The role of pure shear strain on the site of crack initiation in notches
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C RACK initiation in r e a l m a t e r i a l s can be the d i r e c t r e s u l t of intense localization of p l a s t i c flow. A det a i l e d evaluation of the s t a t e s of p l a s t i c s t r e s s and s t r a i n that surround the site of c r a c k initiation should, t h e r e f o r e , provide insight into the l o c a l i z a tion phenomenon. In p a r t i c u l a r , an i m p o r t a n t i s s u e concerning c r a c k initiation is the elucidation of the c r i t e r i o n or c r i t e r i a which govern the location of the initiation site along a notch contour, e s p e c i a l l y since e x p e r i m e n t a l o b s e r v a t i o n s b e a r out the fact that c r a c k s do not initiate at the notch c e n t e r l i n e . F o r s t r u c t u r a l m e t a l s it is g e n e r a l l y accepted that as load is s t e a d i l y i n c r e a s e d , the sequence of events leading to f a i l u r e a r e : 1) e l a s t i c action; 2) p l a s t i c action; 3) c r a c k initiation; 4) c r a c k propagation; and 5) f r a c t u r e . Often a distinction between any of these events is not p r e c i s e . F o r example, the ability to det e c t c r a c k initiation can be a function of the p a r t i c u l a r e x p e r i m e n t a l technique employed. One o b s e r v a tion, however, is consistent, i.e. p l a s t i c action must occur p r i o r to f r a c t u r e . In r e a l m e t a l s , t h e r e a r e too many p o s s i b i l i t i e s for the activation of r e l a x a t i o n p r o c e s s e s to achieve the t h e o r e t i c a l strength in bulk materials.~ Based upon extensive r e s e a r c h into the r o l e of p l a s t i c i t y in f r a c t u r e by Spretnak and his c o w o r k e r s at the Ohio State University, a c l e a r e r understanding of the significance of p l a s t i c action p r i o r to f r a c t u r e has e m e r g e d . It is stated ~ that the two b a s i c r e q u i r e ments in the f r a c t u r e p r o c e s s in m e t a l s and alloys a r e : 1) plastic flow; and 2) l o c a l i z a t i o n of p l a s t i c flow.
VINCENT J. RUSSO is Chief, Materials Behavior Branch, Air Force Materials Laboratory, Wright-Patterson Air Force Base, OH 45433, AM1YA K. CHAKRABARTI, member ASM, is Research Metallurgist, Systems Research Laboratory, Dayton, OH, and JOSEPH W. SPRETNAK, member AIME, is Earl M. Tilton Professor of Mechanical Metallurgy, Ohio State University, Columbus, OH. This paper is portion of a dissertation submitted by V, J, Russo in partial fulfillment of the requirements for Ph.D. degree, Ohio State University. Manuscript submitted March 19, 1976. METALLURGICAL TRANSACTIONS A
Based on numerous studies employing a v a r i e t y of m a t e r i a l s and t e s t conditions, Spretnak 2 p r o p o s e d that, at a c r i t i c a l s t r a i n in the p l a s t i c zone under s t r e s s
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Fig. 1--Specimens showing small amount of c r a c k opening and typical cracking p a t t e r n s in V-notch s p e c i m e n s . VOLUME gA, MAY 1977 729
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(b) Fig. 2--The position of crack from n
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