Fatigue crack propagation in iron and Fe-Mo solid solution alloys (77 to 296 K)
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r e c e n t y e a r s both d e s i g n e r s and r e s e a r c h w o r k e r s have r e c o g n i z e d the a d v a n t a g e s of dividing the fatigue p r o c e s s into individual s t a g e s of c r a c k initiation, p r o pagation and final f r a c t u r e . This p a p e r will c o n s i d e r only c r a c k propagation. F a t i g u e c r a c k growth r a t e s a r e often e x p r e s s e d in t e r m s of f r a c t u r e m e c h a n i c s p a r a m e t e r s , u s u a l l y in a f o r m s i m i l a r to that of the P a r i s equation, ~
da/dN = A( AK) n
[1]
w h e r e da/dN is the a v e r a g e c r a c k growth i n c r e m e n t p e r s t r e s s cycle, •K is the r a n g e of s t r e s s i n t e n s i t y f a c t o r e x p e r i e n c e d d u r i n g that s t r e s s cycle and A and n a r e m a t e r i a l s c o n s t a n t s . In the s i m p l e f o r m of Eq. [1], the c o n s t a n t s A and n include effects of m e a n s t r e s s , e n v i r o n m e n t , and so forth, and m a t e r i a l p r o p e r t i e s such as yield s t r e s s , modulus, and f r a c t u r e toughness. It is our i n t e n t i o n to define m o r e e x p l i c i t l y the i n f l u e n c e of these m e c h a n i c a l p r o p e r ties. The m a t e r i a l s used in p r e v i o u s e x p e r i m e n t a l s t u d i e s of m e c h a n i c a l p r o p e r t y effects u s u a l l y have been c o m p l e x e n g i n e e r i n g alloys whose t e n s i l e p r o p e r t i e s could be v a r i e d by heat t r e a t m e n t and which w e r e fatigue tested at r o o m t e m p e r a t u r e . R e s u l t s of such s t u d i e s have produced somewhat conflicting c o n c l u s i o n s . Higher f r a c t u r e toughness and yield s t r e s s have b e e n a s s o c i a t e d with r e d u c e d v a l u e s of A in Eq. [1] 2,3 and the value of n has been found to be i n v e r s e l y r e l a t e d to f r a c t u r e toughness. 2'4 In other s t u d i e s , s'6 however, these effects have not b e e n o b s e r v e d . It has also been pointed out 7 that the mode of fatigue c r a c k propagation, as r e f l e c t e d by f r a c t u r e s u r f a c e f e a t u r e s , can affect the i n f l u e n c e of m e c h a n i c a l p r o p e r t i e s on fatigue c r a c k growth rate.
L. H. BURCK, formerly Research Assistant at Materials Science and EngineeringDepartment and Materials Research Center, Northwestern University, Evanston, Ill., is now Assistant Professor, Materials Department, University of Wisconsin-Milwaukee,Milwaukee, Wl 53201. J. WEERTMANis Walter P. Murphy Professor of Materials Science, Materials Science and EngineeringDepartment and Materials Research Center and Professor, Geological Sciences Department, Northwestern University, Evanston, IL 60201. This paper is based on a thesis submitted by L. H. Burck in partial fulfillment of the requirements of the degree of Doctor of Philosophy at Northwestern University. Manuscript submitted March 26, 1975. METALLURGICALTRANSACTIONSA
It is the intent of the p r e s e n t study to r e l a t e fat
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