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= c~ to e v a l u a t e the r e l a t i v e i n f l u e n c e of c r e e p and fatigue, where v i s the f r e q u e n c y , t is the time and k and C1 are c o n s t a n t s that depend on the m a t e r i a l and t e s t i n g cond i t i o n s . If k = 0, the t e s t is t i m e - d e p e n d e n t , as in pure c r e e p , and if k = 1, the t e s t is cycle dependent, a s in pure fatigue. F o r m o s t m a t e r i a l s tested at e l e v a t e d t e m p e r a t u r e s , k i s b e t w e e n 0 and 1, in a g r e e m e n t with o b s e r v a t i o n s that the n u m b e r of c y c l e s to f a i l u r e gene r a l l y i n c r e a s e s with f r e q u e n c y . Coffin 6 h a s developed the concept of a f r e q u e n c y modified life in o r d e r to extend use of the M a n s o n Coffin equation: ,", s

= c~

to e l e v a t e d t e m p e r a t u r e s , where AEp is the p l a s t i c s t r a i n r a n g e , N( the n u m b e r of c y c l e s to f a i l u r e and C2 a c o n s t a n t . The f r e q u e n c y - m o d i f i e d equation is: A ~ p N f v k - 1 = Cs

which is in a g r e e m e n t with some e l e v a t e d t e m p e r a t u r e fatigue data. More r e c e n t l y an equation h a s been d e veloped that r e l a t e s total s t r a i n r a n g e , f r e q u e n c y and cyclic life .7 M a n s o n e t a l . 8 have r e c e n t l y p r o p o s e d a l i n e a r c r e e p fatigue damage r u l e in which the fatigue damage is c a l culated by the method of u n i v e r s a l s l o p e s ~ and the c r e e p damage is d e t e r m i n e d by r u n n i n g cyclic c r e e p - r u p t u r e t e s t s at v a r i o u s s t r e s s l e v e l s . The c r e e p damage p e r cycle is then obtained f r o m the c y c l i c work h a r d e n i n g c h a r a c t e r i s t i c s of the alloy and the r e s u l t s of the c y c l i c c r e e p - r u p t u r e t e s t s . The effect of f r e q u e n c y is to change the cyclic work h a r d e n i n g or softening c h a r a c t e r i s t i c s and t h e r e f o r e the a m o u n t of c r e e p p e r cycle. The p h e n o m e n o l o g i c a l m o d e l s for high t e m p e r a t u r e fatigue that have been developed to date do not conF. E. ORGAN and M. GELL are associated with the Materials Engineering and Research Laboratory, Pratt & Whitney Aircraft, Middletown, s i d e r the actual c r a c k i n i t i a t i o n and p r o p a g a t i o n b e h a v i o r of m a t e r i a l s and how this b e h a v i o r is modified Conn. by t i m e - and t e m p e r a t u r e - d e p e n d e n t p r o c e s s e s . The Manuscript submitted August 12, 1970. METALLURGICALTRANSACTIONS

VOLUME 2, APRIL 1971-943

287 OAI

I

A --,~1

_

f0, o

--t1

-5 AI~.~

SECT-A-A (a)

~0.165 DIA I

i

-

Fig. 1--Mierostrueture of the Udimet 700 alloy. Magnification 160 Limes.

[ +_..J

._~0.625 IDIA

~/

SECT.8-

1.800 4.300 (b)

Table I. Chemical Composition

Element

NominalComposition

C Mn S Si Cr Co Mo Ti A1 B Ni

0,07 0.15 max, 0.015 max. 0.20 Max. 15.0 18.5 5.0 3.0 4.25 0.030 Remainder

p u r p o s e of this study is to pr