It is usually regarded as a common understanding that fatigue failure would not occur if stress amplitude is kept below the fatigue limit diagram. However, it was shown in this study that fatigue failure occurred in some special cases of variable amplitude loading condition even when stress amplitude was kept below fatigue limit diagram in the case of small-notched specimen. The cause of such a phenomenon was examined using repeated two-step loading pattern. 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. In the case of repeated two-step loading 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. Consequently, fatigue failure occurred even when stresses were kept below the fatigue limit diagram of constant amplitude. It was shown that the constant amplitude fatigue limit diagram does not necessarily guarantee the safety in the case of variable amplitude loading.
