• PDF / 975,376 Bytes
• 4 Pages / 613 x 788.28 pts Page_size
• 72 Downloads / 259 Views

DOWNLOAD

REPORT

---

T::, II.-

~

140

...1 Z IZO

IO01

4 3 4 0 STEEL TEMPERED TO Rc 3 0 " 3 2

,"

~o

~

.'o

go

'o

,'o

NUMBER OF CYCLES x 10-~

Fig. 1--Mean positron lifetime vs number of fatigue cycles for AISI 4340 steel quenched and tempered to Rc30-32.

METALLURGICALTRANSACTIONSA

80

n e s s level and fatigued at a m a x i m u m cyclic s t r e s s of t w o - t h i r d s the yield s t r e s s , will i n i t i a l l y cyclicly h a r d e n and then a l t e r n a t e l y soften and h a r d e n , e s p e c i ally if " c o a x i n g " S is induced by f r e q u e n t " r e s t s " in the fatigue hi.story. The cycles of h a r d e n i n g and softe n i n g have b e e n shown to be a s s o c i a t e d with point defects a n d / o r mobile i n t e r s t i t i a l s t r a i n aging effects r a t h e r than with changes in the d i s l o c a t i o n cell s i z e . T h i s work was supported by the United States Dep a r t m e n t of E n e r g y . The c o m m e n t s of D r . K. G. Lynn on the o r i g i n a l m a n u s c r i p t were g r e a t l y a p p r e c i a t e d . 1. K. G. Lynnand J. G. Byrne:Met. Trans. A, 1976,vol.7A, p. 604. 2. B. E. Warren:Progr. Met. Phys., 1959,vol.8, p. 147. 3. K. G. Lynn:Unpublishedwork, 1975(Physicist,PhysicsDepartment,BrookhavenNationalLaboratory,Upton,LongIsland,NY 11973). 4. T. D. Hadnagy:Ph.D.Thesis,Universityof Utah, 1976. 5. T. Hall:NucL hzst. Meth., 1974,vol. 117,p. 253. 6. T. J. Dolan:Metal Fatigue, G. Sinesand J. L. Waisman,eds.,p. 56, McGrawHill BookCo., Inc.,NewYork, 1959.

The Effect of Intermetallics Composition and Microstructure on Fatigue Crack Initiation in AI 2219-T851 W. L. MORRIS F o r m o s t 2000 and 7000 s e r i e s a l u m i n u m alloys, fatigue c r a c k i n i t i a t i o n o c c u r s at b r i t t l e i n t e r m e t a l l i c p a r t i c l e s located n e a r a surface.l-v The s t a t i s t i c a l d i s t r i b u t i o n of c r a c k i n g s i t e s , on a m i c r o s c o p i c scale, is an i m p o r t a n t f a c t o r in the s c a t t e r in fatigue lifetime for such alloys.8 Although c r a c k i n i t i a t i o n tends to be m o r e p r e f e r r e d for l a r g e r i n t e r m e t a l l i c s , only a s m a l l f r a c t i o n of even the l a r g e s t i n t e r m e t a l l i c s lead to c r a c k i n i t i a t i o n d u r i n g fatigue.~ The p r e s e n t work r e p o r t s on the effect of c o m p o s i t i o n and m i c r o s t r u c t u r e of the i n t e r m e t a l l i c s on c r a c k i n i t i a t i o n for A1 2219T851. A flat, t r i a n g u l a r shaped, constant s u r f a c e s t r e s s , f l e x u r a l fatigue s p e c i m e n , d e s c r i b e d e l s e w h e r e , was used for this i n v e s t i g a t i o n .6 Each s p e c i m e n was m e c h a n i c a l l y polished ending with 0.05 prn a l u m i n a powd e r . S p e c i m e n s w e r e positioned in the test f i x t u r e with the s u r f a c e s t r e s s axis p a r a l l e l to the r o l l i n g d i r e c t i o n and w e r e fatigued in a i r for 5 • 103 cycles at 5 Hz with R = amin/Crmax = - 1, Cmax = 0.9 (