Influence of grain orientation on fatigue crack propagation
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H. W E B E R AND R. W . H E R T Z B E R G
F O R S Y T H1 p r o p o s e d that the f a t i g u e c r a c k p r o p a g a t i o n p r o c e s s be s e p a r a t e d i n t o t w o s t a g e s : S t a g e I , c o n s i s t i n g of f r a c t u r e by c r y s t a l l o g r a p h i c p r o c e s s e s a l o n g p l a n e s o f m a x i m u m s h e a r s t r e s s , a n d S t a g e I I , c o n s i s t i n g of f r a c t u r e a p p r o x i m a t e l y n o r m a l t o the t e n s i l e s t r e s s a x i s a n d c o n t r o l l e d by the n o r m a l s t r e s s c o m p o n e n t . A s e x p e c t e d , faceted* f r a c t u r e s u r f a c e s h a v e been ob*For the present study, a facet is defined as a portion of the fracture surface which is macroscopically planar, irrespective of the microscopic morphology of the surface. s e r v e d i n S t a g e I a n d r e l a t e d t o the o r i e n t a t i o n of s l i p p l a n e s . ' O n the o t h e r h a n d , f a c e t s2 o r f a t i g u e p a t c h e s 3 not n o r m a l t o the s t r e s s a x i s w e r e o b s e r v e d d u r i n g S t a g e II c r a c k g r o w t h s u g g e s t i n g g r a i n o r i e n t a t i o n e f f e c t s . T h e p u r p o s e of the p r e s e n t s t u d y w a s t o i n v e s t i g a t e the r e l a t i o n s b e t w e e n g r a i n o r i e n t a t i o n a n d f r a c ture s u r f a c e appearance. A cold-rolled plate, ~ in. thick, of c o a r s e - g r a i n e d 70 C u / 3 0 Z n b r a s s w a s a n n e a l e d f o r 1 h r a t 1 6 2 5 ° F , a i r c o o l e d , s u b j e c t e d t o a t e n s i l e s t r a i n o f 10 p e t , a n d r e a n n e a l e d f o r 24 h r a t 1 6 5 0 ° F . T h e r e s u l t i n g a v e r a g e g r a i n d i a m e t e r w a s f o u n d by l i n e i n t e r c e p t m e t h o d s t o be 2 . 0 m m . F o u r c e n t r a l l y n o t c h e d f a t i g u e s p e c i m e n s , 2 i n . wide a n d 9 i n . l o n g , w e r e c u t f r o m the s h e e t m a t e r i a l w i t h the r o l l i n g d i r e c t i o n b e i n g p a r a l l e l t o the l o n g i t u d i n a l a x i s of the s p e c i m e n s . T o t a l l e n g t h o f the s t a r t e r n o t c h e s w a s 0.4 i n . S a m p l e s were t e s t e d u n d e r tension-tension loading on a S o n n t a g F a t i g u e T e s t i n g m a c h i n e a t 30 H z . S t r e s s a m p l i t u d e s of 6000 p s i a n d 8100 p s i w e r e e m p l o y e d w i t h
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Fig. l-Stercographic triangle showing gross fracture facet orientation. Numbers in parentheses are the angle (in degrees) between fracture facet and tensile axis.
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G r a i n o r i e n t a t i o n s w i t h r e s p e c t t o the t e n s i l e a x i s w e r e o b t a i n e d by b a c k - r e f l e c t i o n L a u e p h o t o g r a p h s t a k e n on a G e n e r a l E l e c t r i c X R D - 4 u n i t w i t h u n f i l t e r e d , white radiation from a tungsten target. F r a c
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