Lehigh UniversityLehigh Preserve

Fritz Laboratory Reports Civil and Environmental Engineering

1969

Fatigue crack propagation in A514 steel, November1969R. Hertzberg

H. Nordberg

Follow this and additional works at: http://preserve.lehigh.edu/engr-civil-environmental-fritz-lab-reports

This Technical Report is brought to you for free and open access by the Civil and Environmental Engineering at Lehigh Preserve. It has been acceptedfor inclusion in Fritz Laboratory Reports by an authorized administrator of Lehigh Preserve. For more information, please contactpreserve@lehigh.edu.

Recommended CitationHertzberg, R. and Nordberg, H., "Fatigue crack propagation in A514 steel, November 1969" (1969). Fritz Laboratory Reports. Paper405.http://preserve.lehigh.edu/engr-civil-environmental-fritz-lab-reports/405

3SS.1

Low Cycle Fatigue BehaviorOf Joined Structures

FATIGUE CRACKPROPAGATION IN

AS14 STEEL

byR. WI Hertzberg

HR Nordberg

Fritz Engineering La boratory Report No, 358.7

i

1

Low-Cycle Fatigue

FATIGUE CRACK PROPAGATION IN

A514 STEEL

by

Richard Hertzberg

Hans Nordberg

This research was sponsored by the Office of Naval Research,Department of Defense, under Contract N0014-68-A-0514;NR064-509. Reproduction in whole or part is permitted forany purpose of the united States Government.

Department of Metallurgy and Material ScienceLehigh University

Bethlehem, Pennsylvania

November, 1969

Fritz Laboratory Report No. 358.7

358.7

TABLE OF CONTENTS

Page

SYNOPSIS i

1. INTRODUCTION 1

2. EXPERIMENTAL PROCEDURES 7

1. Weld Preparation 7

2. Specimen Preparation 7

3. Experimental Testing 8

4. Fractographic Techniques 9

3. EXPERIMENTAL RESULTS AND DISCUSSION 10

1. Effect of Specimen Thickness on 10Fatigue Crack Propagation

2. Effect of Mean Stress on Fatigue 11Crack Propagation

3. Effect of Specimen Orientation 12on Fatigue Crack Propagation

4. Effect of Weldments on Fatigue 14Crack Propagation

5. Effect of Environment on Fatigue 19Crack Propagation

6. Fractographic Observations 20

4. SUMMARY 21

5. NOMENCLATURE 23

358.7

6. ACKNOWLEDGEMENTS

7. TABLES AND FIGURES

8. REFERENCES

Page

24

26

35

358.7 -i

SYNOPSIS

This paper presents one phase of a major research

program designed to provide information on the behavior and

design of joined structures under low-cycle fatigue. This

paper reports on a detailed study of the fatigue crack

propagation process in the A5l4 steel base plate, heat

affected zone and weld metal. These data are evaluated in

terms of fracture mechanics concepts.

In the early studies of this phase, several

variables of potential importance in the fatigue process

were examined. It was observed that fatigue crack

propagation in A514 steel is strongly dependent upon the

stress intensity factor range and only moderately dependent

upon the maximum stress intensity level over a range of A =. Krnax

1.0 - 4.0 where A =~. The growth rate is independent

of specimen thickness (up to one inch). Fatigue crack

propagation rates were determined in A5l4F and A5l4J steel

in the RW and WR orientations. No directionality in fatigue

behavio~ was noted in A514F while A5l4J did exhibit some-

what faster crack growth in the WR direction as compared to

that observed in the RW direction. The above results are

consistent with data previously published for other materials.-

358.7 -ii

A particularly striking effect was noted in

crack propagation studies of A514F steel containing

weldments. Fatigue crack propagation rates in transverse

weld metal and the heat affected zone were substantially

less than in the base metal. For example, at a stress

intensity level of 40,000 psi in., the fatigue crack

propagation rate in the weld metal was almost an order of

magnitude less than that displayed by the base plate. Growth

rates in the three metallurgically distinct regions tended

to converge at large stress intensity levels. After a heat

treatment of 110QoF for one hour, the crack growth rates in

the HAZ and weld metal increased significantly and were then

comparable to the growth rate in the base plate. Stated

differently, the dependence of crack growth rate on the

stress intensity level in the weld metal and HAZ decreased

as a result of the heat treatment. Heat treatment of the

base plate did not affect fatigue behavior. Similar results

were found in fatigue studies of longitudinal welds though

the magnitude of growth rate differences was considerably

less than was observed in the transverse welds. Again, heat

treatment eliminated the major differences in crack growth

in the different weld regions. These results are discussed

with reference to residual stress.

358.7 -1

1. INT~ODUCTION

When formulating the experimental program for a

study of the fatigue behavior of a material, it is

important at the outset to establish the relative importance

of the initiation and crack propagation stages in the fatigue

life of a component. It has been well established that in

many engineering applications where metallurgical and/or

manufacturing flaws are present, the crack propagation stage

is dominant in the fatigue process. This is particularly

true under conditions of high stress resulting in a relatively

small number of cycles to failure. Consequently, the main

thrust of the following remarks will deal with an evaluation

of the fatigue crack propagation behavior of ASTM type A514

steel and the effect of welding upon the fatigue crack

propagation process.

While most of the earlier studies of fatigue

crack propagation were performed with aluminum alloys,

thereby eliminating a direct comparison of these results

with the present investigation, these earlier results did

isolate the main variables affecting fatigue crack

propagation. Most of the earlier crack propagation laws

358.7 -2

described the crack growth rate as being related to some

function of applied stress· and crack length. (1-7) . In 1963

Paris and Erdogan(8) presented a comprehensive compilation

of fatigue crack propagation data and showed that the

fatigue crack growth rate was strongly dependent upon

changes in the stress intensity factor range. Hence, the

concepts of fracture mechanics which have contributed much

to the understanding of static fracture can also be used in

evaluating the response of a material under cyclic loading.

Compiling the results of several investigators who looked

at the response of both ferrous and non-ferrous materials,

Paris and Erdogan(8) showed that the fatigue crack growth

rate could be described by the following relationship,

~~ = C~Kn, where a is crack length, ~K is stress intensity

factor range C, n are material constants. In their earlier

studies they found the exponent "n" to be equal to 4. It

is noted that the fatigue crack propagation rate is

primarily a function of the stress intensity factor range·1

with all other variables being combined in the constant

term. For a more accurate evaluation of the fatigue response

it is obviously necessary to incorporate in any given formula.

as many of the variables as possible. In attempting to

arrive at a more comprehensive relationship, Forman, Kierney

and Engle(9) ha~ proposed a relationship of the following

form:

358.7

da AmliKndn =

-XAKKc

where A Kmax== LrK

Krnax = maximum stress intensity factor

K = fracture toughnessc

m,n = material constants

-3

It is to be noted that this relationship includes the stress

intensity factor range, 6K, a measure of the maximum stress

intensity factor,A= ~~x , and an evaluation of the material

fracture toughness in the previous relationship incorporates

the role of metallurgical factors. At low growth rates

associated with low stress intensity conditions it has been

found that metallurgical factors exercise only a secondary

effect on fatigue crack propagation. This observation was

found by weber(lO) for the case of austenitic stainless

steel and 70-30 type brass, by Heiser (11) for the case of

banded steel and by Miller(12) for the case of 4340 steel.

However at high stress intensity factor values associated

with much higher growth rates, the metallurgical structure

plays an important role in the fatigue process as shown

by Miller(12) and Heiser. (11) It has been argued that

the increasing importance of metallurgical effects at higher

stress intensity factors is associated with the increasing

358.7 -4

amount of unstable crack extension caused by locallized

fracture of inclusions. It is to be noted that the rapid

increase in fatigue crack propagation rates at high stress

intensity levels can be predicted by the Forman et ale

relationship under conditions where the maximum applied

stress intensity factor approaches the material fracture

toughness level.

As mentioned above, the most important variable

in the fatigue crack propagation behavior of a material

has been shown to be the stress intensity factor range.

The influence of mean stress on fatigue crack has also been

investigated. Frost(13) and Yokabori et al. (14) found

that the effect of mean stress upon fatigue crack propagation

in steel alloys was of second order importance. However,

since these tests were performed at relatively low stress

levels, they do not assess the influence of high mean stress

levels which approach the fracture toughness level of the

material. It is to be expected that the mean stress level

should playa more important role under these conditions.

The effect of environment on fatigue crack growth

has been the subject of recent reviews by Achter,(15)

Johnson and Paris, (16) and Wei. (17) For the case of a

358.7

material sensitive to environment, stress corrosion

cracking can play an important role ~n fatigue at

-5

intermediate stress levels where K is larger thanmax

K and where the crack propagation time is sUfficientlyI sec

long.

In a recent review article, McMillan and

Hertzberg(18) summarized the important role of electron

fractographic observations in fatigue studies. They showed

that fatigue striations found on fatigue fracture surfaces

are a source of considerable quantitative information.

While extensive use of fractography has been made in studies

of non-ferrous alloy fatigue behavior, it is unfortunately

far more difficult to develop similar data for ferrous

materials since fatigue striations are not nearly as easily

resolved on steel fracture surfaces as they are in aluminum

alloys.

The objective of this program is to evaluate the

fatigue response of welded structures. Since welded

components almost always contain metallurgical flaws

resulting from the welding process, and since the welded

structures of interest are subject to relatively few cycles

at high stress levels, the point of view taken in this

358.7

investigation will be that the fatigue crack propagation

stage is dominant.

-6

The fatigue crack propagation behavior of type

A514 steel will be examined as a function of specimen

thickness, mean stress at both low and high crack growth

rates, specimen orientation with respect to the rolling

direction, and in the presence of weldments. In the latter

case, fatigue crack propagation studies will be performed

in welded panels where the fatigue crack will be made to

grow both across the weld (longitudinal weld) and along

both of the weld metal and the heat affected zone,

(transverse welds).

358.7 -7

2. EXPERIMENTAL PROCEDURES

2.1 Weld Preparation

Four welded plates were prepared from 3/4 inch

thick plates of T-l steel. All weldments were made by a

Metal Inert Gas process using 60 degree joints. The filler

metal used was AIRCO AX-90. Longitudinal plates having

dimensions of 6" x 18" x 3/4" with the weld running

parallel to the long direction were welded with five passes

of the filler metal. The heat input for these passes

~anged from 35 to 42 Kj/inch. Transverse plates having

dimensions of 18" x 14" x 3/4" were welded in the long

direction in eight passes with heat inputs varying from

22 to 56 Kj/inch. During the welding process all weld

passes were allowed to cool in air approximately 1 1/2

hours before the next pass was made.

2.2 Specimen Preparation

The fatigue crack propagat,ion studies were performed

on single edge notch specimens 12 11 x 3 11 X 1/8" with 3/4"

loading pin holes, Fig. 1. Specimens .061, and .266 inches

thick were also prepared for determination of thickness effects.

358.7 -8

Notches were introduced to the specimen· with a machine saw

cut followed by a jeweler's saw cut., The total length of

the starting crack was approximately 3/8".

Specimens containing weldments were prepared

from panels obtained by slicing the original plate in half

along the mid-thickness plane. For the studies involving

crack propagation entirely in one region (for example

along the weld metal on HAZ) specimens were prepared from

the transverse plates such that the weld was perpendicular

to the long direction of the specimen. For longitudinal

samples where a crack propagates across different metallurgical

structure, specimens were prepared from the longitudinal

plates with the weld parallel to the loading direction.

2.3 Experimental Testing

~atigue crack propagation tests were performed on

an MTS machine at a frequency of lOcps. With the aid of a

travelling microscope, the instantaneous crack length was

recorded with respect to the corresponding number of cycles

applied. These data along with ,applied loads and dimensional

data were subsequently analyzed with the aid of a CDC 6400

computer.

358.7 -9

2.4 Fractographic Techniques

Electron fractographic techniques were applied

to the fatigue fracture surfaces. Samples suitable for

viewing on the electron microscope were prepared by the

two stage replication technique. Fractured surfaces were

cleaned in acetone and then firmly covered with an acetone

softened strip of cellulose acetate. After drying, the

tape was stripped off, placed on a slide face up, and

shadowed in a vacuum using platinum carbon pellets as the

shadowing material. A layer of carbon, 400-500 Angstroms

thick, was then deposit~d upon the shadowed replica. These

samples were then cut into approximately 1/8" squares and

placed in an acetone bath where the cellulose acetate

replica was allowed to dissolve. The carbon films with the

shadowing material were mounted on 200 mesh grids for

examination in the electron microscope.

Viewing of the replicas was done with an RCA

EMU-3G electron microscope. Photomicrographs were taken

of various specimens at magnifications ranging from 5,400X

to 35,OOOX.

358.7 -10

3. EXPERIMENTAL RESULTS AND'DISCUSSION

In the initial studies of this phase of the

overall program, a pilot study was initiated to examine

several variables that might affect the fatigue crack

propagation behavior ofA514 steel with and without welds.

It will be shown in the following comments that while some

variables are of only mild importance other factors

contribute significantly to the fatigue behavior of the

material.

3.1 Effect of Specimen Thickness on Fatigue Crack propagation

To examine the effect of specimen thickness on

fatigue crack propagation in type A514J alloy steel, fatigue

studies were performed on panels of 0.061, 0.126, and 0.266

in. thickness. In all cases the crack was oriented normal

to the rolling direction of the panel. As shown in Fig. 2,

there was no observed effect of thickness on crack

propagation rate. One may conclude from this observation

that the crack propagation rate was independent of anyI '

metallurgical variation associated with the specimen

thickness and/or that the fatigue crack propagation rate

was independent of the relative size of the plastic zone

358.7

with respect to the sheet thickness for the range of

thickness examined.

with the crack oriented parallel to the rolling

direction of the panel, fatigue crack propagation rate in

1/8" thick type A514F steel was similar to that observed

by MCHenry(19) in 111 thick panels. Therefore, in a range

of .06 to 111 thick panels there was no observed effect of

thickness on fatigue crack propagation in A514 steel.

3.2 Effect of Mean Stress on Fatigue Crack Propagation

-11

To evaluate the effect of mean stress on fatigue

crack propagation in type A5l4J the fatigue crack propagation

rates were determined as a function of the A value, a

parameter defined as the ratio of the maximum stress

intensity factor to the stress intensity factor range. The

data given in Fig. 3 show the material response as a function

of A over a range of 1.07 to 4.3. It was found that the

data could best be portrayed by two straight line segments

in a log stress intensity factor range -- log growth rate

plot. It is not clear whether such a discrete discontinuity

should exist based upon either continuum or microscope

considerations. Nevertheless, it is possible to treat the

growth rate data from each linear segment as

358.7 -12

where C, ro, n = material constants.

In the linear segment at lower growth rates

the estimated values of m and n are 0.4 and 2.4, respectively,

while in the upper segment of the data the m and n values

are higher. These exponent values reveal that while fatigue

crack growth rates are very sensitive to 8K there is less

dependence on Kmax or Kmean For example, a four-fold

increase in the stress intensity factor range can effect a

thirty-fold increase in the fatigue crack propagation rate

whereas a four-fold increase in the ~ value gives rise to

less than a factor of 2 increase in the fatigue crack

propagation rate. The greater dependence of K on growthmax

rate at high K levels, may be related to a greater degree

of local cracking of inclusions during the fatigue process.

Consequently the fatigue process at high stress intensity

levels may consist of a summation of a pure fatigue process

combined with local static fracture associated with broken

inclusion particles.

3.3 Effect of Specimen Orientation on Fatigue CrackPropagation

To determine the extent of metallurgical

358.7 -13

anisotropy upon fatigue crack propagation, panels were

prepared such that fatigue. cracks were grown both parallel

and perpendicular to the sheet rolling direction. In Fig.

4 it is seen that no directionality effect was observed in

A5l4F material. The fatigue crack propagation rates were

identical with the crack traversing both parallel and

perpendicular to the rolling direction. In A514J material,

it was observed that fatigue crack propagation was faster

when the crack traversed parallel to the rolling direction.

This effect was negligible at low stress intensity levels

but did produce higher crack growth rates at the higher

stress intensity levels. For example, at a stress intensity

range of 50,000 psi in the fatigue crack propagated twice

as fast when running parallel to the rolling direction than

when it was oriented perpendicular to the rolling direction.

As mentioned in the previous section, local fracture of

oriented inclusions could have given rise to the faster

growth rate when the crack was oriented parallel to the

rolling direction. Since multiple tests were not conducted

to evaluate the role of crack orientation with respect to

the rolling direction, it is not possible to conclude that

the A5l4F materials was definitely superior in transverse

behavior to that in the A5l4J material. More testing would

be necessary to'validate this point. In any case, the factor

358.7

of two different in growth rate observed at high K levels

in this study is not considered to be a major effect and

suggests the metallurgical effect of crack plane to ~e

of second order importance.

3.4 Effect of Weldments on Fatigue Crack Propagation

-14

Having determined the role of specimen thickness,

mean stress, and specimen orientation on fatigue ,crack

propagation of the base metal, it was possible to evaluate

the effect of weldments on fatigue crack propagation in

theA5l4F material. Tests were conducted with both

longitudinal and transverse welds.

Crack propagation rates in the heat-affected

zone and weld metal in transverse welds were considerably

different than that displayed by the base metal. In Fig.

5, it is seen that fatigue crack propagation in the weld

metal was substantially less than in the base metal. This

effect was most pronounced at low stress intensity levels;

for example, at a stress intensity level of 40,000 psi

in the fatigue crack propagation rate in the weld metal

was almost an order of magnitude less than that displayed

by the base metal. It was observed that the relative

advantage of the weld metal over the base metal tended to

358.7

be eliminated at the higher stress intensity levels.

Stated in another way, the ·power dependence of the growth

rate on stress intensity range was much higher in the weld

metal where n 4.6 than in the base metal where n 2.4.

-15

As shown in Fig. 6, a similar observation was

found with respect to fatigue crack propagation in the

heat-affected zone as compared to the base metal. Here

again, fatigue crack propagation rates were considerably

lower in the heat-affected zone than in the base metal,

especially at low stress intensity levels. It is possible

that the marked superiority of the heat-affected zone and

weld metal in the transverse specimens was due to some k~nd

of favorable residual stress pattern associated with the

welding process and the subsequent machining procedure to

produce the specimens. If, indeed, the marked superiority

of the heat-affected zone and weld metal at low crack growth

rates was due to a residual stress effect, it had to be of

a compressive nature. It is not clear at this time how

such a favorable residual stress pattern could have been

produced. It is important to note that an argument based

on residual stresses supports the observation that the

heat-affected zone and weld ,metal lose their superior fatigue

behavior at high stress intensity levels. This would be

358.7

associated with the residual stress pattern being swamped

by the much higher stress intensity level being applied.

-16

To test the supposition that a favorable residual

stress was present in the weld and heat-affected zones in

transverse specimens, duplicate samples were subjected to

a stress relief heat treatment of one hour at llOQoF. As

a control, a specimen containing only the base metal was

also subjected to a one hour 11000F stress relief heat­

treatment. As shown on Figs. 5 and 6, the heat treatment

did not alter the fatigue response of the base metal. On

the other hand, the marked low crack growth rate superiority

of the weld and heat-affected zones was almost eliminated

by the heat treatment. Since metallurgical changes

resulting from the heat treatment were not likely to alter

the fatigue response of the material to the extent observed,

it may be concluded that the heat treatment did remove a

favorable residual stress pattern in both the weld metal

and the heat-affected zones.

One additional parameter was evaluated with

respect to fatigue crack propagation in transverse welds.

Crack propagation studies were conducted with the crack

propagating both upstream and down stream with respect to

the solidification direction in the welding process. No

358.7

effect on fatigue crack propagation rate was observed for

-17

this variable. It is possible that the multi-pass welding

process eliminated or at least minimized the possibility

of anisotropic effects due to the solidification process.

Since the transverse weld fatigue test results

have led to some suprising conclusions concerning the nature

of. the residual stress patterns in the welded plate, additional

tests are planned to verify the reproductibility of these

findings. In addition, it has been suggested by Coffin(20)

that the observed difference in fatigue behavior between the

base metal HAZ and weld metal could be the result of strain

aging effects. This possibility will also be explored.

For longitudinal welds the response of the various

metallurgical structures in the welded specimen (that is,

the base metal, heat affected zone and weld metal) to cyclic

loading was evaluated by fatigue tests which were conducted

at a constant stress intensity factor range; therefore, as

the crack traversed the three metallurgically different

regions of the specimen it was possible to observe the crack

extension rate under the same stress intensity condition.

Recalling that one-eighth inch test panels were machined from

the top and bottom multiple pass one-inch plate, fatigue tests

358.7 -18

with longitudinal welds were performed with both top and

bottom plates. with a top plate speqimen, at a stress

intensity range of 70,000 psi in the growth rate in the

base metal was approximately 50 percent greater than that

of the weld metal. At 60,000 pis in, the growth rate in

the base metal was about twice that in the weld metal. In

both cases, the crack growth rate in the weld metal was

higher than that observed in transverse welds for comparable

stress intensity levels. As the crack traversed the heat

affected zone, considerable scatter was observed in the

fatigue crack propagation rate at both 60,000 psi in and

70,000 psi in. The results did reveal that fatigue crack

propagation in the heat-affected zone was roughly 50 to 100

percent higher than in the base metal.

To examine whether the slower crack growth rate

in the weld metal and the scatter in test results in the

HAZ were due mainly to a residual stress effect as suggested

in the transverse weld studies, an additional fatigue test

of heat treated material were conducted. At a constant.~

stress intensity range of 60,000 psi in, the one hour

110QoF heat treatment increased the crack growth ·rate in

the weld metal from 1.25 - 1.50 ~ 10-5 in/eye. to 2.50 x

10-5 in/eye. in the base metal. In addition, the scatter

358.7 -19

of fatigue results in the heat affected zone was essentially

eliminated as a result of the heat ~reatment. Though only

one test of heat treated material with a longitudinal weld

W~S conducted, the results suggest that residual stress

effects may 'have contributed to the fatigue crack

propagation process. For the case of the longitudinal

welds, the residual stresses parallel to the applied stress

direction appear to be of a compressive nature in the weld

metal and tensile in the heat affected zone.

Longitudinal fatigue. tests were also performed

in top and bottom plates at a constant stress intensity

range of 30,000 pis in. While the growth rate in the weld

metal was again lower than in the base metal, the crack

propagation rates in the three regions were observed to be

approximately five times larger than expected on the basis

of previous results (Figs. 5,6). At the present time, it

is not possible to explain this discrepancy. More tests

are being planned to explore this question.

3.5 Effect of Environment on Fatigue Crack Propagation

To determine the possible effect of water,

fatigue crack propagation studies were carried out in both

air and in water in A514J base plate and in AIRCO AX-90

weld metal. In both cases no effect was observed of water

358.7 -20

on fatigue crack propagation (Fig. 7). The relative humidity

in air for the AIRCO AX-gO study was 65 percent which suggest

that this level is equivalent to a saturated condition in

terms of moisture effects on fatigue crack propagation. (17)

Since all fatigue tests were conducted in a relative humidity

range of 35 - 80 percent, the preliminary results suggest

that humidity variations in testing conditions from day to

day probably plays a relatively minor role in the fatigue

crack propagation process.

3.6 Fractographic Observations

An evaluation of fractographic features of the

fatigue fracture surfaces was started. As expected,

relatively few areas of clearly defined fatigue studies

were observed. The spacing between these striations were

recorded and compared with the associated macroscopic

fatigue growth rate. As shown in Table I, there is general

agreement between the two growth rate measurements. More

fractographic studies are planned to establish this

correlation with greater confidence.

358.7 -21

4. SUMMARY

This paper reports on a detailed study of the

fatigue crack propagation process in A514 base plates,

heat-affected zone, and weld metal. Several variables of

potential importance in the fatigue process were examined.

It was observed that fatigue crack propagation in A514 steel

was strongly dependent upon the stress intensity factor

range and only mode~ately dependent upon the mean stress

intensity level. In addition, the growth rate-was ind~pendent

of specimen thickness (up to one inch). The above results

are consistent with data previously published for other

materials.

A particularly striking effect was noted in crack

propagation studies of A514F steel containing weldments.

Fatigue crack propagation rates in transverse weld metal and

the heat affected zone were substantially less than in the

'base metal. For example, at a stress intensity level of

40,000 psi in., the fatigue crack propagation rate in the

weld metal was almost an order of magnitude less than that

displayed by the base plate. Growth rates in the three

metallurgically distinct regions tended to converge at large

stress intensity levels. After a heat treatment at llOOoF

358.7 -22

for one hour, the crack growth rates in th~ HAZ and

weld metal increased significantly and were then

comparable to the growth rate in the base plate. Heat

treatment of the base plate did not affect fatigue behavior.

It was concluded that a favorable residual stress was present

in th- stress direction in both HAZ and weld metal regions

of transverse weldments.

Residual stress effects were also indicated in

fatigue studies of longitudinal welds. The magnitude of

this effect was considerably less than was observed in the

transverse welds. Again, heat treatment eliminated the

major differences in crack growth rate in the different

weld regi9nso

358.7 -23

5 • NOMENCLATURE

dadn

K

AK

Kmax

Kc

C,m,n

crack growth rate, inches/cycle

stress intensity factor, psi in

stress intensity factor range (Kmax-Kmin) psi in

maximum stress intensity factor, psi in.

plane stress fracture toughness, psi in.

ratio of KmaxL\K

material constants

358.7 -24

6. ACKNOWLEDGEMENTS

This paper presents the results of a detailed

study of the fatigue crack propagation process in A514

steel base plate, heat-affected zone and weld metal. The

investigation is one phase of a major research program

designed to provide information on the behavior and

design of joined structures under low-cycle fatigue.

The investigation was conducted in the Mechanical

Behavior Laboratory of the Materials Research Center and in

the Metallurgy and Materials Science Department, Lehigh.

University, Bethlehem, Pennsylvania. The authors gratef~lly

acknowledge the financial support of the Office of Naval

Research, Department of Defense under contract N 00014-68­

A-514; NR 064-509. The program manager for the overall

research project is Lambert Tall.

The guidance of, and the suggestions from, the

members of the speci~l Advisory Committee on Low-Cycle

Fatigue is gratefully acknowledged.

The authors are grateful to Michael Parry for his

-25

contributions to the preparation of this paper. The

secretarial services of Betty Brader and Louise Valkenburg

are sincerely appreciated. George P. Conrad II is

Chairman of the Department of Metallurgy and Materials

Science, and Joseph F. Libsch is Director of the Materials

Research Center, and Vice-President for Research, Lehigh

University.

358.7 -26

7. TABLES AND FIGURES

358.7

TABLE 1

Fatigue Crack Growth Rate Measurements

-27

Specimen L\K Macro Micro Structure

13 30KSi in -6 -6 Weld Metal7xlO in/eye 9.3xlO in/eye

30 7xlO- 6 -6 HAZ9.3xlO

30 8xlO- 6 -6 Base Metal6.8xlO

60 3xlO- 5 -5Base Metal4.3x10

16

19

28.5

38

30.8

39.6

-65.6xlO

-67.7xlO

9xlO- 6

1.2xlO-5

Base Metal(Long)

Base Metal(Long)

Base Metal(Trans}

Base Metal(Trans)

'a 358.7

.....-...-----12.0" -------.....

-28

o

0--0- - - , ,- -

o

-0- 0·-

3.0"

-----.........- 6£)'-'---........

oo

..-....-2D'---'~

.-..--- 32';------

2A"

Fig. 1 Goemetry of Single Edge Notch and CompactTension Fatigue Specimens. Dashed linesrepresent relative position of weld metal.

358.7

, ,. T I

-29

2 '- x .265 IN ~ -0 .126 II 0

A ,061 " 00.fiO

10A

8

164~ 0)(A -0

y(6)(

~

da>(

dN5~

~ -(lN/CY)

~;j

)A~

62 ~ .;« -

A:<~.a A

~)(

Xx)(

)('1

105 ... (4I~ -'lt' j(

XA

>A. • 1 I IV

80 10040 606K (KS1VfN)

Fig. 2 Effect of Specimen Thickness Upon FatigueCrack Propagation in AS14 Steel.

358.7 -30

5

~dN

(IN/CY) 2

5

2

~ =2.14

20 50 100l1K (KSI/iN)

Figa 3 Effect of A (Kmax/AK) Upon Fatigue CrackPropagation on A514J steel.

358.7

103

A514F RW1WR5 A514 J RW

A514J

2

1(j4

5dadN

(IN/CY) 2

10

5

2

-31

120 50 100

6K (KSI v'fN)Fig. 4 Fatigue Crack Propagation in AS14 Steel

in RW and WR Orientations.

358.7

5

10

dadN

(LN/CY) 2

2

A514F AIRCOAX-go

-32

20 50 1006K (KSIIiN)

Fig. 5 Comparison of F~tigue Crack Propagation BehaviorBetween Transverse Weld Metal (AX-gO) and Base Plate(h514F) in Heat-treated and Unheat-treated Conditions.

358.7 -33

5

5

1,I

II

II

II

I I

21

H~Z1100·F/1h

106tm.---------......---....----..

sii..dN

(IN/CY) 2

20 50 1006 K (KSI./iN)

Fig. 6 Comparison of Fatigue Crack Propagation BehaviorBetween HAZ and Base Plate (A514F) in Heat­treated and Unheat-treated Conditions.

358.7 -34

5 x AIR

o WATERtJ

20

)(

't

AiReD AX-90 x)(

,;<

5cP0

da A514J WR )(I-----dN )(

)(8

(IN/CY) 2 0 fI )(

tl )(

165~ f

0 I)(,..0

00

5 xl,0

R'Io0

f2

106

20 50 1006K (KS I v'!N)

Fig. 7 Effect of Water on Fatigue Crack Propagation inA514J and AIReo AX-90 Alloys.

35807 -35

8 0 REFERENCE~

Is Head, A. KGTHE GROWTH OF FATIGUE CRACKS,Phil~ MagG 44, (1953) 925.

2Q Head, A. K.THE PROPAGATION OF FATIGUE CRACKS,Trans ASME, Sera (1956) 407 Q

30 Frost, No E~ and Dugdale, D. S.THE .PROPAGATION OF FATIGUE CRACKS INSHEET SPECIMENS, J. Mech. Phys. Solids6, (1958) 92.

4. McEvily, A. Jo and Illg, W.THE RATE OF CRACK PROPAGATION IN TWOALUMINUM ALLOYS, NACA TC 4394, (1958)0

5. Frost, N. EG, et aleEXPERIMENTAL STUDIES INTO THE BEHAVIOROF FATIGUE CRACKS, Prac. Crack Prop.Symp. Cranfield (1961) 166.

6. Liu, Hit W.FATIGUE CRACK PROPAGATION AND APPLIEDSTRESS RANGE - AN ESSAY APPROACH, Trans.ASME, Sere D 85 (1963) 116.

7fJ Weibull, w.A THEORY OF FATIGUE CRACK PROPAGATION INSHEET SPECIMEN, Acta Meto 11 (1963) 745.

8Q Paris, Pe C. and Erdogan, FoA CRITICAL ANALYSIS OF CRACK PROPAGATIONLAWS, Trans. ASME, Sere D 85 (1963) 5280

9~ Forman,Ro Go, Kearney, V. E. and Engle, R. MoNUMERICAL ANALYSIS OF CRACK' PROPAGATIONIN CYCLIC-LOADED STRUCTURE, Trans ASME,Sere D 89 (1967) 459.

100 Weber, Jo H.EFFECTS OF CRYSTALLOGRAPHS AND THERMO­MECHANICAL TREATMENT ON FATIGUE CRACKPROPAGATION, Ph.D. Dissertation, LehighUniversity, 1969.

358.7 -36

11. Heiser, F. A.ANISOTROPY OF FATIGUE CRACK PROPAGATION INHOT ROLLED BONDED STEEL PLATE, Ph.D.Dissertation, Lehigh University, 1969.

12. Miller, C. A.THE DEPENDENCE OF FATIGUE CRACK GROWTH RATEON STRESS INTENSITY FACTORS AND THE MECHANICALPROPERTIES OF SOME HIGH STRENGTH STEELS, ASMTransactions, Vol. 61, No.3, (1968) 442.

13. Frost, N. E.EFFECT OF MEAN STRESS· ON THE RATE OF GROWTHOF FATIGUE CRACKS IN SHEET MATERIALS, J. Mech.Eng. Sci. 4 (1962) 22.

14. Yokobori, T., et aleFATIGUE CRACK PROPAGATION OF MILD STEEL ANDHIGH STRENGTH STEELS, Rep. Res. Inst. Strength,Fracture of Materials, Tohoku Univ. 3 (1967) 39.

15. Achter, M. R.EFFECTS OF ENVIRONMENT ON FATIGUE CRACKS,ASTM STP, 415 p. 181.

16. Johnson, H. H. and Paris, P. c.SUB-CRITICAL FLAW GROWTH, J. Eng.Fract. Mech. Vol. 1, No.1, (1968) 3.

17. Wei, R. P.SOME ASPECTS OF ENVIRONMENT-ENHANCED FATIGUECRACK GROWTH, to be published in EngineeringFracture Mechanics

18. McMillan, J. C. and Hertzberg, R. W.APPLICATION OF ELECTRON FRACTOGRAPHYTO FATIGUE STUDIES, ASTM STP 436, p. 89, (1968).

19. McHenry, H., private communication.

20. Coffin, L., private commurnication.

Securaty Classification

DOCUMENT CONTROL DATA • R &0(SOt'urlty ClttBsJllcat~onof Hilt', body of abstrltct tflld il1do"M~ snnoJt'f/o'" nHlNt hi) tJtlteff1cJ wlul(I tI,O ovorall report Is dIJ!lBI/Jed)

1 ORIGINATING ACTIVITY (Corporate lJuthor) , 20. REPORT SECURITY CLASSIFICATION

3 REPORT TITLE

Lehigh UniversityUnrestricted

2b. GROUP

FATIGUE CRACK PROPAGATION IN A5l4 STEEL

4. DESC Rt P T I V E NOT E S (Typo of report and Inc/us/ve dates)

'0. AU THO RIS) (Firat neme, middle Inillal, laet name)

Richard Hertzberg and Hans Nordberg

6. REPORT DA TE

November 1969Sa. CONTRACT OR GRANT NO.

N0014-68-A-0514; NR064-509h. PROJECT NO. 358

c.

d.

10. DISTRIBUTION .STATEMENT

11, SUPPLEMENTARY NOTES

13. ABSTRACT

74. TOTAL ~~. OF PAGES rb. NO. OF R~S

Ott. ORIGINATOR'S REPORT NUM8ER(S)

9b. OTHER REPORT NO(S. (Any other numbers that may ba lIsslgnedthl s report)

12. SPONSORING MILITARY ACTIVITY

This paper reports on a detailed study of the fatigue crackpropagation process in the A5l4 steel base plate, heat affected zoneand weld metal. These data are evaluated in terms of fracturemechanics concepts.

It was observed that fatigue crack propagation in A5l4 steel isstrongly dependent upon the stress intensity factor range and onlymoderately dependent upon the max~mum stress intensity level over arange of A = 1.0 - 4.0 where A = ~ax. The growth rate is independent

LU<of specimen thickness (up to one inch). F~tigue crack propagationrates were determined in A514F and A514J steel in the RW. and WRorientations. No directionality in fatigue behavior was noted in A514Fwhile A5l4J did exhibit somewhat faster crack growth in the WR,direction as compared to that ovserved in the RW direction.

Fatigue crack propagation rates in transverse weld metal and theheat affected zone were substantially less than in the base metal. Forexample, at a stress intensity level of 40,000 psi in., the fatiguecrack propagation rate in the weld metal was almost an order of magni­tude less than that displayed by the base plate. Growth rates in thethree metallurgically distinct regions tended to converge at largestress intensity levels. After a heat treatment of 1100°F for one hour,

Security Classification

Security CJRRsificl:ltion

, 4: LINK A LIN K B LIN K CKEV WORDS

...

the crack growth rates in the HAZ and weldmetal increased significantly and werethen comparable to the growth rate in thebase plate. stated differently, thedependence of crack growth rate on thestress intensity level in the weld metaland HAZ decreased as a result of the heattreatment. Heat treatment of the baseplate did not affect fatigue behavior.

ROLE WT ROLE WT ROLE WT

Security Cla8sification