The effect of strain rate on the cyclic response of metals
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T H E c y c l i c s t r e s s - s t r a i n r e s p o n s e of a m e t a l a p p e a r s to b e the c o n t r o l l i n g f a c t o r in i t s f a t i g u e b e h a v i o r . Both fatigue s t r e n g t h~ and fatigue c r a c k : g r o w t h r e s i s t a n c e z can b e a p p r o x i m a t e l y p r e d i c t e d f r o m a k n o w l e d g e of cyclic response. The c y c l i c s t r e s s - s t r a i n r e s p o n s e of a m e t a l g e n e r a l l y f o l l o w s a p o w e r law r e l a t i o n s h i p :
A(7 = g , ( _ ~ ) n' 2
[1]
w h e r e Aa i s the s t r e s s r a n g e , Aep i s the p l a s t i c s t r a i n r a n g e , and K ' and n ' a r e the c y c l i c s t r e n g t h c o e f f i c i e n t and c y c l i c s t r a i n - h a r d e n i n g exponent, r e s p e c t i v e l y . Eq. [1] is a n a l o g o u s to the t e n s i l e (monotonic) s t r a i n - h a r d ening r e l a t i o n s h i p :
o
[2]
w h e r e a i s the t e n s i l e s t r e s s , ep the p l a s t i c s t r a i n , and K and n a r e the monotonic s t r e n g t h c o e f f i c i e n t and m o n o t o n i c s t r a i n - h a r d e n i n g exponent, r e s p e c t i v e l y . It is w e l l known that the t e n s i l e s t r e s s - s t r a i n r e s p o n s e of m o s t m e t a l s is s t r a i n - r a t e s e n s i t i v e at high s t r a i n r a t e s ; 3 and such high s t r a i n - r a t e s a r e not unus u a l at the tip of an a d v a n c i n g fatigue c r a c k . ~ This r e s e a r c h p r o g r a m was d e s i g n e d to d e t e r m i n e w h e t h e r s t r a i n - r a t e e f f e c t s in the c y c l i c s t r e s s - s t r a i n r e s p o n s e could a c c o u n t f o r the o b s e r v e d s t r a i n - r a t e (frequency) e f f e c t s in fatigue c r a c k growth r a t e s and fatigue l i v e s of m e t a l s . EXPERIMENTAL M a t e r i a l s : The five a l l o y s t e s t e d r e p r e s e n t a v a r i e t y of m e t a l l u r g i c a l s t r e n g t h e n i n g m e c h a n i s m s . The fcc m e t a l s (6061-T651 a l u m i n u m , 7075-T6 a l u m i n u m and c o p p e r - l . 9 wt p c t b e r y l l i u m ) , m a r t e n s i t i c s t e e l (bodyc e n t e r e d t e t r a g o n a l 4340E) and a t w o - p h a s e b r a s s ( f c c b c c ) w e r e s t u d i e d to e v a l u a t e p r e c i p i t a t i o n - h a r d e n e d , m a r t e n s i t i c , and d u p l e x - h a r d e n e d s y s t e m s . The c o n d i tion and c o m p o s i t i o n s of the a l l o y s a r e given in T a b l e I. DAVID K. BENSON is Senior Physicist, Midwest Research Institute, Kansas City, Missouri 64110. JAMES R. HANCOCK, formerly with Midwest Research Institute, is now with the H. C. Price Company, Bartlesville, Okla. 74003. Manuscript submitted July 2, 1973. METALLURGICAL TRANSACTIONS
C y l i n d r i c a l s p e c i m e n s 0.300 in. (0.762 cm) d i a m . (Fig. 1) w e r e m a c h i n e d f r o m 0.75 in. d i a m e t e r r o d to a s u r f a c e f i n i s h of 16 pin. (4.1 • 10 -5 cm). The b r a s s s p e c i m e n s w e r e m a c h i n e d f r o m 0.75 in. (
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