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D. E. BUSCH

The effects of s u r f a c e roughness on the l o w - c y c l e fatigue life of Type 304 s t a i n l e s s s t e e l at 593~ in a i r have been investigated. It is o b s e r v e d that, at a s t r a i n r a t e of 4 x 10 -z s "1 and a total s t r a i n r a n g e of 1 pct, the fatigue life (Nf cycles) d e c r e a s e s with an i n c r e a s e in s u r f a c e r o u g h n e s s . Information on c r a c k growth vs s t r a i n c y c l e s has been generated, as a function of s u r f a c e r o u g h n e s s , by the m e a s u r e m e n t of s t r i a t i o n s p a c i n g on f r a c t u r e d s u r f a c e s of s p e c i m e n s t e s t e d to f a i l u r e . Crack propagation follows the In a ~ : N (where a is the c r a c k length after N s t r a i n c y c l e s ) r e l a t i o n for longer s p e c i m e n fatigue l i v e s (Nf > 2700 c y c l e s ) and d e p a r t s f r o m In a c c N for s h o r t e r fatigue l i v e s . A quantitative e s t i m a t e is made of the number of c y c l e s No(R) to g e n e r a t e a c r a c k length equal to 0.1 mm (~ 1 grain diam). The initial s u r f a c e roughness significantly affects only the initiation component of s p e c i m e n life time. The effect of roughness on c r a c k initiation is d e s c r i b e d by No(R) = 1012R -~ where R is the s u r f a c e roughness ( r o o t - m e a n - s q u a r e value) in m i c r o n s .

F A T I G U E c r a c k s a r e initiated, in m o s t c a s e s , at the s u r f a c e s of s t r u c t u r a l components, and fatigue life is, t h e r e f o r e , s e n s i t i v e to s u r f a c e conditions. The i m p o r tance of tile r o l e of s u r f a c e s in fatigue c r a c k initiation has been d i s c u s s e d by s e v e r a l i n v e s t i g a t o r s . ~'2 I m p r o v e m e n t of s u r f a c e finish prolongs the fatigue life. %4 The magnitude of s u r f a c e - r o u g h n e s s effects on fatigue strength, however, depends on many v a r i a b l e s , such as fatigue t e s t i n g conditions (low-cycle or h i g h - c y c l e fatigue), method of producing s u r f a c e finish, t e m p e r a ture, and testing environment. Raymond and Coffin 5 have o b s e r v e d that, in a smooth s p e c i m e n subjected to axial push-pull cyclic loading, l o w - c y c l e fatigue f a i l u r e a p p e a r s to be g r e a t l y influenced by the changes in s u r f a c e topography, which include intensification in notch g e o m e t r y , s u r f a c e - r o u g h n e s s effects, and l o c a l ized s u r f a c e s l i p s t e p s . F o r example, they w e r e able to i m p r o v e the fatigue life significantly by i n t e r m i t t e n t machining to r e m o v e the changes in s u r f a c e topography that developed during fatigue testing. Because the s u r face is the c r a c k - i n i t i a t i o n site, with a few exceptions, it is important to quantitatively delineate the r o l e of s u r f a c e i r r e g u l a r i t i e s in the c r a c k - i n i t i a t i o n p r o c e s s . A g r e a t m a j o r i t y of the l o w - c y c l e fat