A non-linear damage evolution model for mode II fatigue delamination onset and growth

Abstract A novel approach is proposed describing both the onset and growth of delaminations in fibre-reinforced laminates under pure mode II constant amplitude loading with the same damage evolution rule, unifying these two aspects of the material behaviour that are normally treated separately. A scalar damage variable is introduced to represent the fraction of overall fatigue endurance used up at ply interfaces as a function of the number of accumulated cycles. The damage rate equation is postulated in a generic power law format, which also includes the effect of the load stress-ratio. The material SN curves for pure mode II loading are obtained in closed form by a simple integration of the assumed damage evolution law. The material delamination propagation rate as a function of the energy release rate and the stress-ratio is similarly obtained combining the aforementioned damage evolution law with a regularized expression for the stress field at the crack tip. Two independent fatigue related parameters are sufficient for describing both the delamination onset and its growth. This modelling approach is validated by means of experimental fatigue delamination data for IM7/8552 carbon fibre/epoxy, demonstrating that the unified modelling strategy is able to describe both fatigue initiation and propagation and the associated effect of the stress-ratio.