Frequency and wave-form effects on the fatigue crack growth behavior of alloy 718 at 298 K and 823 K
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c r a c k i n g which was o b s e r v e d at low f r e q u e n c y was e m phasized. The fatigue c r a c k growth b e h a v i o r of this alloy has equally b e e n e x a m i n e d by s e v e r a l a u t h o r s 9 '7 Popp and Coles 6 have i n v e s t i g a t e d the influence of t e n s i l e hold time on the high t e m p e r a t u r e FCG r a t e of Alloy 718. T h e s e a u t h o r s have shown that, at 810 K, the FCG r a t e was i n c r e a s e d as the h o l d - t i m e was i n c r e a s e d f r o m 90 s to 7200 s. Most of t h e i r r e s u l t s were obtained u s i n g thin sheet s p e c i m e n s and large v a l u e s for the s t r e s s i n t e n s i t y factor (K) such that the t h i c k n e s s was lower than the ASTM plane s t r a i n c r i t e r i o n B -> 2.5 (K/~y) ~. More r e c e n t l y , Coles e t al s have used 25.4 m m thick compact s p e c i m e n s and v a r i o u s d e s i g n s of s u r f a c e flawed t e n s i l e s p e c i m e n s to study the effect of t e n s i l e hold t i m e on FCG r a t e . They showed that, in the r a n g e of FCG r a t e s l a r g e r than about 1 p r a / c y c l e the hold time effect was m o r e i m p o r t a n t with the CT type s p e c i m e n than with the o t h e r s . They a t t r i b u t e d this d i f f e r e n c e to the load e x c e n t r i c i t y in the c o m p a c t s p e c i m e n which might m o r e easily p e r m i t c r e e p - l i k e p l a s t i c s t r a i n d u r i n g the h o l d - t i m e . Although this diff e r e n c e in loading g e o m e t r y may be i m p o r t a n t for the study of the FCG behavior at elevated t e m p e r a t u r e s other p a r a m e t e r s such as the loading r a t e u s i n g a s q u a r e wave s i g n a l can also have an i m p o r t a n t effect, as shown in the p r e s e n t study. The influence of this p a r a m e t e r was not i n v e s t i g a t e d nor c l e a r l y d e s c r i b e d in the study by Coles e t al.7 The p r e s e n t study r e p o r t s additional r e s u l t s on the FCG b e h a v i o r of Alloy 718 at 298 and 823 K. T h i s work was u n d e r t a k e n in o r d e r to extend the p r e v i o u s l y obtained r e s u l t s c o n c e r n i n g the LCF p r o p e r t i e s . The inM. CLAVEL and A. PINEAU are with Centre des Matdriaux, Ecole fluence of f r e q u e n c y was i n v e s t i g a t e d at 823 K u s i n g des Nines de Paris, BP87-91003 EVRY-Cedex France. Manuscript submitted June 21, 1977. a s i n u s o i d a l wave f o r m s i g n a l between 5 910 -2 Hz and ISSN 0360-2133/78/0410-0471500.75/0 METALLURGICALTRANSACTIONSA 9 1978AMERICANSOCIETYFOR METALSAND VOLUME 9A, APRIL 1978-471 THE METALLURGICALSOCIETYOF AIME
A L L O Y 718 is a high strength, weldable wrought nickel base alloy which was developed for use at t e m p e r a t u r e s up to about 920 K. T h i s m a t e r i a l is used ext e n s i v e l y in the gas t u r b i n e i n d u s t r y for a c e r t a i n n u m b e r of applications such as the f a b r i c a t i o n of d i s c s where the r e s i s t a n c e to low cycle fatigue (
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