Hypo-viscoelastic modelling of in-plane shear in UD thermoset prepregs

In-plane shear of prepreg material is one of the key deformation modes that affects defect generation in composites manufacturing processes such as Automated Fibre Placement (AFP) and Thermoforming. Recent characterisation of the evolution of uncured prepreg in-plane shear behaviour during manufacturing has revealed a strong influence of deformation rate and temperature on the measured response. In the present contribution, a phenomenological viscoelastic model for the observed behaviour is formulated using rate constitutive equations. Each of the components is related to an assumed micro-mechanical phenomenon (e.g. friction between the fibres, tensile loading of the fibres, etc). The proposed model is implemented into a user material subroutine for the Finite Element (FE) package Abaqus/Explicit. Model validation is performed by modelling experiments with loading programmes distinctly different from those used for material parameter extraction and it is concluded that state-of-the-art numerical models for AFP need to better represent in-plane shear.