1975,Orange, TW - Fracture Testing With Surface Crack Specimens

8/12/2019 1975,Orange, TW - Fracture Testing With Surface Crack Specimens

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T W Orange

Fracture Testing with Surface rack Specimens

REFERENCE: Orange, T. W., F r a c t u r e Te s t i n g w i t h S u r f a c eC r a c k S p e c i m e n s , o u r n a l of Testing and Evaluation JTEVA,Vol. 3, No. 5, Sept. 1975, pp. 335-342.

A B S T R A C T: This paper is a report of ASTM Task GroupE24.01.05 on Part-Through Crack Testing. It includes recom-mendations for the design, preparation, and static fracture testingof surface crack specimens based on the current state of the art.The recommendations are preceded by background information in-cluding discussions of stress intensity factors, crack opening dis-

placements, and fra cture toughness v alues associated with surfacecrack specimens. C yclic-load and s ustaine d-load tests ar e discussedbriefly. Recommendations for further research are included.

K E Y W O R D S : fracture tests, surface defects, cracks, fractureproperties, fracture strength, toughness, mechanics

Ear ly in 1920, A. A. Griff i th repo rted [1] tha t In the courseof an invest igat ion of the effect of surface scratches on themecha nical s t re ngth of sol ids , some general conclusions werereached which appear to have a d i rec t bea r ing on the p rob lemof rup tu re . . . . A l though he was o r ig ina l ly concerned wi thprob lems o f su r face de fec t s , Gr i ff i th used the more t r ac tab le

ana ly t i ca l mode l o f a th rough- th ickness c rack to deve lop h istheory. As amend ed by I rwin [2] and Orowan [3 ], th i s theorybecame the founda t ion o f modern l inea r e l as t i c f r ac tu re me-chanics .

In 1959 an AS TM special commit te e was formed [~i] o ass is t inso lv ing f rac tu re p rob lems invo lv ing h igh s t r eng th so l id rocke tmotor cases . Al though these f rac tu res were o f t en t r aceab le tosmall surface cracks, the commit tee , l ike Griff i th near ly 40years ea r l i e r, t u rned to more t r ac tab le ana ly t i ca l mode l s andtest specimens. Since 1959 the ten-m emb er special comm it tee hasbecome ASTM Commi t t ee E 24 on Frac tu re Tes t ing o f Meta l sand has deve loped severa l AST M tes t me thods and recom mendedprac t i ces . The commi t t ee has a l so fo rmed Task Group 5 onPar t -Through Crack Tes t ing o f Subcommi t t ee 1 on Frac tu reMechan ics Tes t Method s to deve lop gu ide l ines fo r the eva lua t ionof f r ac tu re cha rac te r i s t i c s o f ma te r i a l s th rough t ens ion t e s t s o fspec imens con tain ing pa r t - th rough c racks . Th i s i s a r epor t o ftha t t a sk g roup .

The p r imary purpose o f th i s r epor t i s to p ropose un i fo rm pro-cedures for the design and s ta t ic tes t ing of surface crack speci-mens based on the cur ren t s t a t e o f the a r t . Th i s r epor t i s a l soin tended to no te the a reas where fu r the r sys temat ic r e sea rch i sneeded to more fu l ly unders tand the p rob lem and to deve lopmore de f in i t ive t e s t me thods . A secondary purpose i s to en -courage the t ak ing o f exper imenta l measurements which a re

Chairman of ASTM Task Group E24.01.05 on Part-Through Crack

Testing and research engineer, NASA Lewis Research Center, 21000Broo kpark R d., MS 49-3, Cleveland, Ohio 44135:

not direct ly useful now but which are expected to be so in thenear future .

The scope o f th i s r epor t wi l l be l imi ted to the res idua l s t r eng thtes t . The ob jec t o f such a t e s t i s to de te rmine the res idua l t ens i les t r eng th o f a homogeneous shee t o r p la te spec imen con ta in ing asemiel l ipt ical surface crack of specif ic dimensions or, b y means ofa ser ies of such tes ts , to d eterm ine residua l s t rength as a funct ion

of c rack s i ze and shape . T he spec imen and in s t rumen ta t ion tobe descr ibed wil l be usable (within cer ta in l imitat ions) forcycl ic- load or sustain ed-loa d tes ts as wel l , but these wil l bediscussed only br ief ly.

B a c k g r o u n d

Historical Milestones

The h i s to ry o f the su r face c rack spec imen inc ludes a number o fmilestones in tes t ing and analysis . These serve to ident i fys ign if i can t s t eps toward the so lu t ion o f the p rob lem and to p lacethem in h i s to r i ca l pe r spec tive .

The f i rs t surface crack specimen tes ts to be reported wererunindependen t ly and concur ren t ly a t the Nava l Resea rch Labora -

t o r y [5 6] and a t the Douglas Ai rc ra f t Co . [7 8] around 1960.Randal l [9] in 1966 s tudied the effect of crack s ize and shape onapparen t p lane s t r a in f r ac tu re toughness (K~: ) va lues . He a l soused c rack open ing d i sp lacement measurements a s qua l i t a t iveind ica to r s o f c rack t ip de fo rmat ion phenomena . In 1968 , Corn[10] a t t em pted to cha rac te r i ze the na tu r a l shape t endenc ies o fsurface cracks propagat ing under cycl ic loading. Hal l[11] in1970 compare d app aren t K~o values f rom surface crack speci-mens wi th those ob ta ined f rom o the r spec imen types .

The ana lys i s o f su r face c rack da ta accord ing to f r ac tu remechanics pr inciples was mad e pos sible by Ir win [12] in 1962.From an ea r l i e r work by Green and Sneddon[13] he de r ived thes t ress in tens i ty f ac to r for an e l l ip t ica l c rack embedded in an

in f in it e sol id and e s t ima ted th e max imum s t ress in tens i ty f ac to rfor a sere[el l ipt ical surface crack in a p late . P ar is a nd Sih [14] in1964 a t t emp ted to improve the app l i cab i l i ty o f I rwin ' s e s t ima teto plates of f ini te thickness by means of analogies to exis t ingtwo-dim ensional solut ions. In 1966, F. W. Smith[15] solved theproblem of a sem icircular surface crack in a f ini te thickness plateby a numer ica l me thod . Ayres [16] appl ied a f ini te differenceelastopla st ic solut ion to one semiel l ipt ical surface crack geom etryin 1968. In 1970, Miya mo to and Miy oshi[17] a n d L e v y a n dM a r c a l [18] presented f ini te-element analyses , again for specif icgeomet r i e s . Mar r s and Smi th [19] presen ted a me thod o f de te r-min ing s t r e s s in tens i ty f ac to r s in p las t i c mode l s by th ree -dimensio nal photoelas t ic i ty in 1971. In 1972, Cruse [20] ana-lyzed a semic i rcu la r su r face c rack us ing boundary in tegra lequat ions.

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3 3 6 J O U R N A L F T E S T IN G N D E VA L U A

S t r e s s I n t e n s i t y F a c t o rs

A s y e t t h e r e i s n o e x a c t s t r e s s i n t e n s i t y s o l u t i o n f o r t h e g e n e r a lp rob lem of a semie l l ip t i ea l su r face c rack in a p la te o f f in i t ed imens ions . I rwi n [12] p resen ted a n exac t so lu t ion fo r the e l l ip t i-c a l c r a c k e m b e d d e d i n a n i n f i n i t e s o l i d u n d e r t e n s i o n . H e a l s og a v e a n a p p r o x i m a t e e x p r e s s i o n f o r t h e m a x i m u m s t r e s s i n t e n s i t yfac to r fo r a semie l l ip t i ca l su r face c rac k in a p la te , w hich wasb a s e d o n a n a n a l o g y t o t h e p r o b l e m o f a n e d g e c r a c k i n a h a l fp l a n e . H e a s s u m e d t h a t h i s a p p r o x i m a t i o n w o u l d p r o v i d e ause fu l s t ress in tens i ty es t imate fo r a ~ c and a ~ t /2 , w here ai s c rack de p th , c i s ha l f c rack l eng th , and t i s th ickness ( seeF i g . 1 ) . I n d e e d , h i s e s t i m a t e d i d p r o v i d e f a i r l y c o n s t a n t f r a c t u r et o u g h n e s s v a l u e s f r o m t e s t s o f s m a ll s u r f a c e c r a c k s i n r e l a t iv e l yb r i t tl e , h i g h s t r e n g t h r o c k e t m o t o r c a s e s te e l s[6,21].

A n u m b e r o f i n v e s t i g a t o r s[1~,15,22-29] h a v e a t t e m p t e d t oe x t e n d t h e a p p l i c a b i l i t y o f I r w i n ' s a p p r o x i m a t i o n t o s u r f a c ec r a c k s d e e p e r t h a n h a l f t h i c k n e s s . E a c h m e t h o d i n v o l v e s s o m ek i n d o f a n a l o g y t o a n a l t e r n a t e c r a c k c o n f i g u r a t i o n w h i c h h a ss o m e p h y s i c a l s i m i l a r i t y a n d f o r w h i c h a s o l u t i o n i s a v a i l a b l e .T h e s e a p p r o x i m a t e m e t h o d s d i f f e r o n e f r o m a n o t h e r, a n d i nsom e cases the ca lcu la ted s t ress in ten s i ty fa c to rs d i ffe r con-s i d e r a b l y[29,30].S e v e ra l m e t h o d s h a v e b e e n c o m p a r e d[27-29,31]o n t h e b a s i s o f t h e i r a b i l i t y t o p r o d u c e c o n s t a n t f r a c t u r e t o u g h -n e s s v a l u e s f r o m s e l e c t e d s e ts o f e x p e r im e n t a l d a t a , b u t n o o n ea p p r o x i m a t i o n h a s y e t b e e n s h o w n t o b e c l e a r l y s u p e ri o r.

A t t e m p t s t o d e v e l o p t h r e e - d i m e n s i o n a l s o l u t i o n s h a v e p r o -d u c e d o n l y l im i t e d r e s u l ts . S m i t h ' s n u m e r i c a l s o l u t i o n s f o r th esemic i rcu la r [15] a n d c i r c u l a r s e g m e n t [ 2 8 ] s u r f a c e c r a c k s a r et h o u g h t t o b e f a i rl y a c c u r a t e ( e x c e p t n e a r t h e i n t e r se c t i o n o f t h ec r a c k a n d t h e c r a c k e d s u r f a c e ). O t h e r n u m e r i c a l m e t h o d s[16-18,20] h a v e t r e a t e d o n l y s p e c i f i c g e o m e t r i e s, a n d t h e i ra c c u r a c i e s a r e r e s t r i c t e d b y c o m p u t a t i o n a l l i m i t a t i o n s .

M o s t o f th e s o l u t i o n s a n d a p p r o x i m a t i o n s i u s t d i s c u s s e dc o n s i d e r o n l y t h e c r a c k e d p l a t e u n d e r u n i f o r m t e n s il e lo a d . O n l y

~ ~ E S ectio n A

t

a = c r a c k d e p t hc h a l f c r a c k l e n g t h

L t e s t s e c t i o n l e n g t ht = t h i c k n e s s

W = test sectionw i d t hFIG. 1--Typical surface crack specime n grip detai ls omit ted) .

a few [15,24,26,29]h a v e c o n s i d e r e d t h e c a s e o f l i n e a r ly v a r y i n g( b e n d i n g ) s t r e s s , a n d n o n e h a v e y e t c o n s i d e r e d h i g h e r o r d e rvar ia t ions .

C r a c k O p e n i n g D i s p l a c e m e n t

E x p e r i m e n t e r s h a v e l e a rn e d t h a t v a l u a b l e i n f o r m a t i o n c a n b eo b t a i n e d f r o m c r a c k o p e n i n g d i s p la c e m e n t ( C O D ) m e a s u r e -m e n t s o n s u r fa c e c r a c k s p e ci m e n s. T h e t e r m C O D i s u s e dh e r e i n t o d e n o t e t h e d i s p l a c e m e n t o f th e c r a c k f a c es o n t h ec r a c k e d s u r f a c e a t t h e m i d p o i n t o f t h e c r a c k ( se e F i g . 2 ) ; t h es a m e t e r m i s s o m e t i m e s u s e d b y o t h e r s t o d e n o t e d i s p l a c e m e n ta t t h e c r a c k t i p . R a n d a l l [ 9] w a s a p p a r e n t l y t h e f i r s t t o m e a s u r es u r f a c e c r a c k C O D . H i s s p e c i m e n s w e r e i n s t r u m e n t e d p r i m a r i l yt o o b s e r v e p o s s i b l e p o p - i n b e h a v i o r, b u t f r o m t h e l o a d - C O Dr e c o r d s h e w a s a b l e t o i n f e r t h e p r e s e n c e o f s i g n if i c a n t c r a c k t i pp l a s t ic f l o w. Ti f f a n y e t a l [ 3 2 ] u s e d C O D m e a s u r e m e n t s a sq u a l i t a t i v e i n d i c a t o r s o f s u b c r i ti c a l c r a c k g r o w t h . C o l l i p ri e s t[33,3~]a n d E h r e t [35] h a v e a t t e m p t e d t o m a k e q u a n t i t a t i v e u s eo f C O D m e a s u re m e n t s , a n d s o m e re c e n t C O D d a t a a r e i n c lu d e d

i n R e f 36.E x a m p l e s o f C O D t r e n d s f o r s e v e r a l p o s s i b l e f r a c t u r e p h e -

n o m e n a a r e s h o w n i n F ig . 2 . S t r a i g h t - l in e s e g m e n t s ( O B , D C , G J )ind ica te e las t i c de f lec t ions , and the i r s lopes a re func t ions o fc r a c k s iz e a n d s h a p e . N o n l i n e a r i ti e s ( A C , A F, J L ) m a y b e d u e t oc r a c k g r o w t h , c r a c k t i p p l a s t i c f l o w, o r a c o m b i n a t i o n o f b o t h .T h e c a u s e o f t h e n o n l i n e a r i t y c a n s o m e t i m e s b e d e t e r m i n e d b yu n l o a d i n g t h e s p e c i m e n p r i o r t o f r a c t u r e ; a c h a n g e i n s l o p e( u n l o a d i n g v e r s u s l o a d i n g ) i s i n d i c a t i v e o f a c t u a l c r a c k g r o w t ha n d z e r o o f fs e t i s i n d i c a t i v e o f c r a c k t i p p l a s t ic f l o w. P a t h O B H i nFig . 2 represen t s the c lass ic Gr i ff i th type o f b r i t t l e f rac tu re .Wi t h r e a l m a t e r i a l s , s e v e r a l o t h e r p a t h s a r e p o s s i b l e . C r a c kt i p

: ~ s ti c fl ow, s u b c r i t i c a l c r a c k g r o w t h , o r a c o m b i n a t i o n o f b o t h

m a y r e s u l t i n t h e n o n l i n e a r p a t h A C . I f t h e c r a c k i s s u f f i c i e n t l yd e e p a n d t h e m a t e r i a l s u f f i c i e n t l y t o u g h s ot h a t t h e c r a c k t i p

o a d

k ,--- , ,

F B C /

; i . . / - / .

COD

FIG. 2--~u rface c rack opening displacement CO D) trends for severalpossible ractur e phenomena



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ORANGE ON SURFACE CRACK SPECIMENS 7

plastic zone penetrates the thickness prior to fracture, a pathsuch as AF may result . Or, the crack may pop in (AD) to a newstable shape (DC). Once an instabil i ty point (C, B, or F) hasbeen reached, the crack may then propagate catastrophical ly(CH, BH, or FH) or i t may arrest (G) as a through-thicknesscrack; with further loading the specimen behaves (GJL) as athrough-crack specimen.

Figure 3 shows an a ctual load-C OD record from R ef 341, and aphotograph of the fracture face is inset . The f irs t major loadcycle exhibits linear behavior on loading, nonlinearity due tocrack growth and plasticity, change in slope on unloading, andzero offset. The specimen was the n load cycled at a low stress toproduce a visible marking band on the fracture surface. Theprocess was then repeated three more t imes before the specimenfai led. The marking bands are clearly visible on the fracturesurface (inset) and delineate the four regions of stable sub-critical growth.

Quanti tat ive analysis of COD measurements from surfacecrack specimens is ham pere d by several factors. One is the lack of

an elastic solution for COD as a function of crack size and shape.Green and Sneddon [13] give the complete displacement solut ionfor the elliptical crack in an infinite body, and Smith[15] givesan approximate COD value for the semicircular surface crackin a half space. There are no other analytical expressions avail-able, but one empirical expression[34 35] has been developed.Another problem is that the effect of measurement-point gagelength has not been identif ied. Analyses have shown tha t CODfor the center crack [37] and the three-point bend[38] specimensare functions of gage length, but there is no corresponding ana ly-sis for the surface crack. This problem can be minimized bymaking the gage length as near zero as possible.

I t is hoped that this report wil l point out the need for moreanalyt ical and experimental work in this area. In the mean time,experimenters should not be discouraged by the lack of analyt icaltools . COD measurements can provide valuable quali tat iveinsight into fracture phenomena. If test records are preservedand adequately docum ented, they may be analyzable in the nearfuture.

Fracture Toughness

The basic concept of fracture toughness has undergone con-siderable evolution. Originally it was hoped that the critical

0,001:0. i02 0.0~)3 0.00 4 0,:005 0~9I 5 :

F I G . 3--Lo ad-C OD record and fracture face Ti-6A 1-4Vso lut ion t reatedand aged, ~ in. or 6 .35 mm thick, [34]),

strain energy release rate (9) would be a unique m aterialproperty that would characterize al l sharp-crack fractures. I tsoon became apparen t [21] that fracture toughness (based onmaximum load) decreases with increasing specimen thickness,reaching a nearly co nstant m inimum value as conditions of planestrain are approached. The designation KI, was given to thislower l imit. B rown and Srawley[39] poin ted ou t tha t c rack t ipplast ici ty must be highly constrained in order to properlysimulate a s tate of plane strain. In order to provide such con-strain t th ey suggested ce rtain empirically developed size require-ments for bend and compact specimens. These size require-ments became the foundation of the ASTM Test forPlane-Strain Fracture Toughness of Metal l ic Materials (E399-74), which now provide s an opera tional definition of Kic.

The applicat ion of fracture toughness concepts to surfacecrack specimen test ing has also been evolving. The second reportof the AS TM special com mittee [21] suggested tha t Kz~ could bedetermined from surface crack specimen tests , and the f if threpo rt [410] suggested th at Kie values could b e used to p redict

fai lure loads for surface-cracked structural components. Thesesuggest ions were based on the very l imited data available andon the concept of KI~ as a vaguely defined lower limit. Subse-quent s tudies have indicated that they represent ideal izat ionsof what can be a very complex fracture process.

Ran dall [9] studies the effect of crack size and sh ape o nappare nt fracture toughness values from surface crack specimensof D6AC steel and of Ti-6A1-4V. He concluded that apparentfracture toughness was nearly independent of crack size andshape for these two materials in their high strength conditions,but not for the same materials in much tougher heat t reatmentconditions. Shortly thereafter the concept of a plane strain sizerequirement was advanced [39] which at least part ly explains

some of R andall s results . To ugher materials require largerspecimens to pro vide the same degree of plane strain simulation,but Randall s specimens were al l the same size. I t appears th athis specimens were not large enough to simulate plane strainconditions for the tougher materials.

The size requirements of ASTM E 399-74 were developedspecifically for the bend and compact specimens. Hall[11]attempted to determine empirical ly size requirements forsurface crack specimens. In his tests, calculated fracture tough-ness was reasonably constant as long as the crack depth a andthe uncracked ligament depth ( t - a , Fig. 1) were both greaterthan about 0.5 (KIE/a ys) 2,where O ys is the m aterial yield strength.The designation K~E is customari ly given to ap paren t toughnessvalues obtained from surface crack specimens, as distinguishedfrom KIo values determined according to AS TM E 399-74. Thetests reported in Re f 41 support at least part of Hall s findings.

I t is not a s imple, s traightforward matt er to o btain a constantfracture toughness value (or to correlate fracture stress withcrack dimensions, which is equivalent) over a wide range ofcrack size and shape. One impediment to analysis is the inter-act ion between the stress intensi ty analysis and the fai lurecri ter ion, both of which have part icular uncertaint ies whenapplied to real materials. It is often difficult to determinewhether fracture d ata t rends are due to inexactness of the stressintensity calculation or to deficiencies in the failure criterion.In addit ion to the basic Irwin cri ter ion, several semiempiricalfai lure cri teria [~-/~5] have been advanced. T hese methods show

varying degrees of abil i ty to produce constant toughness valuesfrom selected sets of data, but none has yet proven to be uni-



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8 J O U R N A L O F T E S T IN G A N D E VA L U AT IO N

versally applicable. Anoth er complicating factor is stable sub-critical crack growth. When stable crack growth occurs, thestress intensity factor associated with instability (fracturetoughness) will vary with absolute crack size and also withcrack size relative to specimen dimensions [~6]. Also, if the crackgrows a significant amount it may no longer be semielliptical,which further confounds analysis.

Hall [11]also compared Kz~ values from surface crack speci-mens with Kzc values determined from bend and compact speci-mens according to ASTM E 399-70T. The specimens weremachined from thick plate so that all cracks propagated in thethickness direction. Although encouraging, Hall' s results werenot conclusive nor entirely consistent. Actually, one should notexpect KzE and Kz~ values to be identical, since they are dif-ferently defined. KIE values are based on maximum load, whileKI, values are based on the load corresponding to 2 crackextension. This cons ideration was pointed out earlier by Colli-priest [33].

In summary, the concept of fracture toughness associated with

surface crack specimens is still evolving. It appears that, ifuncerta inties associated with the stress intens ity analysis can beminimized, appa rent frac ture toughness KIE will be fairlyconstant, provided that the crack depth and the ligament depthare both grea ter t han 0.5 (KiE/ays) 2. At present, KIE is vaguelydefined as the limiting value of toughness that is reached asspecimens are made larger and larger. It also appears th at,under directly comparable test conditions, KIE and KI valuesmay be numerically similar, even though they are no t (andshould not be expected to be) identical. It should be noted tha tthese summary statements are based on limited data and shouldbe considered as tentative.

e c o m m e n d a t i o n s

Rationale and Test Planning

Surface crack specimens were originally chosen because theywere very good models of the types of flaws found in service. Butbecause the surface crack specimen is such a realistic model, it issubject to the same complicated phenomena that often occurwith natural cracks in real structures. In spite of these obstaclesthe original rationale is still valid, even if (or especially when)linear elastic frac ture mechanics considerations are not appli-cable. That is, surface crack specimens can be used in a simplemodeling test even if fracture stresses are above yield or if thespecimen thickness is not large enough to simulate plane strain.However, one should not attempt to generalize such test data,for example, for crack sizes or shapes or mater ial thicknessesoutside the test range.

It is reasonable to choose the surface crack configuration mostclosely resembling the typ e of flaw likely to occur in service.For example, lack of penet ration in a one-pass weldment mightbest be modeled by a long shallow surface crack, or a smallfatigue crack grown from an etch pit by a nearly semicircularsurface crack. The range of crack size and shape that must becovered will depend on the ultimate purpose of the test. A cracksize range which results in a fracture stress range from nearultimate tensile strength to about 80 of hardware operating

stress will generally be adequate for design purposes.

Specimen Design

A typical surface crack fracture specimen and the notation andconventions used herein are shown in Fig. 1. Grip details havebeen omitted, since grip design may depend on specimen size andthe available test fixtures. Small specimens (width W _< 4 in.or 10 cm, approximately) are usually loaded through a singlepin and clevis on each end. Larger specimens are usually bolted(using a multiple-hole pattern) to adapte r plates, which in tur nare loaded through large single pins. In general, the only re-quirements are that the gripping arrangements be strongenough to carry the maximum expected load and tha t they allowuniform distribution of load over the specimen cross section.

Since surface crack specimens are usually tested to model aflaw in an actual or intended structure, the specimen thicknessshould be the same as in the intended application. The specimentest section should be long enough and wide enough to simulatean infinite plate, since corrections for finite length and width arenot available. If the specimen is too narrow, the stress distribu-tion around the crack will be altered and the fracture stress will

usually be lowered. However, being overly conservative maydrastically increase testing machine load requirements. Un-fortunately, only a few systematic tests have been reported[35 36 ~7].These tests suggest that a specimen width five timesthe crack length will be adequate for practically all surfacecrack tests. Earlier recommendations [39 40]based on analogy tothrough crack specimens, are prbbably inadequate. Test sectionlength is seldom a practica l problem, and no sys tematic mini-mum length tests with surface crack specimens can be found inthe literature. However, a test section length twice the sectionwidth is generally considered sufficient.

In summary, the following criteria should ensure a validsimulation of an infinite pla te with a surface crack: t = service

thickness, W >_ 5 2c, and L >_ 2 X W. Specimens somewhatshorter or narrower may provide equally valid simulations.However, there are not enough systematic test data to provideaccurate guidelines, and the burden of proof must necessarilyrest on the experimenter. Should these width and length criteriaexceed the actual service dimensions, then of course the servicedimensions should be used, but one should not then attempt togeneralize data from such tests.

Specimen PreparationHere the object is to produce a fatigue crack whose con-

figuration is regular (tha t is, a half ellipse or a segment of a circle),whose depth and length axe fairly close to predetermined tar get

values, and whose subsequent fracture behavior will not beinfluenced by any detail of the preparation process. Regularityof crack configuration is primarily a function of material homo-geneity and fatigue load uniformity. The former is usuallybeyond control but the latter is straightforward.

Although crack starters can be produced in some materials byarc burns [9], or by localized hydrogen embrittlement [5],machining methods are preferred today because they offerbetter dimensional control and can be used on almost all ma-terials. Slitting with thin jewelers' saws of various diameters andelectrical discharge machining (EDM) with shaped electrodesare the most popular methods.

Fatigue crack size and shape control is more of an art than ascience at present. While the crack length can be monitored



5/8

ORAN GE ON SURFACE CRACK SPECIMENS 9

visua l ly, the c rack dep th canno t . Di ffe ren t exper im enters hav eeach deve loped the i r own techniques , genera l ly based on a con-s iderab le h i s tory of t r i a l and e r ror. However, there appear to bebas ica l ly two techniques .

O n e a p p r o a c h i s t o v a r y t h e s t a r t e r s i z e a n d s h a p e o r t h e s t r e s sf ie ld or bo th to ach ieve the des i red f ina l conf igura t ion . Fore x a m p l e , C o r n[10] d e t e r m i n e d p r e f e r r e d p r o p a g a t i o n p a t h s(p lo ts o f c rack depth aga ins t c rack length) for c racks grown inaxia l t ens ion or in bending fa t igue f rom smal l s ta r te rs . Cracks( o r s t a r t e r s ) n o t o n t h e s e p a t h s s h o u l d t e n d t o a p p r o a c h t h e mwith fur ther cyc l ing . In ax ia l t ens ion , c racks grown f r o m s i m u -la ted po in t defec ts t end to remain near ly semic i rcu la r as theygrow; in bending , the ra t ioa/2c t ends to decrease wi th increas ingc y c l i c p r o p a g a t i o n . T h e p r o p a g a t i o n p a t h f o r a g i v e n s t a r t e rc o n f ig u r a ti o n c a n b e d e t e r m i n e d e x p e r i m e n t a ll y b y a l t e r n a t e lyfa t igue cyc l ing and mark ing ( low s t ress cyc l ing) . Then thes p e c i m e n i s b r o k e n a n d p o i n t s o n t h e p r o p a g a t i o n p a t h a r eo b t a i n e d b y m e a s u r i n g t h e m a r k i n g b a n d s o n t h e f r a c t u r e f ac e .W h e n p r o p a g a t i o n p a t h s h a v e b e e n d e t e r m i n e d f o r s e v e r a l

s ta r te r conf igura t ions , the s ta r te r s ize which should g ive thedes i red f ina l s ize and shape c an be se lec ted and the c rack dep thinfer red fa i r ly c lose ly f rom m easurem ents o f the c racklength.

The o ther ap proac h to c rack s ize and sha pe cont ro l i s to use avery sharp s ta r te r o f very near ly the des i red f ina l d imens ions .I f the fa t igue c rack i s then grown only a shor t d i s tance , thecrack shape wi l l no t change very much. Al though th i s approachwould seem to be s impler, i t s p roper use requi red cons iderab leexper ience . The fa t igue c rack i s somet im es res i s tan t to in i t i a t ionaroun d the en t i re per iphery of the s ta r te r. I f a c ircu la r segm ents ta r te r i s ex tended only a shor t d i s tance , the fa t igue c rack wi l lusua l ly be a segment of a c i rc le ra ther than a semie l l ipse , andth is may in t roduce addi t iona l uncer ta in t ies in to the da taanalysis .

I t s h o u l d b e n o t e d t h a t c r a c k p r o p a g a t i o n p a t h s m a y b emater ia l dependent even i f the mater ia l s a re i so t rop ic , and wi l lbe wid th de pend ent i f the spec imen i s no t wide enough to s imu-la te an in f in i te p la te . I t should a l so be no ted tha t compl iancem e a s u r e m e n t s[33-35] and ul traso nic m easu rem ents [418,]e9] m aybe used to g ive a t l eas t a qua l i ta t ive rea l - t ime ind ica t ion ofcrack depth change .

F o r m o s t t h r o u g h - c r a c k f r a c t u r e s p e c i m e n s t h e p r o c e d u r e stha t mus t be fo l lowed to produce an e ffec t ive sharp fa t iguecrack a re wel l es tab l i shed . Analyses and exper iments haved e f i n e d t h e m a x i m u m p e r m i s s i b l e e n v e l o p e w i t h i n w h i c h t h efa t igue c rack and i t s s ta r te r mus t l i e . Exper iments have es tab-l i shed the maximum a l lowable s t ress in tens i ty fac tor dur ingfa t igue c rack ing Kt as a f rac t ion of the p lane s t ra in f rac turetoughness Kic or as a func t ion of the e las t ic modulus E , anda lso the amount of fa t igue c rack ex tens ion needed to e l imina tethe in f luence of the s ta r te r geomet ry. S t ress in tens i ty fac torana lyses (K ca l ibra t ions) p rovide the bas i s by which thesef ind ings can be t ransfer red f rom one spec imen type to another.U n f o r t u n a t e l y, t h e r e h a s b e e n n o c o m p a r a b l e e f f o rt t o d e t e r m i n eprop er c rack prepara t ion proced ures for sur face c rack spec imens .Thus , we mus t re ly wherever poss ib le on procedures deve lopedf o r th r o u g h c r a c k s p ec i m e n s. C e r t a i n r e q u i r e m e n t s f r o m A S T ME 399-74 should, in principle, be applicable to surface crack

spec imens as well , and these (wi th the ap prop r ia te sec t ionnumber in paren theses) a re paraphrased in the fo l lowing .

1 . The fa t igue c rack and i t s s ta r te r m us t l ie en t i re ly wi th in animaginary 30-deg wedge whose apex i s a t the c rack t ip (7 .2 .2) .

2 . The fa t igue c rack ex tens ion sha l l be no t l ess tha n 5 % o f the

f ina l c rack length and not l ess tha n 0 .05 in . (1.3 mm ) (7.2 .3) .3 . Fa t igue c rack ing sha l l be conducted wi th the spec imenfu l ly hea t - t rea ted to the condi t ion in which i t i s to be tes ted (7.4) .

4 . Fa t igu e c rack ing by can t i lever bending i s p rohib i ted (7.4.1).5 . T h e v a l u e o f K f s h a l l b e k n o w n w i t h a n e r r o r o f n o t m o r e

than 5% (7 .4 .1) .6 . For a t l eas t the f ina l 2 .5% of the f ina l c rack length , the

ra t io K r / E shall not exceed 0.002 in. 1/2 (0.00032 mln ). Fu rthe r-mo re, Kf mu st no t exceed 0.6 X K~c (7.4.2).

7 . Th e load ra t ioR = Kmin/Km~shou ld n ot ex ceed 0.1 (7.4.3).8 . When fa t igue c rack ing i s conducted a t a t empera ture T~

and tes t ing a t a d i ffe ren t t empera ture T2 , thenKf/ay~)T~m u s tnot exceed 0.6 X (Kic/qys)r~ (7.4.4) . Th e require me nt onK f / E i s presumably unchanged , p robably s ince e las t ic modul ia re se ldom as sens it ive to tem pera ture as a re y ie ld s trengths .

I t e m s 1 a n d 2 s h o u l d p r o b a b l y b e a p p l i e d a r o u n d t h e e n t i r eper iphery of the sur face c rack . The f i r s t requi rement of I tem 2is feas ib le , bu t the second can be appl ied on ly when the c rackd e p t h i s t o b e g r e a t e r t h a n a b o u t 0 . 0 6 i n . ( 1 . 5 m m ) . I t e m s 3and 7 can be appl ied d i rec t ly to the sur face c rack spec imen.Since can t i lever bending does no t p resen t a c rack p tanar i typ r o b l e m w i t h s u r f a ce c r a ck s , I t e m 4 n e e d n o t a p p l y. I t e m 5 a n dthe f i r s t requi rement of I tem 6 cannot be s t r ic t ly appl ied a tpresen t for l ack of a r igorous e las t ic s t ress in tens i ty ana lys i s ,b u t a n e s t i m a t e o f K f c a n b e m a d e u s i n g t h e b e s t a p p r o x i m a t ea n a l y s is c u r r e n t l y a v a i l a b le . I t e m 8 a n d t h e s e c on d r e q u i r e m e n t

o f I t e m 6 c a n n o t b e a p p l i e d f o r l a c k o f a n u n e q u i v o c a l o p e r a -t iona l def in i t ion of Kzo (or KI~) for sur face c rack spec imens .From the preceding d iscuss ion the fo l lowing guide l ines for

fa t igue c rack ing of sur face c rack spec imens can be ex t rac ted .

1 . F a t i g u e c r a c k w i t h t h e s p e c i m e n i n t h e h e a t t r e a t m e n tcondi t ion in which i t i s to be tes ted . Axia l t ens ion or can t i leverb e n d i n g a r e t h e m o s t c o m m o n m o d e s o f lo a d in g .

2 . Whenever i t i s phys ica l ly poss ib le , the c rack should beextended a t l eas t 0 .05 in . (1 .3 mm); in any event the fa t iguec r a c k e x t e n s i o n m u s t n o t b e l e s s t h a n 5 % o f t h e f i n a l c r a c klength , and the c rack and i t s s ta r te r mus t l i e en t i re ly wi th in ani m a g i n a r y 3 0 - d e g w e d g e w h os e a p e x i s a t t h e c r a c k t i p . T h e s et w o ~ i m e n s i o n a l d e s c r i p t i o n s s h a l l a p p l y a r o u n d t h e e n t i r e

crack f ron t , tha t i s , in a l l p lanes norm al to tangen ts to a l l po in tson the c rack per iph ery ( see F ig . 4 ) .

3 . The load ra t io R sha l l no t be grea te r than 0 .1 .4 . For a t l eas t the f ina l 2 .5% of the to ta l c rack depth , the

ra t io K f f E sho uld no t ex ceed 0.002 in. 1/2 (0.00032 ml/~). Un tilmo re exac t s t ress in tens i ty so lu t ions a re ava i lab le , use the bes tapproximat ions cur ren t ly ava i lab le ( such as Ref 28 for c i rcu la rsegme nt c racks or Ref 29 for semie l l ip t ica l c racks) and docu men tthe fa t igue c rack ing loads and c rack d imens ions .

Instrumentation

A n i n s t r u m e n t a t i o n s y s t e m f o r s u r f a c e c r a c k C O D m e a s u r e -ments should meet the fo l lowing genera l requi rements . Sys temgain , reso lu t ion , and s tab i l i ty mus t be suff ic ien t to provide an



6/8

3 4 JOURNAL OF TESTING AND EVALUATION

/ Starter s lot

Sect iorJ A A

F I G 4 - - F a t i g u e c r a c k a n d s t ar t e r d e t a il s . N o t e : S e c t i o n A - A r e f e rs t oth p lane norm al to any t ange n t to the c rack pe r iphe ry a nd con ta in ing thepo in t o f t angency. )

i n t e r p r e t a b l e t e s t r e c o rd . I f C O D m e a s u r e m e n t s d u r i n g c y c l icl o a d i n g o f t h e s p e c i m e n a r e a n t i c i p a t e d , t h e s y s t e m g a i n s h o u l dn o t c h a n g e n o r s h o u l d t h e z e r o s e t ti n g s h i f t s i g n i f i c a n tl y f o r t h ed u r a t i o n o f t h e t e s t o r b e t w e e n r e c a l i b r a ti o n s . G a g e l e n g t hs h o u l d b e a s s m a l l a s p o ss i b le . T h e m e t h o d o f a t t a c h i n g t h e g a g eo r c l i p s ) t o t h e s p e c i m e n m u s t n o t a l t e r e i t h e r t h e m a t e r i a l

p r o p e r t i e s i n t h e v i c i n i t y o f t h e c r a c k t i p o r t h e s p e c i m e nc o m p l i a n c e .

T h e c l ip g a g e d e s c r i b e d i n A S T M E 3 9 9 - 74 s h o u l d b e a na d e q u a t e t r a n s d u c e r. M o d e r n d - c a m p l i f ie r s c a n s u p p l y m o r et h a n e n o u g h g a i n , a n d s t a b i l i t y s h o u l d b e a d e q u a t e i f t h e s t r a i ng a g e e x c i t a t i o n l e v e l i s p r o p e r l y c h o s e n[50]. I f th i s c l ip gage i s tob e u s e d c y c l i c a ll y, s o m e a t t e n t i o n s h o u l d b e g i v e n t o t h e f a t i g u el if e o f t h e s t r a i n g a g e s[51]. I n o r d e r t o m a k e t h e g a g e l e n g t h a ssmal l a s poss ib le , smal l b rack e t s o r c l ips F ig . 5 ) wi th in tegra lk n i f e e d g e s a re o f t e n m i c r o s p o t w e l d e d t o t h e s p e c i m e n a s n e a r

a s p o s s i b l e t o t h e c r a c k ; t h e e f f e c t i v e g a g e l e n g t h i s t h e n t h ed i s t a n c e b e t w e e n t h e s p o t c e n t e r s . I f t h e c r a c k i s l a rg e e n o u g h ,k n i f e e d g e s c a n s o m e t i m e s b e m a c h i n e d in t o t h e c r a c k e d f a c e o ft h e s p e c i m e n i t s e l f[11]. T h e k n i f e e d g e g e o m e t r y s p e c i f i e d i nA S T M E 3 9 9 - 7 4 s h o u l d b e a p p r o p r i a t e . W h i l e o t h e r t r a n s d u c e r sa n d a t t a c h m e n t m e t h o d s a r e p o s s i b l e , t h e c l i p g a g e a n d s p o t -w e l d e d b r a c k e t a r e c u r r e n t l y t h e m o s t p o p u l a r.

A n e x p e r i m e n t a l l y d e t e r m i n e d p a r a m e t e r w h i c h i s c o n s i d e re dusefu l in the ana lys i s o f nuc lea r reac to r p ressure vesse l s i s theg r o s s s t r a i n c r a c k t o l e ra n c e . G r o s s s t r a i n i s d e f i n e d a s t h e s t r a i n

Fig 5 ~Tyln:cal experimental set for COD measurement.

a t t h e c r a c k l o c a t i o n , n o r m a l t o t h e c r ~ c k p l a n e , t h a t w o u l de x i s t i f t h e c r a c k w e r e n o t t h e r e . M e t h o d s o f m e a s u r e m e n t a n da p p l i c a t i o n a r e d e s c r i b e d i n R e f s 5 2 a n d53.

P r o p a g a t i o n o f a s u r f a c e c r a c k e n t i r e l y t h r o u g h t h e s p e c i m e nt h i c k n e s s b r e a k t h r o u g h ) u n d e r e i t h e r m o n o t o n i c o r c y c l i c l o a di s o f te n a n e v e n t o f i n t e re s t . I f t h e t e s t i s c o n d u c t e d i n r o o m a i r ,v i s u a l o b s e r v a t i o n u n d e r o b l i q u e l i g h t in g i s s o m e t i m e s s u f f ic i en t .B u t f o r m o s t e n v i r o n m e n t a l t e s t s t h a t i s , i n c r y o g e n i c l i q u id s o ra g g r e s s iv e f lu i d s) , r e m o t e r e a d i n g i n s t r u m e n t a t i o n i s n e c e s s a r y.O n e a p p r o a c h i s t o b o n d a f r a n g i b l e w i re t o t h e b a c k f a c e o f t h es p e c i m e n i m m e d i a t e l y b e h i n d t h e c r a c k a n d c o n n e c t it t o a si m p l ec o n t i n u i t y c i r c u i t . A n o t h e r m e t h o d i s t o c l a m p a p r e s s u r e o rv a c u u m c h a m b e r t o t h e b a c k f a c e ; w h e n b r e a k t h r o u g h o c c u rs ,p ressure o r vacuum i s los t , caus ing a sens i t ive p ressure swi tch tob e a c t u a t e d .

Test Procedure

C u s t o m a r y t e s t p r o c e d u r e t o d a y i s s i m i l a r t o g o o d c o n v e n -t i o n a l t e n s il e t e s t i n g p r a c t i c e . E x a m p l e s o f s u c h p r a c t i c e m a y b ef o u n d i n t h e A S T M M e t h o d s f o r Te n s i o n Te s t i n g o f M e t a l l i cM a t e r i a ls E 8 - 69 ) a n d f o r S h a r p - N o t c h Te n s i o n Te s ti n g o fH i g h - S t r e n g t h S h e e t M a t e r i a l s [ E 3 3 8- 6 8 1 9 7 3 )] . T h e r e a p p a r -e n t l y h a s b e e n n o s y s t e m a t i c s t u d y o f t h e e f f e c t s o f l o a d t r a i nm i s a l i g n m e n t o n s u b s e q u e n t s u r f a c e c r a c k f r a c t u r e b e h a v i o r.T h u s , i t i s i m p o r t a n t t o a l i g n t h e s p e c i m e n a s c a r e f u l l y a sposs ib le . Universa l jo in t s o r o ther se l f -a l ign ing dev ices in thel o a d t r a i n a r e d e s i r a b l e . W h e n t e s t i n g l a rg e s p e c i m e n s w i t hm u l t i p l e - b o l t g r i p s i t i s s o m e t i m e s h e l p f u l t o t e m p o r a r i l ya t t a c h a n e x t e n s o m e t e r t o e a c h e d g e o f t h e s p e c i m e n t o v e r i f yu n i f o r m i t y o f l o a d in g .

T h e r e i s n o c l e a r - c u t p r e f e r en c e f o r e i t h e r l o a d c o n t r o l o r d i s -p l a c e m e n t c o n t r o l i n t e s t i n g , n o r a n y i n d i c a t i o n t o d a t e o f as ign i f i can t d i ffe rence in t e s t r esu l ts . H ow ever, i t seems reasonab let o u s e t h e t y p e o f c o n t r o l m o r e c l o s e l y r e s e m b l i n g t h e a n t i c i -p a t e d s e r v ic e c o n d i ti o n s . L o a d o r c r o s s h e a d r a t e s a r e c u s t o m a r i l yc h o s e n s o t h a t f a i l u r e o c c u r s w i t h i n o n e t o t h r e e m i n u t e s a f t e rt h e s t a r t o f lo a d i n g .

I n p o s t f r a e t u r e e x a m i n a t i o n s , p o l a r i z e d l i g h t i n g [ 5 ~ ] o f t e nb r i n g s o u t s u b t l e d e t a i ls o f c r a c k g r o w t h h i s t o r y o n t h e f r a c t u r ef a c e. O p t i c a l m i c r o m e t e r s t r a v e l in g m i c r o s c o p e s) a r e o f t e n u s e dt o m e a s u r e c r a c k d i m e n s i o n s , o r e n l a rg e d p h o t o m a c r o g r a p h s o ft h e f r a c t u r e f a c e c a n b e m e a s u r e d w i t h a p r e c i s i o n s c a l e i f t h em a g n i f i c a t i o n i s a c c u r a t e l y k n o w n . I f a n ir r e g u l a r c r a c k t h a t i s,o n e h a v i n g a s h a p e o t h e r t h a n a h a l f e l li p se o r a s e g m e n t o f ac i r c l e ) i s n o t p h o t o g r a p h e d , e n o u g h d i m e n s i o n a l m e a s u r e m e n t s

s h o u l d b e t a k e n s o t h a t t h e c r a c k f r o n t c o n t o u r c a n b e r e c o n -s t r u c t e d .

Ana lys i s and Repo r ti ng

A s p ec i fi c m e t h o d o f d a t a a n a l y s i s c a n n o t b e r e c o m m e n d e d a tp resen t s ince , a s d i scussed ea r l i e r, the ana lys i s o f su r face c rackf r a c t u r e d a t a i s f a r f r o m a c l o s e d i ss u e . A p l o t o f g r o s s f r a c t u r es t r e s s a g a i n s t s o m e m e a s u r e o f c r a c k s i z e i s t h e s i m p l e s t a n dm o s t g e n e r a l l y u s e f u l w a y t o d i s p l a y d a t a . I n m o s t c a s e s t h ep a r a m e t e r a / ,~ 2 where i s a d imens ion less func t ion o f c racke l l ip t i e i ty[12,13] i s a s g o o d a m e a s u r e o f c r a c k s iz e a s a n y. F o rtes t s invo lv ing deep c racks in th in sec t ions , i t is somet ime s use fu lto p lo t g ross f rac tu re s t ress aga ins t c rack l eng th [M] .

I n r e p o r t i n g t e s t r e s u lt s , a n a l y s i s o f t h e d a t a i s n o t n e a r l y a si m p o r t a n t a s t h e r e p o r t i n g o f ed l p e r t i n e n t i n f o r m a t i o n . A t t h e



7/8

ORA NGE ON SURFACE CRACK SPECIMENS 34

present time, analysis of the data is essentially optional. As aminimum, the following should be reported:

(1) Material and heat treatment. If the toughness of thematerial is known to be sensitive to heat-treatment parameterssuch as quench rate (D6AC steel) or annealing history (t itanium

alloys), these should be described in detail.(2) Crack and load orientation with respect to material grain

direction.(3) Conventional tensile proper ties and elastic modulus using

specimens from the same lot of material (if the material isuncommon, a full stress-strain curve is desirable).

(4) Crack sta rter depth, length, shape, and me thod ofproduction.

(5) Type of loading for fatigue cracking, maximum load orstress, R ratio, and cyclic frequency.

(6) Initial (starter plus fatigue) crack depth, length, aildshape.

(7) Width, length, and thickness of specimen test section.(8) Test temperature, environment, and method of control.(9) Maximum load or corresponding gross section stress.(10) Estimated precision and accuracy of the measurements

above and of all major instrumentation.

If the following information is available, it should also bereported:

(11) Chemical analysis of material and source of analysis.(12) Number of fatigue-cracking cycles from first visible

microcrack to finished size.(13) Elast ic compliance (COD + load) for each test, COD

gage length, and at least typica l load-COD curves. If space orothe r considerations prohibit the presentation of all availableload-COD curves, the curves should be preserved and thoroughlydocumented for future reference.

(14) Loads or stresses corresponding to observed significantevents such as pop-in and breakthrough.

(15) Gross strain t o fracture and measurement gage length.(16) Miscellaneous measurements such as hardness and

Poisson's ratio.

Other Tests Using Surface Crack Specimens

As stat ed earlier, the specimen configuration, prepara tion,and instrumentation that have been described are usable forother than residual strength tests. However, certain constraintsare peculiar to each test.

Surface crack specimens have been used to determine crackpropagation characteristics under sustained load in both benignand aggressive environments [32,55,56]. The maximum cyclicload (or stress intensity) during fatigue cracking must besubstantiall y less than t hat to be sustained during the test ;otherwise, an erroneously hi gh apparent threshold w il l beindicated. Also, if the material exhibits any stable crack growthon rising load, the sustained test load should be applied with thespecimen already in the test environment. If the specimen isloaded in air and then introduced to the environment, a higherthreshold may be indicated.

Surface crack specimens have also been used to determinecrack propagation characteristics under cyclic load. A majorproblem in such testing is that the parameter of interest is the

change in crack depth, which usually cannot be measured directly.Ultrasonic [$8] and compliance derivative [34] methods have been

used to infer the depth of a propagating crack in real time. Notethat the specimen size requirements discussed earlier should beestablished based on the largest final surface crack size of interest.

Recommended Further Research

Further systematic studies to determine minimum test sectionwidth and length are desirable. Additional studies of surfacecrack shape change during fatigue cracking would greatlyreduce the amount of trial and error needed b y experimenters newto the field. Experimental determination of the maximum crackstarter envelope and .the minimum fatigue crack extension wouldalso be valuable. The maximum fatigue cracking load (or K f / Eneeds to be determined, and the effect of load train misalign-ment needs to be examined.

An exact stress intensity and displacement solution for thesemlelliptical surface crack in a finite plate would be extremelybeneficial. Unt il such is available, a series of fracture tests cover-ing a wide range of crack size and shape in a brittle metal mightallow the endorsement of one of the available stress intensityapproximations. Further analytical*and experimental com-pliance studies would be most valuable. The maximum allowableCOD gage length must be determined, and alternate transducersand methods of at tachment should be Considered.

Finally, the phenomenon of stable crack growth under risingload deserves concentrated study. I t is possible that some of theR-curve concepts [57] developed for thin sheet testing can beapplied to surface crack specimens.

Acknowledgments

This document is a report of ASTM Task Group E24.01.05 onPart-Through Crack Testing and was prepared by the taskgroup chairman. It is based on responses to a survey question-naire on current testing procedures submitted by the followingtask group members: G. E. Boekrath (McDonnell DouglasAstronautics Co.), R. M. Bonesteel (Lockheed Palo AltoResearch Laboratory), L. R. Hall (The Boeing Co.), C. L. Ho(Rockwell International Science Center), J. C. Newman, Jr.(NASA Langley Research Center), H. S. Pearson (Lockheed-Georgia Co.), P. N. Randall (U.S. Atomic Energy Commission),and W. E. Witzell (General Dynamics/Convair). Valuable andconstructive written criticisms of preliminary drafts were madeby W. G. Reutter (Aerojet Nuclear Co.) and Messrs. Hall andRandall. Additional oral comments by various task groupsmembers are gratefully acknowledged.

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8/8

3 4 JOURN L CbF TESTING ND EV LU TION

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