Fretting fatigue under variable loading below fretting fatigue limit

The fatigue limit diagram provides the critical condition of non-failure against fatigue under constant amplitude loading. The fatigue limit diagram is usually considered to give the allowable stress if every stress component is kept within the fatigue limit diagram. In the case of variable amplitude fretting fatigue, however, this study showed that fatigue failure could occur even when all stresses were within the fatigue limit diagram. An example of such a condition is a repeated two-step loading such as when the first step stress is R = - 1 and the second step stress has a high mean value. The reason why such a phenomenon occurs was investigated. A non-propagating crack was formed by the first step stress even when well below the fatigue limit. The resultant non-propagating crack functioned as a pre-crack for the second step stress with a high mean value. Consequently, fatigue failure occurred even when every stress was within the fatigue limit diagram of constant amplitude loading. The fatigue limit diagram obtained in constant amplitude fatigue test does not necessarily guarantee safety in the case of variable amplitude loading in fretting fatigue.