Fatigue Failure under Varying Loading within Fatigue Limit Diagram
Although fatigue limit diagram is defined in principle for constant stress amplitude condition, it is often considered that fatigue failure would not occur even in varying loading if applied stresses were kept within the fatigue limit diagram. However, it was shown in the case of small-notched specimen and fretting fatigue that fatigue failure occurred in some special case of variable amplitude loading condition even when all stress amplitudes were kept within the fatigue limit diagram. The cause of this phenomenon was examined using two-step and repeated two-step stress patterns in which the first step stress was with zero mean stress and the second step stress had a 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. Consequently, fatigue failure occurred even when all stress amplitudes were kept within the fatigue limit diagram. It was an unexpected fracture caused by the interference effect of non-propagating crack and mean stress change.
Y. Kondo et al., "Fatigue Failure under Varying Loading within Fatigue Limit Diagram ", Materials Science Forum, Vols. 567-568, pp. 1-8, 2008