Phenomenological approach to the study of hierarchical damage mechanisms in composite materials subjected to fatigue loadings

In this paper it is shown that the strength degradation of fiber reinforced composites subjected to constant amplitude (CA) fatigue loadings can be described by three distinct functions derived from a recently developed two-parameter residual strength model and associated with the sequence of damage mechanisms. Despite the phenomenological prerogative of the model, from the analytical approach it results that under moderate loadings the multiple damage mechanisms develop with different kinetics and manifest their effectiveness at different time scales, accordingly. This highlights the three-Stage hierarchical nature of damage accumulation in composites, from diffuse matrix cracking (I), to fiber/matrix interface failure (II) to fiber and ply rupture and delamination (III). It is also shown that by increasing the loading severity the timescales become comparable witnessing the simultaneous co-existence of the different damage mechanisms.