• PDF / 652,511 Bytes
• 6 Pages / 612 x 792 pts (letter) Page_size
• 27 Downloads / 184 Views

DOWNLOAD

REPORT

---

A N D Y. T. C H O U

The e l a s t i c e n e r g y of a t w o - d i m e n s i o n a l s l i t c r a c k in b c c r e f r a c t o r y m e t a l s ( F e , Nb, Ta, C r , V, Mo and W) i s a n a l y z e d a s a function of c r y s t a l l o g r a p h i c o r i e n t a t i o n . A p p r o x i m a t e e n e r g y e x p r e s s i o n s a r e o b t a i n e d f o r c r a c k s l y i n g on {100}, {110}, and ~111} p l a n e s . The d i s t i n c t i o n b e t w e e n the m o d e of f r a c t u r e and the mode of l o a d i n g in g e n e r a l a n i s o t r o p i c s o l i d s is c l a r i f i e d . C l e a v a g e f r a c t u r e in bcc m e t a l s is d i s c u s s e d b a s e d on the e n e r g y data.

IN

s t u d y i n g c r a c k p r o p a g a t i o n in c r y s t a l l i n e s o l i d s , it is a d v a n t a g e o u s to u s e s i n g l e c r y s t a l s . The s i m p l i c i t y of u s i n g s i n g l e c r y s t a l s in an e x p e r i m e n t a l i n v e s t i g a t i o n , h o w e v e r , i s not c o m p l e m e n t e d b y s i m p l e t h e o r e t i c a l a n a l y s e s . In fact, the a n i s o t r o p i c e l a s t i c i t y for a three-dimensional crack remains analytically int r a c t a b l e . Thus, m o s t a n i s o t r o p i c t r e a t m e n t s of c r a c k p r o b l e m s have been confined to the t w o - d i m e n s i o n a l c a s e , and a n a l y t i c a l r e s u l t s have b e e n o b t a i n e d by s e v e r a l i n v e s t i g a t o r s . 2-s H o w e v e r , the r e s u l t i n g e x p r e s s i o n s a r e g e n e r a l l y i m p l i c i t and thus i n c o n v e n i e n t for application. M o r e r e c e n t l y , the p r e s e n t a u t h o r s 6'7 t r e a t e d the e n e r g y of a t w o - d i m e n s i o n a l s l i t c r a c k in a n i s o t r o p i c m e d i a with c r y s t a l l o g r a p h i c s y m m e t r y and o b t a i n e d e n e r g y e q u a t i o n s in c l o s e d f o r m . It was noted that on a given c r y s t a l l o g r a p h i c plane the e n e r g y of the c r a c k i s o r i e n t a t i o n - d e p e n d e n t , and that, e x c e p t f o r a few s i m p l e c a s e s , the e n e r g y v a l u e s have to be d e t e r m i n e d n u m e r i c a l l y . It was a l s o noted t h a t the e v a l u a t i o n of the c r i t i c a l s t r e s s f o r c r a c k p r o p a g a t i o n in t r a n s g r a n u l a r f r a c t u r e should be b a s e d on the m i n i m u m e n e r g y value on the plane of p r o p a g a t i o n , r a t h e r than the a v e r a g e value d e r i v e d f r o m i s o t r o p i c e l a s t i c i t y t h e o r y . These ' r e s u l t s s u g g e s t the need f o r d e t a i l e d i n f o r m a tion on c r a c k e n e r g i e s in o r d e r to u n d e r s t a n d the b a s i c m e c h a n i s m of the f r a c t u r e p r o c e s s . In t h i s p a p e r we s h a l l e x a m i n e the o r i e n t a t i o n d e p e n dence of t w o - d i m e n s i o n a l s l i t c r a c k s in c o m m o n b c c r e f r a c t o r y m e t a l s , n a m e l y , F e , Nb, Ta, Mo,