Effective strain–fatigue life data for variable amplitude fatigue

The usual analysis procedure for variable amplitude fatigue calculates fatigue damage based on constant amplitude strain controlled fatigue tests of smooth specimens. The resulting predictions are typically nonconservative due to a load interaction effect in variable amplitude fatigue. This paper reviews recent work which shows that large loads in a service load history decrease the crack opening stress and as a result increase the effective strain range for subsequent small cycles. A new strain–life fatigue test is introduced in which periodic large strain cycles reduce the crack opening stress for subsequent smaller cycles. The overloads are applied frequently enough that closure free fully open crack growth is achieved for the small cycles in the long life regime. An effective strain–life curve is derived and a crack opening stress equation calibrated by comparison of constant amplitude and effective strain ranges at given fatigue lives. The use of the effective strain–life curve in predicting fatigue lives is illustrated for service strain histories and for a variable amplitude load sequence applied to notched specimens. The predictions are good but somewhat conservative.