Multi-scale study of tensile properties and large deformation mechanisms of polyethylene terephthalate/polypropylene knitted composites

This paper is concerned with large deformation tensile properties and energy absorption mechanisms of thermoplastic weftknitted composites, fabricated by compression molding co-knitted preforms consisted of polyester (polyethylene terephthalate) and polypropylene continuous filament yarns. The large deformation mechanisms have been studied by observing morphological changes during quasi-static tensile loading in meso- and micro-scales. In-situ SEM observations have identified critical points including brittle matrix crack initiation from imperfect composite surfaces, fibre/matrix debonding, serious loop distortion along the loading directions and fibre straining during the large deformation until composite fracture. The energy absorption mechanisms have been identified as localized matrix compression and extension, matrix crack propagation, localized loop distortion, and fibre straining during the deformation process. Effects of fabric structure, fibre volume fraction and plasma treatment on fibre surface have been also studied.