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 σos (≅ 1.5 σw, σw : fatigue limit) which was applied periodically during the repetitions of the stress of fatigue limit σw was investigated through the behavior of micro-crack. The main results obtained are as follows ; 1) When σos is repeated in nos cycles at every 107 cycles of σw and the value of nos 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 σw. 2) The value of Σ(n/N) depends on the size of a non -propagating microcrack formed by the first 107 cycles of σw. 3) The repetition of 107 cycles of σw increases the fatigue life of the specimen under σos.