Application of random process statistics to the rate sensitivity of plane-strain fracture toughness
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HETTCHE
Correlation functions of random process theory are used to intercompare available fracture toughness (Kic) vs loading rate (/~) data of three different mild steels. The analyses confirm that large fluctuations in fracture strength are dependent in an extremely sensitive yet statistically reproducible way upon/~, and apparently insensitive to temperature and neutron irradiation. The emergence of a common pattern of rate sensitivity Kic (Is in these ferritic steels provides a discriminating index of strain rate in the initiation stages of the crack growth process, helpful in aligning fracture models.
T H E u s e f u l n e s s of the K c and Kic i n d ic e s of f r a c t u r e toughne s s in evaluating h i g h - s t r e n g t h a l l o y s is now g e n e r a l l y r e c o g n i z e d . Since t h e r e w e r e p r o b l e m s with the h i g h - s p e e d a l t e r n a t o r r o t o r s of the late 1950's, ~'2 t h e r e has been a p e r s i s t e n t effort to extend the " F r a c t u r e M e c h a n i c " m e a s u r e m e n t s of KIc to m e d i u m - and l o w - s t r e n g t h s t e e l s . T h e r o t o r s w e r e so l a r g e that safety by v i r t u e of c o n v e n t i o n a l f r a c t u r e m o d e t r a n s i tion t e m p e r a t u r e could not be a s s u r e d . A d e c a d e l a t e r , s i m i l a r q u e s t i o n s a r e posed with r e g a r d to the cons t r u c t i o n of v e r y t h i c k - w a l l e d p r e s s u r e v e s s e l s for n u c l e a r p o wer p l an t s , this t i m e f o r t u n a t e l y without the p r e c e d e n c e of c a t a s t r o p h i c f r a c t u r e f a i l u r e s , s Such c o n s i d e r a t i o n s have m o t i v a t e d Kic testing of heavy s e c tions n e a r a m b i e n t t e m p e r a t u r e s . ~ ' 2 ' 4 ' s Most a v a i l a b l e data, h o w e v e r , a r e f o r s m a l l e r , l e s s c o s t l y s p e c i m e n s e m b r i t t l e d by low t es ti n g t e m p e r a t u r e s , e.g., R e f s . 6 thr ough 9. In a l l , s o m e two dozen d i f f e r e n t low- and m e d i u m - s t r e n g t h s t e e l s have been studied. It is f a i r to say that in m o s t c a s e s the s c a t t e r of the Kic data has been f ar g r e a t e r than d e s i r e d , often an o r d e r of ma gnit u d e g r e a t e r than e x p e c t e d f r o m e x p e r i e n c e with h i g h - s t r e n g t h a l l o y s . V a r i a t i o n s in KIc as l a r g e as a f a c t o r of two a r e o b s e r v e d b e t w e e n o s t e n s i b l y valid, r e p l i c a t e m e a s u r e m e n t s . It s e e m s unlikely that s o u r c e s of v a r i a t i o n such a s i n h o m o g e n e i t y of the m a t e r i a l o r d i f f e r e n c e s in t e s t i n g m e t h o d o lo g y could be r e s p o n s i b l e f or the fluctuations in o b s e r v e d f r a c t u r e s t r e n g t h . This u n c e r t a i n t y is n a t u r a l l y a s o u r c e of c o n c e r n in e v a h i a ting the f r a c t u r e safety of thick m i l d s t e e l s t r u c t u r e s . One p o s s i b l e explanation of
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