過小応力に含む2段繰返し疲労試験の寿命予測における微小き裂の役割(<特集>疲労特集)

概要

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The effect of understress in two-step stress fatigue tests on fatigue life was investigated with emphsis on the behavior of small cracks. In fatigue under variable amplitude loading, two critical stresses play important roles in fatigue damage prediction. One is the fatigue limit σ_w defined by the failure of a specimen containing a small crack, and the other is the critical stress σ_<wi> which is the threshold value for fatigue crack initiation from the small crack. It should be noted that both σ_w and σ_<wi> are the function of crack size and accordingly the degree of contribution of understress σ_L to fatigue damage varies at every moment with the variation of crack size, i.e. crack propagation. The equations to predict σ_w and σ_<wi> were derived here as the function of crack size. If the value of σ_L is lower than σ_<wi> defined on the basis of crack length l, the repetition of σ_L does not contribute fatigue damage. However, if the crack propagates due to the repetition of overstress σ_ decreases below σ_L, σ_L starts to contribute to fatigue damage. This fact explains the reason for a wide scatter of the values of cumulative fatigue damage D of modified Miner's rule obtained in previous studies. A method is also discussed to predict the lower limit of D.
社団法人日本材料学会の論文
1987-10-15