Size Effect on Fatigue Strength of Machined Carbon Steel : X-Ray Detection of Fatigue Damage in Machine Parts

概要

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An X-ray observation was made during the fatigue process of 100 mm, 20 mm and 10 mm dia. rotating bending fatigue specimens machined by a lathe. The results obtained from this experiment are as follows : (1) It became clear that there exists a thin surface layer which has been work hardened by machining and is sensitive to repeating stress. This surface layer yields throughout the specimen surface fairly uniformly before crack initiation. (2) When the reduction rate of surface residual stress at the number of cycles N=10^4 and N=10^5,Δσ_R/σ_R_0 (where Δσ_R is the changed value of residual stress and Δσ_R is the changed value of residual stress and σ_R_0 is the residual stress of initial state), was plotted as a function of number of cycles to failure, all the points lie close to a single line. Thus, the change in residual stress during the fatigue process depends mainly on fatigue life itself irrespective of specimen size.
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