NATIONAL BUREAU OF STANDARDS REPORT

9996

MICROPLASTICITY I - MEASUREMENT OF SMALL

MICROSTRAINS AT AMBIENT TEMPERATURE

To

Materials and Processing Branch

Naval Air Systems Command

Department of the Navy

U.S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
NATIONAL BUREAU OF STANDARDS

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NATIONAL BUREAU OF STANDARDS REPORT

NBS PROJECT

3120410

NBS REPORT

9996

MICR0PLAST1CITY I - MEASUREMENT OF SMALL

MICROSTRAINS AT AMBIENT TEMPERATURE

By

G. W. Geil

and

I. J. Feinberg

Engineering Metallurgy Section

Metallurgy Division

To

Materials and Processing Branch

Naval Air Systems Command

Department of the Navy

IMPORTANT NOTICE NATIONAL BUREAU OF STA

for use within the Government. E and review. For this reason, the whole or in part, is not authors Bureau of Standards, Washington

the Report has been specifically p

Approved for public release by the

Director of the National Institute of

Standards and Technology (NIST) on October 9, 2015

> accounting documents intended objected to additional evaluation

listing of this Report, either in Office of the Director, National

the Government agency for which

pies for its own use.

U.S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

MICROPLASTICITY I - MEASUREMENT OF SMALL

MICROSTRAINS AT AMBIENT TEMPERATURE

INTRODUCTION

This paper presents some important features observed in a study of

microplasticity in metals. Investigations of microplasticity usually are

concerned with very small strains of the order of 10 ^ to \ 0

~'> in

materials that are stressed, generally, much below their normal yield

strengths. The various microstrain measuring techniques used by investigators and the microstrain behavior of selected materials have been

reviewed extensively in a very recent publication (l). Microstrain

measurements generally have been made using electrical resistance gages,

variable inductance transformers, Tuckerman strain gages, or capacitance

gages. The capacitance gage technique was selected for the measurement

of microstrains in this study primarily because it has a very high

— V _ A

sensitivity, on the order of 10 ( to 10 in. /in.

Microstrain measurements limited to the gage length portion of a

tensile specimen was a prime requirement in the present study. In many

of the previous investigations (2 to 9 )

i n which a capacitance gage was

used for the determination of microstrains introduced into a tensile

specimen, the gage was usually attached to the shoulders of the specimen.

The recorded strain also included the strains in the fillet sections and

those portions of the reckjced section outside of the gage length. Thus,

these readings did not provide the more desirable measurement of the

actual strain over the gage length.

As the study progressed, certain effects related to temperature

changes caused by loading and unloading specimens became apparent, which

seem to have been neglected in most reported studies dealing with microplasticity. These effects have now been studied quantitatively and

their importance in reporting microplasticity data established by the

findings in this paper.

SPECIMEN AND GAGE DESIGNS

A tensile specimen and a capacitance gage holder were specially

designed to permit measurement of the actual strain over the gage length.

The specimen (Fig. l) is a modification of ribbed specimen designs previously reported (10, ll). The reduced section of 0.450 inch diameter

has two 45° angle circumferential ribs of 0.05 inch height. These ribs

are spaced two inches apart for attachment of the capacitance gage

holders. The capacitance gage holders are designed for positioning

three gages spaced 120° apart on the specimen. The gages, as installed

on a specimen are shown in Fig. 2. The capacitance probes are positioned

in the upper holder and micrometers fitted with small plate caps on the

micrometer spindles are held in alignment with the capacitance probes by

the lower holder. This micrometer arrangement facilitates positioning of

the micrometer plate caps very close, normally within 0.001 inch or less,

to the ends of the capac itance . probes . Each holder consists of two parts

with an inner groove machined to match the rib on the specimen with about

0.01 inch clearance between the hofder and the reduced section of fhe

specimen. The two halves of each holder are he 1 d 'together by two guide

- 2 -

pins with adjustable compression springs. This holder design was

selected in preference to a knife edge or set screw type holder, since

some slippage generally occurs with the latter types. Even though

slippage might be very small, it would definitely invalidate microstrain

measurements

.

Any bending of the specimen during a test will be magnified greatly,

as revealed by different strain readings of the three capacitance gages

whose axes are 0.75 inch from the axis of the specimen. Moreover, the

average strain of the specimen can be determined readily from the strain

readings of the three gages. The microstrain values reported in this

study are average strain values, except the strains determined within

one minute after unloading of the specimen. These latter strains were

obtained from readings of one gage.

TEST PROCEDURES

Preliminary tests demonstrated that reliable and reproducible strain

measurements, within ± 1 x 10”^, could be made with the NBS capacitance

gage assembly provided special precautions were taken in the testing

procedures and in maintaining precise control of the temperature of the

specimen, gages, and other components of the test equipment. To achieve

this it was found necessary to enclose the specimen, specimen holders

and pull rods, and the- capacitance gages in a thermally insulated chamber

that could be maintained at a constant temperature. The temperature

selected, 2b.2° C, was 1 to 2° C above the thermostat-controlled temperature of the laboratory, and it was maintained to within ± 0.01° C. with a

propor t i ona 1 -power type thermistor controller. The precision variablecapacitor used in the capacitance gage bridge circuit also was enclosed

- 3 -

in a temperature controlled, therma 1 1 y- i nsu 1 ated chamber, as it was found

to be temperature sensitive. The coaxial wires, connectors and switches

used in the measuring circuit of the capacitance gages were shielded.

In addition, metal foil was placed on the inside of the thermal chamber

enclosing the specimen and gages to minimize pick up of stray electrical

fields.

The microplastic behavior of a specimen was determined by employing

a step loadi ng-unloadi ng procedure, loading to successively higher peak

stresses. Measurements of the residual (plastic) microstrain were

repeated after complete unloading following each loading to successively

higher peak stresses. Consistent measurements of the microstrain of a

specimen to within ± 1 x 10“^ while attempting to maintain a constant

low load with the hydraulic tensile machine used in these tests were not

attainable; a very slight variation of only one fourth of a pound in the

load on a steel specimen of 0.450 inch diameter varies the elastic

extension of a 2 inch gage length by approximately 1 x 10” ^ inch.

EXPERIMENTAL RESULTS

Initial Tests on Type 4340 Steel . Preliminary results were obtained

on a tensile specimen machined from a bar of normalized 4340 steel that

had been heated at 900 ° C (I65O 0 F) for one hour and air cooled. The

4340 steel was vacuum melted and had the following chemical composition

in percent by weight; carbon 0 . 40 , manganese 0.68, phosphorus < 0.01,

sulphur < 0.01, silicon 0.33u nickel 1 .80, chrorni urn 0.79* -molybdenum

0 . 24 , balance iron.

. 4 -

The microstructure of the normalized steel consisted mainly of lower

bainite and martensite. Its yield strength (0.2<jo offset) was determined

to be 151 ks i

.

The initial test data revealed that the observed microstrain readings

depended upon the interval between complete unloading of the specimen and

the strain readings. Consistent strain readings at zero load generally

were obtainable only after a waiting period of about 10 minutes, as a

small continuously decreasing strain (contraction of the specimen) with

increase in time was observed during this period. This characteristic

is illustrated in Fig. 3 for three tests on a single specimen. The

specimen was loaded to stresses of 51 > 52 and 53 ks i , representing about

one-third of the yield strength. The specimen was loaded and unloaded

at a rate of approxi mate 1 y 10 ksi/min. In the two tests represented by

curves 1 and 2, the first strain measurements were obtained 50 and 30

seconds, respectively, after complete unloading of the specimen, whereas,

in the test represented by curve 3> (Fig. 3 part c) the first strain

measurement was obtained 10 seconds after unloading of the specimen. As

an appreciable decrease in strain apparently had occurred prior to the

first strain measurements in the two tests represented by curves 1 and 2,

the curves were norma l i zed wi th curve 3 at the 1 minute value and the

normalized turves are shown in pa.rt c of Fig. . 3,.

The observed total decreases i.n strain with time suggested to the

t authors that a decrease of about 0 . 06 '° C in the temperature of the

specimen probably had occurred during the time involved' in each of the

three series of strain readings. This suggested temperature change was

- 5 -

based upon a value of 11.3 *

10"^’ in/in/°C for the coefficient of linear

thermal expansion of 4340 steel. Some cooling of the specimen during

loading within its elastic range and heating during unloading is to be

expected from thermodynamic considerations (12). Elastic extension of

a material with a positive coefficient of linear thermal expansion under

adiabatic loading conditions should decrease the temperature of the

material. Conversely, elastic contraction that occurs during elastic

unloading of the material should increase its temperature.

Temperature Changes of AlSi 4340 Steel Specimen Accompanying Loading

and Unloading within its Elastic Range . A literature search did not

reveal any data on the specific temperature change of steel during

loading and unloading within its elastic range. Consequently, a series

of tests were conducted on the normalized 4340 steel specimen in which

its temperature changes during loading, unloading, and the subsequent

zero load condition were measured. A 22 gage Chrome 1 -A 1 ume 1 thermocouple

was taped tightly to the surface of the specimen at the midpoint of the

gage-length section and a zero suppression, multi-range milli-microvoltmeter (ranges varying from 0 - 0.1 pV full scale, to 0 - 100 qV

full scale) was used to determine the temperature changes.

Iri the first series of tests the specimen was loaded within its

elastic range at selected loading rates to a stress of 50 ks i . This

stress was maintained until the specimen regained its initial temperature.

The specimen then was unloaded at the same rate. Representative data

are presented in Fig. 4.

- 6 -

The data obtained in a test at a relatively slow loading rate of

3.1 ksi/min. are shown in part a. Special attention should be directed

to the temperature decrease observed during loading and again to the

temperature increase during unloading. The temperature changes were

appreciable, even though testing at this slow loading rate did not

approach adiabatic conditions, and there was a considerable heat flow

into and out of the specimen through its holders and its environment.

Data obtained in a test at a higher loading rate of 19 ksi/min.

are shown in Fig. 4, part b. The heat transfer was less and consequently the temperature changes were about three times greater than

those observed in the test conducted at the loading rate of 3*1 ksi/min.

Larger temperature changes were observed (Fig. 4, part c) in the

test at a high loading rate of 137 ksi/min. This test closely approached

an adiabatic test condition with very little heat transfer. A slight

time lag was observed in the decrease of the surface temperature of the

specimen on loading. This is indicated by the occurrence of a minimum

surface temperature several seconds after application of the maximum

loading stress of 50 ksi. Attention also should be directed to the time

(10 to 20 minutes) that was required for the specimen to regain its initial

temperature, both after loading to and holding at 50 ksi, and after

complete unloading. Similar data (not shown) were obtained in additional

tests at other loading rates.

A second series of tests was conducted in which the specimen was

stressed within its elastic range at selected loading rates to 50 ksi and

then immediately unloaded at the same rate. (Loading followed by

immediate unloading is the method usually employed in mi crop 1 as t i c i t

y

- 7 -

studies.) Representative data are presented in Fig. 5.

At a slow loading rate of 4.7 ksi/min. (Fig. 5 > a ) heat transfer

into the cooled specimen was sufficient for it to regain its initial

temperature value early in the unloading stage at about 44 ks i

.

Further heating of the specimen generally occurred during the remainder

of the unloading stage producing a temperature at 12.5 ks i that was

nearly 0 . 05 ° C above its initial temperature. This was followed by

slow cooling during the further unloading of the specimen; the heat

transferred out of the specimen to its holders and environment was

greater than the heat generated in the specimen by its additional

elastic contraction.

The temperature changes of the specimen during a test at a loading

rate of 19 ksi/min. (Fig. 5,b) were greater than those observed in the

test at the slower loading rate. However, the increase in temperature

of the specimen above its initial value at complete unloading was much

less than the temperature decrease on loading to the 50 ks i stress.

The temperature changes observed in a test at a very high loading

rate of 3^0 ksi/min. are presented in Fig. 5c. This test closely

approaches adiabatic conditions with very little heat transfer during the

loading and unloading. Moreover, the change in the surface temperature

of the specimen apparently lagged behind that in the interior, and this

resulted in the maximum decrease in the surface temperature occurring

at the 49 ksi stress during unloading. At the instant of complete

unloading of the specimen its surface temperature was slightly below

its initial value and it continued to increase for about 40 seconds,

- 8 -

attaining a value slightly above its initial temperature. This increase

in the surface temperature after complete unloading can be attributed to

the temperature gradient existing in the cross section of the specimen.

Moreover, even though the duration of the loading and unloading stages

was very short (about 20 seconds) there was a very small transfer of

heat into the specimen since its temperature during loading and unloading

was below the initial temperature. The observed increase in temperature

above the initial temperature can be attributed to this heat transfer.

The time required for the specimen to attain temperature equilibrium (to regain its initial temperature) after unloading in these tests

(Fig. 5 ) and in similar tests (not shown) at other loading rates was

generally about 10 minutes.

The data presented in Figs. 4 and 5 and those obtained in similar

tests are summarized in Fig. 6. These graphs depict the influence of

the loading rate on the maximum changes in temperature of the specimen

during loading to 50 ks i stress followed by either immediate or delayed

un 1 oad i ng

.

The two curves in Fig. 6 for tests in which unloading was delayed

until the specimen had regained its initial temperature reveal that the

decrease in temperature of the specimen by the loading was numerically

equal to the increase in the temperature of the specimen by unloading

at the same rate. These curves also illustrate the dependence of the

maximum temperature changes on the approachab i 1 i ty to adiabatic test

conditions. Adiabatic conditions apparently were approached closely at

loading rates of about 100 ksi/min. and higher, as indicated by the

nearly constant values of the maximum temperature changes at these rates.

- 9 -

The two curves in Fig. 6 for the tests in which the specimen was

unloaded immediately after loading to 50 ks i stress also illustrate

indirectly the approachab i 1 i ty to adiabatic test conditions. If the

test conditions were completely adiabatic the temperature of the specimen

after unloading should be the same as its initial value. Thus, the

increases in temperature above the initial temperature of the specimen

that were observed following the complete unloading is a relative

indication of the heat transfer into the specimen during the test.

Temperature Changes of Invar Accompanying Loading and Unloading

Within its Elastic Range . The degree of cooling and heating of a specimen during loading and unloading within its elastic range should depend

upon its coefficient of linear thermal expansion. For conformation of

this characteristic a test was conducted at 24.2°C on a specimen of

annealed Invar with a very low positive coefficient of linear thermal

expansion at ambient temperatures (approximately 1 x 10“^ in/in/°C which

is less than one-tenth that of the 4-340 steel). This specimen was

machined from a sample of Invar that had been given the prescribed

annealing treatment of 830° C (1525° F) for one hour followed by quenching

in water. Its yield strength (0.2 percent offset) was determined to be

43.1 ks i . The specimen was prestressed in microplasticity tests to 15

ks i with a total microstrain of 15 x 10"u

. This prior treatment was

selected to minimize any plastic microstraining of the specimen during

subsequent tests in which temperature changes were measured. The temperature changes observed during loading and unloading of the specimen within its elastic range at a rate of approxi mate ly 10 ksi/min. to stresses

of 10, and 15 ks i are presented in Fig. 7 . A very slight, although

10

significant, cooling occurred during loading of the specimen and heating

during unloading. These temperature changes conformed to expected

values. For example, when stressed at 15 ks i the elastic strain of the

Invar specimen which has a modulus of elasticity about two-thirds that

of the 4340 steel, should be about one-half that of the 4340 steel specimen stressed to 50 ks i . Furthermore, with the coefficient of linear

thermal expansion of the Invar specimen equal to approxi mate 1 y one-tenth

that of the 4340 steel specimen, its observed temperature change of

-0.007° C at the 15 ksi stress (Fig. 7 ) should be, and was, approximately

one-twentieth of the temperature change of -0.13° C observed for the

4340 specimen stressed to 50 ksi at a similar elastic strain rate. It

should be noted that to obtain similar elastic strain rates, loading of

the Invar specimen at 10 ksi/min. corresponds to loading of the 4340

specimen at about 15 ksi/min.

Temperature Changes of Normalized 4340 Steel Specimen during Loading

into Microplastic Deformation Range and Unloading . Measurements were made

of the changes in temperature of the normalized 4340 steel specimen

accompanying the loading into the microplastic deformation range,

unloading, and the subsequent zero load condition. The specimen prior tc

this test had been stressed in step loading-unloading microplasticity

tests to a peak stress of 110 ksi with a total residual microstrain of

830 x 10

-l°. The temperature changes observed in a test conducted at a

loading rate of 18 ksi/min. to a stress of 129 ksi, followed by immediate

unloading at the same rate are presented in Fig. 8.

The temperature of the specimen decreased during the elastic

loading to a stress of approximately 110 ks i . As the stress increased

-f from 110 to 129 ks i a plastic strain of 3^7 x 10” occurred, accompanied

by heating of the specimen. The heating effect due to the plastic

deformation on increasing stress from 110 to 129 ks i was much greater

than the cooling effect due to the accompanying increase in elastic

strain. The temperature continued to increase during unloading, reaching

a maximum of 0.2° C above the initial temperature of the specimen. The

time required for the specimen to cool to its initial temperature after

unloading was about 30 minutes.

The above test and the previously reported tests in which the temperature changes of specimens of normalized 73^0 steel and annealed Invar

were monitored, clearly demonstrate that reliable plastic microstrains

usually cannot be readily determined from readings taken immediately or

very soon after complete unloading of the specimen, due to the thermal

strains that occur during the interval required for the specimen to regain

its initial temperature. These thermal strains can be minimized, but not

completely eliminated, by step loading and unloading (l) at very low

loading rates, probably requiring an hour or more for the I oadi ng-un 1 oad i ng

step or ( 2 ) at very high loading rates approaching adiabatic test

conditions. Thus, any reported data obtained in microplasticity tests

at ambient temperature in which the microstrain readings were taken

immediately or very quickly after unloading of the specimen may be of

questionable value due to the inclusion of unmeasured thermal strains.

Moreover, any microstrain readings during the loadi ng-unloadi ng stage,

such as those obtained through automatic recording of tests at ambient

temperature, will include effects of thermal contractions and

12

expansions along with the elastic strains as well as plastic strains

if the specimen had been stressed into the microplastic deformation range.

SUMMARY

A tensile specimen and a capacitance gage assembly were designed

for determination of small residual microstrains to a sensitivity of

± 1 x lO

-

'

7 over a 2-inch gage-length section.

A study was made of the microplastic behavior of a specimen of

normalized 4340 steel, employing a step loading-unloading procedure,

loading to successively higher stresses. The residual microstrains were

determined at zero stress after each unloading of the specimen. In

order to obtain consistent and reliable measurements of residual microstrains to within ± 1 x 10”7, special test procedures were required;

these and some observed characteristics follow:

(1) The specimen and capacitance gage assembly must be maintained

at a constant temperature to within ± 0.01° C prior to loading the

specimen and during the gage readings following the complete unloading

of the specimen. This required the enclosing of the specimen, specimen

holders and pull rods, and the capacitance gage assembly in an insulated

thermal chamber controlled to within ± 0.01° C.

(2) Satisfactory measurements of residual microstrains were not

plausible while maintaining a small load on the specimen with the tensile

machine. For example, a slight variation of only 4 ounces in the load

on the 0.450 inch diameter specimen of 4340 steel changes the elastic

extension of a 2-inch gage-length section by approximately 1 x 10" (7 inch.

(3) Small changes in temperature that accompany the loading and

unloading of the specimen must be taken into account. The 4340 steel

specimen with a positive coefficient of linear thermal expansion undergoes significant cooling during loading within its elastic range and

heating during unloading.

Appreciable heating also occurs during microplastic deformation,

even though the microstrain of the specimen is very small. Thus, the

specimen at or immediately after unloading is not at its initial

temperature, i.e., the controlled temperature of the thermal chamber.

An interval of 10 or more minutes was generally required for the specimen

to regain its initial temperature.

Microstrain measurements determined during loading or unloading of

specimens will include thermal strains due to temperature changes, along

with the elastic strain and any plastic strain of the specimen. Moreover, microstrain measurements taken immediately after unloading of the

specimen will include some thermal strains. An interval of 10 or more

minutes following unloading usually was required prior to taking microstrain readings to eliminate thermal strains and provide reliable

residual microstrains; when the plastic microstrain during the test was

very large, such as 3^7 x 1 0“^’ or greater, a waiting period of 30 or more

minutes was required.

Thermodynamical theory indicates that the temperature changes during

loading within the elastic range and unloading should be relatively small

for a material which has a low coefficient of linear thermal expansion.

To confirm this behavior temperature changes were monitored in a test

conducted at 24.2° C on a specimen of annealed Invar which had a very

14 -

low coefficient of linear thermal expansion at ambient temperatures.

The temperature changes of the specimen were very small, less than

0.01° C, during loading and unloading within its elastic range to

stresses of 5, 10 and 15 ks i . The specimen regained its initial temper

ature within one minute after unloading. The accompanying thermal

strains of the specimen were too small to be readily measured.

ACKNOWLEDGMENTS

The authors wish to express their appreciation for the partial

financial support of this investigation by the Naval Air Systems

Command of the Navy Department. The authors also wish to thank

W. D. Jenkins for his assistance in the measurement of the temperature

changes of the specimens and to Dr. R. D. Cutkosky of the NBS Institute

for Basic Standards for his assistance in the design of the capacitance

gage bridge circuit. The authors acknowledge their appreciation for

advice and helpful discussions of Dr. M. R. Meyerson.

- .15 -

REFERENCES

1. "Mi croplastici ty" - book edited by C. J. Mahon, Jr., Interscience

Publishers, Division of John Wiley & Sons (1 968)

2 . J. M. Roberts and N. Brown, Trans. ASHE 28 , (i960).

3 . K. F. Lukens and N. Brown, Acta Metal lurgica 106 (1961).

4. J. M. Roberts and N. Brown, Acta Metal lurgica J_0 , 430 (1962).

5 . N. Brown and R. A. Ekvall, Acta Metal lurgica 1

, 101 (1962).

6 . J. M. Roberts and D. E. Hartman, Trans. ASME 230 ,

1 125 ( 1984 ).

7 . R. Kossowsky and N. Brown, Trans. A I ME 233 , 1 389 (1965).

8. R. Kossowsky and N. Brown, Acta Metal lurgica J_4 , 131 (1966).

9 . N. Brown, Final Report - Office Naval Research, 1 May 1967.

10 . D. H. Fisher, R. L. Carlson and F. C. Holden, ASTM Materials

Research and Standards 2 , 26 (1962).

11 . R. K. Penny, E. G. Ellison and G. A. Webster, ASTM Materials

Research and Standards 6 , 76 (1966).

12 . C. Zener, "Elasticity and Anelasticity of Metals", The University

of Chicago Press ( 1948).

- 16 -

USCOMM-NBS-DC

FIG. I. MICROPLASTICITY SPECIMEN

FIG. 2 CAPACITANCE GAGES INSTALLED ON SPECIMEN

CHANGE

IN

LENGTH,

I0' T

in/in

FIG. 3 . Decrease in total strain with time after complete unloading of normalized

14. 314.0 steel specimen. Specimen loaded within its elastic range at approximately 9 ksi/min (l MN/nrT/sec) and immediately unloaded at same rate.

(a) Stressed to 51 ksi (352 MN/m2

); £b) Stressed to 52 ksi (359 MN/m )

;

and (c) stressed to 53 ksi (3^5 MN/m ).

TEMPERATURE

CHANGE,

G. 4. Temperature changes or normalized 4340 steel specimen during test. Specimen

loaded within its elastic range at selected rates to 50 ksi (345 MN/m2

) with

delayed unloading at same rate.
LL
TEMPERATURE

CHANGE

FIG. 5 . Temperature changes of normalized 43^0 steel specimen during test. Specimen

loaded within its elastic range at selected rates to 50 ksi (3^-5 MN/m2

) and

immediately unloaded at same rate.

MN/m2 /s

J

FIG. 6 . Effect of loading rate on maximum temperature changes of normalized 4340 steel

specimen stressed to 50 ksi (345 MN/rrr) and unloaded.

TIME ,MINUTES

FIG. 7. Temperature changes of annealed Invar specimen during test. Specimen loaded at a rate of 10 ksi/min (l.l MN/m2 /s) to selected stresses within its elastic range

and immediately unloaded at the same rate.

+ 0.25

+ 0.20

+0.15

+0.10

c_^>

Li I +0.05

CJj

c^>

<xcc

0.20

0.25

<8—10

0 ~°I

-0.05

12.5

c

-0.10

CNJ LO T

1

-0.15 _ 50—

75-

oqj o_o_o_o_o^ -o4340 STEEL

-O r, -

STRAIN, 367 X I0"

6

RATE 18 ksi/min (2.1 MN/m2/s)

NUMBERS INDICATE STRESS, ksi

J L j I i I i L

0 20 40 60 80 100 120 140 160 TIME , min

FIG. 8. Temperature changes of normalized 4340 steel specimen during loading

into its microplastic range. Specimen loaded at a rate of 19 ksi/min

(2.2 MN/m /s) to 129 ksi (890 MN/rr+) and immediately unloaded at the same rate.

J]]J]]]J]J]]]]]]