Optimal S-N testing for variable-amplitude fatigue applications

A new methodology is developed that can identify the optimal testing procedure to be used in generating S-N curves for predicting damage accumulation under variable-amplitude service loads. This is a significant extension of existing knowledge, which includes a solution to this problem only for situations in which the service loads are constant amplitude. The methodology has the potential to improve significantly the predictions of fatigue damage and reliability under realistic service loads. In general, finding the truly optimal solution involves solving a quartic equation, but it is often possible to use a simplified form that involves only a quadratic equation. The new results are identical to the previously developed formulation for the special situation of a constant-amplitude service load. The information needed to use the general procedure is the distribution of stress amplitudes in the service load and the variability of the service life among identical load/specimen combinations. The procedure is illustrated here by its application to the special case of bilevel service loads, for which numerical results are presented. This includes information on both the optimal S-N testing procedure, and the sensitivity of the mean-squared error to changes in the testing procedure.