Non-linear elastic longitudinal behaviour of continuous carbon fibres/epoxy matrix composite laminae: Material or geometrical feature?

Carbon fibres and unidirectional continuous carbon fibre composites exhibit a non-linear elastic behaviour. There has long been a debate on the physical origins of such a behaviour for these materials. Indeed, two main mechanisms have been proposed: either the reorientation of graphene sheets in the carbon fibre or that of the fibre itself due to initial fibre waviness arising from composite manufacturing. This paper addresses this issue by performing, on the one hand, specific mechanical tests to extract the non-linear elastic coefficients in a reliable way, and, on the other hand, using specific finite element analyses with a non-local model. It is shown first that the proposed experimental method is adequate. Besides, the simulations allow us to show that, while both contributions for non-linear elasticity indeed come into play for the non-linear elasticity of carbon fibres, the intrinsic behaviour of fibres can be considered as the main contributory factor. Composites with glass fibres stay fully linearly elastic, due to the limited stiffness of these fibres.