Cyclic creep of type 304 stainless steel during unbalanced tension-compression loading at elevated temperature
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b e e n known for many y e a r s that c y c l i c s t r a i n ing can be used to p r o m o t e the a c c u m u l a t i o n of net s t r a i n in one d i r e c t i o n at low m e a n s t r e s s l e v e l s . Many d i f f e r e n t t y p e s of e x p e r i m e n t s have b e e n done which r e l a t e to the a c c e l e r a t i o n of s t r a i n by s t r a i n or s t r e s s c y c l i n g . M o y a r and S i n c l a i r , ~ Wood and c o w o r k e r s , e-4 and R o n a y s have made e x t e n s i v e s t u d i e s of the a c c u m u l a t i o n of a x i a l s t r a i n in c y l i n d r i c a l s a m p l e s s u b j e c t e d to s m a l l a x i a l loads d u r i n g c y c l i n g b e t w e e n fixed s t r a i n l i m i t s in t o r s i o n . Coffin has r e p o r t e d on v a r i o u s shape i n s t a b i l i t i e s which o c c u r d u r i n g t e n s i o n - c o m p r e s s i o n cycling6'v and on e x p e r i m e n t s where s a m p l e s were s u b j e c t e d to s m a l l , steady t o r q u e s d u r i n g t e n s i o n - c o m p r e s s i o n c y c l i n g , s Coffin has a l s o p e r f o r m e d u n i a x i a l e x p e r i m e n t s in which s a m p l e s were cycled b e t w e e n s t r a i n l i m i t s with a s u p e r i m p o s e d m e a n s t r a i n - r a t e s and b e t w e e n s t r e s s l i m i t s with a m e a n s t r e s s . 9 A l l of this work g i v e s the s a m e q u a l i t a t i v e r e s u l t : Net s t r a i n s of s e v e r a l p e r cent can be a c c u m u l a t e d u n d e r the i n f l u e n c e of a s m a l l m e a n s t r e s s if the m a t e r i a l i s s i m u l t a n e o u s l y cycled through a r e v e r s i b l e p l a s t i c s t r a i n c y c l e . A n o t h e r type of e x p e r i m e n t a l s i t u a t i o n in which r e Luted b e h a v i o r is o b s e r v e d has b e e n c a l l e d c y c l i c c r e e p # 6-1s In this type of e x p e r i m e n t , a c r e e p t e s t i s r u n with p e r i o d i c unloading (either p a r t i a l or total), but the d i r e c t i o n of the s t r e s s is not r e v e r s e d . U n d e r c e r t a i n c o n d i t i o n s it is found that this load p a t t e r n p r o d u c e s a g r e a t e r s t r a i n r a t e t'haa would r e s u l t if the load were always m a i n t a i n e d at its m a x i m u m (cyclic c r e e p a c c e l e r a t i o n ) . U n t i l r e c e n t l y , the focus of a t t e n t i o n in t h i s type of t e s t i n g was on the load v a r i a t i o n s and the n e t elongation, and little a t t e n t i o n was paid to the s t r a i n which o c c u r r e d d u r i n g u n l o a d ing and r e l o a d i n g . R e c e n t l y , however, i n v e s t i g a t o r s have c a r e f u l l y m e a s u r e d the cyclic c o m p o n e n t of the
s t r a i n and have found that c y c l i c c r e e p a c c e l e r a t i o n is o b s e r v e d only when a r e v e r s e p l a s t i c s t r a i n is obs e r v e d on unloading.Is T h i s i n d i c a t e s that even for c a s e s of unloading only, w h e r e the sign of the s t r e s s is
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