Shear response of 3D non-woven carbon fibre reinforced composites

We experimentally and numerically investigated the shear response of a three-dimensional (3D) non-woven carbon fibre reinforced epoxy composite with three sets of orthogonal tows and approximately equal fibre volume fractions in the orthogonal directions. Shear tests on two orientations of dogbone specimens showed significant strain hardening and an increasing unloading stiffnesses with increasing applied strain. Unloading was also accompanied by considerable strain recovery, with X-ray tomographic scans revealing minimal damage accumulation in specimens until near final failure at shear strains in excess of 50%. To understand the origins of this unusual mechanical response of the 3D carbon fibre composites, we developed a micro-mechanical model wherein all tows and matrix pockets in the composite are explicitly considered. The tows were modelled using a pressure-dependent crystal plasticity approach to capture texture evolution under large deformations and the model replicated many of the experimental observations with a high degree of fidelity. Importantly, the model illustrated the role of the 3D architecture in not only suppressing delamination but also enhancing the strain hardening response due to a 3D confinement effect of the tow architecture. On the other hand, a model wherein the tows were modelled using an anisotropic Hill plasticity framework (absent plastic spin) failed to replicate the observed strain hardening response or capture the associated strain recovery upon unloading. This highlights the importance of accounting for the evolution of the material substructure within the tows of these high ductility 3D composites. The results of this work illustrate the unique mechanical behaviour of 3D non-woven fibre composites and provide insight into how 3D fibre architecture can be used to enhance the mechanical performance of fibre composites.