History influence exponent in cumulative fatigue damage determined using two‐step loading experiments

ABSTRACTThe only material parameter, which appears in quadratic phenomenological hypothesis of cumulative fatigue damage, is the history influence exponent ϑ reflecting the material sensitivity to loading sequence and loading level interactions. Methods of its determination on the basis of two‐step loading experiments are presented. One method is based on the least‐squares method using linearizing transformation, other two ones are based on concept of individual fatigue life curves and statistical concept of inexact structural model. Parameter ϑ has been evaluated in 24 cases representing various materials and different test conditions. Possibility of existence of a common ϑ value holding for all materials is discussed. Such a value does not follow from analysed experimental data. However, ϑ = 0.5 is recommended for improving the reliability of engineering fatigue life estimates in case when required experimental material data are not available.