Fatigue crack propagation under varied mean stress conditions

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s p i t e of the t e c h n o l o g i c a l i m p o r t a n c e , few a t t e m p t s have b e e n m a d e to i s o l a t e m e a n s t r e s s e f f e c t s in the c y c l i c h a r d e n i n g - c r a c k n u c l e a t i o n - c r a c k growth s t a g e s of fatigue f a i l u r e in m e t a l s . The m o s t c o m p r e h e n s i v e s t u d y of m e a n s t r e s s e f f e c t s on r a t e s of fatigue c r a c k growth is t h a t r e p o r t e d b y F r o s t and c o - w o r k e r s , 1'2 who m e a s u r e d c r a c k growth r a t e s in a v a r i e t y of s h e e t m a t e r i a l s . F o r m a t e r i a l s such a s a n n e a l e d m i l d s t e e l , c o p p e r , b r a s s , t i t a n i u m , and so f o r t h , t h e y found that the r a t e of c r a c k growth was i n s e n s i t i v e to the l e v e l of the m e a n s t r e s s . The o b s e r v e d g r o w t h r a t e , d L / d N , was e x p r e s s e d a s d L =AaSaL dN

[11

w h e r e a a is the a l t e r n a t i n g s t r e s s s e m i - r a n g e , L is the c r a c k length, and A is a c o n s t a n t independent of the m e a n s t r e s s , a m . On the o t h e r hand, f o r m a t e r i a l s such a s the h a r d e n e d a l u m i n u m a l l o y s , c o l d - r o l l e d m i l d s t e e l and s t a i n l e s s s t e e l , the p r o p o r t i o n a l i t y c o n s t a n t A tn Eq. [1] was a l i n e a r function of the m e a n s t r e s s . In a s u b s e q u e n t a n a l y s i s of t h e i r data, F r o s t and c o - w o r k e r s d e m o n s t r a t e d t h a t an equation of the f o r m d..~L = C(AK)m dN

[2]

(where AK is the c y c l i c s t r e s s i n t e n s i t y e x p r e s s i o n and C is a c o n s t a n t which m a y o r m a y not depend on the m e a n s t r e s s ) a l s o p r o v i d e d an a d e q u a t e fit to t h e i r d a t a without a l t e r i n g t h e i r c o n c l u s i o n s r e g a r d i n g m e a n s t r e s s e f f e c t s . Although the functional f o r m d i f f e r s R. A. YESKE, formerly Research Assistant at Materials Science Department, Northwestern University, Evanston, I11.,is now Assistant Professor, Department of Metallurgical and Minerals Engineering, University of Illinois, Urbana, Illinois 61801. J. WEERTMAN is Walter P. Murphy Professor of Materials Science, Materials Science Department and Professor, Geological Sciences Department, North western University. This paper is based on a portion of a thesis submitted by R. A. Yeske in partial fulfillment of the requirements of the degree of Doctor of Philosophy at Northwestern University. Manuscript submitted August 8, 1973 METALLURGICAL TRANSACTIONS

f r o m c a s e to c a s e , F r o s t ' s o b s e r v a t i o n s of the s e n s i t i v i t y of growth r a t e s to m e a n s t r e s s l e v e l s have b e e n v e r i f i e d in s u b s e q u e n t i n v e s t i g a t i o n s ( s e e , f o r e x a m p l e , R e f s . 3, 4). A t t e m p t s to extend t h e s e o b s e r v a t i o n s to include i n t e r a c t i o n s with o t h e r p e r t i n e n t fatigue v

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