On the subsequent yielding of an aluminum alloy following cyclic prestraining
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the t h r u - t h e - t h i c k n e s s w a v e s " r i n g d o w n " l e a v i n g r e s i d u a l i n - p l a n e s t r e s s and d e f o r m a t i o n s t a t e s that i n f l u e n c e the l a t e t i m e g r o s s s t r u c t u r a l m o t i o n f l C l e a r l y then, the s u b s e q u e n t y i e l d i n g of the s t r u c t u r a l m a t e r i a l following the e a r l y t i m e n o n s t e a d y l o a d i n g m a y influence the n a t u r e of the final d e f o r m a t i o n of the s t r u c t u r e a s a whole. The r e s p o n s e of a m e t a l to c y c l i c l o a d i n g d e p e n d s upon s e v e r a l f a c t o r s : the m a t e r i a l and i t s i n i t i a l s t a t e , the magnitude and h i s t o r y of the c y c l i c s t r a i n , and the m a t e r i a l t e m p e r a t u r e d u r i n g d e f o r m a t i o n . In t h i s s t u d y it w i l l b e shown t h a t the i n i t i a l s t a t e of the m a t e r i a l , n a m e l y t e x t u r e and h a r d n e s s , and the h i s t o r y of c y c l i c p r e l o a d i n g both influence the " s u b s e q u e n t c r o s s y i e l d i n g " of a c o m m e r c i a l A1-Mg-Si a l l o y . Thin w a l l e d t u b u l a r s a m p l e s w e r e f i r s t c y c l e d in the a x i a l d i r e c t i o n b e t w e e n r e v e r s e d s t r e s s l i m i t s . It i s well known that following an i n i t i a l t r a n s i t o r y p e r i o d the r e s p o n s e of a m a t e r i a l o r s t r u c t u r e will g e n e r a l l y " s h a k e d o w n " to a s t e a d y s t a t e under c y c l i c l o a d a p p l i c a t i o n . The s a m p l e s w e r e c y c l e d s u f f i c i e n t l y to a c h i e v e such a s t e a d y s t a t e condition. F o l l o w i n g a x i a l c y c l i c loading, i n t e r n a l p r e s s u r e was a p p l i e d so that a m e a s u r e m e n t could be m a d e of the s u b s e q u e n t c i r c u m f e r e n t i a l , o r hoop, y i e l d s t r e n g t h of the t u b u l a r s a m p l e s . In t h i s way we obtain a point on the s u b s e q u e n t y i e l d s u r f a c e in t w o - d i m e n s i o n a l p r i n c i p a l s t r e s s s p a c e (i.e., a x i a l s t r e s s / h o o p s t r e s s in a t h i n - w a l l e d tube), which is 90 deg f r o m the s t r e s s a x i s u s e d f o r c y c l i c loading, and thus the t e r m " s u b s e q u e n t c r o s s y i e l d i n g " a s u s e d above i s a p p r o p r i a t e . The c h a n g e s o b s e r v e d in the c i r c u m f e r e n t i a l y i e l d s t r e n g t h a r e e x p l a i n e d in t e r m s of the c r o s s effect which is shown to be a n a l o g o u s to the B a u s c h i n g e r effect f a m i l i a r in u n i a x i a l s t r e s s e x p e r i ments. The s a m p l e s u s e d in t h i s s t u d y w e r e m a c h i n e d f r o m 6061-T651 aluminum. The -T651 temper designation means that the alloy is in its peak hardness condition and that it has undergone a stress relief operation in manufacturing consisting of stretching the material to a permanent deformation of 1.5-3.0 pct
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