Formation of damage and its effects on non-crimp fabric reinforced composites loaded in tension

Non-crimp fabric (NCF) composites, manufactured by resin infusion techniques are one of the most promising next generation composite materials. They offer large potential for application in primary structures as they give excellent performance at low production costs. However, before NCF composites will be efficiently used in design, detailed understanding of governing micro mechanisms must be accumulated and described by predictive models. In the present study, NCF cross-ply laminates have been tested in tension. Intralaminar cracks caused in the 90° fibre bundle layers and their effect on laminate mechanical properties have been monitored. Occurrence of ‘novel’ type of cracks propagating in the load direction (longitudinal cracks) is explained by a thorough FE analysis using an Representative Volume Element (RVE) approach, revealing stress concentrations caused by 0° fibre bundle waviness. Effects of damage on mechanical properties are modelled using modified micro mechanical models developed for analysis of conventional laminated composites. The analysis reveals mechanical degradation to be ruled by the crack opening displacement (COD). However, unlike traditional composites, transverse cracks do not generally extend through the entire thickness of the 90° layer, but are rather contained in single fibre bundles, limiting the COD.