A. Siewert, Pendulum Impact

Machines: Procedures and Specimens for Verification, ASTM STP 1248,

American Society for Testing and Materials, 1995, 91-100.

3."Notched Bar Impact Testing Standards have Yielded Widespread Benefits

for Industry," M.P. Manahan, C.N. McCowan, T.A. Siewert, J.M. Holt, F.J. Marsh, and E.A. Ruth, Standardization News, 27 (2): 30-35, February 1999.

4."Performance Verification of Impact Machines for Testing Plastics," T.A.

Siewert, D.P. Vigliotti, L.B. Dirling, and C.N. McCowan, NIST Journal of

Research, 104(6): 1-9, November-December 1999.

5."The History and Importance of Impact Testing," T.A. Siewert, M.P.

Manahan, C.N. McCowan, J.H. Holt, F.J. Marsh, and E.A. Ruth, 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, 2000.

6."Maintaining the Accuracy of Charpy Impact Machines," D.P. Vigliotti, C.N.

McCowan, and T.A. Siewert, 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, 2000.

7."Reference Specimens for Plastics Impact Machines," C.N. McCowan, D.P.

Vigliotti, and T.A. Siewert, 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, 1999.

8 "Experimental Evidence of the Difference Between Total Absorbed Energy

Measured Using an Instrumented Striker and that Obtained Using an Optical

Encoder," M.P. Manahan, Sr., T.A. Siewert, C.N. McCowan, and D.P. Vigliotti, in preparation for Charpy Centenary Conference -CCC2001, Poitiers, France,

2001.

17


October 2000