Micro-mechanical modelling of longitudinal compression in unidirectional composites: effect of misalignment

Unidirectional composites exhibit their best performances when loaded in the longitudinal direction, exploiting the high stiffness and strength of the reinforcements. In longitudinal compression, the strength is significantly lower if compared to tension. This is due to the inherent material imperfections, such as fibre misalignment, that trigger the formation of kink bands and micro-buckling of fibres under compression. The phenomena involved in longitudinal compression are very complex. According to some authors, failure is governed by the matrix yielding and the fibre-matrix debonding, promoted by fibre misalignment. Predictive models in the literature consider these phenomena to capture the kink band formation. In this work, a micromechanical model based on finite elements is used to simulate the longitudinal compressive behaviour of a unidirectional composite. The microstructure is generated based on an algorithm, extended to include misalignment. The features of the model are: i) fibre-matrix interfaces modelled with cohesive elements, ii) matrix plasticity and damage, to account for the non-linear behaviour of the matrix; iii) sinusoidal misalignment and iv) random geometrical arrangement in the transverse direction. The results of this investigation are compared with other similar works in the literature, both with finite element and analytical formulations. The results of this work are the first part of a broader research, aimed to investigate more realistic 3D misalignments and its effect on compressive strength.