Frequency effects in low-cycle fatigue
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The p h e n o m e n o n of l o w - c y c l e fatigue was i n v e s t i g a t e d with total s t r a i n r a n g e s f r o m 1 to 2 pct and f r e q u e n c i e s f r o m 0.2 to 100 cpm. Changes in e n d u r a n c e life u n d e r v a r y i n g s t r a i n and f r e quency conditions w e r e d e t e r m i n e d . D u r i n g the e n d u r a n c e t e s t s m o m e n t and s t r a i n v a l u e s w e r e r e c o r d e d . F r o m the continuous t i m e r e c o r d i n g s of t h e s e p a r a m e t e r s , e l a s t i c and p l a s t i c t i m e r a t i o s of s p e c i m e n l i v e s were defined and t h e i r d e p e n d e n c e on s t r a i n and f r e q u e n c y l e v e l s found. The change in c r a c k p r o p a g a t i o n r a t e with f r e q u e n c y was also s t u d i e d . F i n a l l y , a c u m u l a t i v e fatigue hypothesis, i n c o r p o r a t i n g both f r e q u e n c y and s t r a i n p a r a m e t e r s , was f o r m u l a t e d and shown to be v a l i d u n d e r v a r y i n g s t r a i n and f r e q u e n c y c o n d i t i o n s . M A C H I N E c o m p o n e n t s and s t r u c t u r a l m e m b e r s which fail b y fatigue a r e n e a r l y a l w a y s s u b j e c t e d to loads of v a r y i n g f r e q u e n c i e s . In p r a c t i c e t h e s e f r e q u e n c i e s r a n g e f r o m t h o u s a n d s of c y c l e s p e r m i n u t e down to t h o u s a n d s of m i n u t e s p e r c y c l e . An u n d e r s t a n d i n g of the f r e q u e n c y effects in fatigue is i m p o r t a n t not only b e c a u s e it m a y throw light on the b a s i c m e c h a n i s m o r m e c h a n i s m s of fatigue f a i l u r e , but a l s o b e c a u s e of the n e e d of the d e s i g n e r for such data in d e s i g n i n g s t r u c t u r e s o r in d e s i g n i n g a c c e l e r a t e d t e s t s for t h e s e structures. In r e c e n t y e a r s , a t t e n t i o n h a s i n c r e a s i n g l y b e e n d r a w n to this p r o b l e m . However, m o s t of the work has b e e n c a r r i e d out in the r a n g e of f r e q u e n c i e s of c o n v e n t i o n a l fatigue m a c h i n e s , which is r o u g h l y 500 to 10,000 cpm, and under loads giving long e n d u r a n c e l i v e s . U n d e r t h e s e conditions f r e q u e n c y effects have b e e n found to be s m a l l , u s u a l l y of the o r d e r of a few p e r c e n t . In the i n v e s t i g a t i o n s above this r a n g e a c o n t i n u o u s i n c r e a s e in fatigue s t r e n g t h , with or without a m a x i m u m was o b s e r v e d . Below this r a n g e , t h e r e is not a g r e a t deal of data a v a i l a b l e , and the r e s u l t s of the i n v e s t i g a t i o n s a r e s o m e t i m e s c o n t r a d i c t o r y . R e v i e w s of l i t e r a t u r e d e a l i n g with the effect of f r e q u e n c y on fatigue have b e e n g i v e n b y Van Leeuwen, 1 B r a d s h a w , 2 and W e l l e r . s It a p p e a r s that f r e q u e n c y effects o r i g i n a t e f r o m s p e c i m e n h e a t i n g due to high d a m p i n
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