Institute for Materials Science and Engineering

FRACTUREAND

DEFORMATION

NAS-NRC

Assessment Panel

January 21-22, 1988

NBSIR 87-3613

U.S. Department of Commerce

National Bureau of Standards

Technical Activities

1987

Institute for Materials Science and Engineering

FRACTUREAND

DEFORMATION

H.l. McHenry, Chief

R.P. Reed, Chief until 9/1/87

NAS-NRC

Assessment Panel

January 21-22, 1988

NBSIR 87-3613

U.S. Department of Commerce

National Bureau of Standards

October 1987

Technical Activities

1987
For a uniaxial graphite-fiberreinforced epoxy-matrix

composite, this polar diagram

shows the angular variation of

the Poisson ratio V32. It shows

results for the matrix (0.0), a twenty-volume-percent

composite (0.2), and the pure

fiber (1.0). The dashed lines

reflect negative values; in these directions, a tensile

force would cause a transverse expansion.

Reported by R.D. Kriz and H.M.

Ledbetter, “Elastic

Representation Surfaces of

Unidirectional Graphite/Epoxy

Composites,” in Recent

Advances in Composites in the

United States and Japan,

ASTM STP 864, (Amer. Soc.

Test. Mater., Ph i la., 1985), pp.

661-675.
CONTENTS

INTRODUCTION 1

RESEARCH STAFF 3

TECHNICAL ACTIVITIES

ELASTIC-PLASTIC FRACTURE MECHANICS 7

Fracture Mechanics 9 Nondestructive Evaluation . . . . 16

Welding 22

Thermomechanical Processing 24

FRACTURE MECHANISMS AND ANALYSIS 29

Physical Properties 31

Fracture Physics . 34

Composite Mechanics 36

Cryogenic Materials 45

Material Performance 50

OUTPUTS AND INTERACTIONS

Selected Recent Publications 53

Selected Technical and Professional Committee Leadership ... 62

Industrial and Academic Interactions 65

APPENDIX - ORGANIZATIONAL CHARTS

National Bureau of Standards 71

Institute for Materials Science and Engineering 72

Harry I . McHenry

Chief (from 9/1/87)

(303) 497-3268

DIVISION ORGANIZATION

Richard P. Reed

Chief (until 9/1/87)

(303) 497-3870

ELASTIC-PLASTIC FRACTURE MECHANICS

Fracture Mechanics

Nondestructive Evaluation

Welding

Thermomechanical Processing

FRACTURE MECHANISMS AND ANALYSIS

Fracture Physics

Time -Dependent Properties

Composite Mechanics

Mechanical Metallurgy

Physical Properties

Material Performance

Cryogenic Materials

ADMINISTRATIVE SUPPORT

Administrative Officer

Division Secretary

Group Secretaries, Boulder

Group Secretary, Gaithersburg

David T. Read

(303) 497-3853

Alfred V. Clark, Jr.

(303) 497-3159

Thomas A. Siewert

(303) 497-3523

Yi-Wen Cheng

(303) 497-5545

Ing-Hour Lin

(303) 497-5791

Richard J. Fields

(301) 975-5712

Ronald D. Kriz

(303) 497-3547

George E. Hicho

(301) 975-5707

Hassel M. Ledbetter

(303) 497-3443

Bruce W. Christ

(303) 497-3424

Richard P. Reed

(303) 497-3870

Kathy Sherlock

(303) 497-3288

Heidi Mueller

(303) 497-3251

Virginia Candelaria

(303) 497-5338

Chris King

(303) 497-3290

Judith Vaughan

(301) 975-5704
'

.
INTRODUCTION

Our division conducts research to expand knowledge of the deformation and

fracture behavior of materials, mainly structural alloys, composites, and

weldments. Other research is directed toward improving structural safety

and industrial productivity. Research in fracture mechanics, nondestructive evaluation, and welding technology contributes to structural safety.

The development of fitness - for- service criteria, quality control sensors,

and deformation processing aids industrial productivity. Our research

and technical activities respond to the diverse requirements of many

sponsoring organizations:

Base Program (STRS)

NBS programs and initiatives

Research for other governmental agencies

Technical services

The base program supports the scientific base that underlies the division's goals to improve structural safety and industrial productivity.

Research topics include nonlinear fracture mechanics, acoustoelasticity,

arc physics, fracture physics, composite mechanics, and physical property

modeling

Current NBS programs and initiatives help us to respond to national

needs. Recently, we have been working with the Office for Nondestructive

Evaluation in applying our ultrasonics capabilities to the development of

quality control sensors for monitoring formability of aluminum sheets.

For the NBS Steel Initiative, we have used our simulation and control

techniques to build a thermomechanical processing simulator, which is

being used to study the physical metallurgy of high-strength steels during controlled rolling.

Research for other governmental agencies uses our technical resources to

address division goals and benefit the nation:

Cryogenic Materials : Development and evaluation of materials

for cryogenic service for DoE, NASA, and the Air Force.

Railway Safety : Use of nondestructive evaluation and fracture

mechanics research on wheels and tank cars for the Federal

Railway Administration.

Pressure Vessel Safety. Development of procedures for hazard

recognition, assessment and mitigation for the Navy and for

the Occupational Health and Safety Administration.

Nuclear Safety : Evaluation of the pressurized thermal shock

resistance of steel plates through large-scale crack-arrest

testing for the Nuclear P.egulatory Commission.

1
In addition to our research programs, we provide technical services to

many industrial, academic, and governmental organizations.

Testing Expertise : We assisted the MTS Corporation in

setting up a cryogenic materials testing laboratory at

the Societe Europeen Aerospatiale in France.

Failure Analysis : We investigated failures of power generation equipment for the Potomac Electric Power Company

and of a ship -propuls ion rotor for NOAA's Office of Marine

Operations. We assisted Center for Building Technology in

the investigations of the Moscow embassy and the Bridgeport,

Connecticut building failure.

Test Services : We provide testing services in areas where

we have exceptional capabilities, particularly measurements

at cryogenic temperatures and mechanical tests in the 52 -MNload-capacity machine.

Consultation : In addition to routine interactions with

governmental and private organizations, we provide in-depth

consultation services to the U.S. Navy in the area of

thermomechanical processing of steels, to the Occupational

Health and Safety Administration regarding pressure vessel

safety, and to several DoE, NASA, and DoD laboratories in

the area of cryogenic materials.

This report provides a brief description of our research staff, technical

activities, publications, and interactions. Our technical staff is

interdisciplinary, evenly distributed among the materials, physics, and

engineering disciplines. The technical activities are presented according to the NBS tasks assigned to our division: Elastic—Plastic Fracture

Mechanics and Fracture Mechanisms and Analysis. The publications are another summary of the year's efforts. Leadership positions in professional committees are listed, and numerous industrial and academic interactions are briefly described.

H . I . McHenry

Chief

Fracture and Deformation Division

2
RESEARCH STAFF
_
Austin, Mark W.

Cheng, Yi-Wen

Christ, Bruce W.

Clark, Alfred V. , Jr.

Delgado, Luz M.

deWit, Roland

Fields, Richard J.

Fitting, Dale W.

Heyliger, Paul R.

Hicho, George E.

Kira, Sudook

Kriz, Ronald D.

• Elastic properties

• X-ray diffraction applied to stacking- fault

energy, internal strain, texture

• Fatigue of metals

• Fracture behavior of surface flaws

• Fatigue- life predictions

• Thermomechanical processing of steels

• Data-base management

• Failure analysis of engineering structures

• Mechanical properties of structural metals

• Mechanical durability of motor vehicles

• Theoretical and experimental ultrasonics

• Engineering mechanics

• Nondestructive evaluation

• Mechanical testing

• Compilation of low- temperature reference data

• Defect theory

• Fracture mechanics

• Mechanical properties

• High- temperature materials

• Sensor arrays for NDE of anisotropic and

inhomogeneous media

• Ultrasonic and radiographic NDE

• Signal and image processing

• Computational mechanics

• Finite-element methods

• Mechanics of composite materials

• Mechanical metallurgy

• Ferrous metallurgy

• Failure analysis

• Preparation and certification of standard

reference materials

• Ultrasonic testing and analysis of elastic

properties

• Low- temperature physical properties

• Development of finite-element analysis for

composites

• Mechanics of composite materials

• NDE of composite materials

3
Ledbetter, Hassel M. Physical properties of solids

Theory and measurement of elastic constants

Physical properties of austenitic steels

Measurement and modeling of physical

properties of composite materials

Martensite- transformation theory

Lin, Ing-Hour Fracture - toughening mechanisms

Ductile—brittle transition

Elastic interactions of cracks and defects

Dynamic deformation and fracture

Theory of dislocations and strengthening

mechanisms

Low, Samuel R., Ill Mechanical testing

Instrumentation for dynamic measurements

McCowan, Chris N. Welding metallurgy

Mechanical properties at low temperatures

Metallography and fractography

McHenry, Harry I. Fracture mechanics

Low- temperature materials

Fracture control

Purtscher, Pat T. Fracture properties of high-strength steels

Metallography and fractography using light

and electron microscopy

Mechanical and fracture toughness test

methods

Read, David T. Physics of deformation and fracture

Elastic-plastic fracture mechanics, analysis

and test methods

Mechanical properties of metals

Reed, Richard P. Mechanical properties

Low- temperature materials

Martensitic transformations

Reno, Robert C. Neutron diffraction studies of texture

Electron microscopy

Mossbauer spectroscopy

Schramm, Raymond E. Ultrasonic NDE of welds

Ultrasonic measurement of residual stress

Design of EMATs

Shepherd, Dominique Failure analysis

Metallography

Fractography

4
Shives, T. Robert

Shull, Peter J.

Siewert, Thomas A.

Simon, Nancy J.

Tobler, Ralph L.

Walsh, Robert P.

• Hardness test methods

• Mechanical properties

• Failure analysis

• Materials characterization with capacitivearray and eddy-current sensors

• Electronics for EMATs

• Welding metallurgy of steel

• Gas—metal interactions during welding

• Welding data-base management

• Information retrieval and data-base management

of properties of materials at low temperatures

• Compilation of structural alloy properties at

low temperatures for handbook

• Fracture mechanics

• Fatigue crack growth resistance at low

temperatures

• Low- temperature test standards

• Mechanical properties at low temperatures

• Design and development of mechanical test

equipment

5
.
TECHNICAL ACTIVITIES

ELASTIC-PLASTIC FRACTURE MECHANICS

This task addresses the establishment of a scientific basis for the

development of fracture prevention requirements for structural materials.

Safety and durability are primary considerations in the design and construction of engineering structures. Sophisticated methods to assess structural

safety and durability have been developed on the basis of linear-elastic

fracture mechanics. These analytical methods are applicable to the highstrength alloys used for aerospace vehicles and to the heavy- section steels

used for nuclear pressure vessels. Unfortunately, the methods of linearelastic fracture mechanics do not apply to a wide range of metal structures,

including bridges, pressure vessels, ships, offshore structures, and pipelines, because in these applications, the materials of construction (typically steels and aluminum alloys) are highly plastic before fracture.

Consequently, the assessments of their structural safety and durability are based on empirical methods and prior experience. Although current methods

usually provide a reasonable record of structural safety, improved efficiency and productivity could be achieved if more rational methods were used

to establish material- toughness requirements, allowable stress levels, minimum service temperatures, and weld-quality standards.

Over the years, we have expanded the scope of this task beyond the development of fracture mechanics models to evaluate the significance of flaws in

materials. In 1980, we began research in nondestructive techniques to detect and size flaws and to measure residual stress, the largest uncertainties in the analysis of structural safety. In 1984, the task was expanded

to include welding technology, because welds often have low fracture resistance, a high incidence of flaws, and high residual stress levels.

As the nation became more concerned about international competitiveness, we began to apply the technologies developed in this task to the improvement of

industrial productivity. Using seed money from the Office of Nondestructive

Evaluation, the steel initiative, and our base program, we are redirecting

part of our resources as follows:

Thermomechanical Processing : The servohydraulic equipment,

the computer- simulation and material-modeling capabilities,

and the environmental controls developed for advanced fracture testing are being applied to research on the thermomechanical processing of steel.

Process Control Sensors : The sensor developments, the dataacquisition and signal-processing capabilities, and the

acoustoelastic models developed for nondestructive evaluation are being applied to the development of process control

sensors

Welding Automation : The adaptive controls, the laserimaging and video-processing capabilities, and the process

models used to study the formation of weld defects are being

applied to welding automation and process control.

7
Representative Accomplishments

• In crack-arrest tests on nuclear pressure vessel steels, very large

(10 x 1 x 0.15 m

3

) single-edge-crack tensile plates had crack-arrest

temperatures above the onset of upper shelf behavior. These data show

a rapidly increasing arrest toughness with temperature and provide the

technical basis to extend the ASME Section XI design curve to higher

temperatures, the region of current engineering interest.

• A new model to describe plastic fracture of low- temperature alloys has

been proposed. The fracture of austenitic steels has been examined at

temperatures from 4 to 295 K in tensile and fracture toughness tests.

A critical stress criterion for nucleation that depends on both the applied stress and strain accounts for the increase in fracture toughness

with alloying and the dependence of fracture toughness on yield strength.

• Progress has been made in the application of electromagnetic-acoustic

transducers (EMATs) to materials characterization and defect detection.

EMATs have been fabricated and used to measure ultrasonic velocities in

rolled aluminum- alloy sheet. These velocities correlated well with a measure of formability. In addition, construction is proceeding on two

EMAT-based systems for field application. One system is designed for

measurement of residual stress in the rims of railroad wheels; the other

is intended for roll-by inspection of the treads of railroad wheels.

• A capacitive - array probe was designed, fabricated, and evaluated for use

in materials characterization and nondestructive evaluation. The probe was used to characterize surface features (such as slots and steps) in

dielectric and conducting media. Results were in good agreement with

finite-element calculations, in which the probe's response to a flaw is

computed using a line- integral technique. The probe had good sensitivity

to artificial cracks in dielectric materials and to the thickness and

porosity of thermal barrier coatings.

• The natural frequency for short-circuiting transfer in gas -metal - arc welding has been measured over a wide range of current and voltage. A

stable current regulator was used to eliminate 60-Hz line ripple. The

maximum frequency on the response surface corresponds to the empirically

derived optimum weld condition now used in the welding industry. The

existence of this natural frequency may be the basis for development of

a control algorithm for closed- loop process control.

• A simulator has been built to study the thermomechanical processing of

steels. Its features are: (1) vacuum to 1 x 10' s torr, (2) deformation

rate to 20 mm/mm/s , (3) load capacity of 250 kN, (4) cooling rate (using

helium gas) to 25 K/s , (5) multip le - strike capability, and (6) a 450-kHz

induction heater. The simulator is being used to develop improved procedures for controlled rolling and accelerated cooling of high-strength

steels

.

8
Fracture Mechanics

D. T. Read, R. deWit, R. J. Fields, D. E. Harne , J. Heerens,*

S. R. Low III, J. D. McColskey, H. I. McHenry, P. T. Purtscher,

R. P. Reed

Fracture mechanics relates the fracture resistance of a material containing a crack to the stress and strain fields in the vicinity of the crack.

Fracture occurs when the crack-tip strain fields (the driving force for

fracture) reach a magnitude that exceeds the material's resistance to

fracture. Determination of the critical conditions is complicated by the

numerous variables that influence the fracture process. Because modern

structural metals are sufficiently tough that fracture rarely occurs under linear-elastic strain conditions, we have concentrated on fracture

conditions of plastic strain at and near the crack tip.

Our research this past year can be categorized as studies of fracture

driving force, material fracture resistance, and application of fracture

mechanics to structural safety.

New testing facilities

The new fracture testing laboratory is now operating. We have used the

1-MN machine for a wide-plate test in four-point bending. We have used

the 100-kN machine for many fracture toughness tests, and a servo con- troller has been ordered for the 5-MN (lx 10 6 -lbf) machine.

Driving force for fracture

Weldments with residual stresses. The experimental technique of direct

measurement of the J- integral was adapted to measure J during the cutting

of a notch in a large, thick plate containing a weld in a groove. The

groove weld simulated a repair weld in a pressure vessel (damage, such as pitting or cracking, found during inspection is repaired by grinding out

and welding). The welded plate was instrumented with strain gages, and

then the notch was cut with a special circular saw designed for minimum

interference with the strain gages. After each increment of cut depth,

the strain gages were scanned and the strain relaxations were recorded.

Analysis of the strain data to produce J- integral values had to be approximate because J is not path- independent when the path includes both

base plate and weld metal. However, the measured relaxations of the

strains on the back face of the specimen (opposite the notch) were small.

*Guest worker from GKSS Research Center, Geesthacht, West Germany.

9
This result supported our assumption that the strain changes at the

interface between the weld and base metal were also small. Thus, the

change in the J- integral during the cutting of the notch could be calculated from the measured front- and back- face strains, after adjusting them for their initial value, which was the residual strain from

welding. The results showed that the increase in the J- integral with

crack depth was much larger in the as-welded plate than in the stressrelieved plate. With the mean of the yield strengths of the weld metal

and the base plate used as the residual stress value, the measured

J- integral compared reasonably well with values calculated for surface

cracks in bending.

Fracture mechanics analysis of weldments. In the last two years, the

effects of strength mismatch between weld metal and base plate were studied experimentally in tensile panels. This year, two efforts were

made at modeling the results: the experimental results for surface flaws

in two undermatched welds in tensile panels were compared with simple,

closed-form predictive formulas; and finite-element analyses were performed on two-dimensional, center- cracked tensile panel configurations

with mismatched welds.

The simplified line spring model was used to calculate the J-integral,

and the modified critical COD model, the crack mouth opening displacement. Comparison of the calculations with experimental results showed

that the general behavior was predicted properly by the models. The

models also predict the effect of decreasing the weld yield strength,

namely, a much higher crack driving force at constant applied stress

level. However, the weldment behavior was far from elastic—perfectly

plastic, as assumed by the models. The next modeling effort in these

systems should include the strain-hardening effect, at least in the

undermatched weld metal.

The finite-element analyses considered center-cracked tensile panels

with small cracks (relative length, 0.017) and large cracks (relative

length, 0.10). Each was analyzed with the weld undermatched by 20%,

matched, and overmatched by 20%. Center-cracked panels were chosen to

try to model the main features of the behavior economically, without

concern for the three-dimensional complexities of surface cracks. The

benefit of overmatching was greatest for the small cracks. The applied

J-integral was reduced by up to a factor of 5 at strains several times

yield and by a factor of 2 at yield strain. In the configurations with

large cracks, the benefits of overmatching were much less, because the

effect of the crack outweighed the benefits of overmatching, even though

the overmatching was 20% and the crack was only 10% of the specimen

width. Standard fracture toughness tests do not demonstrate the benefits of overmatching because the specimens have very deep cracks (relative size, 0.5). The driving force studies for the small cracks,

however, show clearly that weld overmatching can be very beneficial.

10
Finite-element analysis of surface cracks in tensile panels. Threedimensional elastic-plastic finite-element analysis of a fully instrumented

test of a tensile panel with a small surface crack gave two significant

results: (1) The direct experimental measurement of the J- integral for

a surface flaw is approximately accurate, even though it cannot take into

account the third-dimensional term that results from unmeasurable strains

in the interior of the plate. The third- dimensional term accounts for only

15%, or less, of the total J-integral. (2) State-of- the-a^t finite -element

analysis can predict the linear-elastic, ligament-yielding ^ and grosssection-yielding portions of the behavior of the specimen, JJ>ut , so far,

cannot predict the observed Liiders band effect. This study was done by

R. H. Dodds, a professor at the University of Illinois, who worked with

D. T. Read this past summer.

Wide-plate crack-arrest testing. We studied dynamic crack propagation and

arrest in wide plates at temperatures up to the upper shelf region with the

goal of predicting the behavior of a nuclear pressure vessel undergoing

pressurized thermal shock.

Three groups of single-edge-cracked tensile specimens were scheduled for

testing in a thermal gradient on the 26 -MN universal testing machine at NBS

The first series of tests was on specimens of HSST A533 Grade B, Class 1,

quenched- and- tempered steel. The second series of tests, now in progress,

is on low upper-shelf 2kCr—IMo steel. A low upper-shelf weld material will

be tested in the third series. Of the twenty-two tests scheduled, twelve

have been completed. The edge-notched plates welded into the middle of the

specimen were supplied by Oak Ridge National Laboratory. The pull plates

and tabs were designed and constructed at NBS.

The thermal gradient in which the specimens are fractured extends from about

—100 to 260°C (—150 to 500°F) across the 1-m specimen width. This is done

so that the crack will initiate in a cold, brittle region and arrest in a hot, tough region. To establish this gradient, a heating and cooling system

was constructed.

The test conditions, specimen configuration, and instrumentation used have

been different for each test. Changes were made in attempts to obtain the

desired crack- run and arrest behavior and to improve the quality of the data

collected. In particular, efforts were made to initiate crack propagation

at lower and lower stress- intensity factors. The results, which show a rapidly increasing arrest temperature, provide the technical basis for ex- tending the ASME Section XI design curve to higher temperatures.

Each test yields a wealth of information. Strain gages were combined and

located for the best determination of crack position as a function of time.

The tape recorder enables recording 28 signals simultaneously. Of these,

20 were strain-gage signals; others were information about loads, crack

mouth opening displacements, accelerometers, timing wires, and acoustic

emission. Generally, these signals are recorded at 76 cm/s (30 in/s) over

the half-hour period of loading until the specimen breaks in two. Hence,

we have recorded signals with microsecond time resolution on about 300 m

(1000 ft) of tape for each test.

11
The strain-gage data were analyzed to determine stress intensity as a function of crack position. This enabled evaluation of the crack-arrest tough- ness whenever the crack stopped. Since the crack arrested and reinitiated

several times, crack arrest toughness values were obtained over a wide range

of temperatures.

The analytical approach was to fit the measured strains to the mode-I cracktip strain singularity, using stress intensity, crack length, and remote

strain as fitting parameters. Despite the complexity of the test and the

rather large distances between most of the strain gages and the crack tip

at any given instant, the first term of the singularity represented the mea- sured strains surprisingly well. The agreement between the form of the

static, linear-elastic, singular, strain peak and the measured strain peak

continued, even though strains several times yield were reached. The

strains were definitely plastic because they were irreversible. A correc- tion procedure was developed to relate the magnitude of the plastic strain

peak to the stress - intensity factor through the use of the J- integral. The

stress - intensity- factor values obtained concurred with handbook values.

The measured strain peak was wider than the peak calculated by a static,

elastic—plastic finite-element analysis. Thus, it appears that the linearelastic singularity coincidentally describes the strains near a moving crack

in an elastic—plastic material better than does the static, elastic—plastic

singularity. These results show the importance of the extensive test instrumentation in enabling an understanding of the phenomena.

Material resistance to fracture

Fracture in the ductile- to-brittle transition region. The fracture tough- ness values of many steels in the ductile - to -brittle transition temperature

range are considerably scattered. Values at the low end of the scatter band

are so low that it is amazing that the steel structures providing the

world's infrastructure survive so well. If the lower-bound toughness values were applied simultaneously with the largest credible cracks and stresses,

failure would be predicted for many structures that have performed well for

many years. 'This paradox can be considered to be an indictment of the usual

fracture mechanics practice, which uses the worst-case conditions. The

challenge is to learn how to account for the scatter in toughness values

when assessing structural safety.

J. Heerens, a guest worker from GKSS Research Center, and D. T. Read investigated the reasons for the scatter of the fracture toughness in the transition region as well as the mechanisms leading to cleavage fracture. The

test material was a quenched-and- tempered pressure -vessel steel, DIN 20

MnMoNi 55. The fracture surfaces of the specimens indicated that cleavage

fracture starts at one small area in the ligament. Cleavage initiation

occurs ahead of the crack tip at the location of the maximum normal stress.

Fractography and metallography show four different types of initiation

sites

:

12
Type A: Cleavage facet at the initiation site

Type B: Unfractured inclusion at the initiation site

Type C: Cluster of inclusions at the initiation site

Type D: Initiation at a local zone of ductile tearing

In the fracture toughness specimens, cleavage crack initiation occurs at the

location of maximum normal stress ahead of the crack tip. The scatter of

fracture toughness values in the upper transition region is due to the scatter of r c , the distance between the fatigue crack tip and the cleavage initiation site.

New ductile fracture model. Over the last fifteen years we have accumulated

a considerable data base on the fracture toughness of austenitic stainless

steels at 4 K. Our experience and understanding of these materials is now

the foundation of attempts to relate toughness to inclusion spacing, interstitial contents, nickel content, and grain size. During the course of discussions of the effect of these material characteristics on toughness, the

need for a mathematical-physical model became evident.

We have compared current theories for the micromechanisms of fracture and

continuum mechanics with the results of our previous fracture mechanics

tests on nitrogen- strengthened austenitic stainless steels at 4 K. These

materials have a relatively high strength at this low temperature; nevertheless, they fracture in a ductile manner, retaining much of the high

toughness that is characteristic of austenitic stainless steels at room

temperature. Metallographic cross sections through the crack tip of tested

fracture mechanics specimens revealed very limited regions of microscopic

cracking in the plastic zones. Nucleation of microvoids occurs during the

blunting stage of the fracture mechanics test at large plastic strains, and

is limited to the region directly ahead of the advancing crack tip. It

accounts for the majority of the energy absorbed in breaking the specimen.

We have developed a model that relates the stress-intensity factor for

nucleation to the inclusion spacing, the critical stress for void nucleation, the material's strength, and the local stress at inclusions produced

by dislocation pile-ups. The model assumes (1) nucleation of a microvoid

is the critical step in the ductile fracture process, (2) void growth

and coalescence occur with little or no additional applied energy, and

(3) nucleation occurs when the interfacial stress between the inclusions

and the matrix exceeds a critical value. This critical stress, ac , is com- posed of a macroscopic applied stress, om , and a local stress, o^, that is a function of the applied strain, e:

°c = a S. + °m

where o$ = ajje 2 and is a coefficient relating strain to stress near

inclusions

^Theories of LeRoy, Embury, Edwards, and Ashby; Hutchinson; and Rice and

Rosengren.

13
From the continuum mechanics analyses by Hutchinson, and Rice and Rosengren,

we know the distribution of strain in the plastic zone ahead of the fatigue

precrack. If we equate the strain at the point of nucleation in the fracture mechanics test to the assumed nucleation condition, we can solve for

a stress-intensity factor for nucleation (Knuc )

:

ac - 3.5ay

Knuc “ (4.5ayEL) li

where L is the average inclusion spacing, Oy is the yield strength, and

E is Young's modulus.

Using values for ct c and a^ estimated from exiting test results, we were

able to calculate the effect of increasing the yield strength on the Knuc

of austenitic stainless steels at 4 K and to compare the prediction to the

measured effect (see figure 1). The slope of the predicted curve and mea- sured data match well for the higher strength levels (> 600 MPa)

.

Figure 1. Measured and predicted dependence of fracture toughness

on yield strength.

14
This indicates that the effect of yield strength on nucleation can account

for the variation of fracture toughness with yield strength in the higher

strength steels. At the lower stress levels (< 600 MPa), the model predicts

a smaller contribution of nucleation to the measured toughness. This agrees

in principle with work done earlier on ductile fracture of low-strength

materials, in which the fracture toughness is related to the growth of

microvoids and the effect of nucleation is assumed to be negligible.

Application of fracture mechanics to repair welds of pressure vessels

Pressure vessels used for navy ship propulsion boilers and for civilian

power plants are always given a post-weld heat- treatment (PWHT) after

fabrication, to relieve residual stresses from welding and to improve the

metallurgical properties of the weldment. Navy boilers are inspected and

repaired repeatedly during their lifetimes, but PWHT of the repair welds

is often difficult, time-consuming, and expensive. Therefore, the Navy

has sponsored research into the benefits of PWHT after repair welding. An

approach based on elastic—plastic fracture mechanics was used to determine

and compare the crack- driving force in the PWHT condition and in the as- welded (AW) condition. Fracture toughness measurements were used to compare

the material fracture resistance in the PWHT and AW conditions. Because we had both toughness and driving force data, we were able to calculate critical crack sizes in both the AW and PWHT condition. The results showed that

the PWHT condition was definitely better because it could tolerate larger

cracks. However, in the AW condition, cracks on the order of a few millimeters in depth could be tolerated. This approach seems to provide a means

for making fitness -for- service decisions on PWHT, which in the past have

been based on "engineering judgment."

15
Nondestructive Evaluation

A. V. Clark, D. W. Fitting, P. R. Heyliger, D. V. Mitrakovid ,

J. C. Moulder, R. C. Reno, R. E. Schramm, P. J. Shull

Substantial progress has been made in application of electromagnetic - acoustic transducers (EMATs) to the practical problems of materials

characterization and defect detection. EMATs are noncontacting devices

(requiring no acoustic coupling); they use a transduction mechanism that

is less efficient than conventional (piezoelectric) transducers. This low

efficiency has limited the use of EMATs.

The signal- to -noise ratios of EMATs have been improved recently; in some cases, they approach the values of piezoelectric transducers. This improved

performance is achieved through the use of high-current amplifiers and lownoise receivers designed and built chiefly by D. V. Mitrakovid, a guest

worker from the University of Belgrade.

This improved EMAT system capability has been exploited in a number of

projects; measurement of formability and texture, measurement of residual

stress, and defect detection in railroad wheels.

Ultrasonic measurement of texture and formability

In the project to measure formability, several sheets of commercial grade

aluminum alloy (used as stock for beverage cans) were obtained from actual

production runs. Data on formability of these sheets was provided by the

manufacturer, with the condition that the proprietary nature of the material

be respected.

The velocities of bulk and guided ultrasonic waves were measured by the NDE

Group and others at NBS -Gaithersburg and Iowa State University who collaborated on this project. Measures of texture were calculated from the ve- locity measurements of each sheet. These calculations were based on recent

theories that model the effect of texture on ultrasonic propagation. As a separate check on both the theoretical models and the ultrasonic measurements, the texture of the sheets was measured with neutron- diffraction pole

figures made by R. C. Reno in Gaithersburg. Measures of texture, such as the quantity u> 420 shown in figure 2, obtained by the different methods were

in good agreement.

As a practical matter, it is more significant that the ultrasonic determination of texture correlates with formability. The texture parameter w420 , as determined by EMAT measurement, correlated reasonably well with the

formability, as shown in figure 3. The formability data were plotted in

arbitrary units to protect the proprietary interest of the aluminum

*Guest Worker from the University of Belgrade, Belgrade, Yugoslavia.

16
manufacturer. An elementary error analysis of the data of figure 3 revealed

that the uncertainty is about the same as the uncertainty in the destructive

measurements currently performed in production.

|cd-420 | x 103 (neutron diffraction)

Figure 2. Comparison of ultrasonic and neutron-diffraction

measurements of one texture parameter. If the data

points agreed exactly, they would fall on the solid

line. The broken line is the linear fit of the data.

|W42O|x(10?)

Figure 3. Comparison of the texture parameter with formability.

The solid line is the linear fit of the data.

17
EMAT inspection of railroad wheels

A new project has been initiated on use of EMATs for roll-by inspection of

railroad wheels. The ultimate goal is construction of an EMAT system for use in a rail yard. For this application, an EMAT unit, consisting of a transmitter and receiver, would be mounted in a rail. As a wheel passes

over the unit, ultrasonic signals would be generated by the EMAT transmitter. These signals, in the form of surface waves, will propagate around

the tread of the wheel and be reflected by planar defects. The reflected

signals will be detected by the receiving EMAT, amplified, and analyzed for

features that correlate with defect size.

To date, several transmitting and receiving EMATs have been designed and

tested for this purpose. For compactness, the transmitting and receiving

EMATs have been integrated into a single search unit. The beam pattern of

the acoustic wave generated by the transmitter has been characterized. A

high-current pulser has been built to drive the transmitter at the frequency

range of interest, and a low-noise amplifier has been constructed that gives

about 80 dB of amplification of the microvolt- level signals detected by the

receiving EMAT.

Tests have been performed on railroad wheels with artificial defects (saw

cuts) in the treads. These defects have been detected by the search unit,

with the unit mounted directly on the wheel and also with the unit mounted

in a rail.

Residual stress measurement with EMATs

To measure residual stress, shear waves are propagated through the stressed

region, and the difference of arrival times of orthogonally polarized waves

is measured. Since the effect of stress on ultrasonic velocity is small, it

is necessary to measure arrival times with a precision of the order of 100

parts per million or better. This, in turn, requires a good signal- tonoise ratio (S/N) in the ultrasonic signals. The low S/N achievable with

most EMAT systems has typically limited their use for measurement of stress

in structural components.

With the new electronics, it has been possible to rapidly characterize the

state of stress in thick steel components, such as welded plates and rims

of railroad wheels. Residual stresses measured on 51-cm- thick welded steel

plates were within 20 percent of the values determined by destructive methods using strain gages. The residual stress in a heat-treated railroad

wheel subjected to drag-braking was also measured. The thickness - averaged

stress was found to be compressive, a result verified by a saw-cutting procedure performed by the American Association of Railroads (AAR)

.

Further research is being done to obtain a method of quantitative residual

stress determination in railroad wheels. If the method is successful,

an EMAT-based system will be built and delivered to the Department of

18
Transportation Test Center (run under contract by the AAR) . Then the system

will undergo further testing by AAR researchers before being placed in

service in a railroad-yard wheel shop.

Capacitive -array sensor

A new type of NDE probe, a capacitive-array sensor, is being developed that

is capable of operating on both dielectric and conductive materials. During

the past year, work on the new probe included (1) design and characterization and (2) investigation of possible applications.

Unlike the eddy current or inductive probe, which responds to the interaction of the magnetic field with the workpiece, the capacitive -array probe

interrogates the material with an electric field. Thus, it can interrogate

surface features in conductive materials and both surface and subsurface

features in dielectric materials.

To characterize the probe, we designed a series of tests on well-defined

surface and subsurface flaws. Notches were cut on the surfaces of a con- ductor and an insulator; their depths ranged from 0.025 to 0.5 mm and their

widths varied. The effects of notch width, lift-off, dielectric constant,

sample grounding, and probe shielding were determined. The effects of notch

depth as the probe scans the surface are shown in figure 4. A model was developed by P. R. Heyliger to predict and interpret these experimental

results. Examples of the model's predictions for probe response to notches

in a dielectric and in a conductor are shown in figure 5.

The response of the probe to subsurface flaws was determined on notches

located from 0.3 to 4.0 mm below the surface (figure 6). The probe not only

detected the flaws, but also small machining defects on the edges of the

flaws 4.0 and 0.3 mm below the surface.

To determine the potential capabilities of this probe and the direction

of this project in the future, we performed some preliminary studies of

applications

1. Ceramics — On 40-/im-wide through cracks in alumina and 4.0-^mwide through cracks in glass, we observed variations in the

signal response due to changes in thickness and porosity of

thermal barrier coatings.

2. Dielectric constant monitoring - We observed two processes:

the curing of five-minute epoxy and the recrystallization of

phenolsolycelate

3. Subsurface flaws in insulators — In plexiglass, we interrogated a flat, disc-shaped flaw that was 5 mm in diameter,

0.1 mm wide, and approximately 2 mm below the surface.

In these preliminary studies, the flaws or processes were clearly observed

with signal- to-noise ratios greater than 10.

19
'
,03mm ,12mm

t i ,25mm .37mm

i ,50mm

1 — -1.5mm !

t

1 t Work Piece

12mm

I J

Figure 4. Response of the capacitive- array probe to a series

of surface notches in a dielectric and a conductor.

Probe to Notch Distance

Figure 5. Calculated response of capacitive-array probe to a notch

(a) in a dielectric (b) in a conductor.
Figure 6. Response of the capacitive-array probe to

subsurface notches in a dielectric.

21
Welding

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

Arc physics

The natural arc frequency in short-circuiting transfer was mapped over a range of current and voltage, as shown in figure 7. Using the current

regulator to filter the line frequency component, we were able to eliminate

the problem that had prevented these data from being developed. The region

of highest transfer frequency corresponds to the empirically derived opti- mum welding conditions that are now used in the welding industry. The

existence of this natural frequency enables the development of an algorithm

for closed- loop control of welding power sources.

The laser backlighting system and high-speed video camera were used to make

precise (±0.5 mm) measurements of the arc length during spray transfer.

These measurements, at a variety of current and voltage combinations (180 A,

27 V to 280 A, 30 V), are being used to refine the models for anode drop

potential in the arc. The high-speed videos also reveal the gas flow

patterns over the weld and the change in electrode shape as a function of

current density.

Figure 7 . Map of natural arc frequency for

short-circuiting droplet transfer.

22
Our exceptional laboratory facilities (current regulator, laser backlighting system, and high-speed video camera) continue to generate cooperative

programs. During the past year, S. Liu of Pennsylvania State University

and M. Shepard of Vanderbilt University have used our laboratory to study

droplet transfer in gas-metal-arc (GMA) welding.

Stainless steel weldments

Our research indicated that inclusion spacing is the major cause of lower

toughness in weldments than in base metal at cryogenic temperatures. To

understand this effect better, two welds were evaluated. An experimental

GMA electrode that had produced a weld with a strength of 1015 MPa and a fracture toughness of 203 MPa-m^ at 4 K was used to produce the welds under

conditions that minimize the oxygen content in welds. Instead of using a standard shielding gas tip with an Ar—2wt.%0 2 gas mix, a special tip was constructed to eliminate the aspiration of air into a 99.995-weight-percent

pure argon atmosphere. The 50-percent reduction in weld oxygen con- centration resulted in a weld with 40 percent better toughness (280 MPa-m^)

at the same strength level (1000 MPa). The toughness of the welds produced substantially exceeded the goals of the U.S. fusion energy program

(150 MPa-m^ at 1000 MPa yield strength), and matched the toughness goals

for base metals (200 MPa-m^ at 1000 MPa yield strength).

Modification of the weld-pool deoxidation products is another way to reduce

the number of inclusions. Aluminum is being evaluated as a deoxidizer and

a possible replacement for the usual silicon deoxidizer. Preliminary tests

indicate that sufficient strength can be obtained by using carbon instead of

nitrogen as the primary strengthener , thus precluding the formation of AiN,

which would weaken the weld.

Real-time radiology

The division hosted a workshop in April to develop a ranked list of national

research priorities in real-time radiology. The twenty-three participants

identified radiation transfer standards, source characterization standards,

and image quantification as the top three needs. These priorities will

guide NBS activities in standard development over the next five years.

23
Thermomechanical Processing of Steels

Y.-W. Cheng, F. J. Fields, H. I. McHenry, D. Shepherd

Increasing the strength of a steel usually reduces its toughness. However,

strength and toughness, especially the low- temperature toughness, can be

increased simultaneously if the strength is realized through ferrite grain

refinement. Further strength improvement with little sacrifice in toughness

is obtained with bainitic structure and copper-precipitation strengthening.

To achieve the optimum combination of strength and toughness of a steel

through copper-precipitation strengthening, it is essential to select and

control the time—temperature—deformation schedule used for hot rolling, the

cooling after rolling, and the subsequent aging cycle. On the basis of fundamental metallurgical principles, a study of the thermomechanical process

can produce the best rolling schedule.

Simulator evaluation

A simulator has been built in the division's laboratory to study the thermomechanical processing of steels. The simulator, shown in figures 8 and 9,

consists of a servohydraulic loading system, a vacuum system, an induction

heater with gas cooling capacity, and a computer system for process control

Figure 8. Hot-deformation simulator.

24
Figure 9. Interior of the vacuum chamber.

and data handling. It has the following features: 250-kN load capacity

with a maximum actuator traveling speed of 50 cm/s; multistrike capability

at different displacements and strain rates; a maximum heating rate of

150°C/s (for a cylindrical steel specimen 1.2 cm in diameter and 19.3 cm

high); a maximum cooling rate of 25°C/s; and vacuum to 1 x 10* 5 torr.

The simulator has been used to study the hot -deformation behavior of the

A710 high-strength, low-alloy steel. Preliminary results indicate that

under moderate straining, the steel does not undergo static recrystallization at 950°C.

Technology Implementation

The Department of Defense (DoD) plans to introduce thermomechanical processing into the U.S. steel industry. Steel companies participating in the DoD

program* will be given incentives to install the necessary equipment in the

form of purchase commitments to purchase thermomechanical ly processed steel

plates

.

Defense Production Act Title III program: Accelerated Cooling Processing

of DoD Steels.

25
We are assisting the U.S. Navy in the design of this program. Also, we are collecting technical data on the thermomechanical processing equipment used in Japan and the properties of the steel plates produced by this

process

Automated. Quantitative Microscopy

Thermomechanically processed steels will be characterized by two commercial,

automated, television—microscopy systems. The systems have been programmed

to measure the content, shape, and size distributions of nonmetallic inclusions in steel. Programs to measure grain shape and size distribution have

also been developed.

Tests have demonstrated the accuracy of these programs for a resulfurized

stainless steel containing a relatively high concentration of MnS inclusions. Inclusion content was measured about twelve hundred times to obtain

a statistically appropriate distribution function (see figure 10) . Neither

of the conventionally assumed distribution functions (normal or log normal)

was valid. At this time, a three -parameter Weibull distribution fits the

data best. The correct distribution function is the basis for effective

use of a smaller number of measurements or for the prediction of a rare

event, such as the occurrence of a deleteriously large inclusion or inclusion agglomeration.

The two-dimensional data collected by this technique have been transformed

by analysis into three-dimensional information. The algorithm worked cor- rectly on an artificially constructed distribution of ellipsoidal inclusions. It has been successfully applied to sulfide inclusions in stainless

steel. This approach is necessary to characterize the prior-austenite grain

shape in steels rolled below the recrystallization temperature.

26

10,000

9,000-

8 ,000 -

7,000-

6,000-50%-

4 th Roof of 4 th Moment

i

5,000-

-8 40%-

4,000-

3,000-25%-

2, 000-

1 , 000-

Toil Detail Enlorged 100* Vertically

0 12 3 4 5 ****** « « — ift m t

O O

-

o

0 i i l

1'* 3

1'* 4 >2

Nominal Area Percent of JK-A (t) Classes

Figure 10. Distribution of inclusion concentration in resulfurized steel

with various statistical measures or distributions for comparison.

175%

FRACTURE MECHANISMS and ANALYSIS

The goals of this task are to improve our understanding of the deformation

and fracture behavior of materials and to apply this understanding to the

assessment and improvement of the performance of materials. The research

is distributed about equally among three groups of materials: metals and

alloys, advanced composites, and cryogenic materials.

Mechanical deformation is composed of shear and dilation components. The

elastic constants, which relate these deformations to stress, are physical

properties; they, in turn, are related to various fundamental solid-state

phenomena, including interatomic potentials, equations of state, and phonon

spectra. Thermodynamics links elastic constants with specific heat, thermal

expansivity, and the Debye temperature. Thus, the cornerstones of our re- search on deformation are measurements and modeling of elastic constants and

related physical properties for the three groups of materials plus the new

oxide superconductors

Dislocation theory, which provides the theoretical basis for plastic deformation, has been helping us to understand fracture. Our work has emphasized

two concepts: (1) dislocations can shield the crack tip from remote stresses

and thereby toughen the material and (2) dislocation emission from the crack

tip blunts the crack and forces crack growth to occur by ductile processes.

Certain techniques used in these studies are proving to be powerful tools

for modeling the fracture behavior of composite materials.

Composite materials have a wide range of mechanical properties that depend

on the individual properties, amounts, and orientations of the constituents.

Optimizing the formulation of a specific composite requires a model that

relates the constitutive properties to the bulk performance. Our research

has compared measurements and observations with the predictions of analytical models for a wide range of composite materials. Recent work has emphasized: (1) determining the residual stress state due to variations in the

thermal expansivity of the components of a composite, (2) predicting the

elastic properties of a composite on the basis of constitutive properties,

and (3) studying the damage accumulation processes that precede fracture.

Characterization of mechanical performance is essential for making the best

choice of structural materials for a given application. Over the years, we have evaluated many materials for various government agencies, and we have

made many contributions to the development of standardized test procedures.

This is particularly true in the area of cryogenic materials, which we have

studied continuously for over thirty years. Our recent work has emphasized

structural and insulating materials for superconducting magnets, copper for

cryogenic magnets, indium for solder joints, and titanium for aerospace

tankage

29
Representative Accomplishments

• Using scattered-piane-wave ensemble - average methods, we developed a new theoretical model to explain the elastic properties of porous ceramics.

Predicting the porous -material elastic constants requires three ingredients: matrix elastic constants, pore volume fraction, and pore shape.

For alumina, assuming disc-shape pores in the model yielded good agreement with measurements. Used inversely, the model yields the void-freematerial elastic constants.

• The new high- critical - temperature metal -oxide superconductors,

R 1 Ba 2 Cu3 0 7 . x(R denotes a trivalent rare-earth cation), were studied by

x-ray diffraction and ultrasonic waves. Superconductor properties depend

strongly on the details of crystal structure and on the species of R

atom, which lies at the unit-cell center. Against expectation, some of

the superconductors, especially Ho—Ba—Cu—0, contain a magnetic phase

together with the superconducting phase.

• By including the effect of applied stresses, which is neglected in traditional dislocation mechanics, the dislocation zone size was found to be

inversely proportional to the net shear resistance of atomic motion. An

expression for the activation energy required to nucleate a dislocation

loop at crack tips in intrinsically brittle materials was derived. The

results will be used to treat the transition from brittle - to-ductile

behavior

.

• As a part of the United States-Japan Cooperative Program for the Development of Cryogenic Mechanical Test Standards, a round-robin test program was conducted to evaluate the draft standards. Tensile and fracture

tests of a Fe—13Cr—22Mn austenitic stainless steel at 4 K were conducted

at seven laboratories in the United States and Japan. Drafts of proposed

standard 4-K tensile and fracture toughness test procedures were written,

reviewed, and revised. A matrix of tests was executed at four laboratories to provide data on test procedures.

• A new method has been used to study the cooling and heating in aluminum

and steel under tensile and cyclic loading; it is based on the measure- ment of infrared emission from the surface of a loaded body. Test procedures have been developed to measure energy changes near the crack tip

and the immediately surrounding area. The technique is also used to

determine the position of the crack tip during cyclic loading.

30
Physical Properties

H. M. Ledbetter, M. W. Austin, S. A. Kim, M. Lei*

Physical properties studied by measurement, analysis, and modeling include

sound velocities, internal friction, thermal expansivity, specific heat,

magnetic susceptibility, volume (mass density), stacking- fault energy, phase

transformation, internal strain (residual stress), and especially, elastic

constants. We study many types of materials: metals and alloys, polymers,

intermetallic compounds, ceramics, and composites, which include particle,

fiber, and cloth reinforcement and both metallic and nonmetallic matrices.

For many studies, the temperature ranges between 295 and 4 K. During the

past year, we focused on three types of materials: austenitic steels,

structural composites, and the new high-critical- temperature metal-oxide

superconductors

Austenitic steels

We developed a new correlation between stacking- fault energy, 7 , and the

monocrystal elastic stiffnesses, C^j . This correlation may permit us to

publish several experimental studies by M. W. Austin: alloying effects of

nickel, manganese, and carbon-plus -nitrogen in Fe—Cr—Ni alloys. If verified, the correlation provides many opportunities for further alloying

studies

We measured and modeled the effect of molybdenum on Fe—Cr—Ni-alloy volume.

We ascribe the large volume change, unexplained by atomic volumes or elastic

constants, to a 4d-electron bcc-paradigm metal located in a 3d-electron fee

crystal structure.

Composites

Working with B. K. D. Gairola of the University of Stuttgart, we used an elastic -continuum model to predict the polycrystalline quasi- isotropic elastic constants of graphite. These calculations provide useful reference

states for studying two materials: graphite fibers in composites; graphite

nodules and flakes in cast iron.

With S. K. Datta of the University of Colorado, we published several studies

that model composite-material interfaces, especially their effect on ultrasonic -wave velocity and attenuation. These results provide a basis for

nondestructive study of interface properties.

*Guest Worker from the Institute of Metal Research, Shenyang, China.

31
For a uniaxial glass -fiber-reinforced epoxy-matrix composite, we measured

and modeled the torsional modulus and associated internal friction. Our

model considered nonhomogeneous fiber distribution.

For SiCp/Al composites, we measured and modeled elastic constants and internal friction. The model included particle -shape effects. Against expectation, internal friction increased with increased particle volume fraction.

We developed a model for the elastic constants of porous ceramics that applies also to porous superconductors (discussed below) . Unlike all previous

studies, our model considered nonspherical pores. For the test case, alumina, we obtained good agreement between observation and prediction if we assumed oblate-spheroidal (disc- shaped) pores (see figure 11). Both spherical and prolate-spheroidal pores fail to explain observation.

CO

Figure 11. For Young's modulus versus pore volume fraction of alumina,

this figure compares three sets of modeling results—for

spheres, prolate- limit cylinders, and discs (c/a - 1/9)

with observation (dashed line)

.

Superconductors

By ultrasonic methods, we measured the elastic constants of the new highcritical- temperature metal-oxide superconductors, especially R 1 Ba2 Cu3 0 7 . x (where R denotes a trivalent rare-earth cation), at temperatures between

295 and 4 K. Below and above the critical temperature, the elastic con- stants behaved irregularly. Some materials showed hysteresis. Figure 12

shows representative results.

32
Figure 12. Relative shear modulus G - pv£ between 275 and 4 K. Above Tc (65 K) , behavior is normal. Below Tc , contrary to expectation,

G increases. Magnetic susceptibility, x> shows an abrupt de- crease below 79 K, a d*/dT maxmimum between 60 and 65 K, and a zero in d

2 x/dT2 at 62 K.

To complement the above study, we measured the low- temperature elastic

constants of BaTi03 , a building block for the superconductor. [To obtain

R1 Ba2 Cu3 0 7 _ x crystal structure, stack three cubic BaTi0 3 unit cells, re- move one 0 atom at (0,0, 1/2), remove another at (1/2, 0,0), and allow re- laxations.] The BaTi03 results showed many surprises, which remain

noninterpreted

As mentioned above under Composites, we successfully applied our plane-wave

scattering ensemble -average models to the porous superconductors. This

approach enabled us to determine the pore- free -superconductor elastic stiff ness values , which are much softer than those for BaTi03

.

Other materials

This year, other physical-property studies included beryllium, indium, a graphite—magnesium composite, a glass—epoxy composite, and cast iron.

33
Fracture Physics

I.-H. Lin, R. M. Thomson*

Research of the Fracture Physics Group is directed toward understanding

(1) the toughness or brittleness of materials in terms of their fundamental

properties and the interactions between cracks and dislocations and (2) the

effects of external chemical environments on cracks in brittle materials.

Brittle- to-ductile fracture transition

We have recently developed a theory of dislocation loop-nucleation under

the influence of applied shear stresses. In it, we have shown that the

activation energy required to nucleate a dislocation loop on a slip plane

is strongly affected by the dislocation core size. For a given applied

shear stress, the dislocation core size, which is inversely proportional to

that stress, alters the critical dislocation loop size, and the activation

energy is sensitive to the dislocation loop size.

This theory has been applied to the study of dislocation- loop emissions

from sharp cracks. Work under way should establish the conditions under

which dislocation- loop emissions occur throughout a zone surrounding the

crack tip during the brittle - to-ductile transition in intrinsically brittle

materials. Furthermore, with these ideas we hope to be able to understand

and model the brittle- to-ductile transition temperature, because heat

assists dislocation- loop emission processes at crack tips. We suggest that

one important factor in the determination of the brittle- to-ductile transition temperature is the competition among the dislocation- loop emissions,

dislocation propagation after nucleation, and shielding crack breakaway.

Model of composite ductile failure

J. P. Hirth and I.-H. Lin have derived an expression for the thermodynamic

force on line- force defects in which line forces describe the incompatibility strain field of a plate-like inclusion, precipitate, or composite

laminate in a stressed matrix. We have begun to study the elastic interactions among inclusions, dislocations, and other defects. With this

approach, we hope to achieve a better understanding of the ductility improvement in particle-reinforced aluminum composites, which we recently

examined by a failure model of composites. In this model, we have assumed

that voids nucleate at the particle—matrix interface and grow to a critical

size at fracture. One adjustable parameter, the critical void size, was

fit to results of uniaxial tensile tests. The model quantitatively predicts

the tensile strains at fracture as a function of the SiC volume fraction.

Preliminary comparisons of the model with observation look promising.

*Institute Scientist, Institute for Materials Science and Engineering.

34
Mixed-mode fracture

Knowledge of crack initiation and stable crack growth in combined mode -I and mode -III fracture is needed to understand ductile fracture in tough

materials. The slope, M, in Jjh-Ji toughness space is a new parameter for

characterizing mixed-mode fracture. To understand this failure, we have

developed a stable-crack-growth model, in which the crack growth is both

controlled by the applied mixed J- integrals and the injection of mixed

superdislocations into the ductile materials from the crack tip. The emitted dislocations account for both the crack- tip plasticity and the crack- tip

opening displacements. The value of M predicted by our simple model is 0.3

for stable crack growth, which is in good agreement with experimental re- sults reported by Schroth et al.

Dynamic crack velocity

We have developed a quasi -static crack model to predict two loci of the

mode -I dynamic crack tip. At these loci, the crack tip induces corresponding maximum and zero difference strains, which are observed by strain gages

located at some height above the crack plane. This calculation is applied

to obtain instantaneous crack velocities in crack-arrest tests. The crack

velocity calculated by this method is very accurate when it is less than

about half the Rayleigh wave speed.

Modeling the effects of a chemical environment

Generally, when modeling a crack-tip configuration, one relies on the

Barenblatt approximation when the continuum is valid or on discrete treatments otherwise. When large external molecules enter a crack, they cannot

penetrate it all the way to the tip. Then a combination of discrete and

continuum views is desirable. When penetration of the external molecules

is incomplete, some very interesting physical consequences occur in the

structure of the tip owing to a wedging effect of the external molecules.

A new theory has been developed for the observed activated crack growth,

which incorporates the discrete effects at the crack tip with the longrange surface forces

35
Composite Mechanics

R. D. Kriz, P. R. Heyliger,* H. M. Ledbetter, M. Lei,t J. c. Moulder

The emphases of our research are (1) the mechanics of fracture, (2) the

mechanics of deformation, (3) nondestructive evaluation of laminated and

fiber-reinforced composite materials, and (4) physical -property measurement

and modeling. We develop models to predict the mechanical behavior of

these materials, and whenever possible, we compare theory with experiment.

Unlike conventional homogeneous isotropic materials, composite materials

can be designed for specific applications by controlling their wide range

of parameters, which include fiber volume fraction, interface properties,

fiber orientation, laminate stacking sequence, and fiber-matrix elastic

properties. We study how the design parameters relate to the mechanical

and physical behaviors of the composites.

Composite fracture mechanics

Fracture mechanics is not as well established for composites as it is for

homogeneous isotropic materials. Therefore, our studies concentrate on the

development of new methods to study the physical significance of different

types of damage in laminated composites (i.e., ply cracks, interface cracks,

edge stresses) that exist near bimaterial interfaces. We also measure the

static strength and fatigue life of materials that are candidates for me- chanical performance improvements.

Mixed quarter-point elements in linear-elastic fracture mechanics. The

accuracy and efficiency of mixed finite elements in linear-elastic fracture

mechanics problems were examined. Mixed elements possess displacement and

stress degrees of freedom and generally have more rapid strain-energy con- vergence rates than displacement elements have. Conventional and quarterpoint-element configurations were used to determine mode-I stress- intensity

factors (Kj) and total strain-energy values for several two-dimensional test

geometries. The goals of this effort were to determine the effect of the

quarter-point configuration on the behavior of the mixed element and also

to determine the relative accuracy of different methods of computing Kj

.

Three techniques were used to evaluate the stress - intensity factors:

stiffness derivative, crack extension, and J- integral. The conventional

and quarter-point mixed elements yield lower and upper bounds, respectively,

of the strain-energy values for the geometries considered. The stressintensity factors computed using conventional mixed elements were always

less than those computed using mixed quarter-point elements; only when

using the J- integral to compute Kj are the mixed quarter-point values more accurate

.

*NBS Postdoctoral Research Associate.

tGuest worker from the Institute of Metal Research, Shenyang, China.

36
In general, both configurations of the mixed elements gave very accurate

(within 2 percent) values of Kj for very coarse meshes. The combination of

these configurations enables rapid calculation of the bounds on the strain

energy for a cracked body (see figure 13).

4.0

_q q

I o Displacement Conventional Rectangles _

e Displacement Quarter-Point Rectangles

Mixed Conventional Rectangles

_8 0

i a Mixed Quarter-Point Rectangles

& Nixed Conventional Triangles a Mixed Quarter-Point Triangles - 10.0 1 I I I 0.1 0.2 0.3 0.4

NORMALIZED MESH SIZE

0.5

Figure 13. Strain-energy convergence for a double-edge-cracked panel

for various finite-element configurations.

We also studied enriched mixed elements. Unlike the enriched displacement

method, the enriched mixed method results in numerical singularities at

crack- tip nodes. We are studying modifications of the present method to

eliminate this problem.

Analytical methods of calculating free-edge singularities. We have success- fully calculated the singularity at the bimaterial interface of a stress

free laminate by using Stroh's method. This singularity will be included

in a generalized plane -strain enriched element. The total boundary value

problem will be solved with the displacement method, where the coefficients

of the singular terms are solved numerically along with the nodal displacements. Preliminary results indicate that these coefficients and their singular terms are extremely sensitive to the anisotropic elastic properties

near the bimaterial interface. For isotropic materials, these coefficients

are the well-known stress- intensity factors associated with the r*^ singular

terms. If possible, the mixed enriched method will also be used to study

this problem. Eventually, these methods will be extended to study the more complicated ply-crack problem.

37
Edge stresses in woven composite laminates. A finite-element model with

generalized plane -strain elements was used to study the effect of weave geometry on the edge stresses. No singularities are included in this model;

hence, stresses can be predicted only near the stress- free edge. Results of

this study showed that the weave has a beneficial effect—it suppresses the

initiation of damage at the stress-free edge. This prediction is supported

by experimental observations: for some weave geometries, growth of damage

that initiates inside the laminate is arrested near the free edge. For

nonwoven laminates, the opposite is true: damage usually initiates at the

stress -free edge and grows rapidly into the laminate. Future studies will

include the interfacial stress-free-edge singularities.

New composite material for thermal isolation straps. In a study of advanced

fiber- reinforced composites, we compared two thermal- isolation strap con- figurations, identical except for their fiber reinforcements: S2-glass and

Al 2 0 3

. A popular cryogenic grade resin was used in both composites. Static

and cyclic mechanical tests and thermal conductivity measurements indicated

that the straps with Al 2 0 3 fibers were superior in performance. The fatigue

test results for both materials are shown in figure 14. Other results of

this study indicated that failure initiates in a region where the load is

transferred by shear and compression.

00 w*

CO

CO

LUC

C

H

CO

2

D

5

X

<

2

K.

Composite deformation mechanics

Nonlinear response of rectangular beams from a theory of higher order shear

deformation. The Timoshenko theory has been used almost exclusively in the

study of the static and dynamic behavior of shear deformable rectangular

beams. The primary deficiency of the Timoshenko theory is the presence of

the shear coefficient k, which is introduced to account for the contradic38
tory shear distribution over the cross section of the beam. The search for

the best shear coefficient has been described as a "small research industry."

In the literature, typical values for beams of rectangular cross section

range from 0.822 to 0.870.

Higher order beam theory was used to study moderately large deflections and

moderately large amplitude vibrations of rectangular beams with various boundary conditions. It allows for shear deformation and correctly accounts for

the stress -free boundary conditions on the upper and lower surfaces of the

beam. This higher order theory results in a model that is more theoretically

sound than the Timoshenko theory because the need for a shear coefficient is

eliminated. The resulting finite element equations are based on the minimum

continuity requirements of the higher order theory. They do not require the

reduced integration that is essential in the use of Timoshenko beam elements

to eliminate numerical difficulties (locking), which can occur when the beam

is thin.

Figure 15. Nondimens ional tip displacement of

a uniformly loaded cantilever beam.

39
Solutions were obtained for static and dynamic response problems. The results

obtained using the higher order theory are in excellent agreement with existing

solutions for isotropic beams (see figure 15). The theory may be extended to

analyze composite beam components consisting of layered isotropic materials.

Mechanics of nondestructive evaluation for composites

Our nondestructive evaluation studies concentrate on (1) measuring and mapping the mechanical deformation near damage in a composite laminate by optical

methods and (2) developing a numerical model of a capacitive - array sensor to

monitor cracks in conductors (graphite fibers) and dielectrics (epoxy matrix).

Determination of whole-field strain in a composite panel using coherent optical processing. A newly devised video-optical experimental technique enables

automated determination of the in-plane, plane stress components of the infinitesimal deformation tensor at discrete locations over an area of interest

in a loaded specimen. It was used to evaluate a square area enclosing the

central crack in a G10-CR woven glass-epoxy panel loaded in tension. A

finite-element analysis modeled the strain tensor field surrounding the cen- tral crack. The experimental and the finite-element-analysis results were compared along the plane of the crack, along a plane above and parallel to

the plane of the crack, and finally, along a plane perpendicular to the plane

of the crack close to the singularity at the crack tip. The experimental and

finite-element-model results are qualitatively equivalent and show the localized effect of high strains close to the crack tip. This localized effect in

the composite panel is illustrated in figure 16, where the results are com- pared with a similar center- cracked aluminum panel.

Numerical modeling of capacitive- array sensors. A numerical model was developed for a capacitive- array sensor, which can be used in detecting flaws in

dielectric and metallic plates. The simulated probe response was computed

from the change in admittance (figure 17a), which is calculated from a line

integral that is a function of the scalar potential and its normal derivative

with respect to the specimen surface. The electrostatic potentials (figure

17b) and their gradients were computed from a two-dimensional finite-element

model

.

Parametric studies were performed on homogenous, isotropic specimens to determine the relative effects of flaw size, specimen dielectric constant, and

probe lift-off distance. In general, the responses to flaws in dielectric

specimens saturate for much smaller flaw sizes than do similar curves in

conducting specimens.

Future applications of this model will include the detection of subsurface

graphite fiber (conductor) breaks in an epoxy (dielectric) base for the

nondestructive evaluation of graphite - epoxy composites. Because the electrostatic field penetrates the dielectric medium, subsurface flaws in the

composite resulting from a material or field discontinuity may be readily

detected using the present model.

40
Load

Direction

UUMKAHIbUN Uh o l HAHN rlfcLUb l-UH!

Figure 16. Comparison of strain fields for (a) center-cracked aluminum

plate and (b) center-cracked woven laminate plate. The light areas indicate regions of high strain.

41
X/B

(a) (b)

Figure 17. Response of the capacitive array probe

for a flaw in a dielectric specimen.

(a) change in admittance;

(b) potential for step in specimen.

Physical-property measurement and modeling

We focus on several composite-material physical properties: sound velocities, mass density, elastic constants internal friction, thermal expansivity, and specific heat. These properties depend on constituent

properties (both occluded phase and matrix) and occluded-phase geometry.

For the occlusion, phase geometry includes volume fraction, shape, orientation, size, and distribution nonhomogeneity. We developed theoretical

models and accompanying computer programs that apply to a wide variety of

composites: particle, fiber, cloth, laminated reinforcements; plastic or metal matrices; rod-shaped or disc-shaped particles; composites with voids.

Modeling. We considered the phase velocity and attenuation of plane elastic waves in a particle -reinforced composite medium. Our study considered

effective -plane -wave propagation, both longitudinal and shear, through a medium containing a random distribution of spherical inclusions. We assumed

that the particles and matrix are separated by a thin layer of elastic material with different properties. For some systems, we predict measurable

effects for the thin layers. Especially, we considered Pb/epoxy and SiC/Al,

laboratory and practical materials, respectively.

Using theoretical models, we considered the elastic constants of ceramics

containing pores. As an example, we considered alumina. However, the

42
approach applies to all ceramics. As a point of departure, we considered

spherical pores. For all the usual elastic constants—Young's modulus,

shear modulus, bulk modulus, Poisson's ratio—we derived relationships for

both the forward and inverse cases: predicting the porous ceramic properties

and estimating the pore-free ceramic properties. Following a suggestion by

Hasselman and Fulrath that sintering or hot pressing can produced cylindrical pores, we derived a relationship for the elastic constants of a distribution of randomly oriented long cylinders (c/a = «>, the prolate - spheroid

limit) . This model predicts elastic constants lower than those for spherical pores, but well above observation. We obtained agreement with observation by assuming the pores are oblate spheroids. For alumina, the necessary

aspect ratio equals one-ninth. Using this oblate - spheroid pore-shape model,

we calculated all the elastic constants versus pore volume fraction for

alumina. Besides pore aspect ratio, the model requires only the pore-free

alumina elastic constants. It contains no adjustable parameters.

Measurement and modeling. Using dynamic methods, we measured the Young's

and shear moduli of aluminum-matrix particle-reinforced composites containing up to 55 volume percent SiC particles. Using a scattered-plane -wave ensemble -average model, we calculated four elastic constants: Young's,

shear, and bulk moduli and Poisson's ratio. The model uses four kinds of

information: matrix elastic constants, particle elastic constants, particle

volume fraction, and particle shape. Between measurement and calculation,

we found good agreement. As expected, the elastic stiffnesses fall well

below a rule-of -mixture prediction; the Poisson's ratio falls above. Also, we measured the Young' s -modulus -mode internal friction, which, contrary to

expectation, increases with increasing particle content.

By a kilohertz - frequency resonance method, we determined the torsion modulus

and internal friction of a uniaxially fiber-reinforced composite. The com- posite comprised glass fibers in an epoxy-resin matrix. We studied three

fiber contents: 0, 41, and 49 volume percent. The internal friction failed

to fit a classical free-damped-oscillator model where one assumes a linear

rule of mixture for three quantities: oscillator mass, force constant, and

mechanical-resistance constant. The torsion modulus fit approximately a plane-wave- scattering ensemble -average model. The microstructure showed

strong fiber-distribution nonhomogeneity. Considering this nonhomogeneity

yielded better agreement between model and observation. Thus, torsionmodulus measurements provide a method to detect and quantify fiberdistribution nonhomogeneity.

Analysis. We considered the directional variation of elastic constants of

a uniaxial graphite-fiber-reinforced magnesium-matrix composite. Unidirectional graphite -magnesium composites show high elastic anisotropy and un- usual geometrical features in their elastic-property polar diagrams (see

figure 18). From the five -component transverse- isotropic elastic-stiffness

tensor we computed and graphed representation surfaces for Young's modulus,

torsional modulus, linear compressibility, and Poisson's ratios. On the

basis of Christoffel ' s equations, we described some unusual elastic-wavesurface topological features. For graphite -magnesium, we predicted some

43
new results: a shear-wave velocity that exceeds a longitudinal -wave velocity

in the plane transverse to the fiber, a wave that changes polarization from

longitudinal to transverse as the propagation direction sweeps from the

fiber axis to the perpendicular axis. Our study emphasizes the importance

of obtaining the complete elastic-stiffness tensor. We considered examples

of errors resulting from popular elastic-constant approximations.

Figure 18. Polar diagram of Poisson's ratio i/ 32

for various fiber volume fractions.

44
Cryogenic Materials

R. P. Reed, L. M. Delgado, J. K. Han,* T. Ogata, t p. T. Purtscher,

N. J. Simon, R. L. Tobler, R. P. Walsh

Adiabatic heating calculations

Internal specimen heating effects during low- temperature tests of mechanical properties have been questioned for a long time. The very low specific heat and thermal conductivity of metals at low temperatures lead one

to anticipate near-adiabatic conditions. Discontinuous yielding, a phenomenon in which very high strain rates are experienced in local regions

(similar to Luders band regions in steel at ambient temperatures), complicates the question, and the answer is prerequisite to the establishment of

test standards.

Tensile stress—strain curves at differing strain rates are shown in fig- ure 19. At the lower two strain rates, discontinuous yielding is prominent. At the highest strain rate, discontinuous yielding is less evident,

the curves have an erratic appearance, and reduction of ultimate strength

is apparent. Clearly, the nature of the deformation process has changed.

The onset of discontinuous yielding is dependent on both alloy and strain

rate

The temperature measurements of thermocouples attached to the central portion of the specimens surfaces are also included. Notice the change from

thermal spikes at lower strain rates to thermal waves at the highest

strain rate. The thermal spikes are associated with individual discontinuous yielding events. The thermal waves, which gradually build up to

temperatures as high as 96 K, are associated with the lack of discrete

discontinuous yields and with the wavy nature of the stress—strain curves. Reduction of tensile strength is associated with this thermal transition.

Our calculations have been completed. They estimate the work put into

a specimen during deformation, the stored energy in terms of dislocations

and dislocation interactions, and the dissipated heat. These contributions, equated in terms of a specimen heat balance, indicated that the

reduction of strength at high strain rates is associated with specimen

warming, which is caused by the transition from nucleate to film-boiling

heat transfer on the specimen surface.

*Guest worker from Korea Advanced Institute of Science and Technology,

Seoul, Korea.

jGuest worker from the National Research Institute for Metals, Ibaraki,

Japan.

45
Figure 19. Stress—strain curves and specimen surface temperatures

at various strain rates for alloy 304L.

Effects of loading rate during tensile tests at 4 K

We investigated the effects of loading rate on the tensile properties of

AISI 304L, 310, and 316LN steels through load-control tensile tests at 4 K. Usually, the tensile properties of structural materials at 4 K are obtained

from displacement—control tensile tests in which discontinuous deformation

occurs accompanied by load drops and temperature rises. Discontinuous

deformation is different in load-control tensile tests: (a) no load drop occurs during the deformation because the amount of displacement is not

limited by the testing machine and (b) the measured ultimate strength is

46

TEMPERATURE.

K

TEMPERATURE,
lower. This deformation behavior affects the mechanical properties of

structural materials at low temperatures under load-controlled deformation.

From load-control tensile tests in which the loading rates were varied from

0.

5.N/s to 5 kN/s, we obtained the following results:

1. Ultimate tensile strength decreased with increased loading rate, re- flecting increased specimen heating; at 5 kN/s it was 65 percent of

the ultimate strength obtained in displacement- control tests. Thus,

a freely deforming material at low temperatures will fail at a much

lower stress than one whose deformation is controlled.

2. AISI 310 specimens deformed 2 percent at just 3 percent above the yield

strength when 500 N/s was applied. At 5 N/s, AISI 310 specimens deformed significantly—15 mm—near 40 percent strain; rapid deformation

occurred along the entire specimen length. Average deformation velocities were 26 mm/s (7.5 x 10' 1 s' 1 ). This deformation was accompanied

by local specimen heating, which stopped when a balance of the local

strength and work hardening was reached. Subsequent discontinuous

deformations occurred in other areas of the specimen where there had

been less local strain hardening.

3. AISI 304L specimen elongation was not affected by either the loading

rate or the control mode. This is due to its lower yield strength and

its high work hardening accompanied by martensitic transformation. The

transformation caused interrupted and unstable deformation.

Effects of specimen variations on J-integral test results

We cooperated with researchers from the National Research Institute for

Metals in Ibaraki and Tohoku University in a study of the J-integral test

at 4 K, in support of the development of a standard test procedure.

The tests used a computer-controlled unloading- compliance technique, a single-specimen method that is especially useful at 4 K, on compact speci- mens of AISI 316LN alloy. The test variables were:

• specimen size, 12.5, 25, and 38 mm thick (0.5TCT, 1TCT, and 1 . 5TCT)

• fatigue precracking temperature, 293, 77, and 4 K at a stressintensity factor of about 24 MPa-m^

• plain and side- grooved specimens

The effects of the test variables are shown in figure 20. Toughness decreased with increasing specimen thickness, at least from 25 to 38 mm. The

toughness of the side-grooved specimens was less than that of plain specimens, but the effect was small—less than 10 percent in the largest specimen. The effects of specimen thickness and side - grooving are related to the

constraint exerted at the crack tip. Fatigue precracking temperature had a significant effect on the size of the plastic zone of the specimen, but had

no measurable effect on the fracture toughness at 4 K.

47
05T IT 15T

Figure 20. Kj c (J) estimates at 4 K as a function of thickness for proportionally sized specimens (W/B - 2) , showing the effects of side

grooves. Symbols indicate fatigue precracking temperatures:

A — 293 K, o or • — 77 K, and — 4 K. Open symbols indicate

plain specimens; closed symbols, specimens with side grooves.

Copper processing for cryogenic performance

In the annealed condition, copper is usually not strong enough for use as the stabilizing element for low- temperature superconductors or as a highfield magnet conductor. It must be strengthened or reinforced. Two con- ventional ways to strengthen copper are cold working and reduction of grain

size. Yet quantitative low- temperature property data on the effects of

these metallurgical variables are not available. Therefore, we studied the

effects of grain size variation and cold rolling on the tensile properties

and electrical resistivity of copper at 4, 76, and 295 K.

We found that cold rolling dramatically increases yield strength, ultimate

strength, and electrical resistivity; it reduces elongation, but does not

affect reduction of area. Cold rolling to 10 percent reduction in thickness

increases the yield strength by a factor of about 8 at all low temperatures;

subsequent rolling to 60 percent increases the yield strength by a factor of

only about 1.5. Tensile strengths are related to d‘^, where d is the grain

size, following the Hall—Petch relationship. This dependence increases at

low temperatures. Grain size has little effect on resistivity. On the basis

of optimal strength and minimal resistivity, a modest amount of cold rolling

(about 10 percent) produces the best ratio of strength to resistivity at low

temperatures

.

48
United States—Japan program for the development of test standards

A United States—Japan cooperative program is developing tensile and fracture

toughness testing standards for austenitic stainless steels (superconducting

machinery construction materials) at 4 K. Sponsors of the program are the

U.S. Department of Energy and the Japan Atomic Energy Research Institute

(JAERI) . Contributing organizations include NBS and four Japanese laboratories: JAERI , Tohoku University, Kobe Steel, and the National Research

Institute for Metals.

During the past year, cryogenic materials scientists were interviewed at

government, industrial, and academic institutions in Japan. Workshops were held in Sendai, Tokai-mura, and Reno. The first round- robin tensile and

fracture tests of a Fe— 13Cr-22Mn austenitic stainless steel at 4 K were conducted at seven laboratories in the United States and Japan. Five drafts

of a proposed standard 4-K tensile test procedure were written and submitted

to the workshops for review. Five drafts of a proposed 4-K fracture standard were also reviewed, and a matrix of tests was executed at four laboratories to answer questions about test procedures. Six joint research papers

have been accepted for publication. Following final revisions, the test

standards will also be submitted to ASTM for consideration as an ASTM

standard

Material properties handbook

The Materials Handbook for Fusion Energy Systems (MHFES) is being developed

to provide an authoritative common source of material properties for the

fusion energy community to use in concept evaluation, design, safety analysis, and performance prediction and verification of fusion energy systems.

This single source for properties is essential to ensure a common basis for

comparing the eventual performance and economics of the various fusion systems currently under study, in operation, or under construction.

We are continuing to work on handbook pages for the MHFES covering lowtemperature properties of structural alloys for the superconducting magnets

used for plasma confinement in fusion energy devices. Three nitrogenstrengthened austenitic stainless steels that are the most promising near- term candidates for structural use in superconducting magnets were chosen

for initial coverage. Initial work for preparation of handbook pages on aluminum alloys has been completed. Current work involves a review of

properties of several high-purity coppers and copper alloys: CDA 101, 102,

104, 105, 107, 155, and 170 through 175.

49
Material Performance

R. J. Fields, C. H. Brady, B. W. Christ, R. deWit, G. E. Hicho , Y. Huang,*

T. R. Shives, L. C. Smith

Structure and properties of a high-strength, low-alloy steel

Our efforts to understand the relationship between the properties and structure of alloy A710 are continuing. To be acceptable for naval applications,

the A710 steel must meet the bulge- test specification requirements. This

test is usually performed after all the mechanical tests, and occasionally

the plate material fails the bulge test. Plates of A710 steel that had

passed or failed the bulge test were subjected to strain-hardening experiments; their texture was studied by means of pole figures. The 22 steel

plates were statistically analyzed: each element and selected pairs of

elements were compared with each other, and the overall composition was compared with that of the A710 steel that passed or failed the bulge test.

Statistical results indicated that a correlation exists between the chemical

composition of the steel and its ability to pass the bulge test, enabling

naval engineers to predict success or failure prior to the extensive

mechanical tests.

Infrared method of studying crack growth

A new thermographic method has been developed that is based on the measure- ment of infrared emissions from the surface of a loaded body. It measures

the thermoelastic cooling phenomenon during the tensile process and cooling

and heating during cyclic loading. This method has been recently used to

determine crack tip position during cyclic loading. It has improved the

accuracy of crack-growth measurements, which are the basis for evaluating

the fracture toughness of aluminum and steel plates used in tank cars.

Compression tests of copper -Inconel laminates

With the successful explosive bonding of C10700 copper layers in between two

layers of Inconel 718 and the promising progress in numerical modeling of

the composite material, a program of composite material characterization and

model verification was initiated.

The program consisted of a series of face compression tests, in uniform

axial compression, at room temperature. The procedures of this program

were

:

1. Examine the single-cycle mechanical characteristics. Establish

the 0.2-percent yield point. Explore well into the plastic range.

*Guest Worker from the Institute of Metal Research, Shenyang, China.

50
2. Examine the low- and moderate -cycle behavior (at 10 and 3000

cycles) at loads approaching 75 percent of yield.

Tests were performed on the 53-MN (12 x 10 6 -lbf) NBS universal testing

machine. Support equipment included displacement gages, bridge amplifiers,

a digital oscilloscope, a wide-band FM magnetic tape recorder, and a microcomputer .

Test specimens were 18, 20, or 23 cm (7, 8, or 9 in) in diameter; their

thickness varied from 2 to 3.5 cm (0.8 to 1.4 in). They were supplied by

Princeton Plasma Physics Laboratory.

The general experimental procedure consisted of a number of static and two

cyclic tests in each series. In step 1, the stress—strain curve for a given

configuration was established by gradual loading (about 7 MPa/s, 1 ksi/s)

while monitoring both diametric and axial displacements in real time. The

tests were usually carried to an axial displacement of 0.38 mm (0.015 in),

well into the plastic range. From these test results, the 0.2-percent yield

point was established. For the 20-cm (8-in) diameter specimens, this

occurred at about 20 MN (4 x 10 6 lbf) or 550 MPa (80 ksi)

In step 2, the load level representative of 75 percent of yield was established from the results of step 1 for two new specimens of the same type.

The specimens were then cycled at 0.03 Hz for three thousand times at this

load. For the 20-cm-diameter specimens, this was a load range from 2 to

10 MN (0.5 to 3 x 10 6 lbf). For the second specimen in each series, the

load range was increased in periodic steps until it was beyond the yield

point. In these cases, the displacement gages revealed a significant amount

of cyclic creep at the higher loads. Nevertheless, these specimens sus- tained loads well above the yield strength (69 to 370 MPa, 10 to 53 ksi)

and tensile strength (220 to 455 MPa, 32 to 66 ksi) of copper.

Failure examinations for the Potomac Electric Power Company

During the past year, we have presented to the Potomac Electric Power

Company (PEPCO) twenty- three reports on failures that occurred in their

power generating equipment. Our failure analyses were diverse: from the

quality of welding in two water-wall tubes that were joint to whether an existing crack followed an electrical resistance weld. Some of our failure

analyses had to be done very quickly to keep the nonoperating time of the

power generating unit to a minimum. In addition to determining the causes

of these failures, we have assisted PEPCO as consultants on replacement

materials for those that failed.

Mechanical Failures Prevention Group

At the request of the Naval Air Test Center (NATC) , the Mechanical Failures

Prevention Group (MFPG) held its 41st meeting in October 1986 at NATC in

Patuxent River, Maryland. The subject of the three-day symposium was Detection, Diagnosis, and Prognosis of Rotating Machinery to Improve Reliability,

Maintainablity , and Readiness through the Application of New and Innovative

51
Techniques. About 235 people attended. The meeting was sponsored by NBS

and the Office of Naval Research in cooperation with the American Helicopter

Society.

The 42nd meeting of the MFPG was held in September 1987 at NBS -Gaithersburg

.

The theme of the meeting, Technology Innovation — Key to International

Competitiveness, was intended primarily for the small business community.

About 95 people attended. The three-day meeting was cosponsored by NBS and

the Office of Naval Research. Cooperating organizations included the Small

Business Administration, Battelle Pacific Northwest, and the University of

Maryland.

52
OUTPUTS

and

INTERACTIONS

SELECTED RECENT PUBLICATIONS*

Cardenas-Garcia, J. F. ; Moulder, J. C.; Kriz , R. D. Video optical determination of a whole-field strain in composite panel. Optical Methods in

Composites. Bethel, Connecticut: Soc . Exper. Mech. ; 1986 October. 48—57.

Cheng, Y.-W. Fatigue crack growth analysis under sea-wave loading. Int

J. Fatigue; accepted.

Cheng, Y.-W.; Broz, J. J. Cycle-Counting Methods for Fatigue Analysis with

Random Load—Time Histories: A Fortran User's Guide. Nat. Bur. Stand.

(U.S.) NBSIR 86-3055; 1986 August. 52 pp

Clark, A. V.; Fukuoka, H.; Mitracovid, D. V.; Moulder, J. C. Ultrasonic

characterization of residual stress and texture in cast steel railroad

wheels. Thompson, D. 0.; Chimenti, D. E., eds . Review of Progress in

Quantitative Nondestructive Evaluation, vol. 6B. New York: Plenum, 1987;

1567-1575.

Clark, A. V.; Govada, A.; Thompson, R. B.; Smith, G. F. ; Blessing, G. V.;

DelSanto, P. P.; Mignogna, R. B. The use of ultrasonics for texture

monitoring in aluminum alloys. Thompson, D. 0.; Chimenti, D. E., eds.

Review of Progress in Quantitative Nondestructive Evaluation, vol. 6B.

New York: Plenum; 1987. 1515—1524.

Clark, A. V.; Moulder, J. C.; DelSanto, P. P.; Mignogna, R. B. A comparison

of several ultrasonic techniques for absolute stress determination in the

presence of texture. Achenbach, J. D.; Rajapakse, Y. , eds. Proceedings,

Symposium on Solid Mechanics Research for Qualitative NDE . Dordrecht, The

Netherlands: Martinus Nijhoff; 1987. 345—360.

Clark, A. V.; Moulder, J. C.; Mignogna, R. B.; DelSanto, P. P. Ultrasonic

determination of absolute stresses in aluminum and steel alloys. Proceedings, International Conference on Residual Stress; accepted.

Clark, A. V.; Moulder, J. C.; Mitracovid, D. V.; Fukuoka, H. Ultrasonic

characterization of residual stress and texture in a heat-treated steel

railroad wheel. Mater. Eval.; accepted.

Clark, A. V. ; Read, D. T. Ductile Tearing Stability Analysis of a Ship

Structure Containing a Crack Arrester Strake. Nat. Bur. Stand. (U.S.) NBSIR

85-3038; 1986. 48 pp.

Danko, B. A.; Low, S. R. , III; deWit, R. ; Fields, R. J. Wide plate crack

arrest tests: instrumentation for dynamic strain measurements. Shives

T. R. , ed. Use of New Technology to Improve Mechanical Readiness, Reliability, and Maintainability. London: Cambridge University Press; 1987;

178-192.

^Papers that were published or accepted for publication by NBS Editorial

Review Boards during fiscal year 1987.

53
Datta, S. K. ; Ledbetter, H. M. Ultrasonic-velocity studies in metal-matrix

composites: measurements and modeling. San Antonio, Texas: Nondestructive

Testing Information Analysis Center; Santa Barbara, California: Metal Matrix

Composites Information Analysis Center; Columbus, Ohio: Metals and Ceramics

Information Center; 1986. 41—62.

Datta, S. K. ; Ledbetter, H. M. ; Shindo, Y. ; Shah, S. H. Interface effects

on attenuation and phase velocity in metal-matrix composites. Thompson,

D. 0.; Chimenti, D. E., eds . Review of Progress in Quantitative Nondestructive Evaluation, vol. 6. New York: Plenum; 1987. 1075—1984.

Datta, S. K. ; Schramm, R. E. ; Abduljabbar, Z. Plate modes generated by

EMATs for NDE of planar flaws. Thompson, D. 0.; Chimenti, D. E. , eds.

Review of Progress in Quantitative Nondestructive Evaluation, vol. 6A.

New York: Plenum; 1987. 101—108.

Datta, S. K. ; Shah, A. H. ; Ledbetter, H. M. Ultrasonic scattering and NDE

of materials and cracks. Proceedings, 0NR Symposium on Solid Mechanics

Research for Quantitative NDE. Dordrecht, The Netherlands: Martinus

Nijhoff ; 1986. 361-376.

DelSanto, P. P.; Clark, A. V. Rayleigh wave propagation in deformed orthotropic materials. Thompson, D. 0.; Chimenti, D. E., eds. Review of Prog- ress in Quantitative Nondestructive Evaluation, vol. 5B, New York: Plenum;

1986. 1467-1414.

DelSanto, P. P.; Mignogna, R. B.; Clark, A. V.; Mitracovit, D. Acoustoelastic determination of residual stresses. Proceedings, International

Conference on Residual Stress; accepted.

deWit, R. ; Fields, R. J. Wide plate crack arrest testing. Nucl. Eng. Des.

98: 194-155; 1987.

deWit, R. ; Read, D. T.; Low, S. R. , III; Harne , D. E. ; McColskey, J. D.;

Hicho, G. E. ; Smith, L. C.; Danko, G.; Fields, R. J. Wide plate crack ar- rest testing: a description and discussion of the first two wide plate tests

and the results of six, full thickness, bend bar tests. Interim report to

the Naval Research Center. Nat. Bur. Stand. (U.S.) NBSIR; in press.

Glazer, J.; Morris, J. W. , Jr.; Kim, S. A.; Austin, M. W. ; Ledbetter, H. M.

Temperature variation of the elastic constants of aluminum alloy 2090-T81.

AIAA J. 25: 1271-1272; 1987.

Grong, 0.; Siewert, T. A.; Edwards, G. R. Effects of deoxidation practice

on the transformation behavior and toughness of steel welds. Weld. J. 65:

179-s—188-s ; 1986.

Grong, 0.; Siewert, T. A.; Martins, G. P.; Olson, D. L. A model for the

silicon—manganese deoxidation of steel weld metals [86]. Metall. Trans.

17A: 1797-1807; 1987.

54
Heyliger, P. R. ; Shull, P. J.; Moulder, J. C.; Gimple, M. ; Auld, B. A.

Numerical modeling of capacitive array sensors using the finite element

method. Thompson, D. 0.; Chimenti, D. E. , eds . Review of Progress in

Quantitative Nondestructive Evaluation. New York: Plenum; in press.

Hicho, G. E. ; Brady, C. H. ; Smith, L. C.; Fields, R. J. Effects of heat

treatment on the mechanical proerties and microstructures of four different

heats of a precipitation hardening HSLA steel. J. Heat. Treat. 5: 7—19;

1987.

Hirth, J. P. ; Lin, I. H. The thermodynamic force on line force defects.

Philos. Mag. A. 56(1): 89—92; 1987.

Jones, E. R. ; Datta, T.; Almasan, C.; Edwards, D. ; Ledbetter, H. M. Lowtemperature magnetic properties of fee Fe—Cr—Ni alloys: Effects of man- ganese and interstitial carbon and nitrogen. Mater. Sci. Eng. 91:181—188;

1987.

Kasen, M. B. A strain-controlled torsional test method for screening the

performance of composite materials at cryogenic temperatures. J. Mater.

Sci . ; accepted.

Kasen, M. B. High quality organic matrix composite specimens for research

purposes. J. Compos. Technol. Res. 8(3): 103-106; 1986.

Kohn, G.; Siewert, T. A. The effect of power supply response characteristics on droplet transfer of GMA welds. David, S. A., ed. Advances in

Welding Science and Technology. Metals Park, Ohio: Amer. Soc. Met.; 1986.

299-302.

Kriz, R. D. ; Ledbetter, H. M. Elastic representation surfaces of unidirectional graphite—magnesium. San Antonio, Texas: Nondestructive Testing

Information Analysis Center; Santa Barbara, California: Metal Matrix Composites Information Analysis Center; Columbus, Ohio: Metals and Ceramics

Information Center; 1986. 63—76.

Kriz, R. D.; Ledbetter, H. M. Elastic-wave surfaces in anisotropic media.

Huet, C., ed. Proceedings, 19th Annual Colloquium on Rheology of Anisotropic Materials; 1984 November 28—30; Paris, France. Toulouse, France:

Cepadues; 1986. 79—91.

Kriz, R. D. ; Sparks, L. L. Performance of alumina/epoxy thermal isolation

straps. Clark, A. F. ; Reed, R. P. , eds. Accepted for publication in

Advances in Cryogenic Engineering — Materials, vol. 34. New York: Plenum.

Ledbetter, H. M. Acoustoelastic residual - stress measurements: Role of

anisotropic dislocation arrays. J. Appl. Phys . ; accepted.

Ledbetter, H. M. ; Austin, M. W. Deformed-polycrystalline-copper elastic

constants. Phys. Status Solidi; forthcoming (1987 November).

55
Ledbetter, H. M. ; Austin, M. W. Internal strain (stress) in an SiC/Al

particle-reinforced composite: An x-ray diffraction study. Mater. Sci. Eng.

89: 53-61; 1987.

Ledbetter, H. M. ; Austin, M. W. Molybdenum effect on volume in Fe—Cr—Ni

alloys. J. Mater. Sci.; accepted.

Ledbetter, H. M. ; Datta, S. K. Effective elastic constants of materials

with inclusions. Huet, C., ed. Rheology of Anisotropic Materials.

Toulouse, France: Cepadues; 1986. 291—306

Ledbetter, H. M. ; Kim, S. A. Manganese contributions to the elastic con- stants of face -centered-cubic Fe—Cr—Ni stainless steel. J. Mater. Sci.;

accepted

.

Ledbetter, H. M. ; Kim, S. A. Molybdenum effect on Fe—Cr—Ni- alloy elastic

constants. J. Mater. Res.; forthcoming.

Lin, I.-H. Ductility improvement in particle reinforced aluminum com- posites: A ductile - fracture model based on void nucleation and growth.

Yan, M. G.; Zhang, S. H.; Zheng, Z. M. , eds . Mechanical Behavior of

Materials — V. Oxford: Pergamon; 1987. 1261—1264.

Lin, I.-H.; Thomson, R. M. The influence of dislocation density on the

ductile—brittle transition in bcc metals. Scr. Metall. 20(10): 1367—1371;

1986.

McCowan, C. N. Cryogenic Strength, Toughness, and Ferrite Content of Stainless Steel Welds. M.S. Thesis No. T 3349. Golden, Colorado: Colorado

School of Mines; 1987.

McCowan, C. N.; Siewert, T. A. Inclusions and fracture toughness in stainless steel welds at 4 K. Clark, A. F. ; Reed, R. P., eds. Advances in Cryogenic Engineering — Materials, vol. 34. New York: Plenum; accepted.

McCowan, C. N.; Siewert, T. A.; Reed, R. P.; Lake, F. B. Manganese and

nitrogen in stainless steel SMA welds for cryogenic service. Weld. J.

66(3): 84s—92s ; 1987.

McCowan, C. N.; Siewert, T. A.; Tobler, R. L. Tensile and fracture properties of an Fe—18Cr—20Ni—0 . 16N fully austenitic weld metal at 4 K. Trans.

ASME 108(4): 340-343; 1986.

McHenry, H. I.; Purtscher, P. T.; Shives, T. R. Observations of hydrogen

damage in a failed pressure vessel. Corrosion Sci.; in press.

McHenry, H. I.; Read, D. T.; Shives, T. R. Failure analysis of an amineabsorber pressure vessel. Mater. Perf. 26(8): 18—24. 1987.

Moulder, J. C.; Shull, P. J.; Capobianco, T. E. Uniform field eddy current

probe: Experiments and inversion for realistic flaws. Thompson, D. 0.;

Chimenti, D. E., eds. Review of Progress in Quantitative Nondestructive

Evaluation, vol. 6A. New York: Plenum; 1987. 601—610.

56
Nakajima, H.; Yoshida, K. ; Shimamoto, S.; Tobler, R. L. ; Purtscher, P. T.

Reed, R. P. Round robin tensile and fracture test results for an Fe-22Mn13Cr—5Ni austenitic stainless steel at 4 K. Clark, A. F. ; Reed, R. P., eds

Advances in Cryogenic Engineering - Materials, vol. 34. New York: Plenum;

accepted

Ogata, T.; Ishikawa, K. ; Reed, R. P.; Walsh, R. P. Loading rate effects on discontinuous deformation in load-control tensile tests. Clark, A. F.

Reed, R. P., eds. Advances in Cryogenic Engineering — Materials, vol. 34.

New York: Plenum; accepted.

Ogata, T. ; Ishikawa, K. ; Yuri, T.; Tobler, R. L. ; Purtscher, P. T.; Reed,

R. P. ; Shoji, T. ; Nakano, K. ; Takahashi, H. Effects of specimen size and

precracking temperature on Jj c test results for AISI 316LN at 4 K. Clark,

A. F. ; Reed, R. P., eds. Advances in Cryogenic Engineering — Materials,

vol. 34. New York: Plenum; accepted.

Pugh, C. E.; Naus , D. J.; Bass, B. R. ; Nanstad, R. K. ; deWit, R. ; Fields,

R. J.; Low, S. R. , III. Wide-plate crack-arrest tests utilizing a prototypical pressure vessel steel. Int . J. Pressure Vessel and Piping; in

press

Purtscher, P. T. Absence of stretch zones in austenitic stainless steels

fractured at cryogenic temperatures. ASTM J. Test. Eval. 15(5): 296—298;

1987.

Purtscher, P. T.; Read, D. T. Effect of prior deformation on the 76-K

fracture toughness of AISI 304L and AWS 208L stainless steels. ASTM

J. Eng. Mater. Technol. 109: 151—157; 1987.

Purtscher, P. T. ; Reed, R. P.; Read, D. T. Effect of void nucleation on fracture toughness of high-strength austenitic steels. ASTM STP . Philadelphia: Amer. Soc. Test. Mater.; in press.

Purtscher, P. T.; Walsh, R. P. ; Reed, R. P. Modification of 316LN-type

alloys. Clark, A. F. ; Reed, R. P. , eds. Advances in Cryogenic Engineering — Materials, vol. 34. New York: Plenum; accepted.

Purtscher, P. T. ; Walsh, R. P. ; Reed, R. P. Fracture behavior of 316LN

tensile specimen at cryogenic temperatures. Clark, A. F. ; Reed, R. P., eds.

Advances in Cryogenic Engineering — Materials, vol. 34. New York: Plenum;

accepted.

Read, D. T. J- integral values for small cracks in steel panels. ASTM STP.

Philadelphia: Amer. Soc. Test. Mater.; in press.

Read, D. T. Strain Hardening and Stable Tearing Effects in Fitness -forService Assessment: First Interim Progress Report. Nat. Bur. Stand. (U.S.)

NBSIR 86-3045; 1986.

Read, D. T.; McHenry, H. I. Post-weld Heat-Treatment Criteria for Repair

Welds in 2-l/4Cr—IMo Steel Superheater Headers: An Experimental Study.

Nat. Bur. Stand. (U.S.) NBSIR 87-3075. 1987.

57
Read, D. T. ; McHenry, H. I.; Petrovski, B. Elastic plastic models of sur- face cracks in tensile panels. Optical Methods in Composites. Bethel,

Connecticut: Soc. Exper. Mech. ; 1986 October. 210—216.

Reed, R. P. Austenitic stainless steels, with emphasis on strength at low

temperatures. Walters, John, ed. Chapter in Alloying. Metals Park, Ohio:

American Society for Metals; accepted.

Reed, R. P., ed. Materials Studies for Magnetic Fusion Energy Applications

at Low Temperatures — X. Nat. Bur. Stand. (U.S.) NBSIR 87-3067; 1987 May;

438 pp

.

Reed, R. P.; Simon, N. J.; Purtscher, P. T. ; Tobler, R. L. Alloy 316LN for

low temperature structures: A summary of tensile and fracture data. Proceedings, Eleventh International Cryogenic Engineering Conference. London:

Butterworth Scientific; 1986. 786—790.

Reed, R. P.; Walsh, R. P. Tensile strain rate effects in liquid helium.

Clark, A. F. ; Reed, R. P., eds . Advances in Cryogenic Engineering — Materials, vol. 34. New York: Plenum Press; accepted.

Reed, R. P.; Walsh, R. P.; Fickett, F. R. Effects of grain size and cold

rolling on the low temperature tensile properties and electrical resistivity

of copper. Clark, A. F. ; Reed, R. P. , eds. Advances in Cryogenic Engineering — Materials, vol. 34. New York: Plenum; accepted.

Reno, R. C.; Clark, A. V.; Blessing, G. V.; Fields, R. J.; Govada, A.;

Thompson, R. B.; DelSanto, P. P.; Mignogna, R. B.; Smith, J. F. Use of

neutron pole figures to calibrate ultrasonic techniques for on-line texture

control of aluminum plates. Wadley, H. N. G.; Rath, B. B.; Wolf, S. M.

;

Parrish, P. A., eds. Proceedings, Conference on Intelligent Processing of

Materials and Advanced Sensors. Metals Park, Ohio: Amer. Soc. Met.; in

press

.

Reno, R. C.; Fields, R. J.; Clark, A. V. Crystallographic texture in rolled

aluminum plates: neutron pole figure measurements. Thompson, D. 0.;

Chimenti, D. E. , eds. Review of Progress in Quantitatitve Nondestructive

Evaluation. New York: Plenum; in press.

Schramm, R. E.; Clark, A. V. , Jr.; Mitracovi6, D. V.; Shull, P. J. Flaw

detection in railroad wheels using Rayleigh-wave EMATs . Thompson, D. 0.;

Chimenti, D. E. eds. Review of Progress in Quantitative Nondestructive

Evaluation, vol. 7. New York: Plenum; accepted.

Schramm, R. E.; Siewert, T. A. Sizing canted flaws in weldments using lowfrequency EMATs. Thompson, D. 0.; Chimenti, D. E., eds. Review of Progress

in Quantitative Nondestructive Evaluation, vol. 6B. New York: Plenum, 1987.

1731-1736.

Scull, L. L. ; Reed, R. P. Tensile and fatigue -creep properties of a copperstainless steel laminate. Clark, A. F.; Reed, R. P., eds. Advances in

Cryogenic Engineering - Materials, vol. 34. New York: Plenum; accepted.

58
Scull, L. L. ; Sparks, L. L. ; Arvidson, J. M. Low temperature mechanical

property and thermal expansion measurements of silica aerogel foam. Clark,

A. F. ; Reed, R. P. , eds. Advances in Cryogenic Engineering — Materials,

vol. 34. New York: Plenum; accepted.

Shimada, M. ; Tobler, R. L. ; Shoj i , T. ; Takahashi, H. Size, side- grooving

and precracking effects on Jj c data for an SUS 304 stainless steel. Clark,

A. F. ; Reed, R. P., eds. Advances in Cryogenic Engineering — Materials,

vol. 34. New York: Plenum; accepted.

Shull, P. J.; Capobianco, T. E.; Moulder, J. C. Design and characterization of uniform field eddy current probes. Thompson, D. 0.; Chimenti, D. E

Review of Progress in Quantitative Nondestructive Evaluation, vol. 6A.

New York: Plenum; 1987. 695—704.

Shull, P.; Heyliger, P. ; Moulder, J. C.; Gimple, M. ; Auld, B. A. Applications of capacitive array sensors to nondestructive evaluation. Thompson,

D. 0.; Chimenti, D. E. Review of Progress in Quantitative Nondestructive

Evaluation, vol. 7. New York: Plenum; accepted.

Siewert, T. A.; Gorni, D; Kohn, G. High-energy beam welding of type 316LN

stainless steel for cryogenic applications. Clark, A. F. ; Reed, R. P., eds

Advances in Cryogenic Engineering — Materials, vol. 34. New York: Plenum;

accepted.

Siewert, T. A.; Schramm, R. E. Compressive stress effects on the ultrasonic detection of cracks in welds. Thompson, D. 0.; Chimenti, D. E., eds.

Review of Progress in Quantitative Nondestructive Evaluation, vol. 6B.

New York: Plenum, 1987. 1593—1600.

Siewert, T. A.; Trevisan, R. E.; Purtscher, P. T. Fusion line shape versus

toughness in HY-80 GMA welds. Nat. Bur. Stand. (U.S.) NBSIR 86-3043; 1986.

43pp.

Siewert, T. A.; Ziegenfuss, H. G. Welding in the Soviet Union — A closer

view. Weld. J. 66: 27-34; 1987.

Simon, N. J.; Reed, R. P. Design of 316LN-type alloys. Clark, A. F.

Reed, R. P. , eds. Advances in Cryogenic Engineering — Materials, vol. 34.

New York: Plenum; accepted.

Simon, N. J.; Reed, R. P. Strength and toughness of AISI 304 and 316 at

4 K. J. Nucl . Mater. 141-143: 44-48; 1986.

Takahashi, H. ; Shoj i , T. ; Tobler, R. L. Acoustic emission and its applications to fracture studies of austenitic stainless steels at 4 K. Clark,

A. F. ; Reed, R. P., eds. Advances in Cryogenic Engineering — Materials,

vol. 34. New York: Plenum; accepted.

Tobler, R. L. ; Reed, R. P. Proposed standard method for tensile testing

of structural alloys at liquid helium temperatures. Reed, R. P., ed.

Materials Studies for Magnetic Fusion Energy Applications at Low Temperatures -X. Nat. Bur. Stand. (U.S.) NBSIR 87-3067; 1987. 387-404.

59
Tobler, R. L. ; Reed, R. P. Proposed standard method for fracture toughness

testing of structural alloys at liquid helium temperatures. Reed, R. P.

,

ed. Materials Studies for Magnetic Fusion Energy Applications at Low

Temperatures - X. Nat. Bur. Stand. (U.S.) NBSIR 87-3067. 1987. 405-421.

Tobler, R. L. ; Shoji, T. ; Takahashi, H. ; K. Ohnishi Fracture, acoustic

emission, and adiabatic heating of austenitic stainless steels at liquid

helium temperatures. Proceedings, Symposium on Acoustic Emission.

Yamaguchi, K; Aoki , K. ; Kishi, T. , eds. The Japanese Society of NDI ; 1986.

453-461.

Tobler, R. L. ; Trevisan, R. E. ; Siewert, T. A.; McHenry, H. I.; Purtscher,

P. T. ; McCowan, C. N. ; Matsumoto, T. Strength, fatigue, and toughness

properties of an Fe—18Cr—16Ni—6 . 5Mn—2 . 4Mo fully austenitic SMA weld at 4 K.

Clark, A. F. ; Reed, R. P. , eds. Advances in Cryogenic Engineering — Materials, vol. 34. New York: Plenum; accepted.

Yoshida, K. ; Nakajima, H.; Oshikiri, M. ; Tobler, R. L. ; Shimamoto, S.;

Miura, R. ; Ishizaka, J. Mechanical tests of large specimens at 4 K. Clark,

A. F. ; Reed, R. P., eds. Advances in Cryogenic Engineering — Materials,

vol. 34. New York: Plenum; accepted.

60
SELECTED TECHNICAL AND PROFESSIONAL COMMITTEE LEADERSHIP

Advanced Materials Institute

1987 National Conference Committee

R. P. Reed

American Society for Metals

Joining Division

T. A. Siewert, Government Liaison

American Society for Testing and Materials

E24: Fracture Testing Committee

Proceedings, 18th National Symposium on Fracture Mechanics

D. T. Read, R. P. Reed, editors

E24.05: Terminology

R. deWit, Secretary

E24.06: Fracture Mechanics Applications

E24.06.04: Weld Testing

H. I. McHenry, Chairman

E28: Mechanical Testing

E28.04: Tension Testing

E28.04.03: Verification of Alignment

B. W. Christ, Chairman

E28.06: Hardness

E28.06.07: Hardness Test Block Intercomparison Task Group

T. R. Shives, Chairman

American Welding Society

Technical Papers Committee

T. A. Siewert

Welding Journal

T. A. Siewert, Reviewer

American Welding Institute

On-line Welding Data Base Committee

T. A. Siewert

Colorado School of Mines

Steel Research Institute Advisory Board

R. P. Reed

Federal Railroad Administration

Railroad Tank Car Safety Task Force

R. J. Fields, G. E. Hicho

International Cryogenic Materials Conference

Board of Directors

R. P. Reed, Finance Officer

61
International Cryogenic Materials Conference, continued

Proceedings

R. P. Reed, Coeditor

1987 Conference Program Committee

M. B. Kasen

Mechanical Failures Prevention Group

T. R. Shives, Executive Secretary

S. R. Low, Committee member

T. R. Shives, Proceedings Editor

Metals Properties Council

Technical Advisory Committee

R. P. Reed, B. W. Christ

Materials for Utilization at Cryogenic Temperatures Task Group

M. B. Kasen

Metallurgical Transactions

Board of Review

H. M. Ledbetter

NBS -Boulder Editorial Review Board

R. L. Tobler

National Academy of Science

Materials Advisory Group of the Marine Board

T. A. Siewert

Society of Automotive Engineers

1987 Symposium: Durability by Design

B. W. Christ, cochairman of symposium

U.S. Department of Energy

Fossil Energy Research Review Panel

R. J. Fields

U.S. Department of Energy, Office of Fusion Energy

Analysis and Evaluation Task Group

N. J. Simon

Joint U. S .—Japanese Exchange Program Advisory Committee

H. I. McHenry, R. P. Reed

Low-Temperature Materials for Magnetic Fusion Energy Committee

R. P. Reed, Coordinator

U. S. Department of Transportation

Technical Hazardous Liquid Pipeline Safety Standards Committee

R. J. Fields

62
Versailles Project on Advanced Materials and Standards Task Group

Cryogenic Structural Materials Working Group

R. P. Reed

Welding Research Council

Data Base Task Group

T. A. Siewert

Materials and Welding Procedures Subcommittee

T. A. Siewert

63
INDUSTRIAL AND ACADEMIC INTERACTIONS

Industrial Interactions

In response to requests from designers in industry, members of our division

have supplied data and references on properties of specific materials and

advice on material selection, data interpretation, measurement techniques,

and welding procedures. Our expertise has been utilized in wind tunnel

design, space instrumentation, accelerator design, cryogenic tankage, aero- space, and advanced aircraft applications. We are a major source of materials data at cryogenic temperatures. We provide handbook data on the

cryogenic properties of stainless steels, copper, and copper alloys to many

industries. Our excellent test facilities are often used by industrial

personnel. These small requests, too numerous to be listed individually, accrue to a significant assistance to industry.

Aluminum Die-Casting Institute

The division is determining the shear and tensile properties of a newly

developed die-casting alloy for the Aluminum Die-Casting Institute.

Test specimens are provided by the die-casting industry, and some testing equipment is being loaned to NBS by Bell Laboratories for this

work. The results will be disseminated through ASTM. Additional

research on the fatigue properties of this new alloy is under way.

Amax Materials Research Center

H. M. Ledbetter is studying the elastic constants of pure molybdenum

for Amax Materials Research Center in Ann Arbor, Michigan. This

material can undergo low- temperature magnetic transitions that may

affect the elastic constants of Fe—Cr—Ni—Mo alloys.

American Association of Railroads

The NDE group is collaborating with the American Association of Railroads on the design of ultrasonic systems for nondestructive evaluation

of railroad wheels. Eventually these systems will be used for on-line

inspection in railroad yards.

American Welding Institute

The division provided assistance to the American Welding Institute (AWI)

during its formative period. The purpose of the organization is to

improve the transfer of the latest welding technology to U.S. industry.

Last year NBS and AWI hosted a workshop on Computerization of Welding

Data to determine the best organization of data for increasing industrial productivity. Results of the workshop are guiding the development

of a national welding data base.

Ball Aerospace

64
M. W. Austin measured the thermal expansivity of beryllium, which is

used in the design of laser mirrors at Ball Aerospace; this information

is essential for attaining maximum dimensional stability in the mirrors.

Combustion Engineering

As a major producer of nuclear power plants, Combustion Engineering (CE)

has an ongoing interest in nuclear pressure vessel integrity. Heavysection steel plate has been supplied by CE for characterization by NBS

test methods. The behavior of these plates will be compared with the

behavior of pressure vessel steels provided by Oak Ridge National Laboratory (ORNL) . ORNL has machined the CE plates and included them in

the ongoing program at NBS. NBS will provide dynamic crack growth,

arrest, and fracture behavior information to CE for design and evaluation purposes.

Hercules Aerospace and Morton Thiokol

The NDE Group has initiated a collaboration with researchers at Hercules

Aerospace and at Morton Thiokol on nondestructive evaluation of composites. Both electromagnetic and acoustical sensors will be tested for

sensitivity to structural defects in composites. In addition, acoustical arrays will be fabricated for material property determination.

MTS Corporation

At the request of the MTS Corporation in Minneapolis, J. D. McColskey

worked six weeks assisting the Societe Europeen Aerospatiale in Paris,

France to set up a state-of-the-art million-dollar laboratory for

cryogenic materials testing, particularly their closed- loop servo- hydraulic fatigue machines that are comparable to the ones in our laboratory.

Materials Research and Engineering

Several members of our division helped Materials Research and Engineering in Boulder, Colorado to establish standard test procedures for

determining fatigue and fracture properties of materials at cryogenic

temperatures

Micromotion

H. M. Ledbetter and S. A. Kim measured the elastic constants of rods of

five commercial alloys. Micromotion in Boulder, Colorado needed these

measurements for the development of equipment to measure acoustic

elastic properties of metal tubes.

Perkin Elmer

H. M. Ledbetter is studying the sound velocities, thermal expansivity,

and electrical resistivity of pure beryllium at low temperatures for

Perkin Elmer in Danbury, Connecticut. Initial sound-velocity results

suggest an unexpected low- temperature transition.

Potomac Electric Power Company

65
The division has performed numerous failure analyses for the Potomac

Electric Power Company (PEPCO) for two reasons: (1) to assist the public

utility in its service to the community and (2) to maintain and upgrade

NBS competence in materials technology for the electric power industry.

As the average age of U.S. plants increases and failures become more

frequent, such competence will be needed to identify and transfer stateof-the-art technology to industry.

Rockwell International

A major low- temperature test program is in progress to measure the

mechanical properties of selected titanium alloys, which are the main

structural material for aerospace containers of liquefied hydrogen. The

effects of different thermomechanical processes on the mechanical properties of these alloys at 20 K are being examined. Research has concen- trated on tensile and notch tensile measurements of Ti—5Al—2 . 5Sn at 20

K, low- temperature elastic constants of Ti—Al—Sn alloys, and low- and

high-cycle fatigue at 20 K. Distinctions between testing in gaseous and

liquid cryogenic environments have been studied.

SCM Metal Products

H. M. Ledbetter is studying the elastic constants of alumina-dispersionstrengthened copper that is manufactured at SCM Metal Products in Cleveland, Ohio. Because the alumina particles are so small (a few hundred

angstroms) , modeling this material presents an intriguing problem.

Welding Research Council

The Welding Research Council (WRC) and NBS have established a joint

program to produce nationally recognized weld procedures. The WRC is

producing a series of welds that are representative of production welds.

These welds, sectioned according to fabrication code requirements, are sent to NBS, which is serving as a nationally known, impartial testing

laboratory capable of meeting the many accuracy and calibration requirements. NBS support for this program has increased from the testing of

200 specimens in 1985 to testing more than 1000 specimens in 1987. The

WRC is currently reviewing the data from these tests and preparing the

first weld procedure specifications. Eventually, these specifications

will eliminate the costly and time-consuming individual tests for structural integrity that are now required of welding contractors.

Joint Industrial and Academic Interactions

Aluminum Company of America and Iowa State University

The NDE Group has worked with researchers of the Aluminum Company of

America and the Ames Center for Nondestructive Evaluation at Iowa State

University on ultrasonic monitoring of texture and formability in rolled

aluminum alloy sheets.

66
Academic Interactions

Davidson College

H. M. Ledbetter collaborates with L. S. Cain of the Physics Department

at Davidson College in Davidson, North Carolina in experimental studies

of elastic constants. The high- temperature elastic-constant studies of

L. S. Cain complement the low- temperature studies of H. M. Ledbetter.

Currently, they are studying the carbon-plus -nitrogen alloying effects

on Fe—Cr—Ni- alloy elastic constants, which are different at different

temperatures

Lehigh University

G. E. Hicho collaborated with G. Sih, Head of the Fracture Mechanics

Institute at Lehigh University in Bethlehem, Pennsylvania, on thermal

effects at crack tips, a subject of ongoing research in the division.

R. J. Fields continued his work with T. Delph on the quantification of

creep damage. R. J. Fields developed techniques for characterizing the

population of cavities on the fracture surfaces of copper bicrystals

that were tested at Lehigh. Comparison with various theories is under

way

Pennsylvania State University

T. A. Siewert is working with S. Liu of Pennsylvania State University in

the study of droplet transfer frequency in welding arcs. This study is

revealing the physical basis for qualitative descriptions of the voltage

and current values that produce acceptable welds. Arc extinction and

reignition patterns determine the arc stability and so the weld spatter, one measure of an acceptable weld.

Princeton University

The welding group is determining the strength of welds in copper alloys

at 76 K and room temperature. Princeton Plasma Physics Laboratory re- quires the data for the design of the Compact Ignition Tokamak.

Several members of the division collaborated with G. Brown of the

Princeton Plasma Physics Laboratory in research on the compressive me- chanical properties of a nickel -alloy—copper laminate for possible use

in fusion reactor magnet windings. The NBS 52-MN testing facility was used to perform static and cyclic loadings of these laminate specimens.

More than ten thousand cycles were applied at loads between 15 and 20 MN

in the first fatigue testing application for this NBS test equipment.

Stanford University

The NDE group has continued collaboration with B. A. Auld of Stanford

University in the area of electromagnetic sensors. Work this year

centered on the comparison of theoretical and experimental results for

characterization of a capacitive -array sensor.

67
Tohoku University

H. M. Ledbetter collaborates with Y. Shindo of the Mechanical Engineering Faculty on problems of waves scattered by interfaces. They use a scattered-plane -wave ensemble -average model.

University of Arkansas

H. M. Ledbetter, M. W. Austin, and S. A. Kim collaborate with A. M.

Hermann of the Physics Department on the physical properties of the new high-critical- temperature metal-oxide superconductors. These properties

include electrical resistivity, magnetic susceptibility, elastic con- stants, and crystal structure (by x-ray diffraction). The studies focus on substituting various trivalent rare-earth cations (R) in the compound

Ri Ba 2 Cu3 0 7 _ x .

University of California

The Physical Properties Group worked with J. Glazer and J. W. Morris,

Jr. in studying the low- temperature constants of an Al—Li alloy.

University of Cambridge

H. M. Ledbetter works with P. Withers and M. Stobbs of the Metallurgy

and Materials Science Department on the theoretical problem of internal

stress in composites, stress that arises from differences in the thermal

expansivities of the particle and matrix components.

University of Colorado

H. M. Ledbetter collaborates with T. Datta of the Physics Department on theoretical and experimental studies of low- temperature austenitic-steel

physical properties, especially elastic constants and magnetic susceptibility. In these materials, physical properties (and perhaps plasticdeformation properties) depend on the magnetic state. They also study

the elastic and magnetic properties of the new high-critical - temperature

metal-oxide superconductors, especially Y 1 Ba 2 Cu3 0 7 _ x , which show large

elastic and magnetic changes at the critical temperature.

H. M. Ledbetter collaborates with R. Edge of the Physics Department

on studies of the low- temperature magnetic state in Fe—Cr—Ni alloys.

Recently, at Oak Ridge National Laboratory, they obtained new neutrondiffraction results, which they are now trying to interpret.

University of Stuttgart

H. M. Ledbetter collaborates with E. Kroner and B. K. D. Gairola on

the theoretical problems of the elastic constants of polycrystals. This

year, they calculated the elastic constants of perfectly random polycrystalline graphite. These new results help to explain the peculiar

elastic behavior of two important technological materials: graphite

fibers (used in composites) and cast iron.

68
Vanderbilt University

T. A. Siewert collaborated with M. E. Shepard and G. E. Cook in modeling

the self -regulation process in gas-metal-arc welding. Precise measure- ments of arc length, obtained in the NBS welding laboratory, are being

used to determine the coefficients in the model.

69
'

.
APPENDIX

Institute

for

Materials

Science

and

Engineering

L.

H.

Schwartz,

Director

R.

B.

Johnson,

Associate

Director
U

S.

DEPARTMENT

OF

COMMERCE

National

Bureau

of

Standards
MBS-1 14A (REV. 2 -80 ) U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET (See instructions)

1. PUBLICATION OR

REPORT NO. NBSIR b7 3613

2. Performing Organ. Report No. 3. Publ ication Date

4. TITLE AND SUBTITLE

Institute for Materials Science and Engineering

Fracture and Deformation Division

Technical Activities 1987

5. AUTHOR(S)

6. PERFORMING ORGANIZATION (If joint or other than N8S. see in struct/on s)

NATIONAL bureau of standards

department of commerce

WASHINGTON, D.C. 20234

7. Contract/Gram No.

8 . Type of Report & Period Covered

9.

SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS ( Street . City. State, ZIP)

Fracture and Deformation Division, 430

National Bureau of Standards

325 Broadway

Boulder , CO 80303

10.

SUPPLEMENTARY NOTES

|

! Document describes a computer program; SF-185, FIPS Software Summary, is attached.

11.

ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant

bi bliography or literature survey, mention it here) This report summarizes the 1987 fiscal-year programs of the Fracture and Deformation

Division of the Institute for Materials Science and Engineering. Its two major programs are Elastic-Plastic Fracture Mechanics and Fracture Mechanisms and Analysis.

Elastic-Plastic Fracture Mechanics includes nondestructive evaluation, welding, and

thermomechanical processing. Fracture Mechanisms and Analysis consists of physical

properties, fracture physics, composite mechanics, cryogenic materials, and material

performance. The report summarizes the principal accomplishments in these research

areas, including wide-plate crack-arrest tests, a new model of plastic fracture,

design and evaluation of an capacitive-array probe, measurement of the natural frequency for short-circuiting transfer in gas-metal-arc welding, and a model for prediction of the elastic properties of porous ceramics.

The report lists the division's professional staff, their research areas, publications, leadership in professional societies, and collaboration in research programs

with industries and universities.

12.

KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semicolon s) austenitic steels; composites; deformation; fracture mechanics; fracture physics;

low temperatures; nondestructive evaluation; physical properties; welding

13.

AVAILABILITY

^Uni imited

| | For Official Distribution. Do Not Release to NTIS

I 1 Order From Superintendent of Documents, U.S. Government Printing Office, Washington. D.C.

20402.

Order From National Technical Information Service (NTIS), Springfield, VA, 22161

14. NO. OF

PRINTED PAGES

80

15. Price

$ 13,95

USCOMM-DC 604 3-P so
.

*

®

TM

• *

'

n' '^

; ®’


IR 87-3614

CANCELLED

NOT AVAILABLE FOR BINDING