Through-thickness crack growth resistance in fibre composites and its role in preventing ply cracking in cross-ply laminates

Abstract

A new mechanism is proposed to elucidate recent experimental observations of a transition from slow, stable through-thickness cracking to unstable growth in the 90° ply of a cross-ply laminate as the ply thickness increases above 40 µm for typical carbon fibre reinforced polymer composites. Herein we have identified that the transition is attributed to a rising crack-growth resistance (or R-curve) of transverse matrix cracks with increasing size. This new explanation is substantiated by obtaining the R-curve using a high-fidelity micromechanical model, followed by employing fracture mechanics principles to predict the progression and stability of through-thickness microcracking in a ply. The benefit of this new approach is that only one simulation is required to generate the R-curve, which can then be employed to predict the crack-growth behaviour for any ply thickness, instead of requiring separate simulations for each ply thickness, thereby reducing the computational burden considerably. This is particularly valuable for parametric studies to investigate the dependence on various material properties and computationally efficient analysis of large-scale structures. As illustrative examples, the dependence on matrix toughness and on volume fraction was investigated and simple linear relationships were identified for the steady-state value of crack-growth resistance. Such relationships can further reduce the computational burden, particularly when the relevant material properties may not be available from direct measurement, but can be reasonably estimated, as for cryogenic applications.