Crack closure inadequacy at negative stress ratios

This paper reports an experimental study related to the influence of different maximum load, Pmax, levels on crack closure, and on fatigue crack growth, at a negative stress ratio, R=−1. It focuses on crack tip plasticity effects and on roughness effects on crack closure. Measurements of roughness and of crack opening loads are made, in order to verify their influence on crack propagation rate. It is confirmed that, at negative stress ratios, crack closure changes with Pmax for the same R ratio. It is also confirmed that roughness is not a relevant mechanism at negative stress ratios. It is demonstrated in this study that the crack closure concept is not adequate to explain crack propagation rate as a function of Pmax at R=−1. In its place, a concept based on plasticity and on cyclic plastic properties, or on internal stresses, should be used to explain fatigue crack propagation.