Microstructures around the tips of fatigue
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a v e r a g e g r a i n s i z e was about 60 # m in d i a m . F a t i g u e t e s t s w e r e m a d e at a r e s o n a n c e f r e q u e n c y of about 120 Hz, using a r e s o n a n c e type a l t e r n a t i n g bending a p p a r a t u s , u When c r a c k s g r e w to about 2.0 m m , the 3,0 2,5 :g 2,0
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Fig. 2 - - F a t i g u e c r a c k length vs n u m b e r of c y c l e s .
EXPERIMENTAL
PROCEDURE
Polycrystalline iron was supplied in the form of 12 mm diam bar; the chemical composition in wt pct being Fe 99.66, C 0.01, Si 0.01, Mn 0.28, Cu 0.02, P 0.009 and S 0.01. After annealing at 950~ [1223 K], these bars were cold rolled to plates of 0.25 mm thickness. Specimens as shown in Fig. i were prepared from the p l a t e s , f u l l y a n n e a l e d at 900~ [1173 K] f o r 1 h in vacuo, and then l i g h t l y e l e c t r o p o l i s h e d p r i o r to t e s t s . The
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40 Fig. 1--Fatigue test spemmen (all dlmensions in ram). J. AWATANI, K. KATAGIRI, and T. SHIRAISHI are Professor, Assistant Professor, and Graduate Student, respectively, at The Institute of Scmntific and Industrial Research, Osaka University, Japan. Manuscript submitted May 6, 1975. METALLURGICAL TRANSACTIONS A
Fig. 3 - - D i s l o c a t i o n s t r u c t u r e s in the m a t r i x of a s p e c i m e n fatigued at a high s t r e s s (17.0 k g / m m z [167 MN/m2]). VOLUME 7A, JUNE 1976-807
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! Fig. 4--DisIocation s t r u c t u r e s immediateIy ahead of the c r a c k tip.
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specimens were removed from the apparatus, and observations of d i s l o c a t i o n s t r u c t u r e s w e r e made around the c r a c k t i p s . In this fatigue t e s t , an exact e s t i m a t e of the s t r e s s applied to the specimen was difficult, but c r a c k growth r a t e s could be obtained from the curve of c r a c k propagation o b s e r v e d on the s p e c i m e n s u r f a c e (Fig. 2); the growth r a t e being 0.01 p m p e r cycle or so at the c r a c k length of 2.0 mm. A r e a s adjacent to the c r a c k s a r e p r e f e r e n t i a l l y p o l ished away in the c o u r s e of o r d i n a r y eIectropolishing. However, it was found that a r e a s as c l o s e as 1 ~m f r o m the c r a c k s were p r e s e r v e d in jet e l e c t r o p o l i s h 808-VOLUME 7A, JUNE 1976
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ing under a s p e c i a l condition ( e l e c t r o l y t e , 20 pct p e r c h l o r i c acid and 80 pct ethanol, voltage, 80V; c u r r e n t density, 4.5 A/cmZ; t e m p e r a t u r e , about 20~ [293 K]; j e t d i a m e t e r , 1.3 mm), and thin foils containing s h a r p c r a c k s could be p r e p a r e d . These foils could be obtained at a d e s i r e d depth from the s u r f a c e of the bulk specimen, by f i r s t e l e c t r o p o l i s h i n g to the d e s i r e d depth f r o m the s u r f a c e b e f o r e beginning the thinning p r o c e s s f r o m the r e a r . Koterazawa 12 r e v e a l e d that
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