Fracture Behaviout of Ni-Fe

8/2/2019 Fracture Behaviout of Ni-Fe

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E L S E V I E RFusion Engineering and De sign 36 (1997) 251-267

F u s i o nE n g i n ga r i d D e s m g n

F r a c tu r e m e c h a n i c s b e h a v i o r o f a N i - F e s u p e r a ll o y s h ea t hf o r s u p e r c o n d u c t i n g f u s i o n m a g n e t s .

P a r t 1. p r o p e r t y m e a s u r e m e n t s 1R . L . T o b l e r a ,, , I .S . H w a n g u, M . M . S t e e v e s c

a Nat ion al Ins t it u t e o f S tandards and Technology, Boulder, CO 80303, U SAb Seou l Nat io nal Univers it y , Seou l 151-742, So uth Ko reac Ma ssachuse t t s Ins t i t u t e o f Technology, Cambr idge, M A 02139, U SA

Received 8 July 1996

Abs t rac t

A s e a m l e s s e x t r u d e d c o n d u i t f o r s u p e r c o n d u c t o r c a b l i n g w a s f a b r i c a t e d a n d s u b j e c t e d t o m e c h a n i c a l t e s t s . T h ec o n d u i t i s m a d e o f a n i c k e l -i r o n a l l o y h a v i n g a g i n g a n d t h e r m a l c o n t r a c t i o n c h a r a c t e r i s ti c s c o m p a r a b l e w i t hN b 3 S n c o n d u c t o r s . T h e c o n d u i t i n l i q u i d h e l i u m a t 4 K r e t a i n s i t s d u c t i l i t y a n d o f f e r s h i g h s t r e n g t h , t o u g h n e s s ,and fa t igue re s i s t ance . Spec im ens wi th su r face c racks in t ens ion o f fe r subs tan t i a l f rac tu re re s i s t ance fo r thep r a c t i c a l c as e o f c r a c k p r o p a g a t i o n i n t h e t h r o u g h - w a l l d i r e c t i o n . F a t i g u e t e st s i n d ic a t e t h a t s u r f a c e c r a c k s a d o p t an e a r l y s e m i c i r c u l a r s h a p e a s t h e y g r o w t h r o u g h t h e c o n d u i t w a l l ( L - S o r i e n t a t i o n ) a t r a t e s i n t h e p o w e r - l a w r e g i o ntha t a re no fa s te r tha n ra te s in the transve rse d i rec t ion (L - T o r ien ta t ion) . T he s e rv iceab i l i ty o f th i s m a te r ia l i sdiscussed.

1 . I n t r o d u c t i o n

A n icke l - ir on s upe ra l loy ( Inco loy 908 ) 2 was r e -cen t ly deve loped fo r u s e in fu s ion magne t s a s as h e a t h f o r c a b l e - i n - c o n d u i t s u p e r c o n d u c t o r s [ 1 -3 ] . A s hea th i s a tubu la r load -bea r ing member

* Corresponding author.Contribution o f NIST.2 Tradenames are used h erei n for material identificationonly and do not imply endorsement of the products or manu-facturers by the authors or by NIST. Oth er materials by othermanufacturers may work as well or better.0920-3796/97/$17.00 1997 Published by Elsevier Science S.A. AllP I I S 0 9 2 0 - 3 7 9 6 ( 9 6 ) 0 0 7 0 1 - 6

t h a t s u p p o r t s N b 3 S n s u p e r c o n d u c t o r s w h i le c a r r y -i n g s u p e r c r i t i c a l h e l i u m a t 4 K . F o r s u c h a p p l i c a -t i o n s t h e s u p e r a l l o y o f f e r s p o t e n t i a l b e n e fi ts .U n l i k e a u s t e n i t i c s t a i n l e s s s t e e l s , t h e s u p e r a l l o y i sn o t d e g r a d e d b y h e a t t r e a t m e n t s t y p i c al f o rN b 3 S n f a b r i ca t i o n . I t s t h e r m a l c o n t r a c t i o n b e l o w1 0 0 0 K i s c o m p a t i b l e w i t h N b 3 S n , s o c o m p r e s s i v es t r a i n s i m p o s e d o n s u p e r c o n d u c t o r s d u r i n g c o o l -i n g a r e o p t i m i z e d .

A s h e a t h m u s t w i t h s t a n d h i g h s t a t i c a n d c y c l i cs t r e s s e s a r i s i n g f r o m L o r e n t z f o r c e s a t t e m p e r a -t u r e s a s l o w a s 4 K a n d i n m a g n e t i c f i e l d s a s h i g ha s 1 4 T . T h e f a t i g u e p r o p e r t i e s o f t h e s h e a t h a l l o yrights reserved.


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2 5 2 R . L . T o b l e r e t a l . / F u s i o n E n g i n eer i n g a n d D es i g n 3 6 ( 1 9 9 7 ) 2 5 1 - 2 6 7will often limit the design life of an advancedfusion system [4]. Mechanical failure of the sheathor helium leakage are both unacceptable sincereplacement or repair is prohibitively expensive.Flaw inspection is also problematic. Therefore, thestructural reliability of magnets is a primary designrequirement in advanced devices such as the Inter-national Thermonuclear Experimental Reactor.Fracture mechanics behavior under the circum-stances is critical to mater ial selection. Relevantdata for the Ni-Fe superalloy are promising, butlimited to developmental heats of material in theform of wrought plates [1-3,5-8]. We report newmeasurements here, extending the database tocommercially fabricated tube products.

2. Experim ental procedureWe obtained a seamless extruded conduit (SEC)

[5] and conducted tension, fatigue, and toughnesstests in ambient air (295 K), liquid nitrogen (76 K),and liquid helium (4 K). Surface-cracked tension(SCT) tests were used to evaluate crack tolerancein the through-wall direction at 4 K. SCT tests aredescribed in ASTM Practice E 740. By definitionthe crack depth is a, the total crack length is 2c,and the aspect ratio is a/c . The quantities deter-mined in SCT tests include the growth rates offatigue cracks and residual strengths for specimenswith part-through cracks on one surface. Theresidual strength of a cracked specimen is a func-tion of test temperature, crack dimensions, andspecimen thickness; it reflects the maximum loadthat can be sustained, and is particularly meaning-ful for conduits where crack propagation throughthe wall is a likely mode of failure in service.2.1. Materh~l and test specimens

The SEC was obtained from a commercial man-ufacturer and tested in the aged condition (923 K,200 h). The chemical composition in mass percentis: 49.42Ni-40.80Fe-3.99Cr-2.94Nb-1.55Ti- 1.03A1-0.16Si-0.04Mn-0.01C-0.01Cu-0.01Mo-0.01Co-0.01Ta-0.004P-0.003B-0.001S. The material was ex-truded, tube-reduced, and hydrogen-annealed in aseries of steps outlined previously [5]. The final

product was rectangular, with external dimensionsof 38 by 52 mm and a wall thickness of 6,7 mm.The nominal grain diameter was 74 gm and theVickers hardness was 295 kg mm 2.Ten pieces of conduit 250 mm long were ob-tained for testing. Each piece had residual coldwork ranging from 8-14%. Specimens were takenfrom strips representing the broad sides of theconduit. The specimens were machined out, heattreated, and tested in that order. Fig. 1 shows thesequence of operations involved in specimen man-ufacture. Fig. 2 shows the layout and relativeorientat ion of the conduit strips and blanks used tomake the three types of pin-loaded test specimens.

Tensile specimens had a longitudinal orienta-tion, a gage length of 30.5 mm, and a 6.2 x 4.6 mmcross section. Compact specimens were 6.2 mmthick and 35.5 mm wide, with standard planarproportions and clip gage retention points locatedat the specimen edge or loadline. The compactspecimen geometry is diagrammed elsewhere [8],and according to ASTM standard notation itsfracture plane orientation is L-T.

1"~----52.5m m - - ~

rrail l ~

255 mm ~ I

, lHeatT~ m ~ t (~O'C ~ 2~ h i~ ~c~uml IF i g . 1 . S e q u e n c e o f o p e r a t i o n s f o r s p e c i m e n m a n u f a c t u r e .


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R.L. Tobler et al. /Fusion Engineering and D esign 36 (1997) 251-2 67 253

38x 126mm

Surface! Flaw #2

Tensile Tensile#1 #2Blanksize :26 x82 mm

Tensile Tensile#3 #4

Compact0 I~ si,,.Blank size= 4 5 x43 mmCompact0 #T~'nsion

0Compact0 #T~lsion

0CompactTe n , ~ o n#4

0CompactTension#5

0

Fig. 2. Pla n for specimen cutting , showing how specimenblanks are sized and oriented in three 250 mm strips from thebroad sides of conduit.

F i g . 3 s h o w s t h e S C T s p e c i m e n g e o m e t r y . I t sw i d t h ( W ) i s 1 2.5 m m a n d i ts r e d u c e d - s e c t i o nl e n g t h i s 5 0 . 8 m m . E i g h t s p e c i m e n s w e r e r o u g h -m a c h i n e d , h e a t t r e a t e d , f i n a l - m a c h i n e d , a n d m e -c h a n i c a l l y p o l i s h e d t o a f in a l t h i c k n e s s ( B ) o f 6 .2m m . T h i s t h i c k n e s s i s e s s e n t i a l l y e q u a l t o t h e w a l lt h i c k n e s s o f a c o n d u i t i n s e rv i ce . H e a t t r e a t m e n tw a s p e r f o r m e d i n v a c u u m ( 0 .0 0 1 3 3 P a ) a t 9 2 3 Kf o r 2 0 0 h , f o l l o w e d b y f u r n a c e c o o l in g . E l e c t r o -d i s c h a r g e m a c h i n i n g ( E D M ) p r o d u c e d t h e s l i ts iz e s s h o w n i n T a b l e 1 . F o u r o f th e s p e c i m e n s h a ds e m i c i r c u l a r s l i t s , a n d f o u r h a d s e m i e l l i p t i c s l i t s .T h e s l it s w e r e 0 . 0 9 4 - 0 . 0 2 m m w i d e a n d c e n t e r e do n a b r o a d s id e o f th e r e d u c e d s e c ti o n i n t h e L - STable 1Nom inal starting flaw sizes for SCT specimensFlaw shape a (mm) 2c (mm) a /c ( - )Sem icircular 0.889 1.778 1.0Sem ielliptic 0.127 2.540 0.1

II~ 38.1 -*q

- .

l6_.21 i i It

Fig. 3. Su rface-cracked tension specimen.o r i e n t a t i o n . T h e t e n s i le a x i s i s t h e r e f o r e p a r a l l e l t ot h e e x t r u s i o n d i r e c t i o n , a n d c r a c k s i n t h e S C Ts p e c i m e n s t r a v e l i n a d i r e c t i o n t h r o u g h t h e c o n -d u i t w a l l .2 . 2 S t r e n g t h a n d f r a c t u r e t o u g h n es s

A S T M p r o c e d u r e s w e r e f o ll o w e d f o r t e n si o nt e s t s ( M e t h o d s E 8 - 9 4 a a n d E 1 4 5 0 - 9 2 ) a n dt o u g h n e s s t e s t s ( M e t h o d s E 3 9 9 -9 0 a n d E 8 1 3 -8 9 ).I n d u c t i l e f r a c t u r e t e s t s , a J - r e s i s t a n c e c u r v e ( p l o to f J v s . c r a c k e x t e n s i o n ) i s d e t e r m i n e d . A q u a n t i -t a t i v e m e a s u r e o f d u c t i le f r a c t u r e t o u g h n e s s J ~c i st h e n o p e r a t i o n a l l y d e f i n e d a s th e p o i n t o f in t e rs e c -t i o n o f t h e r e s i s t a n c e c u r v e r e g r e s s i o n l i n e a n d ab l u n t i n g l i n e , J = 2 ~ r v A a , d r a w n a t a 0 . 2 m mo f f s e t ( A S T M E 8 1 3 -8 9 ). A c o r r e s p o n d i n g f r a c -t u r e t o u g h n e s s v a l u e i n t e r m s o f t h e li n e a r e l a st i cp a r a m e t e r K ~c is e s ti m a t e d f r o m ( E x j~)l,.2, w h e r eE i s Y o u n g ' s m o d u l u s .2 . 3 . Fa t igu e c rac k in i t i a t i on and g row th

F a t i g u e c r a c k i n i t i a t i o n a t n o t c h t i p s h a v i n g ar a d i u s p w a s s t u d i e d u s in g c o m p a c t s p e c im e n s .


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254 R.L . Tobler et al. / Fusion Engineering and Design 36 (1997) 25 1-2 67The number of cycles required to initiate a 0.25mm fatigue crack is defined as the crack-initiationlife Ni and correlates with A K / x / p , a parameterproportional to the change in maximum elasticstress at the notch root. We assume that Ne de-pends on the maximum elastic stress Smax aheadof the notch and that Smax is proportional toK / x / p , where K is the applied nominal stressintensity factor [9,10]. The nominal value of K iscalculated from expressions for the s tandard stressintensity, using the notch length as if it wereequivalent to a sharp crack of length a [10].

Cracking was detected from changes in theload-displacement slope as the specimens werecyclically loaded at a stress ratio R = P min /P r n a x(minimum load/maximum load) = 0.1. During pe-riodic interruptions for single-cycle loadings, theload-displacement data were fed to a computerfor real-time data reduction, and a standard com-pliance function was used to infer the instanta-neous crack length as a funct ion of load cycles N.The resulting plots of a-vs -N were analyzed forcrack initiation and growth. Ne was determined atA a i = 0.254 mm and plotted vs. A K / x / p , whereA K is Kr~x - K~n for the fatigue load cycle.The growth rates da/dN of fatigue cracks werecalculated using short-crack simulation (SCS) andconstant-stress-ratio techniques. The conventionaltest maintains R = 0.1 and is used at moderate orhigh A K values. The SCS test maintains the max-imum stress intensi ty factor Kmax constant while Rgradually increases from 0.1-0.7. The SCS ap-proach is used at low A K (below 25 MPa x x/m)to efficiently generate a conservative thresholdstress-intensity factor AK-rh. We used these tech-niques before, and elsewhere describe the fatigueapparatus and test procedures in more detail [7-121.2.4. Surface crack tensio n tests

The SCT specimens were fatigue precracked at295 K and then fractured under uniaxial tensileloading at 4 K. the fatigue loading at 10 Hz andR = 0.1 was periodically interrupted to measurethe crack length 2c by an optical method. Pre-cracks of various shapes and sizes were created byfatigue in axial tension or three-point bending [13].

Specimens with semicircular slits (a/c = 1) werefatigued in axial tension using maximum fatigueloads equal to 30 or 40% of the ultimate tensilestrength at 295 K. Precracks formed in 111 000cycles or less, and the final a /c ratios variedbetween 0.87 and 0.97.

One specimen with an elliptical slit (a/c = 0.1)was fatigued in tension, but cracks formed at theloading-pin holes as well as at the EDM site. Toprevent this, the other specimens having ellipticalslits were fatigued in 3-point bending using atable-model 100 kN servohydraulic machine and abend-test fixture with roller pins adjusted to a spanof 63.5 mm.

Rates of fatigue crack growth in the L-S orien-tation (295 K) were calculated for two SCT speci-mens. The 2c-vs.-N data obtained duringprecracking were used to estimate da/dN values forcracks moving in the wall thickness direction. Inthis estimation, the flaw aspect ratios were interpo-lated between the initial EDM value Eq. (1) andthe final values (measured on the fracture surfaces)following Newman and Raju's method [14].

After precracking was completed, each SCTspecimen was cooled to 4 K and fractured in asingle loading using a 250 kN servohydraulic ma-chine in displacement control. The loading rate tofracture was within the normal range used instandard 4-K tension tests, and time to fracturewas about 5 min. The load-stroke curves wererecorded, and residual strength was calculatedfrom the maximum load at fracture divided by thespecimen's original cross-sectional area (Method E740, section 7.6).

3 . Resul ts

3.1. Te nsile and fractu re toughness propertiesTable 2 presents the tensile test results. Thetensile strength and ductility parameters for thismaterial are mildly temperature dependent. The

behavior resembles that of other aged superalloys[15-18] in showing a moderate increase of yieldstrength, a larger increase of tensile strength, andsignificant ductility at temperatures between 295and 4 K. The average value of the yield strength


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R.L. Tobler et al./Fusion Engineering and Design 36 (1997) 251 267Table 2Results of u niaxial tension tests at three temperatures.

255

Temperature(K) YS (MPa) UTS (MPa) EL (%) RA (%)295 1020 1270 22 38980 1250 22 35990 1240 19 37Average 997 1253 21 3776 1135 1540 25 401070 1540 29 39Average 1102 1540 33 384 1120 1635 22 281190 1680 26 32Average 1155 1658 24 30

a t 4 K i s 1 1 5 5 M P a , n e a r t h e t a r g e t e d v a l u e o f1200 M P a [2] .

T a b l e 3 l i s t s t h e f r a c t u r e t o u g h n e s s m e a s u r e -m e n t s a n d r e l a t e d p a r a m e t e r s a t 2 9 5 , 7 6 a n d 4K . L i n e a r e la s ti c p r o c e d u r e s ( M e t h o d E 3 9 9 )p r o d u c e d K Q d a t a a s s h o w n , b u t v a l i d K Io d a t ac o u l d n o t b e o b t a i n e d d i r e c t l y , d u e t o s u b s t a n t i a lp l a s t i c d e f o r m a t i o n i n t h e 6 . 2 m m s p e c i m e nt h i c kne s s . W e a pp l i e d e l a s t i c -p l a s t i c p roc e dure s(Me t hod E 813-89 ) i n s t e a d . F i g . 4 a nd F i g . 5s h o w a t y p i c a l r e s i s t a n c e c u r v e a n d t h e s p e c i m e nf r a c t u r e d i n t h a t t e s t a t 4 K . A s s h o w n i n T a b l e3 , t he c r i t i c a l J va l ue s a nd c o r re spond i ng Ki ce s t i m a t e s d e r i v e d f r o m t h e m a r e m i l d l y t e m p e r a -t u r e d e p e n d e n t a n d i n cr e as e m o d e s t l y w i t h t e m -p e r a t u r e r e d u c t i o n b e t w e e n 2 9 5 a n d 4 K . T h ea v e r a g e K i c e s t i m a t e ( 1 9 6 M P a x x / m ) a t 4 Kv i r t u a l l y e q u a l s t h e t a r g e t e d v a l u e o f 2 0 0M P a x / m .

F i g . 5 i s r e p r e s e n t a t iv e o f t h e f a i le d c o m p a c ts p e c i m e n s w h i c h w e r e s i m i l a r i n a p p e a r a n c e r e -g a r d l e ss o f t e st t e m p e r a t u r e . T h e z o n e o f h i g h e s tr e f le c t iv i ty a n d b r i g h t n e s s c o r r e s p o n d s t o t h e f a -t i g u e c r a c k . A b o v e t h a t i s l o c a t e d t h e f r a c t u r es u r f a c e c r e a t e d d u r i n g t h e J - t e s t , w h i c h c o n s i s t so f a z o n e o f f ia t f r a c t u r e , b o r d e r e d o n b o t h s i de sb y s h e a r l i p s w h i c h a r e c u s p s p r o j e c t i n g o u t o ft h e p l a n e o f t h e p h o t o g r a p h . T h e f i a t - f r a c tu r ez one i s t r a pe z o i da l s i nc e t he she a r l i p s i nc re a se i ns i z e a s f r a c t u re p rog re s se s . S c a nn i ng e l e c t ron mi -

c r o s c o p y a t h i g h e r m a g n i f i c a t i o n s r e v e a l e d d i m -p l e s on a l l t he f r a c t u re -z one su r fa c e s , c on f i rmi ngt h e e x i s t e n c e o f a d u c t i l e f a i l u r e m e c h a n i s m a te a c h t e s t t e m p e r a t u r e . T h e d i m p l e s f o r c o m p a c ts p e c i m e n s a r e s i m i l a r i n a p p e a r a n c e t o t h o s e f o rt h e S C T s p e c i m e n s s h o w n l a t e r i n t h e t e x t .

T h e c r i t i c a l J m e a s u r e m e n t s i n t h i s s t u d y a r ed e n o t e d J Q ( n o t J ~ ) b e c a u s e t h e s t a n d a r d r e -q u i r e m e n t s r e g a r d i n g u n i f o r m c r a c k a d v a n c e b e -y o n d t h e f a t i g u e c r a c k f r o n t c o u l d n o t b esa t i s f i e d i n t he se t e s t s . As i l l u s t r a t e d i n F i g . 5 fo rt h e r e p r e s e n t a t i v e s p e c i m e n , f a t i g u e p r e c r a c k i n ga l w a y s p r o d u c e d s a t i s f a c t o r y c r a c k - f r o n t s t r a i g h t -n e s s , b u t q u a s i s t a t i c l o a d i n g t o f r a c t u r e c a u s e dp r e f e r e n t ia l c r a c k a d v a n c e a t t h e c e n t e r o f s p e ci -m e n t h i c k n e s s ( t u n n e l l i n g ) . O w i n g t o t u n n e l l i n g ,t h e r e q u i r e m e n t s f o r A S T M E 8 13 -8 9 p a r a g r a p h8 .4 .3 .10 c a nno t be s a t i s f i e d , a nd c l o se a g re e me n tbe t we e n t he f i na l phys i c a l c ra c k s i z e a nd t hec o m p l i a n c e - p r e d i c t e d c r a c k s i z e a c c o r d i n g t op a r a g r a p h 9 . 4 . 1 . 7 w a s l o s t . S o m e i m p l i c a t i o n sa n d m i t i g a t i n g f a c t o r s a r e n o t e d l a t e r i n t h e D i s -c us s i on .3.2. Fatigue crack initiation

D a t a f o r f a t i g u e c r a c k i n i t i a t i o n a re p l o t t e d i nF i g . 6 . T h e n u m b e r o f c yc l es n e e d e d t o g e n e r a t e af a t i g u e c r a c k i n c r e a se s a t c r y o g e n i c t e m p e r a t u r e s ,e s p e c i a l l y b e t w e e n 7 6 a n d 4 K . F o r a l l t e m p e r a -


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256 R.L. Tobler et al./Fusion Engineering and Design 36 (1997) 251 267T a b l e 3R e s u l t s o f t o u g h n e s s t e s t s a t t h r e e t e m p e r a t u r e sT e m p e r a t u r e T h i c k n e s s W i d t h a/W ( -) Pmax/PQ -- ) KQ JQ Ktc(J(K) (mm) (mm) (MPa x x/m) (kJ/m 2) (MPa x x/m)295 6.044 35.560 0.614 1.02 a 170 a N A N A

6.222 35.510 0.589 1.56 100 NA NA6.172 35.510 0.592 1.42 113 165 171

76 6.274 35.560 0.591 1.62 117 215 2006.324 35.510 0.601 1.46 125 227 206

A v e r a g e 221 2034 6.222 35.560 0.600 1.52 124 210 198

6.122 35.510 0.585 1.80 102 201 193A v e r a g e 205 196a A c c i d e n t a l o v e r l o a d o c c u r r e d i n t h i s t e s t. F o r d i s c u s s i o n o f t e s t v a l id i t y r e q u i r e m e n t s , s e e t e x t .

t u r e s a n d s t r e s s r a n g e s c o n s i d e r e d i n o u r e x p e r i -m e n t s , a p o w e r - l a w e x p r e s s i o n a p p l i e s :U i = A ( A K / x / p ) - b ( 1 )w h e r e A a n d b a r e e m p i r i c a l c o n s t a n t s . A s s h o w ni n t h e f i g u r e , d a t a c o n f o r m i n g t o E q . ( 1 ) d i s p l a yl i n e a r t r e n d s w h e n AK/x/p s p l o t t e d v s N , . u s i n gl o g - l o g c o o r d i n a t e s. F o r s u c h p l o ts , A is t h ei n t e r c e p t o n t h e o r d i n a t e a x i s a t N i = 1 , a n d - bc o r r e s p o n d s t o t h e s l o p e o f t h e s t ra i g h t l in e . F o rt h e t r e n d l i n e s s h o w n i n F i g . 6 , T a b l e 4 l i s t s t h ee m p i r i c a ll y d e t e r m i n e d v a l u e s o f A a n d - b a te a c h t e s t t e m p e r a t u r e .

3.3 . Fat igue crack growthF i g . 7 s h o w s m e a s u r e m e n t s o f d a / d N f o r t h e

L - T o r i e n t e d c o m p a c t s p e c i m e n s a t 2 9 5 K . C r a c k -i n g r a t e s a t R = 0 . 1 f o r t h e S E C w e r e o b t a i n e d i nt h e r a n g e 1 5 < A K < 5 0 M P a x x / m . A l s o i n t h i sf i g u r e i s a b a n d r e p r e s e n t i n g p r e v i o u s S C S d a t a

80C

70

60

'~ 50 0

20C

10C

A l l o y 9 0 8T = 4 K

0.5 1.0 1.~C r a c k E x t e n s i o n , Aa , m m

Fig. 4. J-resistance curve f o r c o m p a c t s p e c i m e n a t 4 K. Fig. 5. C o m p a c t s p e c i m e n f r a c t u r e d a t 4 K.


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R.L . Tob ler e t a l . /Fus ion Engineer ing and Des ign 36 (1997) 251-267 257

500(3Q .

1000

50 0

I I I I

4 K

C o m p a c t S p e c i m e n 2 9 5 K ~B - - 6 2 1 m m ~ b ~ . . ~ . ~ . ~ -W = 3 5 . 6 m m - ~ "an = 1 4 . 5 m m

p = 0 . 0 7 7 m m

5 1 0 ' * s 1 0 5Num ber of Cycles to Ini t iate 0.25 rnnrnCrack

F i g . 6 . C r y o g e n i c e f f e c t s o n f a t i g u e c r a c k i n i t i a t i o n l i f e.

f o r w r o u g h t p l a t e s b e g i n n i n g a t AK = 3 0 M P a xx / m ( R = 0 .1 ) a n d e n d i n g a t A K = 2 M P a x x / m( R < 0 . 7 ) [8 ] . t h e p l a t e a n d c o n d u i t r e s u l t s a g r e e a tA K = 2 0 - 2 5 M P a x x / m , w h e r e R = 0.1 o r n e a r l y0 .1 i n b o t h t y p e s o f te s ts . B e l o w A K = 2 M P a xx / m , d i v e r g e n c e a p p e a r s i n t h e f o r m o f a k i n k o rk n e e i n t h e t r en d l i n e f o r t h e c o n v e n t i o n a l ( c o n s t a n tR - r a t i o ) r e s u l t s o f th i s s t u d y .

F i g. 8 s h o w s m e a s u r e m e n t s o f d a / d N o b t a i n e da t 7 6 K . A g a i n , c o n v e n t i o n a l d a t a f o r t h e S E C o ft h i s s t u d y ( R = 0 . 1 ) a r e c o m p a r e d w i t h e a r l ie r S C Sd a t a f o r w r o u g h t p l a t e s ( v a r i a b l e R ) . T h e d a t a a tR = 0 .1 e x t e n d t o r a t e s a s l o w a s 1 0 - 6 m m p e rc y c l e . E x t r a p o l a t i n g t h e t r e n d t o 1 0 - 7 m m p e rc y c le i n d i c at e s a t h r e s h o l d o f a b o u t 8 M P a x x / ma t t h i s t e m p e r a t u r e , w h e r e a s t h e S C S r e s u l t s i n d i -c a t e a m u c h l o w e r t h r e s h o l d : AKTh = 3 . 5 M P a xx / m . P r e s u m a b l y , 3 . 5 M P a x ~ / m i s t h e i n t r i n s i ct h r es h o l d a n d 8 M P a x , J m is a n a p p a r e n tt h r e s h o l d a f f e c t e d b y th e t e s t p r o c e d u r e a t R = 0 . 1 .I n a n o t h e r s t u d y o f a n n e a l e d 3 1 6 L N s te el , as i m i l a r d i f f e r e n c e i n t h r e s h o l d v a l u e s w a s r e p o r t e d ,a n d c r a c k c l o s u r e a t R = 0 .1 p r o v e d t o b e t h e c a u s eo f t h e h i g h e r t h r e s h o l d i n c o n v e n t i o n a l t e s t s [ 8,1 1] .

F i g . 9 sh o w s t h e m e a s u r e m e n t s o f d a M N o b -t a i n e d a t 4 K . I n t h i s s t u d y , w e o b t a i n e d S C S d a t af o r th e S E C a t A K b e l o w 25 M P a x x / m , a n dc o n v e n t i o n a l d a t a ( R = 0 . 1 ) i n t h e r a n g e 1 7 0.3 or a / ~ 2 > 0.9).

The residual strengths at 4 K for three speci-mens wi th semiel l ip t ic cracks are a l so shown inFig . 13 . For these , the f inal aspect rat ios rangebetween 0 .58 and 0 .74 . The data are too few todef ine a trend , but for s imi lar values of 2c thesemiel l iptic cracks give higher residual strengthsthan semicircu lar cracks s imply because the

EE

o~2(3

c t)

i i i i I i ~ =A x i a lT e n s i o nF a t i g u e

B e n d i n g~ e r ~a t i g u e

S l i t S i z e s . ~ . . . . ~

I i I I I I I I I I0.5 1.0Aspect Rat io a /c

F i g . 1 0 . R o o m t e m p e r a t u r e d a / d N d a t a f r o m c o m p a c t a n d F i g . 1 2 . F l a w d i m e n s i o n c h a n g e s i n 3 - p o i n t b e n d i n g a n d i nS C T s p e c i m e n s , t e n s i o n .


11/17

1817o - -

~:~ 1600)O3" ~ 1 4 0 0gw

1200 Z.;

O-- I

I I l I0 7 " N e a r l y S e m i c i rc u l a r C r a c k , T = 4 K" ~ ( a / c . 0 .9 )

~ " ~ , ~ (a /c = 0 .85 , 0 .74)~ o ~ S e m i e ll ip t i c C r a c k s , T = 4 K

I I I4 5 6

C r a c k L e n g t h 2 c , m m

F i g . 13 . R e s i d u a l s t r e n g t h v s . c r a c k l e n g t h .s e m i e l l i p t i c c r a c k s a r e s h a l l o w e r . J u s t o n e s p e c i -m e n w i t h a s e m i e l l i p t i c c r a c k ( a / c = 0 . 5 8 ) w a st e s te d a t 2 9 5 K ; a s e x p e c t e d , i t p r o d u c e d a s o m e -w h a t l o w e r r e s i d u a l s t r e n g t h c o m p a r e d t o t h es p e c i m e n s t e s t e d a t 4 K .

F i g . 1 5 i s a r e p r e s e n t a t i v e f r a c t u r e t e s t r e c o r df o r a n S C T s p e c i m e n . A l l te s t r e c o r d s a t 4 K w e r en o n l i n e a r a n d f i n e l y s e r r a t e d , w h i c h a r e q u a l i t a -t i v e i n d i c a t i o n s o f d u c t i l i t y .

O t h e r i n d i c a t o r s d e m o n s t r a t i n g m a t e r i a l d u c t il -i t y d e s p i t e t h e e x i s t e n c e o f s u r f a c e c r a c k s a r es h o w n i n F ig . 1 6. A s i n c o n v e n t i o n a l t e n s i o n t e s tso f u n f l a w e d s p e c i m e n s , w e c a l c u l a t ed a ' re s id u a lr e d u c t i o n o f ar ea ' R A f o r t h e S C T s p e c im e n s b yp i e c i n g th e b r o k e n s p e c i m e n h a l v e s b a c k t o g e t h e r ,m e a s u r i n g t h e f in a l d i m e n s i o n s a t t h e n e c k , a n dc a l c u l a t i n g t h e p e r c e n t c h a n g e r e l a t i v e t o t h eo r i g i n a l c r o s s s e c t i o n a l a r e a : ( A i - A r ) / A i x 1 0 0 =R A . S i m i l a r l y , w e t a k e t h e s t r o k e t r a v e l r e q u i r e dt o f r a c tu r e a s p e c i m e n a s a m e a s u r e o f t h e s p e c i-

1800 0]2 1,4

1so0

14o~n-

1200 ~ ~ t0.2 0,3 0,4R e l a t iv e C r a c k D e p t h a /B

P a r a m e t e r a ] ~0.4 0.16 0.8 1.0 1.2i I i l i I i j i

I0.5

Fig. 14, R e s i d u a l s t r e n g t h v s . r e l a t i v e c r a c k d e p t h .

I7

D i s p l a c e m e n t , m m

1 2 5

R . L . T o b l e r e t a l . / F u s io n Engineering and D es i g n 3 6 ( 1 9 9 7 ) 2 5 1 - 2 6 7 26 1

Fig. 15. L o a d - v s . - s t r o k e d i s p l a c e m e n t f o r a n S C T s p e c i m e n a t4 K .

m e n ' s a b i l i t y t o e l o n g a t e d u r i n g t e n s i l e l o a d a p p l i -c a t i o n . A b r i t t le m a t e r i a l w i ll n a t u r a l l y s h o w l o wv a l u e s o f s tr o k e t r a v e l , a s w e l l a s l o w v a l u e s o fr e s id u a l R A w i t h o u t m u c h e f f ec t d u e t o c r a c k

8 1

7EE,c" 615o: 4

3O= = 2

1

o

I I I IS e m i c i r c u l a r C r a c k sT = 4 K

I I I I3 4 5 6C r a c k L e n g t h 2 c , m m

3

11

l O r ,


12/17

26 2 R.L. Tobler et al./Fusion Engineering and Design 36 (1997) 251-267

Fig. 17. Specimen 203:4.5 x v iew of the reduced section after fracture at 4 K , showing front face with surface crack (right) an d ba ckface (left).

s i ze . I n c o n t r a d i s t i n c t i o n , t h e f i n d i n g s f o r o u rS C T s p e c i m e n s w i t h s e m i c i r c u l a r c r ac k s i n F ig . 1 6f e a t u r e s i g n i fi c a n t v al u e s o f r e s i d u a l d u c t i l i t y a n dt h e s t r o k e t r a v e l t o f r a c t u r e a t 4 K . B o t h p a r a m e -t e r s a r e s e n s i t iv e t o , a n d i n v e r s e l y r e l a t e d t o , c r a c ks i z e .

3.5. FractographyF r a c t o g r a p h s i n F ig s . 1 7 - 2 2 a r e r e p r e se n t a t i v e

f o r S C T s p e c i m e n s w i t h n e a r l y s e m i c i r c u l a r c r a c k st h a t w e r e t e s t e d a t 4 K .F i g . 1 7 s h o w s f r o n t a n d b a c k v i e w s o f s p e c i m e n

Fig. 18. Specimen 203 :6. 5 x view , norm al to the fractureplane.

r

Fig. 19. Specimen 201: SEM overview at 33 , showing: theEDM flaw, the 295-K fatigue crack, and the fracture surfaceformed at 4 K.


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R.L. Tobler et al./Fusion Engineer&g and Design 36 (1997) 251-267 263

Fig. 20. S pecimen 201:500 view of the EDM slit surface.2 0 3 ( a = 1 .8 5 4 m m , 2 c = 4 . 2 6 6 m m ) a f t e r d e f o r m a -t i o n a n d f r a c t u r e i n li q u id h e l i u m . T h e f r o n t v i e ws h o w s t h e h o r i z o n t a l s u r f a ce c r a c k a n d t w o 4 5 s l a n t f r a c t u r e s i n t h e l i g a m e n t s a t b o t h s i d e s .L o c a l i z e d d e f o r m a t i o n i s in d i c a t e d b y t h e n e c k e dl i ga m e n t s. U n i f o r m d e f o r m a t i o n b e f o r e n e c k i n ga n d f r a c t u r e i s i n d i c a t e d b y t h e ' o r a n g e - p e e l ' e f f e c tw h i c h a p p e a r s e v e r y w h e r e o n t h e s p e c i m e n ' s r e-d u c e d s e c t i o n e x c e p t i n t h e s t r e s s -f r e e l o c a t i o n s j u s ta b o v e a n d b e l o w t h e s u r f a c e c r a c k ( t h e s e u n d e -f o r m e d a r e a s r e t a i n t h e i r o r i g i n a l p o l i s h ) .

F i g . 1 8 v i e w s s p e c i m e n 2 0 1 ( a = 1 .2 1 8 m m ,2 c = 2 . 5 1 4 m m ) , n o r m a l t o t h e f r a c tu r e p l a n e. T h es e m i c i rc u l a r E D M f la w is s u r r o u n d e d b y a m o r er e f le c t iv e c r e s c e n t - s h a p e d f a t i g u e c r a c k p r o d u c e db y a x i a l l o a d i n g a t R = 0 . 1 . T h e f l a t - f r a c t u r e z o n ei s t r a p e z o i d a l , a n d s u r r o u n d e d o n t h r e e s i d es b y

Fig. 21. Specimen 201:5 00 x view of the 295 K fatigue.

Fig. 22. Specimen 201:7 50 x view of the 4 K fracture surface.s h e a r l i p s p r o d u c e d b y f r a c t u r e s u r f a c e s s l a n t e d a t4 5 t o t h e f a t i g u e c r a c k p l a n e . A p p a r e n t l y , s t a b l ef l a t -f r a c t u re p r o p a g a t e s t o a d e p t h o f 4 m m ( a b o u t6 5 % o f t h ic k n e s s ) b e f o r e u n s t a b l e f a i l u r e o c cu r s .

F i g . 1 9 i s a n S E M v i e w o f t h e s a m e s p e c i m e n ,s h o w i n g t h e s e m i c i r c u l a r E D M f la w , t h e cr e s c e n t -s h a p e d f a t i g u e c r a c k f o r m e d a t 2 9 5 K , a n d t h ef r a c t u r e z o n e f o r m e d a t 4 K . E n l a r g e d i m a g e s a r es h o w n i n F i g s. 2 0 - 2 2 . F i g . 21 r e v e a l s c r y s t a l l o -g r a p h i c f a c e t t in g w i t h s o m e s e c o n d a r y c r a c k i n g o nt h e f a t i g u e s u r f a c e a t 2 9 5 K . F i g . 2 2 s h o w s an e t w o r k o f d i m p l es a n d m i c r o v o i d s o n t h e o v e r -l o a d f r a c t u r e s u r f a c e w h i c h c o n f i r m s a g a i n t h a t ad u c t i l e f a i l u r e m e c h a n i s m i s r e t a i n e d a t 4 K .

4. Di scuss ion

4.1. Material comparisonsT h e t e ns i le a n d t o u g h n e s s p r o p e r t i e s o f t h e

c o n d u i t a r e m i l d l y t e m p e r a t u r e d e p e n d e n t , a n dc r y o g e n i c e f fe c t s a r e f a v o u r a b l e . T o u g h n e s s v a r i a-t i o n s w i t h t e m p e r a t u r e f o r s e l e c t e d s u p e r a l l o y s[ 1 5 - 1 9 ] a r e c o m p a r e d i n F i g . 2 3 . I n c l u d e d a r e t w oh e a t s o f a l l o y 7 1 8 [ 15 ,1 8 ], r e p r e s e n t i n g d i f f e r e n tp r o c e s s i n g h i s t o r i e s . S u c h d a t a a r e s u f f i c ie n t t od e m o n s t r a t e t h a t t h e N i - F e s u p e r a l lo y o f t h is s t u d yb e h a v e s q u a l i ta t i v e ly l i k e o t h e r p r e c i p i t a t i o n - h a r d -e n e d s u p e r a l l o y s o f i t s c la s s.

T h e s t r e n g t h - t o u g h n e s s c o m b i n a t i o n o f t h e c o n -du i t m a te r i a l a t 4 K i s a l so a t t r a c t ive . Th i s i s


14/17

2 6 4 R . L . T o b l e r e t a l . / F u s i o n E n g i n eeri n g a n d D es ig n 3 6 ( 19 9 7) 2 5 1 - 2 6 71 . 4 i I

1 .3i A l I o y 9 0 8/ A l l o y 7 1 E l

1 . 2 _ ~ _ ~ / A U o y 7 6 6~ ~ A l l o y 7 1 8

~ t . t1 . 0 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .0 . 9 ~ o y 7 5 06 .80.7,1 I I0 1 0 0 2 0 0 3 0 6

Test Temperature, KF i g . 2 3 . C r y o g e n i c e f f e c t s o n t h e t o u g h n e s s o f s u p e r a l l o y s .

shown in Fig. 24, where austenitic stainless steelsare represented by a trendline for commercial304-type steels having the T-L fracture plane ori-entation. The conduit specimens of this study hadthe L-T orientation, whereas the plate specimensin a previous study were T-L [8].4 . 2 . Frac ture t e s t v a l id i t y

Although our results do not satisfy all therequirements of the standard J-test, ASTMMethod E 813-89, nevertheless it can be arguedthat the data retain significance for the purposes inview. Standard test criteria are intended to assurethat test results will be size-independent and repro-ducible, whereas we have practically nullified sizeeffects by testing a specimen that is identical in

4 0 0

3o0n:E

20 C= =

03 1 6 (

i i I i i i iT = 4 K

~ O A l l o y 9 0 8 ,received plate A l l o y 9 0 8 ,ged conduit

i A l l o y 9 0 8 ,aged plate

o 8;0 ' t6;o 20002 0 00 0Yie ld S t r e n g t h , M P a

F i g . 2 4. S t r e n g t h - t o u g h n e s s c o m b i n a t i o n s a t 4 K .

thickness to the part that will be placed in service.The design-relevance of the fracture toughnessdata is thereby bolstered considerably.

As mentioned previously, the main problem instandard J-tests of the 6.2 mm compact specimensof is that crack extension at the center of thicknessoutdistances crack extension at specimen edgeswhere sizable shear lips formed. The requirementof section 8.4.3.10 of the J-test standard is vio-lated because the individual crack length measure-ments deviate from the ASTM average cracklength by much more than 7%. As a consequence,the compliance-predicted final crack extensions inour J-tests are about 25% shorter than the physi-cally measured crack extensions, whereas the dis-parity should not exceed 15% according to Section9.4.1.7.The effects of tunnelling are most severe at theend of a test. The reason is that disparities betweenthe ASTM average crack length and the individu-ally measured crack lengths near the specimenedges progressively increase as the shear lips areenlarged during the course of the test. Judgingfrom Fig. 5, shear lips ultimately occupy 60% ofthe overall specimen thickness while the interven-ing flat-fracture zone diminishes in proportion toabout 40%. The fidelity of the compliance-pre-dicted crack lengths is compromised since, in con-tradistinction to the assumptions of complianceanalyses, a considerable portion of material failsby a process of slant fracture occurring out-of-plane and lagging behind the flat-fracture front.Conversely, the effects of tunnelling are lesspronounced at the beginning of the fracture test,where the critical J value is determined. As shownin Fig. 5, fatigue crack fronts in compact speci-mens of the superaUoy are quite straight and fullyacceptable. The minimum crack length measure-ment at the specimen edge typically deviates fromthe ASTM average length by - 3%, whereas themaximum crack length at mid-thickness deviatesby + 1%, and deviations up to 7% for both areallowed by the standard. As a result, the initialcrack lengths are accurately inferred by compli-ance with uncertainty of less than 1%. Similarly,the critical J values reported in this study are notsubject to gross error since they are derived usingan offset procedure applied to the early part ofthe resistance curve at A a = 0.2 mm.


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R.L. Tobler et a l. / Fusion Engineering and D esign 36 (1997) 251-2 67 265100

EE 2O

Z 1 gE

, , i Ij , - - - /M in Size Required for V a l d KIc Test /( A S T M E 3 9 9 )

. . . . . . . . s _ i _ z ? _te_s_t ed__in_ h is _stu_d_y_R . . . . . . . . . . . . . . .

M i n . Size R equired for Val id J le Test /( A S T M E 8 1 3 )

i i i i4 ~ ;~6 2 9 5T e s t T e m p e r a t u re , K

Fig. 25. Thickness requirements for valid toughness tests, andconduit specimen thickness of this study.Regarding size effects, Fig. 25 projects the min-

imum thicknesses required for valid toughnesstests. The thickness required for J-tests is calcu-lated from Method E 813-89, section 9.4.1. Thethickness required for linear elastic response andvalid Kic measurement is calculated from MethodE 399-90, section 9.1.3, using the K i t ( J ) estimates(Table 2). For our 6.2 mm compact specimens,the J-test size criterion is satisfied at all testtemperatures whereas the linear elastic size crite-rion is satisfied at none. The outcome is fa-vourable in that an elastic-plastic response inservice is preferable to a linear elastic one. Thecalculations are of further interest in giving someindication of the material response that may beexpected in future applications. Based on thesedata, linear elastic fractures would be unlikely inservice components at 4 K unless the alloy wereused in section thickness more than 10 timesgreater than the conduit wall thickness tested inthis study.4 . 3 . Re s idua l s t r e ng th

Since our 6.2 mm thick specimens are not sub-ject to linear elastic fracture at 4 K, we calculatedresidual strength values for the L-S orientation

according to Practice E 740. In liquid helium, theSCT specimens fracture before stable cracking canfully penetrate the wall. Appreciable ductile frac-ture resistance is indicated by all criteria of per-formance. Nonlinear loading is associated withplastic deformation and concomitant slow crackgrowth, followed by unstable fracture and shear-ing of ligaments on three sides of the test section.

As shown in the text, the residual strength at 4K decreases regularly with flaw size, ranging from1720-1425 MPa for crack lengths 2c rangingbetween 2.5 and 6.5 mm (nearly semicircularcracks). The 1720 MPa value is actually 5%higher than the average ultimate strength (1658MPa, Table 2) measured in conventional flaw-freetension tests. Thus, there is some material incon-sistency for the two specimen types, even thoughthe same nominal stock and heat-treating condi-tions were used. Scatter in strength measurementscan arise from variations of cold work in thespecimen strips before aging. Or, the inconsis-tency may inadvertently derive from a differencein thermal response, since the SCT specimens andconventional tension test specimens were heat-treated in two separate batches on separate occa-sions.4.4 . Fat igue res is tance

The fatigue resistance of the conduit improvesat cryogenic temperatures, and some fatigueparameters described below show greater im-provement between 76 and 4 K than between 295and 76 K.4 . 4 . 1 . Fa t igue c rac k in i t i a t i onCrack ini tiat ion behavior resembles conven-tional fatigue life behavior in that: (1) the 3 K /x / p - v s . - N ~ curves (Fig. 6) are similar in form toconventional S - N curves, and (2) the crack initia-tion life increases along with the increases oftensile yield and ultimate stresses at cryogenictemperatures. The power laws of Eq. (1) and Eq.(2) for crack initiation and crack growth are alsocomparable, except that: (1) the exponents in thetwo equations have opposite signs, and (2) thecoefficient in Eq. (1) is temperature dependent,whereas the coefficient in Eq. (2) is not.


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266 R.L . Tob ler e t a l . /Fus io n Engineering and Des ign 3 6 (1997) 251 -2674.4.2. Unidirectional crack growth

F a t i g u e c r a c k g r o w t h r a t e s f o r t h e s u p e r a l l o yw e r e o b t a i n e d f o r a w i d e r a n ge o f A K , b u t w e f i n dth e s t r o n g es t t em p er a tu r e e f f ec t s i n t h e n ea r -t h r e s h o l d r e g i o n . L o w e r i n g t h e t e m p e r a t u r e f r o m76 to 4 K ra ises A K T h b y 4 M P a x ~ / m ( F ig . 7F ig . 8 F ig . 9 ) . I n co n t r a s t , an n ea led au s t en i t i cs t a in l e ss s t ee l s sh o w l i t t l e o r n o ch an g e in AK-rhf o r t h e sam e t em p er a tu r e r ed u c t io n [ 8 ]. Th eo r e t i -ca l ly , t h e c r ack in g r a t e t h r e sh o ld d o es in c r ease a tc r y o g e n ic t e m p e r a t u r e s [ 2 0 ]. T h e r e a s o n s f o r am o r e p r o n o u n c e d e f f ec t i n t h e s u p e r a l lo y a r e n o tp e r f ec t ly c l ea r , b u t two l i k e ly co n t r ib u t in g f ac to r sa r e th e ex cep t io n a l ly h ig h y i e ld s t r en g th o f t h esu p e r a l lo y a t 4 K [8 ] an d th e m e ta s t ab i l i t y o f t h eausten i t ic phase in s ta in less s tee ls [11] .

As s t a t ed ea r l i e r , t h e th r e sh o ld im p r o v em en t a tc r y o g en ic t em p er a tu r e s r e l a t e s t o k in k s in t h ed a / d N c u r v e w h i c h m a r k t h e t ra n s i t io n b e t w e e nt h e m i d r a n g e a n d t h r e s h o l d r e g im e s o f c r a c kg r o wth . Th e e f f ec t i s e sp ec ia l ly p r o n o u n ced in th es u p e r a l l o y a t 4 K . V a r i o u s p h y s i c a l m e c h a n i s m sh av e b een ad v an ced to ex p la in s im i l a r b eh av io r i no th e r a l lo y s . Fo r ex am p le , o n e m ig h t ex p ec t t h ec r a c k i n g r a t e d e p e n d e n c e t o c h a n g e w h e n , w i t hd ec r eas in g s t r e s s i n t en s i ty f ac to r r an g e , t h e c r ack -t ip p l a s t i c zo n e r each es a sca l e ap p r o ach in g th eaverag e gra in s ize o f the ma ter ia l . F ur t he r c lari fi -c a t i o n o f t h e b e h a v i o r o f t h e s u p e r a l l o y r e q u i r e sa d d i t io n a l r e s e a rc h b e y o n d t h e s c o p e o f t hep r e sen t s tu d y .4.4.3. Two-dimensional crack growth

F e w d a t a d e s c ri b in g t h e f l a w t o l er a n c e o f c o n -d u i t s i n t h e th r o u g h - th i ck n ess d i r ec t io n h av e b eenp u b l i s h e d b e f o r e [ 6 ]. O u r s t u d y m a i n l y f e a t u r e sp r ec r ack in g in t en s io n , wh ich i s r ep r e sen ta t iv e o fth e ty p e o f lo ad in g cu r r en t ly f o r e seen in se r v i ceap p l i ca t io n s . B ased o n th e l im i t ed o b se r v a t io n sav a i l ab le t o d a t e , t h e p r o p ag a t io n o f su r facec r ack s in t h i s su p e r a l lo y ap p ea r s t o f o l lo w c lo se lyt h e p a t t e r n s d o c u m e n t e d b y C o r n [ 1 3] f o r s e l ec t eds t ee l , a lu m in u m , an d t i t an iu m a l lo y s .

Du r in g ax ia l f a t ig u e , c r ack s in t h e su p e r a l lo yb a s e m e t a l q u i c k l y a d o p t a n d r e ta i n a n e a r l ysem ic i r cu la r sh ap e w i th a sp ec t r a t io s n e a r 0 .9. Th ec r a c k d e p t h c a n n o t b e v i s ua l ly o b s e r v e d d u r i n g a nS C T t e s t , b u t i t c a n b e i n f e r r e d f r o m m e a s u r e -

m e n t s o f 2 c , a s su m in g a = 2 c, b a sed o n th e co r r e -l a t io n sh o wn in F ig . 1 1 . Th e a sp ec t r a t io h as l i t t l eo r n o d e p e n d e n c e o n t h e i n c r e m e n t A a o f g r o w t h .T h i s a p p r o x i m a t i o n c a n b e u s e d i n f u t u r e m a g n e tdesigns , as we shal l i l lus t ra te in a second paper .

F r o m t h is w o r k , t h e g r o w t h r a t e o f fa t ig u ec r ack s in ex t r u d ed an d h ea t - t r ea t ed co n d u i t i sp r ac t i ca l ly i so t r o p ic f o r t h e th i ck n ess an d t r an s -v e r se d i r ec tio n s . Th e u se o f fa t ig u e d a t a o b ta in e df r o m t e s ts o f c o n v e n t i o n a l c o m p a c t s p e c im e n s i sth e r e f o r e ju s t i f ied f o r b ase m e ta l s . Th e b eh a v io ro f su p e r a l lo y we ld s h as n o t y e t b een v e r i f i ed an dwar r an t s f u r th e r s tu d y . A l th o u g h th e r e l a t io n sh ipa = 2 c ap p ea r s t o b e r e l i ab l e f o r b ase m e ta l sp ec i -m en s , i t c an n o t b e ap p l i ed wi th eq u a l co n f id en ceto su p e r a l lo y we ld s . I n two t e s t s ( u n r ep o r t ed ) t h ea /c r a t io s f o r ax ia l f a t ig u e c r ack s in we ld sp ec i -m e n s o f o u r N i - F e s u p e r a l l o y w e r e m e a s u r e d a t1 .0 7 - 1 .1 5 , wh ich i s h ig h e r t h an b ase - m e ta l v a lu esn ea r 0 .9 . S in ce th e t e s t s u sed id en t i ca l p r o ced u r esan d sp ec im en g eo m e t r i e s , t h e l a r g e r a sp ec t r a t io sf o r w e l d s m i g h t b e a t t r i b u t a b l e t o m i c r o s t r u c tu r a lf ac to r s o r r e s id u a l s t r e s se s .

I n th i s s tu d y , f a t ig u e in 3 - p o in t b en d in g p r o -d u c e d p r e c r a c k s w i t h a /c > 0 .58 . Bending fa t iguep e r m i t s t h e u se o f w id e r sp ec im en s an d av o id sc r ack in g a t t h e lo ad in g - p in h o le s o r f i l l e t s . Ho w-ev e r , i n b en d in g , c r ack g r o wth in th e d ep th d i r ec -t i o n in f l a t e s t h e a sp ec t r a t io b e f o r e c r ack in go ccu r s o n th e su r f ace o r 2 c d i r ec t io n . I t i s d if f icu l to r im p o ss ib l e t o ach iev e lo wer a sp ec t r a t io s (a /c < 0 .3) b y th i s a p p r o ach s in ce th e c r ac k d e p thin c r eases r ap id ly r e l a t iv e to c r ack l en g th .

5 . C on c lu s ion s

A n e w N i - F e s u p e r a l l o y w a s u s e d t o m a n u f a c -tu r e a seam less ex t r u d ed co n d u i t . Th i s co n d u i th as a t t r ac t iv e c r y o g en ic p r o p e r t i e s eq u iv a len t t ot h o s e r e p o r t e d e a r l i e r f o r w r o u g h t d e v e l o p m e n t a lp l a t e s . Th e m a te r i a l b eh av es l i k e o th e r p r ec ip i t a -t i o n - h a r d en ed su p e r a l lo y s in t h a t i t s m ech an ica lp r o p e r t i e s a r e m i l d l y t e m p e r a t u r e d e p e n d e n t a n dc r y o g en ic e f f ec t s a r e f av o u r ab le . Th e s t r en g th -to u g h n e ss co m b in a t io n a t 4 K i s ex ce l len t (YS =1 15 5 M P a , J Q = 20 5 k J m - 2 ) , a n d t h e t h r e s h o ldf o r f a t ig u e c r ack g r o wth i s r e l a t iv e ly h ig h ( 1 0


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R.L . Tob ler e t a l . /Fus io n Engineering and Des ign 36 (1997) 251-2 67 267M P a x x / m ) . S u r f a c e - c r a c k e d t e n s i o n t e s ts u s in gs p e c i m e n s o f f ul l t h i c k n e s s ( B = 6 .2 m m ) d e m o n -s t r a t e a d u c t i l e - f r a c t u r e m e c h a n i s m a n d g o o dc r a c k t o l e r a n c e i n t h e t h r o u g h - w a l l d i r e c t i o n . I nf a t ig u e , t h e r e s i s t an c e t o t h e g r o w t h o f c r a c k s i nt h e t h r o u g h - t h i c k n e s s d i r e c t i o n is n e a r l y e q u i v a -l e n t t o r e s i s t a n c e i n t h e t r a n s v e r s e d i r e c t i o n , a n dt h is p r o m p t s t h e f o r m a t i o n o f s e m i c i rc u l a rl ys h a p e d s u r f a c e c r a c k s d u r i n g c y c li c l o a d i n g i na x i a l t e n s i o n . T h e e m p i r i c a l o b s e r v a t i o n s i m p a c tf u s i o n m a g n e t d e s i g n s f a v o u r a b l y , a s s h o w n i n as u b s e q u e n t p a p e r ( P a r t 2 o f t h i s s t u d y ) . T h u s , t h en e w c o n d u i t a p p e a r s t o b e a n a t t r a c t i v e c a n d i d a t ef o r s u p e r c o n d u c t o r s h e a t h in f u s io n m a g n e t s . I m -p l e m e n t a t i o n o f th e m a t e r i a l w i ll u l t i m a t e l y d e -p e n d o n i ts w e l d a b i l i t y a n d r e s i s ta n c e t o s t r e ss -a s s is t ed g ra i n b o u n d a r y o x i d a t i o n ( S A G B O ) ;b o t h o f t h e se t o p i c s a r e c u r r e n t l y b e i n g r e s e a r c h e da n d a r e b e y o n d t h e s c o p e o f t h e p r e s e n t s t u d y .

AcknowledgementsT h e t e s t m a t e r i a l a n d h e a t - t r e a t e d s p e c i m e n s

w e r e p r o v i d e d b y M I T w i t h a s s is t a nc e f ro m L .T o m a . M e c h a n i c a l t es t s w e r e p e r f o r m e d a t N I S Tw i t h fu n d i n g f r o m t h e U S D e p a r t m e n t o f E n e r g y ,O f f ic e o f F u s i o n E n e r g y .

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Steeves, Mechanica l proper t ies of Incoloy 908--an up-date, Adv. Cryog. Eng. Maters., 38 (1992) 1-10.[4] N. Mitchell , Fatigue assessment of the structural steel insuperconducting coils for Tokamaks, Fusion. Eng. Des.,19 (1992) 225-233.[5] L.S. Tom a, I.S. Hwang, M .M. Steeves and R.N . Ran dall ,Thermomechanical Process Effects on Hardness andGra in Size in lncoloy Al loy 908 , Adv. Cryog. Eng.Maters., 40 (1994) 1307-1314.

[6] A. Nyilas, J. Zhang, B. Obst and A. Ulbricht, Fatigueand fa t igue c rack growth proper t ies of 316 LN andIncolo y 908 below 10 K, A dv. Cryog. E ng. Maters., 38(1992) 133-140.[7] A . Bussiba, R.L. To bler and J.R. Berger, Superconductorconduits: fatigue crack growth rate and near-thresholdbehav ior of three alloys, Adv. Cryog. En g. Maters., 38(1992) 167-174.[8] R.L. T obler and I.S. Hwang, F atigue crack thresholds ofa nickel-iron alloy for superconductor sheaths at 4 K ,Adv. Cryog. Eng. Maters., 40 (1994) 1315-1322.[9] A .R. Ja ck and A.T. Price, The init iation of fatigue cracksfrom notches in mild steel plates, Int. J. Fract. Mechs., 6(1970) 401-409.[10] M. Cre ager and P .C. Paris, Elastic field equations forblunt cracks with reference to stress corrosion cracking,Int. J. Frac t. Mech s., 3 (1968) 247-252.[11] R.L. Tobler, J.R. Berger and A . Bussiba, Long-crackfatigue thresholds and short crack simulation at liquidhelium temperature, Adv. Cryog. Eng. Maters., 38 (1992)159-166.[12] K. Suzuki, J. Fu kak ura an d H . Kashiwaya, Near-threshold fatigue crack growth of austenitic stainlesssteels at liquid helium temperature, Adv. Cryo g. Eng.Maters., 38, (1992) 159-162.[13] D.L. C orn, A study of cracking techniques for obtain ingpartial thickness crac ks o f pre-selected depths and shapes,Eng. Frac t. M echs., 3 (1971) 45-5 2.[14] J.C. N ewman and I.S. Raju, A n emp irical stress intensityfactor equation for the surface crack, Eng. Fract. Mechs.,15 (1981) 185-192.

[15] R.L . T obler, Low temp erature effects on the fracturebehavior of a nickel base superalloy, Cryogenics, 16(1974) 669-674.[16] W.A. Logsdon, J.M. Wells and R. Kossowsky, Fracturemechanics proper ties of austenitic stainless steels for ad -vanced cryogenic applications, in: Proc. 2nd Int. Conf.Mechanical B ehavior of M aterials, AS M , 1976, pp. 1283-1289.[17] W .A. Logsdon, Cryogenic fracture mechanics proper tiesof several manufacturing process/heat treatm ent combina-tions of Inconel x 750, Adv. Cryog. Eng., 22 (1977) 47-58 .[18] W.A. Logsdon, J.M. Wells and R. Kossowsky, The influ-ence of processing and heat treatment on the cryogenicfracture mechanics pr operties of Inconel 718, Adv. Cryog.Eng., 24 (1978) 197-209.[19] W.A. Logsdon, J.M. Wells and R. Kossowsky, Cryogenicfracture toughness and fatigue crack-growth rate proper-ties of Inconel 706 base material and gas tungsten-arcweldments, Adv. Cryog. Eng., 26 (1980) 137-150.[20] P.K. Liaw and W .A. Logsdon, Fatigue crack growththreshold at cryogenic temperatures: a review, Eng. Fract.Mechs., 22 (1985) 585-594.

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