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S E V E R A L a u t h o r s 1-u have p o s t u l a t e d the e x i s t e n c e of a critical normal stress criterion for brittle fracture. A c c o r d i n g l y b r i t t l e (often c l e a v a g e ) f r a c t u r e o c c u r s in a m a t e r i a l when the n o r m a l c o m p o n e n t of the s t r e s s r e a c h e s a c e r t a i n c r i t i c a l v a l u e . T h i s condition is s c h e m a t i c a l l y i l l u s t r a t e d by the s t r e s s l i m i t e d line of the f r a c t u r e envelope p r o p o s e d by B e e u w k e s ~ in F i g . 1. T h e value of the l i m i t i n g f r a c t u r e s t r e s s , a ~ , c h a r a c t e r i s t i c of a m a t e r i a l is a s s u m e d to be independent of t e m p e r a t u r e and r a t e of loading, 3'6 but m a y depend o n the s t r a i n h i s t o r y . 7'8 In a t e n s i l e t e s t , n o r m a l s l i p u s u a l l y o c c u r s at s t r e s s l e v e l s c o n s i d e r a b l y l o w e r than this c r i t i c a l s t r e s s ~ and f r a c t u r e is s t r a i n l i m i t e d . 7's'~2 When the t e s t t e m p e r a t u r e is l o w e r e d , the o b s e r v e d o c c u r r e n c e of b r i t t l e f r a c t u r e , p r e d o m i n a n t l y in bcc m e t a l s and a l l o y s , can be a t t r i b u t e d to the i n c r e a s e of the y i e l d s t r e n g t h . T h e s t r e s s t r i a x i a l i t y p r o d u c e d in the v i c i n i t y of n o t c h e s and c r a c k s a l s o c a u s e s an i n c r e a s e in the y i e l d s t r e s s due to p l a s t i c c o n s t r a i n t . T h i s i n c r e a s e can be c a l c u l a t e d f r o m H i l l ' s s l i p line f i e l d t h e o r y . ~3 U n d e r c o n d i t i o n s of plane s t r a i n , n e g l e c t i n g s t r a i n h a r d e n i n g , the y i e l d s t r e s s r i s e s with i n c r e a s i n g d i s t a n c e f r o m the notch r o o t t o w a r d s the e l a s t i c p l a s t i c i n t e r f a c e . If that i n t e r f a c e i s a t a d i s t a n c e b f r o m the notch r o o t of r a d i u s p along the x a x i s , the m a x i m u m n o r m a l s t r e s s in the y d i r e c t i o n (loading d i r e c t i o n ) is given by

ay :2k[l+ln(l+b)l

tle f r a c t u r e below the notch r o o t at the e l a s t i c - p l a s t i c i n t e r f a c e . The c r i t i c a l f r a c t u r e s t r e s s , (r~, could be c a l c u l a t e d f r o m Eq. [1] if the y i e l d s t r e n g t h of the m a t e r i a l , 2k, the notch r o o t r a d i u s and the d i s t a n c e b f r o m the notch r o o t of the f r a c t u r e o r i g i n w e r e a c c u r a t e l y known. T h i s is i l l u s t r a t e d in F i g . 2. H o w e v e r , the value of STRESS UMITEDFRACTURE

TEMPERATURE STRAIN UMITED FRACTURE

PEAK

"'

STRESS

I--

NECK

o'>

/

TEMPERATURE T3

[1]

T h u s t h e r e e x i s t s a p o s s i b i l i t y for i n i t i a t i o n of b r i t A. M. GARDE is Graduate Research Assistant, Department of Metallurgy and Materials Engineering, The University of Florida, Gainesville, Fla. 32601. V. WEISS is Professor of Material