Correlations Between Damage Accumulation and Strength Degradation of Fiber Reinforced Composites Subjected to Cyclic Loading

Composite materials subjected to cyclic loading degrade their strength and/or stiffness due to the accumulation of different overlapping damage mechanisms. Three substantial regimes of damage are commonly recognized. At the early stage of the loading a diffuse matrix cracking and matrix/fiber debonding (Stage I) precipitate in a saturation state termed "characteristic damage state" (CDS) that represents the initiation of fibers rupture and incipient delamination phenomena (Stage II). The final collapse is debited to the coalescence of macro cracks with resulting ply ruptures and delamination (Stage III). In this paper, summarizing the capabilities of a recently developed two-parameter phenomenological model based on residual strength, it is shown that the strength degradation kinetics can be described by three distinct functions associated to the sequence of damage mechanisms. Despite the phenomenological prerogative of the model, from the analytical approach it results that the multiple damage mechanisms develop simultaneously even with different kinetics and manifest their effectiveness at different time scales, accordingly. This highlights the hierarchical nature of damage accumulation in composites, from diffuse matrix cracking, to fiber/matrix interface failure to fiber and ply rupture and delamination. (C) 2016 The Authors. Published by Elsevier Ltd.