Fatigue load sequence effect ranked by critical available energy density

Abstract

The ways with which variable amplitude load spectra are simplified by taking only a portion of the load history as a block and applying it repeatedly make the prediction of fatigue life a nonunique procedure. This is because the process of damage cumulation is sensitive to alteration in load sequence. Variations in load amplitude affect the proportion of energy dissipated and that used to initiate and/or propagate cracks. A weighted wavelength index is defined that can reflect the complex load sequence effect on crack growth rate. Certain combinations of load sequence are shown to enhance crack growth while others tend to retard crack growth. This is demonstrated from a five-cycle load block where the amplitude and frequency are varied. Crack growth is modelled as the coalescence of a series of crack initiation sites, the loci of which are assumed to coincide with the cumulated volume energy density reaching a critical state, which is characteristic of the material in accordance with the strain energy density criterion. Results are based on the incremental theory of plasticity.