疲労限度以下の応力による2段2重変動応力下の疲労破壊に及ぼす微小切欠きの形状と水素の影響

摘要

It is usually regarded as a common understanding that fatigue failure would not occur if all stresses were kept below fatigue limit diagram. However, it was shown that fatigue failure occurred in some special cases of variable amplitude loading condition even when all stresses were kept below fatigue limit diagram in the case of small-notched specimen. The cause of such a phenomenon was examined using two-step stress pattern for low alloy steel SCM440H. In the case of constant stress amplitude loading, non-propagating crack was formed only at low mean stress region and not formed at high mean stress. However, in the case of two-step stress pattern in which the first step stress was chosen as R = - 1 and the second step stress was with high mean stress, a non-propagating crack was formed by the first step stress. This crack functioned as a pre-crack for the second step stress with high mean stress. In this study, the effect of notch shape was examined. The effect of absorbed hydrogen was also investigated. Absorption of 0.3 ppm hydrogen caused more reduction of fatigue limit.%一定応力振幅条件での疲労限度に及ぼす平均応力のrn影響は，疲労限度線図によって評価されることが多い．rn一方，変動応力下では過小応力効果として，疲労限度以rn下の応力振幅も疲労損傷に寄与することが知られていrnる．しかし，疲労限度以下の応力が損傷に寄与すrnるのは，部分的に疲労限度を超える応力が含まれていrnることが前程であり，全ての作用応力が疲労限度線図rnの範囲内に制限されていれば，疲労破壊が生じないとrn判断するのが一般的と思われる．