The effect of grain size and cross-head velocity on the lower yield strength during inhomogeneous discontinuous yielding
- PDF / 468,822 Bytes
- 5 Pages / 612 x 792 pts (letter) Page_size
- 82 Downloads / 225 Views
V~
" lira* . 1"
The validity of the r e l a t i o n , eaF = ei + \ ..v..~---fi----A~A~)
1/m*
(-~)
, has been checked with e x -
p e r i m e n t a l r e s u l t s on the lower yield strength, aaF of v a r i o u s irons and s t e e l s d e t e r m i n e d at r o o m t e m p e r a t u r e o v e r wide r a n g e s in c r o s s - h e a d velocity, Vc, and grain boundary int e r c e p t or g r a i n d i a m e t e r , l. Good a g r e e m e n t was found with the p r o p o s e d r e l a t i o n for m* = 4, where m* is the d i s l o c a t i o n v e l o c i t y - e f f e c t i v e s t r e s s exponent, and ~ = -~. ai, the i n t e r nal stress, ranged between 5000 and 13,000 psi and N ~ O K ' between 4.3 x I0-xs and 19.3 x I0-~s. In the original relation, the value of a was one and K' was given as K, which was assumed to be a material constant. The modification which became necessary indicates that possibly the density of mobile dislocations and/or the resistance of grain boundaries to propagating IMider's bands depend on Ltider's band velocity and strain. The validity of the relation at very high and very low cross-head velocities or strain rates is discussed. The relation should apply in an intermediate range of cross-head or dislocation velocities where m * should remain constant and should show an apparent decrease at both higher and lower velocities. The basis for this prediction is discussed and results are presented at low strain rates.
A
2"1 model b a s e d on dislocation dynamics was p r o p o s e d in an e a r l i e r communication I defining the dependence of the lower yield strength in Iron and s t e e l during L~ider's band propagation on g r a i n size, t e m p e r a t u r e and s t r a i n r a t e , or m o r e p r e c i s e l y , c r o s s - h e a d v e l o c ity. The effect of g r a i n s i z e and t e m p e r a t u r e wa.s e x amined r e c e n t l y a to check the validity of the o r i g i n a l relation. E x p e r i m e n t a l r e s u l t s on the effect of grain s i z e and c r o s s - h e a d v e l o c i t y on the lower yield strength of i r o n and s t e e l during tnhomogeneous, discontinuous y i e l d ing r e p o r t e d in the l i t e r a t u r e a-xx have been employed to check again the o r i g i n a l model. These r e s u l t s indicate that a modification to the initial r e l a t i o n was r e q u i r e d to take into account an a p p a r e n t i n c r e a s e in the mobile dislocation density a n d / o r a d e c r e a s e in the r e s i s t a n c e of g r a i n b o u n d a r i e s to the propagation of I i i d e r ' s bands with i n c r e a s i n g L~ider's band v e l o c i t y and s t r a i n . A good f i t with the modified r e l a tion is found and a g r e e m e n t among e x p e r i m e n t a l r e sults for different i r o n s and s t e e l s is a l s o found. The modification does not affect the r e c e n t r e s u l t s r e p o r t e d e on the effect of grain s i z e and t e m p e r a t u r e on the lower yield strength for iron and s t e e l . Limitations of the r e lation at v
Data Loading...
