A new method for prediction of fatigue crack propagation life under variable amplitude spectrum loading

Abstract

This paper proposes a new method for predicting propagation lives for through-thickness cracks under variable amplitude mode-I spectrum fatigue loads. Only the crack opening, i.e., the increasing parts of the load cycles within spectrum profiles are considered for cycle counting and computing their stress ratios. Moreover, a cycle-by-cycle analysis is also performed throughout the spectrum to compute the so-called Spectrum Overload Index (SOL-Index), which is a multiplier applied on stress intensity factor range used within different crack growth retardation models, in which effects of stress ratio are also included. The SOL-Index is calculated using the average of peak loads for all cycles within the spectrum and their percent deviations from the peaks’ average value. Simulation results using the proposed cycle counting method, the integrated crack growth model and the SOL-Index are compared with those from multiple experiments under two different loading spectrums and another study in the literature. The results show that usage of Wheeler's model embodying the Forman or Walker equation and the SOL-Index together yields the closest results to the experiments.