Tensile properties and meso-scale mechanism of weft knitted textile composites for energy absorption

The investigation on large deformation tensile properties and the relevant meso-scale mechanisms of weft knitted textile composites is presented. The correlation between fabric structure (e.g. loop height and width, number of wale or course per unit length, etc.), matrix damage and material properties are described. Weft knitted fabrics with 1×1 interlock structure were used as the preform for the composites. The materials studied include knitted nylon fabric/unsaturated polyester resin and co-knitted polyethylene terephthalate (PET)/polypropylene (PP) textile composites. The results show that all the nylon/polyester thermoset textile composites samples displayed an ideal bi-linear character in their tensile stress-strain curves, whilst the tensile curves of PET/PP co-knitted thermoplastic samples along the wale, course and 45° directions are all significantly non-linear. The tensile behavior is superior in the wale direction to those in the course and 45° directions. The deformation mechanisms in meso-scale were identified experimentally by in-situ observation of large deformation process for both thermoset matrix and thermoplastic matrix textile composites. For the nylon/polyester composite samples, the non-linear properties mainly come from the change in the configuration of the fabric structure during extension. For the PET/PP co-knitted textile composite samples, the inelastic properties are attributed to the damage evolution in the matrix, interface damage between fiber bundle and matrix, sliding between the wales of the knitted fabric, as well as the change in the configuration of the fabric structure during loading.