Effect of Overstress on the Fatigue Behavior of a Heat-treated 0.54% C Steel under the Stress of Fatigue Limit

Abstract

The effect of definite cycles of overstress σ_os (≅ 1.5 σ_w, σ_w : fatigue limit) which was applied periodically during the repetitions of the stress of fatigue limit σ_w was investigated through the behavior of micro-crack. The main results obtained are as follows ; 1) When σ_os is repeated in n_os cycles at every 10⁷ cycles of σ_w and the value of n_os is within a limited range, the specimen is broken at a small value of the accumulated cycle ratio Σ(n/N). This phenomenon seems to be due to an increase in crack opening ratio under the repetitions of over-stress. In that case, the crack propagates only under the repetitions of σ_w. 2) The value of Σ(n/N) depends on the size of a non -propagating microcrack formed by the first 10⁷ cycles of σ_w. 3) The repetition of 10⁷ cycles of σ_w increases the fatigue life of the specimen under σ_os.