Finite element modeling of crack closure during simulated fatigue threshold testing

Abstract

Elastic-plastic finite element simulations of growing plane stress fatigue cracks are used to study standard fatigue threshold test schemes. Load reduction operations are shown to cause artificial increases in crack closure. The crack still continuously ‘unzips’ during loading, so that the crack surfaces immediately behind the current crack tip are the last to separate. The increases in crack closure are clearly associated with increases in residual displacements in the wake of the crack generated earlier in the load history, when applied stresses were greater. The nondimensional nature of the finite element analysis shows that the dimensional form of the current ASTM standard for fatigue threshold testing admits possible effects of specimen size and applied stress level on closure behavior and hence on the apparent threshold. The current ASTM standard does not appear to be entirely consistent with a continuum mechanics description of plasticity-induced crack closure. The current standard may still give acceptable results when specimens are relatively small and relatively thick and initial applied stresses are a relatively small fraction of the yield stress.