In situ characterization of the damage mechanisms during tensile deformation in sustainable Flax-PP spread-tow woven composites

Abstract Due to current challenges related to global warming and the ecological impact of materials, the industry is considering natural-fibre-reinforced thermoplastic composites for their low weight and sustainability. However, to be considered for structural applications, their mechanical behaviour and damage mechanisms need to be fully understood. Regarding Flax-Polypropylene (Flax-PP) composites, damage mechanisms are successfully identified in the literature; however, their initiation and propagation up to final failure are not well described yet. The present study focuses on a spread-tow woven Flax-PP composite and aims to investigate the damage mechanisms and their evolution in two configurations — namely with fibres of the top ply being either parallel or perpendicular to the loading direction. The damage mechanisms are first identified via SEM analysis of fracture surfaces after standard tensile testing. Then, for both configurations, the evolution of damage mechanisms is studied by in situ SEM micro-tensile testing. In both cases, crack initiation at fibre tips and fibre debonding is the primary failure mechanisms, leading to subsequent fibre pullout and preferred crack path. Results show that the crack path is more in the case of top ply fibres parallel to the loading direction than in the case of transversal fibres. The greater occurrence of fibre pullout compared to fibre breakage is attributed to the weak interfacial strength between the fibres and the matrix, which is typical for natural fibre composites. The present study used in situ investigation to propose a detailed sequence of damage evolution related to tensile loading.