Influence of crack shape on crack opening displacement measurements using charpy specimens
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a d e q u a t e r e p r e s e n t a t i o n of t h e c r i t i c a l C . O . D . f o r ductile crack initiation. T h e o b j e c t of t h i s n o t e i s to d e m o n s t r a t e t h e u n c e r tainty that can arise from a maximum load criterion f o r c r i t i c a l C . O . D . i n p l a i n s p e c i m e n s , a n d to i n d i c a t e how errors may be reduced or eliminated. Charpy specimens with 2 mm deep V notches were t e s t e d in t h r e e - p o i n t s l o w b e n d a t 20~ (8.6 • 10 -3 m m
SPECIMEN
1. A. J. McEvlly,R. C. Boettner, and T. L. Johnston: Ford Scmntific Lab. Rept., No. 66-22. 2. A. J. McEvily,R. G. Davies,C. L. Magee,and T. L. Johnston: Transformations andHardenability in Steel, Chmax MolybdenumCorp., Mich., 1967, p. 179; also availableas Ford ScmntlfiCLab. Rept. No. 67-32. 3. P. Beardmore and C. E. Feltner: International Conferenceon Fracture Brighton, England, April, 1969; also availableas Ford Scienhfic Lab. Rept. No. 68-95. 4. A. J. McEvilyand C. L. Magee: Iron Steel Inst. Publ. LowAlloy Steels, 1968, p. 111 ; also availableas Ford Scientific Lab. Rept. No. 68-30. 5. C. L. Magee: Ford Scientific Laboratory, Dearborn, Mmh.,private communication.
MIRROR
CAMERA
Influence of Crack Shape on Crack Opening DisplacementMeasurements Using Charpy Specimens G. BIRKBECK AND A. E. WRAITH
INTEREST is currently being shown in the measurem e n t of c r a c k o p e n i n g d i s p l a c e m e n t ( C . O . D . ) i n V-notched or fatigue-cracked Charpy specimens before ductile fracture in slow bend and impact fract u r e t e s t s . T e c h n i q u e s f o r t h e m e a s u r e m e n t of o p e n i n g d i s p l a c e m e n t i n c l u d e t h e u s e of a t o r s i o n a l p a d d l e g a g e , 1'2 a s u r f a c e c l i p g a g e , 3 a n d t h e p h o t o g r a p h i c o r o p t i c a l o b s e r v a t i o n of t h e s p e c i m e n s u r f a c e . 4,s O p e n ing displacements have also been determined by calculation from measurements of t h e o v e r a l l s p e c i m e n d e f l e c t i o n .6,7 O b s e r v a t i o n of t h e s p e c i m e n s u r f a c e i n d i c a t e s t h a t t h e a p p e a r a n c e of s u r f a c e c r a c k s in p l a i n C h a r p y s p e c i m e n s c o i n c i d e s w i t h t h e a c h i e v e m e n t of m a x i m u m load; at this load, the crack front is curved suggesting t h a t t h e a p p e a r a n c e of s u r f a c e c r a c k s w a s p r e c e d e d by cracking at the center. 4 This casts some doubt on t h e s e l e c t i o n of d i s p l a c e m e n t a t m a x i m u m l o a d a s a n G. BIRKBECK, formerly Senior Research Associate, Metallurgy Department, University of Newcastle upon Tyne, England, is Senior Scientific Officer, British Rail Engineering Research Center, Derby, England. A. E. WRAITH is Lecturer, Metallurgy Department, University of Newcastle upon Tyne. Manuscript submitted June 30, 1969. 1474-VOLUME 1, MAY 1970
Fig. 1--Details of testing p r o c e d u r e for o b s e r v a t i o n of c r a c k initiation in slow-bend tests.
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