The effects of hydrostatic pressure on the mechanism of tensile fracture of aluminum
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AND
P. F. WEINRICH
The e f f e c t s of h y d r o s t a t i c p r e s s u r e s up to 600 M P a on the t e n s i l e f r a c t u r e m e c h a n i s m s of c o m m e r c i a l a l u m i n u m and a f r e e m a c h i n i n g a l u m i n u m - c o p p e r a l l o y h a v e b e e n i n v e s t i gated. T e n s i l e f r a c t u r e of the c o m m e r c i a l a l u m i n u m was found to o c c u r by a m e c h a n i s m l e a d i n g to d o u b l e - c u p f r a c t u r e at h y d r o s t a t i c p r e s s u r e s up to 125 M P a with the p r e s s u r e c a u s i n g a p r o g r e s s i v e d e l a y in the o n s e t of v o i d d e v e l o p m e n t . Above this p r e s s u r e , v o i d d e v e l o p m e n t was s u p p r e s s e d and all f r a c t u r e s o c c u r r e d at c h i s e l p o i n t s . The c h i s e l point f r a c t u r e was found to i n v o l v e the d e v e l o p m e n t of two o p p o s i n g d i a g o n a l s h e a r z o n e s c r o s s ing through the neck c e n t e r . F r a c t u r e of the a l u m i n u m a l l o y o c c u r r e d by a c u p - c o n e m e c h a n i s m at p r e s s u r e s up to 300 MPa. Within t h i s p r e s s u r e r a n g e the n a t u r a l s t r a i n to f r a c t u r e i n c r e a s e d l i n e a r l y and the a m o u n t of v o i d c o a l e s c e n c e d e c r e a s e d . At h i g h e r p r e s s u r e s f r a c t u r e of the a l u m i n u m a l l o y o c c u r r e d e n t i r e l y by a s h e a r m e c h a n i s m i n v o l v i n g the d e v e l o p m e n t of v o i d - s h e e t s . W H E N t e n s i l e t e s t s on c o m m e r c i a l m e t a l s a r e c a r r i e d out in a h y d r o s t a t i c p r e s s u r e e n v i r o n m e n t the c o m p r e s s i v e h y d r o s t a t i c s t r e s s c o m p o n e n t so r e t a r d s the g r o w t h and c o a l e s c e n c e of v o i d s that the m e c h a n i s m s of t e n s i l e f r a c t u r e a r e g r e a t l y influenced. Studies have b e e n m a d e of the e f f e c t s of s u p e r i m p o s e d h y d r o s t a t i c p r e s s u r e on the t e n s i l e f r a c t u r e m e c h a n i s m s of c o m m e r c i a l c o p p e r I and a n u m b e r of c o m m e r c i a l b r a s s e s 2-4 C o p p e r w a s found to f r a c t u r e by the c u p cone m e c h a n i s m at p r e s s u r e s up to 300 M P a but, at h i g h e r p r e s s u r e s , f r a c t u r e o c c u r r e d by a m e c h a n i s m not i n v o l v i n g void d e v e l o p m e n t which r e s u l t e d in s e p a r a t i o n at a c h i s e l point. The f r a c t u r e m e c h a n i s m in b r a s s e s was found to change, at p r e s s u r e s of about 400 • 50 MPa, f r o m a c u p - c o n e m e c h a n i s m to one i n v o l v ing only s h e a r f r a c t u r e by a v o i d - s h e e t m e c h a n i s m . The change in f r a c t u r e m e c h a n i s m in b r a s s e s was a c c o m p a n i e d by a s u b s t a n t i a l d e c r e a s e in the r a t e of inc r e a s e of n a t u r a l s t r a i n to f r a c t u r e with p r e s s u r e . The e f f
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