Carderock Division AD-A268 605Naval Surface Warfare CenterBethesda. Md. 20084-5000

CRDKNSWC-SSM-61-93/01 April 1993

I Survivability, Structures, and Materials DirectorateResearch and Development Report

The Effect of Electric Discharge MachinedNotches on the Fracture Toughness ofSeveral Structural Alloysby

i J.A. Joyce, U.S. Naval AcademyR.E. Unk, Naval Surface Warfare Center

11 DTICELECTE

U E =

93-19742

cc Approed for puMc relene; dbutwln Is unimled.

IIlo pIl

Carderock DivisionNaval Surface Warfare Center

CRDKNSWCOSM-61-93/01 April 1993

Survivability, Strumc and Matials DepatmentResearch and Development Report

The Effect of Electric Discharge Machined Notches on the

Fracture Toughness of Several Structural Alloys

Accesion For

NTIS CRA&Iby DTIC TAB

U..announced [l

J. A. Joyce, U.S. Naval Academy Justification ...................RE. Link, Naval Surface Warfare Center By

Dist, 7ibut~ion li

Availability Codes

Dist Avail and I orSpecial

Distribution Statement: Approved for public release; distrbution is unlimited.

CONTENTS

Page

LIST OF FIGURES ................................................. iv

LIST OF TABLES .................................................. v

ABSTRACT ....................................................... iii

ADMINISTRATIVE INFORMATION ................................... i

ACKNOWLEDGEMENT ............................................ ix

1.0 OBJECTIVE ................................................... 1

2.0 EXPERIMENTAL DETAILS ....................................... 32.1 Material Description ........... ............................. 32.2 Specimen Details .......................................... 32.3 Test Procedure ............................................ 5

3.0 ANALYSIS ................................................... S3.1 Equations ................................................ 83.2 Analysis Methods .......................................... 10

4.0 DISCUSSION OF RESULTS ....................................... 174.1 Resistance Curve Results ..................................... 174.2 Initiation Toughness Results ................................... 174.3 Improvement of the Common Method ............................ 36

5.0 CONCLUSIONS ................................................ 39

APPENDIX A ..................................................... Al

EER ENCES .................................................... 41

ine

LIST OF FIGURES

No.

Figure 1 EDM notch tip geometry appearance in (a) CS-19 aluminum specimenand (b) A533B steel specimen. ............................... 6

Figure 2 Schematic showing E813 procedure to obtain JQ using an offsetconstruction line procedure ................................. 11

Figure 3 Schematic showing proposed procedure to evaluate the best initial crack

length from unloading compliance results ........................ 13

Figure 4 Schematic of E1290 procedure to obtain 8 ....................... 14

Figure 5 Schematic showing the Common Method offset construction lineprocedure used to obtain 8 ................................. 15

Figure 6 J-R curves for the ASTM A302 alloy showing EDM and fatigueprecracked results ......................................... 18

Figure 7 J-R curves for the ASTM A515 alloy showing EDM and fatigueprecracked results ......................................... 19

Figure 8 J-R curves for the ASTM A533B alloy showing EDM and fatigueprecracked results ......................................... 20

Figure 9 J-R curves for the HY-100 alloy showing EDM and fatigue precrackedresults ................................................. 21

Figure 10 J-R curves for the ASTM A710 (HSLA-80) alloy showing EDM andfatigue precracked results .................................. 22

Figure 11 J-R curves for the CS-19 alloy showing EDM and fatigue precrackedresults ................................................. 23

Figure 12 8-R curves for the ASTM A302 alloy showing EDM and fatigueprecracked results ......................................... 24

Figure 13 8-R curves for the ASTM A515 alloy showing EDM and fatigueprecracked results ......................................... 25

Figure 14 8-R curves for the ASTM A533B alloy showing EDM and fatigueprecracked results ......................................... 26

iv

Figure 15 8-R curves for the HY-100 alloy showing EDM and fatigue precrackedresults ................................................. 27

Figure 16 8-R curves for the ASTM A710 (HSLA-80) alloy showing EDM andfatigue precracked results .................................. 28

Figure 17 8-R curves for the CS-19 alloy showing EDM and fatigue precrackedresults ................................................. 29

Figure 18 Ji evaluation for specimen FYW-512 .......................... 32

Figure 19 81 and S" comparison for specimen FYW-512 .................... 33

Figure 20 81 and 8xm comparison for specimen GFF-33 ...................... 35

V

LIST OF TABLES

No. pae

I Chemical composition and mechanical properties of materials used in this study(Values are in weight percent) ..................................... 4

2 Comparison of the average tearing modulus for fatigue precracked and EDMnotched specimens . ............................................ 30

3 Fracture toughness values for EDM notched and fatigue precrack specimens. 31

4 Comparison of average fracture toughness measured for EDM notched andfatigue precracked specimens ...................................... 33

5 Comparison of Aa at initiation using construction line slopes of 2.0 and 1.4..... 38

vi

ABSTRACT

Recent computatonal studies of the stress and strain fields at the tip of very sharpnotches have shown that the stress and strain fields are very weakly dependent onthe initial geometry of the notch once the notch has been blunted to a radius thatis 6 to 10 times the initial root radius. It follows that if the fracture toughness ofa material is sufficiently high so that fracture initiation does not occur in aspecimen until the crack-tip opening displacement (CLOD) reaches a value from6 to 10 times the size of the initial notch tip diameter, then the fracture toughnesswill be independent of whether a fatigue crack or a machined notch served as theinitial crack.

In this experimental program the fracture toughness (Jk and J resistance (J-R)curve, and CTOD) for several structural alloys was measured using specimenswith conventional fatigue cracks and with EDM machined notches. The results ofthis program have shown, in fact, that most structural materials do not achieveinitiation CTOD values on the order of 6 to 10 times the radius of even thesmallest EDM notch tip presently achievable. It is found furthermore that toughermaterials do not seem to be less dependent on the type of notch tip present. Somematerials are shown to be much more dependent on the type of initial notch tipused, but no simple pattern is found that relates this observed dependence to thematerial strength, toughness, or strain hardening rate.

ADMINISTRATIVE INFORMATION

The work reported herein was performed at the U.S. Naval Academy and the CarderockDivision, Naval Surface Warfare Center under the "Elastic-Plastic Fracture Mechanics of LWRAlloys Program," R.E. Link, Program Manager. The program is sponsored by the Office ofNuclear Regulatory Research of the U.S. Nuclear Regulatory Commission under InteragencyAgreement RES-78-104. The Technical Program monitor is Dr. S.N. Malik at the USNRC. Thiseffort was performed at CRDKNSWC under the supervision of Mr. T.W. Montemarano, Head,Fatigue and Fracture Branch (Code 614).

ACKNOWLEDGEMENT

This work was performed at the Naval Surface Warfare Center, and the U. S. NavalAcademy, both in Annapolis Maryland, under the program, "Elastic-Plastic Fracture Evaluationof LWR Alloys," Mr. RE. Link, Program Manager. The program is sponsored by the Office ofNuclear Regulatory Research of the U.S. Nuclear Regulatory Commission (NRC). The technicalmonitor for the NRC is Dr. Shah N.M. Malik. The authors would also like to acknowledge thehelp of Mr. Charles Roe of the Naval Surface Warfare Center and Mr. Wayne Farmer and Mr.John Hein of the U.S. Naval Academy.

vii

1.0 OBJECTIVE

Standard techniques for evaluating the fiacture toughness of a material involve testing a

notched specimen that contains a real crack at the tip of the notch. The crack is introduced by

fatigue loading the specimen at a load that is a small fraction of that required to initiate stable

tearing. The fatigue cracking procedure results in a very sharp, natural crack that is designed to

provide a high level of constraint and hence, a measurement of the fracture toughness near the

lower bound. The fatigue cracking procedure can be a time-consuming process and adds to the

cost and complexity of conducting fracture toughness tests. For many situations of practical

interest such as the testing of weldments, it may be difficult, if not impossible, to produce a

satisfactory fatigue crack that samples the material of interest. This is particularly true when

trying to measure the fracture toughness of a heat affected zone or local brittle zone in a

weldment. Residual stresses and inhomogeneity of material properties can lead to unsatisfactory

crack fronts that do not sample the desired material or microstructure. Fracture toughness testing

procedures could be greatly simplified if a very sharp, machined notch could be used as the

initial crack in lieu of a real fatigue crack.

Conventional machining methods cannot produce a sharp enough notch that can

adequately simulate a fatigue crack. Work by Joyce and Gudas[1] showed that machined notches

with sharp tips (- 0.001 in. radius) but with included angles of 60W caused the measured J,

fracture toughness to be elevated by a factor of between three and four for an HY130 steel. Over

the past decade, advances in electric discharge machining (EDM) equipment and procedures have

made it possible to produce much narrower notches than previously available with notch tip radii

on the order of 0.002 inches. It is a simple matter to produce slots in typical fracture specimens

that are 0.004 in. wide with a O.002 in. root radius at the tip of the notch. The advantage of an

EDM notch over a fatigue crack is that the EDM notch can be located precisely at the

microstructure of interest and the notch will be perfectiy straight.

A basic assumption of elastic-plastic fracture mechanics is that a single parameter, the

crack tip opening displacement (CTOD) or J integral is sufficient to describe the stress and strain

distribution at the tip of a crack and that crack initiation occurs when this parameter attains a

1

critical value. The fracture toughness is defined as the critical value of this parameter at the

onset of significant ductile crack extension. Recent computational studies of the stress and strain

fields at the tip of sharp notches have shown that the stress and strain fields are very weakly

dependent on the initial geometry of the notch once the notch has been blunted to a radius that

is 6 to 10 times the initial root radius'. It follows that if the fracture toughness of a material is

sufficiently high so that fracture does not occur in a specimen until the CTOD reaches a value

from 6 to 10 times the size of the initial notch tip diameter, then the fracture toughness will be

independent of whether a fatigue crack or a smooth notch served as the initial crack.

The objective of this experimental program was to measure the fracture toughness, 8, and

Jk, and resistance curves (CTOD-R and J-R) for several sructural alloys using specimens with

conventional fatigue cracks and also with EDM notches. The results were then compared in

terms of the ratio of the measured CTOD at crack initiation to the initial notch radius. It is

expected from the preceding argument, that low toughness alloys will demonstrate a dependence

of fracture toughness on the crack tip geometry, while tougher materials will not.

'Private communication, C.F. Shih, Brown University, USA, 1991.

2

2.0 EXPERIMENTAL DETAILS

2.1 Material Description

Six steel alloys and one aluminum alloy were examined in this investigation. Three of

the steels were pressur= vessel steels, ASTM A302, A533 Grade B, and A515. Two were high

strength structural steels, HY-100 and ASTM A710 and the aluminum was a magnesium-

molybdenum aluminum alloy, CS-19. This selection of alloys provides a wide range of strength

and toughness with which to evaluate the effects of EDM notches on toughness. The aluminum

alloy and HY-1O0 steel have a CTOD fracture toughness (using standard fatigue pre-cracked

specimens) on the order of the EDM notch width used in this study. The remaining steels have

a CTOD firactur toughness which is greater than the width of the EDM notch to varying degrees.

The chemical composition and mechanical properties of the materials used in this study are listed

in Table 1. The strain hardening exponent, N, was determined from the relationship2

M. I. 1 (1)

where N-1/n.

2.2 Specimen Details

The specimen geometries used in this investigation were IT C(T) and IT SE(B)

specimens. The C(T) specimens were used for the CS-19 aluminum and A710 steel and wer

1 in. thick. SE(B) specimens were used for all other tests. The A302 and A515 steel specimens

were 2 in. thick and the HY-100 and A533B steel specimens were 1 in. thick. All specimens

were side grooved to a depth of 10% of the specimen thickness on each face. The SE(B)

specimens had a flex bar mounted on one face of the specimen to measure the load-line

displacemenL

2 Anderson, T.L. and Dodds, R.H., Jr., "Simple Constraint Corrections for Subsize Fracture

Toughness Specimens," ASTM International Symposium on Small Specimen Test Techniques andTheir Application to Nuclear Reactor Vessel Thermal Annealing and Plant Life Extension,January 29-30, 1992, New Orleans, LA.

3

Table 1 Chemical composition and mechanical properties of materials used in this study(Values are in weight percent).

I-

ASTM ASTM ASTM HY-100 ASTM CS-19A302,Gr.B A515.Gr.70 A533. Gr.B A710

Cawbn 0.18 0.28 0.19 0.16 0.04

Mang== 1.24 0.82 1.28 0.26 0.59 0.8

Pbasphom 0.012 0.009 0.012 0.003 0.005

Sulfur 0.023 0.028 0.013 0.009 0.004

Silicon 0.22 0.21 0.21 0.19 0.25 0.

NiWJWJ - - 0.64 2.78 0.90

Ciromium - - 1.57 0.70 0.10

Molybdenum 0.47 0.55 0.42 0.19

Aluminsm m - - - - Bal.

hrn Bal. Bal. Bal. Bal. Bad. 0.07

Copper - - - 1.17

Vanadium - - 0.003 0.003

Titanium - - - 0.06

Columbium M- - 0.03-

Magnesium - - 8.42

Beryllium - - 0.001

0.2% YS. Mpg 393 262 443 747 511 251(ksi) (57) (38) (64.7) (109) (74.6) (36.4)

UTS, Mps 538 517 622 877 601 408(ksi) (78) (75) (90.8) (128) (87.7) (59.2)

%EIan. in 50 33 35 26 16.5 33 24mm (2 in.)

Red. of Area, 68 54 60 57 74 29

Hmaning 9 5 9 15 15 7Expoae NL

4

Notches were prepared by wire electric discharge machining to extend the crack starter

slot a minimum of 0.2 in., resulting in a final notch length, a/W, between 0.6 and 0.7. The wire

diameter was 0.004 in. The EDM operation resulted in approximately semi-circular notch tips

with a radius of 0.002 in. Photographs of the notch tip in a CS-19 aluminum and an A533B steel

specimen are shown in Figure 1.

2.3 Test Procedure

Fracture toughness tests were conducted using the unloading compliance technique and

following the guidelines in the relevant ASTM standards, E813, El 152 and E1290. The loading

was carried out until a total crack extension of approximately 0.2 in. was achieved. Some results

for the fatigue pre-cracked specimens waoe obtained from pre-existing data, and these had been

tested to different final crack extensions. AD data sets were analyzed using the equations and

methods described in the following sections. J integral calculations were made using the crack

growth corrected J equations of ASTM El 152, and these calculations ame acceptable for Jk

calculations according to ASTM E813. The ClOD (8) calculations were made using two

different equations so that comparisons between ASTM E1290 and the new ASTM Task Group

E24.08.01 "Common Method"3 procedure could be made. In order to obtain the most accurate

comparison of Jk and 6, values, the initialization procedure that has recently been developed by

ASTM Task Group E24.08.03' was applied to all data. This procedure evaluates an average

initial crack length that is then used for all crack extension estimations. This method avoids

arbitrary "eyeball" data shifts that have characteristically been applied to J-R curves before

evaluation of both Jk and 8, values.

All testing was conducted at temperatures corresponding to the upper shelf for each

3"Standard Method for Measurement of Fracture Toughness," Draft 11, September 1992.Working document of ASTM Task Group E24.08.01, American Society for Testing andMaterials, Philadelphia, PA 19103.

""Standard Test Method for J-Integral Characterization of Fracture Toughness," Draft 8-4,January 1993, Working document of ASTM Task Group E24.08.03, American Society of Testingand Materials, Philadelphia, PA, 19103.

5

10o

CE

C6

esW0

Q %

--, - -

- to..

"• r• .6

matmia. TMh HY-100, A710, A302 and CS-19 alloys were tested at room tempeatu•e. TMw

A533B specimens were tested at 240"F and the A515 specmens were tested at 3027P.

7

3.0 ANALYSIS

3.1 Equations

The I resistance curme were calculated using the equations of El 152-87. The J integral

was caculated using the relationship that

___+____ (2)

where K, is taken from Test Method E399-90 for the SE(B) specin-n:

PIS (3)

with:

Aa/) -3(a/W)" 2 [1.99- (a/ W)(1 -a/') (2.15- 3-93(a/W)+2.7(a/V9) (4)2 (1 + 2al)(I)( -a/W)VR

and IC.) for the CMl is:

with:

,Ka*) C2+ a )[0.866 + 4.64a/W - 13-32(a/W9 + 14.72(a/W? - 5.6(a/WY'Aa/W)-2.a/0 (- a/W?

For both the SE(B) and C( specimens:

+C I.r4Ib .) B . Y b g

where for the CM specimen:

1j = 2.0 + 0.522 bW, and '6 - 1.0 + 0.76 b/W,

and for the SE(B) specimen:

Tj = 2.0 and yj = 1.0.

For CTOD calculations, individual 8 values were calculated in two ways. For the ASTM

E1290 8 calculations the equation used was:

8.r 2(1-V2), ,Wa, 82UnE r,(W-a•)ao÷z

where the center of rotation is defined by r,. with r,=0.44 for the SE(B) and r,=0.4(l+a) for the

CMT) specimen with a defined by:

and vo is the plastic component of the crack mouth opening displacement measured at a

distance z outside of the specimen crack surface. Ibis equation estimates the crack tip opening

displcment at the position of the origina crack tip using the origind crack length for al

9

calculations, i.e. for the calculation of K, r,, and b. = (W - a.).

For the ASTM E24.08.01 "Common Method"5 6 calculations the equation used was:

2,o= + (W-ao+Aajy, (10)2o,,E [r,(w--ad +a +z4

with Aa being the crack extension that has occurred since the beginning of the test.

This "Common Method" equation is estimating the CTOD at the original crack tip using

a specimen center of rotation that is adjusted to account for the true crack length as the testproceeds.

3.2 Analysis Methods

Values of the fracture toughness at the initiation of stable tearing, Jk and 8b were

determined for each specimen in accordance with the procedures in E813 and E1290,

respectively. The Jk procedure of ASTM E813 involves a fit of a two parameter power lawequation to the J-R curve data in an "exclusion zone" just beyond the point of ductile crack

initiation, as shown in Figure 2. The JQ point is evaluated from the intersection of this best-fit

power law and an offset line as shown on Figure 2, and becomes JI if specimen size and other

criteria ane satisfied. This method of evaluating 1k is very sensitive to value of the initial crack

length used to estimate the crack extension of each data point on the J-R curve (or S-R curve).

The ASTM E813 method requires the use of a pre-test initial crack length, which often is not the

best value to use for the evaluation of Jk- A new method has recently been developed by a

'"Standard Test Method for Measurement of Fracture Toughness," Draft 11, September 1992.Working Document of ASTM Task Group E24.08.01, American Society for Testing andMaterials, Philadelphia, PA 19103.

10

300

"I~iI

ItI

2000 0.5 1.0 1i5 2.0

Crack Extension. &a (mm)

Figure 2 Schemati showing ES813 procedure to obtain JQ using an offset construction line

11

working group of ASTM Subcommittee E24.08.03' which fits a construction line to the initial

Ji - a1 data to evaluate a best-fit average initial crack length for use in estimating Aa and hence

the J-R curve and Jk. This procedure has been used for all results presented here. The schematic

in Figure 3 shows how this method fits a "construction" line with the equation J=2crAa to the

J-R curve data with 0.2JQ 5 J1 < 0.6JQ and then extrapolates to the abscissa to evaluate a average

initial crack length. This crack length is used for the evaluation of the J and 8 resistance curves

and then for the evaluation of Jk and Bi.

The procedure for evaluating Bi given in ASTM E1290 involves fitting a three-parameter

power law to the initial region of the 8i-R curve, as shown in Figure 4, and then evaluating the

CTOD at a crack extension of 0.2 mm (0.008 in.) using a vertical fine as shown in Figure 4.

This procedure is simpler than the E813 J,, procedure described above, but it is even more

sensitive to the initial crack length that is used to calculate the Aa, values used to generate the

8-R curve. As discussed further below, this procedure has serious flaws, and often results in

toughness measures that severely underestimate the true toughness of the material.

The Common Method Subcommittee has proposed an alternative procedure for

determining the CTOD initiation fracture toughness for implementation in a common fracture

toughness testing standard under development. The proposed procedure is very similar to the

E813 J1, procedure with a two-parameter power law fit to the data near crack initiation and

defines the initiation point as the intersection of the fitted curve with a line offset from the

blunting line as in the procedure for Jk. A schematic of this method is shown in Figure 5. This

value of CTOD has been denoted as ic8 (subscript CM for Common Method) in this report to

distinguish it clearly from the E1290 8 quantity. One objective of the "Common Method" is to

make this value of CTOD at "initiation" correspond to the J1, initiation point of the E813

procedure. In this work, correspondence is taken to mean that for a given specimen the Jk and

""Standard Test Method for J-Integral Characterization of Fracture Toughness," Draft 12,March 1993, Working Document of ASTM Task Group E24.08.03, American Society for Testingand Materials, Philadelphia, PA, 19103.

12

300

JU

100/

31 32 33

Crad Length a

lFigure 3 Schematic showing proposed procedune to evaluate the best •iniia crack lengthfrom unloading compianc results.

13

0.I I II

I I II

0.3

6mO.165(0.03p'5+"d)0 7

0 0.5 1.0 152.0

Cradc Extnslon, &a% (mm)

Fig=r 4 Schemutic of E1290 procedur to obtain .

14

0.3

*PoMt used for regressionansiysft

d2

0

6, ~0.2mm ~ inelfm x

00 0.5 1.0 1.5 2.0

Cmdc Extnsin. A%, (mrm)

Fig= 5 Schematic showing the Common Method offset constrcdon line procedure used

to obtain

is

6, values obtained would relate to the sane amount of ductile crack extension. In a later section

a modification of the Common Method is proposed which improves the correspondence of the

JI and 8,= crack inimiaton measures.

16

4.0 DISCUSSION OF RESULTS

4.1 Resistance Curve Results

J-integral resistance curves for EDM notched and fatigue cracked specimens are plotted

in Figure 6-11 for each material. Detailed listings of the results from each test are included in

Appendix A. A quick perusal of these figures shows that, as expected, some materials appear

to be very sensitive to the type of notch used, while some materials are quite insensitive.

Unfortunately, it does not appear that a simple toughness criteria amply predicts which materials

are sensitive and which are not. CTOD resistance curves for the same set of specimens ae

presented in Figure 12-17 and they show exactly the same pattern as demonstrated by the J-R

curves. In all cases the resistance curves of EDM specimens are elevated in comparison with

the fatigue precracked specimens. In some instances, the elevation is small and the resistance

curves overlap somewhat as shown by the A515 alloy while the A533B alloy shows a modest,

but clear, elevation, and the CS-19 aluminum shows a dramatic effect, with an elevation by a

factor of 3 at a given value of crack extension.

The material tearing resistance, defined as:

Ta M E dI (11)

is evaluated for each material/notch geometry data set at a crack extension of I mm (0.039 in.)

in Table 2. Both modest increases and dectcases seem to result for the EDM notch geometry.

The low toughness materials, HY-100 steel and CS-19 Aluminum show a 31% decrease and a

55% increase, respectively. The high toughness A710 alloy seems to be unaffected by the notch

geometry, while the intermediate toughness materials are only modestly affected by the presence

of the EDM notch. The numbers in Table 2, for instance, show a toughness decrease of 19% for

the A515 steel, yet this does not seem justified looking at the J-R curves of Figure 7 which

shows that considerable data scatter is present for the four specimens tested in this case. It

appears that the data is too limited to make any clear conclusion except that no strong effect

seems to be present when the fatigue crack is replaced by an EDM notch.

17

300 J-R CURVES FYX A302 ALLOY

2700 ASTM A302 GR. B 21C

2400 4r

1 0 o

' 180 0OBv O

' 150 000150 !0- o )° ýb 0

)! 0 FYX-5160 FYX-527

30c

0 0.03 0.06 0.09 0.12 0.15 0.18 0.21 0.21

CRACK EXTENSION in

igure 6 J-R curves for the ASTM A302 alloy showing EDM and fatigue precracked

re suhts.

18

----- 0 0l i17 FYX 52

n(EDllMI m l Iin i

2000 J-R CURVES FYW A515 ALLOY

1800-

1600 AST As5s GR. "0 15oC 0

1400 0 va a a

CM o , Vovo0 0 °v°0

Ic 1200e cf1000- & 0

.00

100800 v160600 o A FYW-514 (EDM)

V FYW-516 (EDM)

0 FYW-51240 0 FYW-518

20

0 0.03 0.06 0.09 0.12 0.15 0.18 0.21 0.24

CRACK EXTENSION in

Fig=m 7 J-R cwves for the ASTM A515 alloy showing EDM and fadlgue Pnrakd

msulti.

19

500 J-R CURVES A533B ALLOY

4500-- ASTM A533B GR. B 115C

4000& A

at0 A 0~ ~V V% 0 o0 03500- AO v 0 0 0

0 0 0

2v 0 o0

00 0 000d:0

2000 00 0

150C A Al o o

V A3 (EDM)100 0 A4 (ED0)

0 C3

50 0C

0 0.03 0.06 0.09 0.12 0.15 0.19 0.21 0.24

CRACK EXTENSIONI in

Fig=~ 8 J-R cwves for the ASTM A533B alloy showing EDM and fatigu precracked

results.

20

J-R CURVES FYO ALLOY

2000

1800- HY-100 21C o3 r3V

1600 O AoA

140C -

< 120

C 20C V

• 4 1000 C3

80 0 F0O-2 (EDM)Boo 0 V F YO- 6 (EDM]0FY0-8 (EDM)

60( 0 FYO-1500 FYO-151

40C

20(

0 0.03 0.06 0.09 0.12 0.15 0.18 0.21 0.24

CRACK EXTENSION in

Fig=r 9 J-R curves for the HY-100 alloy showing EDM and fadlgm pecrmked ItsuhL

21

12000 J-R CURVES GFF ALLOY

RSLA-80 ALLOY 21C 0

10000 0 0 o(pdo 00 0

0 0

8000-

Cz---N.0

4000 V60000

2010 04000- 4tr0" O GFF-33

0 0.03 0.06 0.09 0.12 0.15 0.18 0.21 0.24 0.27 0.3

CRACK EXTENSION in

Figut 10 J-R curves for the ASTM A710 (HSLA-80) alloy showing EDM and fMigue

prec r .sul.

22

100O0 J-R CURVES FGN ALLOY

900- CS-19 ALUMINUM 21C

800 --I •oc vv90 00

70 130

N n Do

" 600 Za 00

.0 xewDD13

a- 4Qo A 0 0 0 O0

30 0 8ýýA FGN-10 (EDH)V FGN-12 (EDM)

20 0 FGN-16 (EDM)0 FGN-31

10 0 FGN-60FGN-57 (INVALID)

0 0.03 0.06 0.09 0.12 0.15 0.18 0.21 0.24 0.27 0.3

CRACK EXTENSION in

Figu= II J-R curves for the CS-19 alloy showing EDM and fatigue pu cck results.

23

S0.0. ,,CTOD-R CURVES FYX ALLOY

0.045-ASTM A302 GR. B 21C 130.034- O

0.03- 00

00.0 25 0 00 00

S0.02 O; 0 7 0000

0.02 00 0

.01 0 & FYX-511 (EDM)0 V FYX-524 (EDM)

0.0. 0 0 FYX-526 (EDM)0 FYX-516

.00 .. 0 FYX- 521

0 0.03 0.06 0.09 0.12 0.15 0.18 0.21 0.24

CRACK EXTENSION in

Figure 12 8-R curves for the ASTM A302 alloy showing EDM and fatigue prcrackld

results.

24

0.0 CTOD-R CURVES FYW ALLOY

0.04-0.04AST A GR. '70 150C

0.0'

0.03- 0o0 V

4 ov 0• V A aez0.025 00A

o 0.02 ,o VO 0

0.015 fAFYN-514 (EDM)

V FYW-516 (EOD)0.01 0 0 FYW-512

0 0 FYW-518

0.00

0 " I I0 0.03 0.06 0.09 0.12 0.15 .0.18 0.21 0.24

CRACK EXTENSION in

Figure 13 6-R curves for the ASTM A5I5 alloy showing EDM and fatigue premwckd

results.

25

CTOD-R CURVE A533B ALLOY0.0.

0ASTM A533 GR B 115C 0

'~00

0 0 00 0

0.04 0:

V7000

0.02- A

A Al (EDM)V A3 (EDM)

0.01 0A4 (EDo)

0 C3

! I I " , II

0 0.03 0.06 0.09 0.12 0.15 0.18 0.21 0.24 0.27

CRACK EXTENSION in

Figure 14 S-R curves for the ASTM A533B alloy showing EDM and fatigue prerackm d

results.

26

0.0 CTOD-R CURVES FYO ALLOY

0.027

HY-100 21C0.024

0.021- Va 13

0.010Vr~

0.12 0.01

0.009

A FYO-2 (EDM)0.006 FYO-6 (EDM')

a FYO-8 (EDM)0.003 O FYO-150

0 FYO-151

0 0.03 0.06 0.09 0.12 0.15 0.18 0.21 0.24 0.27

CRACK EXTENSION in

FIgum 15 6-R curves for the HY-100 alloy showing EDM and fatigue pmoccked iesults.

27

CTOD-R CURVES GFF ALLOY

0.0o- 00

0.08- HSLA-8o 21C vc

0.017 0

0.06- 0

-4'. 0.05 tVO00o. AV0~

t 0*0.04-0

0.03 0

A GFF-42 (EDM)

0.02 '0 0 V GFF-44 (EDM)0 GFF-33

0.01 0 GFF-34

0 0.03 0.06 0.09 0.12 0.15 0.18 0.21 0.24 0.27

CRACK EXTENSION in

Figure 16 S-R curves for the ASTM A710 (HSLA-80) alloy showing EDM and fatigue

precacked results.

28

CTOD-R CURVES FGN ALLOY0.0'•,

0.021-CS-19 ALUMINUM 21C

0.024 -A FGN-10 (EDM)

0.021-- V FGN-12 (EDM)o FGN-16 (EDM)o0.01 0 FGN-31 c0 FGN-60

-4a FGN-57 (INVALID) 0 0

00.015 .Af 0o o0o

0.012 00 0 00

0.009 % vA 0 30 0

zqn 0. 0 , 01 ! 00.00 0 0)%0 0 90 Cos

0.00

0 0.03 0.06 0.09 0.12 0.15 0.18 0.21 0.24 0.21

CRACK EXTENSION in

Figure 17 8-R curves for the CS-19 alloy showing EDM and fatigue pcawcked sults.

29

Table 2 Comparison of the average tearing modulus for fatigue prcacked and EDMnotched specimens.

MW -1 -m -I_____Puu m)M • dIIr

A32 134. 14L 5.2

A515 134. iU. -19.

HY-ROS 41k5 22. -1.

A~iO 20• 2A710 260. aft 0

CS-19.3 12.9 SS.

4.2 Initiation Toughness Results

JI, 8j, and 8", calculated as descuibed in the previous section, are tabulated in Table 3.

The analysis used for J, for specimen FYW-512 is shown in Figure 18, while the calculations

of Bi and Bm are shown for the sam specimen in Figur 19. The average values of fracture

toughness for each material are tabulated in Table 4. Some material scatter is clearly present

with J values ranging by up to 17% om heaverage, 8 values ranging up to 12% fromthe

average, and 6c values ranging up to 16% from the average. In all cams, the EDM notched

specimens exhibited higher initiation toughness than the fatigue precracked specimens. The

elevation in fracture toughness varied from 11% to 152%, depending on the materi and speific

measure of fracture toughness considered As would be expected, the CrOD toughness

designated in the Common Method is consistently higher than that measured according to ASTM

E1290 and it ranks the materials in exactly the same fashion as does the J, measure of E813.

The E1290 definition of 8, unfairly penalizes higher toughness materials that exhibit

substantial crack tip blunting prior to tearing. This is clearly evident in the case of the ASTM

A710 steel. The A710 steel had the highest J. toughness of all materials tested In term of k

the A710 ranked third in toughness, just below that of the A302 steel, which had a J, of

a maly one-third that of the A710. According to E1290, the critical event occurs at a

fixed amount of crack extension, 0.008 in., regardless of whether the crack has actually begun

to aw. The A710 steel is stl exhibiting blunting behavior at this point, and as shown on

30

Table 3 Fracture toughness values for EDM notched and fatigue precracked specimens.

MMIw SFCM- NaICh k T 6, am Asa-AID Typ" OW.~) .8 h) Om.) Is 4amM

""""in" - - -Oma) 00I.)h

364M 123.m OW 0.00 00096 0.0145 0.0129 -11.0

AMZ 1Y-2 D 77 249. 0.0091 0009 0.0146 0.0131 .10.3

Frx-56 EOM 913 147. 0.06 0.0104 0.014 0&0133 -10.2

FYX-516 Pulp. 566 125. 010061 0.00621 1.0123 0.0112 -8.9

FYX.5Z7 FuIg. 730 144. 1007 0.00751 41.0135 0.0119 -11.9

ASM PW54 EU 614 107. 0.006 41.001147 0.013 M.0123 4&9ASS YW-S16 Imm 709 109. 0.0097 0.0103 0.0149 0.0113 -10.7

FYW-512 Pulp. 534 121 &.0061 0.0070 0.0129 0.0116 -9A

ASTM AtlE 1334 206. 0.0132 &0147 0.0196 &0154 -22.2A 533 - - - - -at a A3 ) 2237 10. 0.0148 1.0196 .027 0.0179 -21.1

A4 E 2020 170. 0.0110 0.0169 &0210 0.0165 -21A4

F C1 Fm*-~ 1292 15m 8.0"6 10117 &0.01 0.01Q0 -14.1

CSm ftigia 3599 1. 0.010 0.013 10133 0014 -19.1

my-too FYO-2 EM929 32.6 0.0050 0.0037 0.0120 1.0106 -11.7

FY06 3D 47 35.3 0.0051 0.0047 0.01127 010105 -10.3

FYO-3 E2II loss 211 0.0054 1.0056W &0126 0.0109 -13-5

FY0-3) Fatqu 748 33! 10039 a.031 a0113 &0100 .11.5

FYO-J4 Foilp 653 431.2 A 0.003 0034 &0109 010096 -10.1

FYO-150 PulfeS 701 XI 1.0034 &.003M 0.0111 010099 10.9

PYO-IS1 Pum*. 715 54A6 0.0035 0.00363 &0111 0.0099 -10.5

HSLA40 OPP.42 EDA 4063 229. 10097 10343 M.0339 10C20C2 -40.

GFFLd4 1DM 3659 250. 0.0103 0.0317 0.0311 0.0139 -39.

OFMlI %ft-. 2336 235. 0.0067 0.0172 1.0256 &0146 -35.

GMP34 paulp. 2010 244. 0.007 0.0125 1.0204 &.0143 -29.9

(3-19 PGN-10 1DM 464 13.6 0.0064 0.006 0.0129 0.0113 -12.4Akb ON-12 EDM 488 12.4 1O" 00662068 &.0133 1.0115 -13.5

VON-Il 1DM 406 12.7 0.0035 0.0056 &0123 010106 -12.2

VON-31 Pal6 .. 175 LS 0003 0.0021A 0.009 10092 -7.6

PON-57 Pulp. 171 7A 000231 0.00241 0.009 10094 -6.7

VON-60 Puti. 200 9.0 00029 0.0027 1.0102 10093 -7.2

31

1200

A FYW-512 A515 STEEL

1000- 0

800-

-, il

400

200w- Exclusion Zone

0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09

CRACK EXTENSION in

Fig=re 18 Jk evaluation for spcime FYW-512.

32

0.01-4 - FYk-512 A515 STEEL

0.0120.o1 -

00. 008-

0 .0 0 6 - B I8 C H

0.004• Exclusion Zone

0,

0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09

CRACK EXTENSION in.

Figure 19 8 and 6c comparison for specitmn FYW-512.

33

Table 4 Comparison of average fracture toughness measured for EDM notched and fatigueprecracked specimens.

- 0-s m , -0 m m -ldalmal

Faftm UM FA WU am rdm SM a

A30 6"8 4 3 06 0.0093 41 0.0069 o.o 45

AS5 594 692 16 .0063 o.0092 44 M.el 0.0094 16

A5335 1445 1917 38 0.0100 0.0135 35 0.0126 0.0171 36

BY-oiO 706 947 34 0.006 0.0053 47 0.0036 0.0052 43

AT10 23 3861 59 0.013 0.01 20 0.0IS m 54

(•-19 II1 453 1 &00 =@ 0.00a 121 t.06 0.0063 W 42

Figure 20, it does not start to tear beyond blunting until approximately 0.4 mm of crack tip

blunting has occurred (as indicated by the change in slope of the data in the early portion of the

R curve). The definition of CTOD initiation proposed in the Common Method is (nearly - see

below) consistent with the definition of Jk, given in E813, and hence makes some allowance for

blunting behavior by specifying that initiation is defined to occur at a fixed offset from the

blunting line.

None of the fatigue precracked specimens had an initiation CTOD toughness that was on

the order of 10 times the initial diameter of the EDM notch tip. The toughest materials, A533B

and A710 had average 8,m's of 0.0126 in. and 0.015 in., respectively, which is from 3.1 to 3.8

times the initial notch tip diameter. It is not clear that structural materials indeed exist which

will demonstrate a 8,, measured in any reasonable fashion, that is on the order of 0.040 in., and

it certainly seems clear that weldments of this toughness are not a likely development in the near

fature. Thus, while the original hypothesis could not be verified, it seems clear that the use of

EDM notches must be expected to result in higher initiation toughnesses, and if they are used

to position a crack tip in a specific microstructure, some procedure would have to be used to

correct the data for the presence of the EDM notch root radius.

It was expected that the tougher materials would show lower elevation than less tough

34

0.0.

0.05-

0.04-/

0.02-

0.01-- Exclusion Zone

A HSLA-90 GFF-33

0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09

CRACK EXTENSION in.

Figuis 20 6j and &cm coaparis for specime OFF-33.

35

materials. This ntnd was not observed in this investigation. The lowest toughness material, CS-

19 aluminum, acted as expected by showing the greatest increase in toughness due to the EDM

notch. For this material, the EDM notch tip diameter was approximately equal to the initiation

toughness of a precracked specimen and the EDM notch specimen had an elevation on the order

of 140% over the precracked specimen. The highest toughness materials, A533B and A710,

showed smaller, but still substantial increases in crack initiation toughness (38% and 59% for Jk

and 75% and 36% for 86.). On the other hand, the A302 and A515 materials, with intermediate

CTOD toughness, showed less sensitivity to the EDM notch, with increases in J,, of 36% and

16% respectively, and increases in 8wm of 45% and 16%. The HY-100 alloy, with the second

lowest toughness in CTOD terms, shows the second lowest dependence on the notch tip radius.

The sensitivity of the initiation fracture toughness to the EDM notch does not seem to

correlate with the strain hardening of the steel either. The HY-100 and A710 steel both have

swain hardening exponents, N-15 and the tougher A710 showed a greater sensitivity to the EDM

notch than the HY-100. On the other hand, the A302 and A533B both have N-9. For these two

materials, the tougher A533B showed less sensitivity to the EDM notch than the A302. These

coaflicting trends indicate that there is not a one-to-one relationship between strain hardening and

sensitivity of the fracture toughness to the presence of an EDM notch.

4.3 Improvement of the Common Method

The slope of the Common Method construction line that is used to evaluate 8,c was set

as 2.0, assuming a circular opening of the crack tip. Comparing the crack extension at crack

initiation that results from this assumption with the crack initiation at J, of E 813, as shown in

Table 3, demonstrates that this definition of 8= is not very consistent with Jk- An improvement

can be made if work of Paris eC al.[2], Shih [3] or Rice et. al.[4] is used giving:

A da (12)da O

36

with a = 0.65 to 0.7, and substituting dJ/da = 2q7 gives

S- 2 - 1.3-1.4 (13)

which implies that the slope of the 8 blunting/onstruction line should have a slope of between

1.3 and 1.4 to be consistent with the slope of 2 0f used by E813 and the Common Method for the

cae of bend type, plane strain ts specimens. Recalculating 6,0 using a construction line slope

of 1.4 gives the results shown in Table 5. It is clear that the B., values have changed only

slightly, but the crack extension at the initiation point has become much more consistent with that

resulting from E813. The differenc between the crack extension at J' and 64., however, is

greatly reduced using the smaller construction loe slope, with the diffeences being reduced from

a maximum of 40% to a maximum of 12.5%.

37

Table 5 Comparison of Aa at initiation using construction line slopes of 2.0 and 1.4.

Uufiumier Spedin a) No"i L. hM As ma 5ý AsM D~iftoType 006) (111) 0h.0

Slonpe a IA

AS2)4 FYX4331 0.0103 0.0145 0601546 ý62A32FYX-534 EDM Moll0 0.0146 0.0158 L.2

FYX-326 EDU 0.0113 &.0149 &.0161 &13

FYX-S16 Pulp.o 0U066 0.0123 U.129 4.9

0.-Z M02O0135 &.0140 3.7

00MFW31 A 135 0.0144 &.7AP1 FW-31G sM &.0109 0.0149 0.0156 &.0

0Y.1 F* LO7 0128 060134 4.7

FY-1 W0 .060.0139 0.0149 7.2

ASTM Al EDM 0.0170 CO0IN O02.1 IsA53A3 EM 0L0211 0.0=7 0 2.2

A4 EM ".194 &.0210 0.0219 4.3

cl Pulp.9 060129 0.0143 U.173 6.3

03 Pulta &03047 0.0133 0.0133 1.1

BY-100 PY062 EDM 0.052.010 U.116 -1.7

PY066 EDM 0.0049 0.01 17 0.01 16 AS.

PY04 EDM U.058 &.0126 060122 .32

FY04) Pulp. U.040 0.01 13 OL0109 345

P70-14 Polpe 0.0036 &.0109 0.0107 -1.8

PYO-ISO PuA*. 0.0037 0.0111 0.0106 -2.7

PYO0151 Pulp. 0.0031 0.011 &03006 -2.7

31SLA40 OFF42E 0.0309 CA0M) U.306 -9.7

O~IEM 0.OM7 0.0311 0028 -9.3

OFF-33 Fawn. G.03M 0.025 &=4 -32.5

0Wq44 Pup. 060151 am*30 0.013 .7.6

CS.l9 POW20 EM006 .190.0123 40.Ahub .I12 EDM 0.007 U.133 O0.031 -ij

FGW.16 EM &.0056 060123 U.121 .1.0

~3.3 ul.0.0= 0.009 0.0097 -2.3

Fat=7 &up.0023 "t0o0 MC0I0 1.0

~~1NO= &u .0601000 0.009 .1-1

38

5.0 CONCLUSIONS

The following are the principal conclusions derived from this effort:

1) Wire EDM notches cannot be substituted for fatigue pre-cracks in fracture

mechanics tests for any of the materials studied in this program without large

changes in the measured initiation toughness resulting. The smallest EDM notch

tip radii that could be cut were appiximately 0.002 in. (0.05 mm) which was on

the order of 1/5 the initiation CTOD measured - using the Common Method

technique. The original precept of the work was that stuctural materials with

initiation CTOD values on the order of 6 to 10 times the initial notch radius

would likely be independent of the initial notch or fatigue crack geomet•y. This

precept was not fully tested since it was found that few, if any, structural materials

were tough enough to meet this criterion.

2) A pattern relating the initiation toughness notch geometry to material toughness

was not found in this work. High toughness alone did not seem to make a

material less sensitive to the initial crack tip geometry. Some materials were

much more sensitive to the initial notch radius, but it was not necessarily the less

tough materials that were the more sensitive.

3) The J-R curves slope, and the general shape of both the J-R and 8-R curves

seemed quite insensitive to the type of notch geometry present in the specimen.

If ductile tearing instability was the mode of failure of principal interest, then the

use of EDM notches might be practical.

4) In general, if EDM notches were to be used, a research study is necessary to

evaluate the effects of the blunt notches, and in all likelihood a correction would

be necessary to estimate the true Jk, 81, or resistance curve that would be present

if fatigue cracks existed in the structural application.

39

5) The initiation ClOD method of the ASTM E24.08.01 Common Method is

strongly preferred in comparison with that of ASTM E1290. The E1290 method

arbitrarily shortchanges the tougher materials by assuming that the onset of ductile

tearing always occurs at 0.2 mm or 0.008 incheL The results of this work show

that this is certainly not the case. The offset blunting line method of the Common

Method document seems to give an initiation point consistent with the Jk

measurement point of ASTM E813. A modification of the Common Method

constuction line slope is recommended, based on these results, which

the correspondence between Ju and the Common Method 8,.

40

APPENDIX A

Dma Tabls for Individu"l Specimn

A-1

~I ~ Jos R3

a4a

-- - -- - -- - ------ ,

d dddddddddddddddl I I codeacoccddddddIdcd Ic

rpfh~ %m AMR-

8R_ flu - I RO

1!if 12H tI dIIldddddddddd

2~~ ~ 9StoI ~ p9 92922ta t

- -- --- -- -- A-2---

si cdo

ldd Hd dd d o ddadddodc ddddddaddddddddddd

-Ot~4- Cv

V0~' ----------

~ '0V~NN e'g-'N ~ - v01 soa- W,0-~

- dd -dddd diiddddadddddddd IdddcJ S

T 0VpR1llRM

l W H R a-..1d::dd:§Citlooo llq ddddddddddddddd

88 ddddddddddddddOdddd OdOddddd

a mlg"P-M0 OfUB'0 fillf0 e4'0WjN l N t

- dg:dddddd do0 ddddddoc - - = - -ddd

A--- 31

Hiciififl ao fool:I

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- -- -- - "AM((4(4(' '

Iy14 Z 4 4 "4 C

1 -- 4q'-'

lb el-ri elo

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A ddd dddddodeddooddoooddddddddddddddddd

--- d-coddddddddodd ooodddddddddooood

et- W. lesI 9Imi ~~ ~ ~ ' Mal 54 B akipla

8Alfll~ddddd~f ddodddd;;gddddddddd

coo ~~~~-4 --------

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VIVO"cco

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Oh ~ h W I Eq~OMENOh

ddddddwdd;;§

],-: III- go 11 HIS t- .13 25

ll:E!idd lllldlidd do diiiid dddddddddcddcc

88 11 ~V I dil dliddiiffill- 1111ifl Ifcl [it 1hS

w ddd7645cd-dwddd

------- ------- ------ A-5---------

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C4CVI~~4~~~~m4 R~ A~~4C~V

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do cc ddddddddddddAdcdd

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83

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A.1

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5! - -- -- -- - -- -- -- - - A-1-3------

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!~Him

r~ 4~ tV ~- w V" W -emm.3.

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83

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REFERENCES

[1] Joyce, J.A. and Gudas, J.P., "Computer Interactive Jk Testing of Navy Alloys," Elasic-

Plastic Fracture, ASTM STP 668, J.D. Landes and G.A Clarke, Eds., American Society

for Testing and Materials, 1979, pp. 451-468.

[2] Paris, P.C., Tada, L, Zahoor, A., and Ernst, H., "The Theory of Instability of the Tearing

Mode of Elastic-Plastic Crack Growth," Elastic-Plastic Fracture, ASTM STP 668, J.D.

Landes and G.A. Clarke, Eds., American Society for Testing and Materials, 1979, pp. 5-

36.

[3] Shih, C.F., "Relationships Between the J-Integral and the Crack Opening Displacement

for Stationary and Extending Cracks," Journal of the Mechanics and Physics of Solids,

Vol. 29, 1981, pp. 305-326.

[4] Rice, J.R. and Rosengren, G.F., "Plane Strain Deformation Near a Crack Tip in a Power-

Law Hardening Material," Journal of the Mechanics and Physics of Solids, Vol. 16, 1968,

pp. 1-12.

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PuJIfic reporting a.Jiefo for this collectl of• information is estimated to average 1 hour 7er resoonse. including the i0me tor rev0ewinc instructions. searciing existing datag.t=erlng m maiitaining the data nede and cooeting arid reviewing the •ollectilon of information. SnO comments regarding this •uienfl estimate c' anv other ascecollection of information. including suggestions for reducing this burden tc wasnington ?ieacauarters services. Directorate for innfrmation Oieationsm and Reports. 1215Danvis Highway. Suite 1204. Arlington. VA 22202-4302. and to the 0Ofice of Management ana Budger. Paperwor, Aeouctsion Proe)-ed (0704.0188) Aashinglon. .C 23503

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I APR 1993 FINAL4. TITLE AND SUBTITLE S. FUNDING NUMBERS

The Effect of Electric Discharge Machined Notcheson the Fracture Toughness of Several Structural Alloys

1-2814-5546. AUTHOR(S) 1-2814-653

J.A. Joyce and R.E. Link7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES) S. PERFORMING ORGANIZATION

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US Naval Academy CRDKNSWCAnnapolis Maryland 21402 SSM-61-93-01Carderock Div, Naval Surface Warfare CtrAnnapolis, MD 21402-5067

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13. ABSTRACT (Maximum 200 wOrOS)Recent computational studies of the stress and strain fields at the tip of very sharp notches haveshown that the stress and strain fields are very weakly dependent on the initial geometry of thenotch once the notch has been blunted to a radius that is 6 to 10 times the initial root radius.It follows that if the fracture toughness of a material is sufficiently high so that fracture initiationdoes not occur in a specimen until the crack-tip opening displacement (CTOD) reaches a valuefrom 6 to 10 times the size of the initial notch tip diameter, then the fracture toughness will beindependent of whether a fatigue crack or a machined notch served as the initial crack. In thisexperimental program the fracture toughness (Ju and J resistance (J-R) curve, and CTOD) forseveral stuctural alloys was measured using specimens with conventional fatigue cracks and withEDM machined notches. The results of this program have shown, in fact, that most structuralmatrials do not achieve initiation CTOD values on the order of 6 to 10 times the radius of eventhe smallest EDM notch tip presently achievable.

14. SUBJECT TERMS 15. NUMBER OF PAGES

Jj,, J R curv.i, constraint, crack-tip opening displacement,CTOD L16. PRICE COOt

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Unclassified Unclassified Unclassified Su as Repor.. -. .. .. - n5 Stanoffaa Orfm 294 (Re ,