Plasticity and damage heterogeneity in fatigue

Abstract

The approach developed demonstrates a framework using both the elastic shakedown concept and the weakest link theory to account for loading mode, loading path and data scatter in high cycle fatigue. The mesoplasticity and damage mechanisms occurring under high cycle fatigue are illustrated by means of observations conducted on a mild steel submitted to purely reversed torsion and push–pull fatigue tests. Plastic glide systems and microcracks are distributed in a very heterogeneous way at the surface of the observed smooth specimens. The microstructure and more specifically the configuration of the grain where the crack initiates and its immediate vicinity (neighbour grains and microstructural barriers) control most of the fatigue life. To account for this heterogeneous feature, a distribution of the elastic shakedown threshold is assumed and probability calculations carried out in the framework of the weakest link concept lead to a new fatigue criterion. In particular, a spatial and a directional heterogeneity factor are introduced. The former represents the way the stress is distributed within a component, while the latter is a measure of the number of highly stressed glide systems. It is then proved that the crack initiation modelling according to mesoplasticity consideration is not enough to reflect the macroscopic fatigue behaviour when changing the loading mode. The directional heterogeneity factor is indeed unable to explain alone the different fatigue limits under purely reversed tension and torsion. A normal stress effect clearly acts and can be readily introduced through a dependence of the scale factor in the initiation distribution. A key feature of this probabilistic mesoscopic approach is its directional formulation. It allows direct extensions regarding the description of glide system severity according to the orientation to the free surface and the non-local effects due to the non-uniform normal stress distribution in a component. This model is shown to be efficient when dealing with complex loading paths like under out-of-phase tension and torsion loading.