• PDF / 3,569,359 Bytes
• 8 Pages / 612 x 792 pts (letter) Page_size
• 29 Downloads / 207 Views

DOWNLOAD

REPORT

---

TRANSACTIONS A

-I

F-CYCLe GJV~ ST.~AT~.S A,e...

~__

F i g . 1--Fatigue s t r i a t i o n s for 2 m m a l c l a d 2 0 2 4 - T 3 a l l o y t e s t e d i n a i r w i t h a p r o g r a m of 10 cycles at R = 0 . 5 and 1 c y c l e at R = 0 w i t h the s a m e m a x i m u m s t r e s s . Courtesy of J . Schijve, t a k e n from Ref. 1 .

It is well to r e c a l l the models in question. The PBP is shown in Fig. 2 and consists essentially of the following. The compressed c r a c k is shown t o have a y shaped tip so t h a t , at the start of the tensile part of the cycle, the branches of the y initially d i r e c t the deformation, but with continued loading, deformation is constrained a l o n g flow bands emanating from the g e n e r a l tip, "blunting" occurs and separation by plastic flow occurs continually up t o the point of m a x i m u m loading. On unloading, the c r a c k closes, but the compressive deformation is not strictly r e versible, and thus a cylindrical hole parallel t o the c r a c k front ( " t r e n c h " in Wanhill's terminology) and directly adjacent to its tip is formed. With full compression, the y-shaped tip is reformed and the c r a c k VOLUME 8A, A P R I L 1977-657

p r o p a g a t i o n c y c l e is c o m p l e t e . T h u s , as W a n h i l l recognized, the PBP predicts the formation of a s h a r p trench d u r i n g c r a c k c l o s u r e . Tomkin's and B i g g s ' m o d e l , on the o t h e r h a n d , p r e d i c t s t r e n c h f o r m a t r o n a t t h e b e g i n n i n g of c r a c k o p e n i n g . W a n h i l l ' s i n t e r p r e t a t i o n of t h i s m o d e l i s s h o w n i n F i g . 3 . At m a x i m u m c o m p r e s s i o n t h e c r a c k tip is a s s u m e d s h a r p . U n d e r a s m a l l tensile load, " p l a s t i c flow o c c u r s a l o n g well d e f i n e d f l o w b a n d s a n d n e w c r a c k s u r f a c e is f o r m e d by a c o m b i n a t i o n o f s h e a r a n d t e n s i l e d e c o h e s i o n ("flow-off") t o produce a blunted c r a c k tip with trenches. U n d e r continued tensile straining t h e flow-off s t o p s at t h e e d g e s o f t h e initial flow b a n d s b e c a u s e of considerable s t r a i n hardening. F r a c t u r e o c c u r s a t t h e c e n t e r of t h e b l u n t e d c r a c k

(a) (d)

(b)

XXNIk

(d

(P F i g . 2--Schematic representation of the p l a s t i c blunting proe e s s of f a t i g u e c r a c k propagation in the S t a g e II m o d e : (a) zero l o a d ; (o) s m a l l t e n s i l e l o a d ; (c) maximum t e n s i l e load of the c y c l e ; (d) s m a l l c o m p r e s s i v e l o a d ; (e) m a x i m u m c o m p r e s s i v e load of the c y c l e ; and {f) s m a l l t e n s i l e load in the s u c c e e d i n g c y c l e . T h e d o u b l e a r r o w h e a d s in (c) and ( d ) s i g n i fy the g r e a t e r w i d t h of slip b a n d s at the c r a c k in t h e s e s t a g e s of the p r o c e s s . T h e s t r e s s axis i s v e r t i c a l . Taken from Ref. 3.

a. MAXIMUM COMPRESSION

b SMALL TENSILE LOAD

(the