Multi-scale study of textile composites under large deformation for energy absorption

This thesis presents a systematic study, in multi-scales, of mechanical properties of newly developed thermoplastic cellular textile composites for energy absorption purposes. This research aims to create a scientific basis for such composite materials with a high specific energy absorption capacity, and to better understand the material system, the fibre architecture and the fabrication techniques for particular applications. The study covers five related aspects: (1) composite fabrication techniques, (2) performance of cellular composites with various material systems under quasi-static compression and impact conditions, (3) deformation mechanism of the cellular structure, (4) in-plane deformation and mechanisms under tensile loading in both macro- and micro-scales, and (5) determination (by employing Raman microscopy) of fibre strain distribution in composites.; Material systems in this study include co-knitting polyester (polyethylene terephthalate, or PET) and polypropylene (PP) continuous filament yarns to be melted as matrix, laminating PET non-woven fabrics and PP matrix films, laminating high performance fibres, ultra-high molecular weight polyethylene (UHMWPE), as knitted fabrics and low-density polyethylene (LDPE) matrix films. The fabrication processes of thermoplastic textile composites, including the corresponding grid-dome with flat-top cellular structure, are described.; The energy-absorption behaviours of these cellular composites are examined under quasi-static compression and impacts. The effects of fibre material, fibre volume fractions and fibre architecture on the energy absorption capacity, are investigated. Cell recovery after impact is also studied. The equivalent cell wall thickness is shown to be a dominant factor governing the energy absorption capacity of the cellular structure made from the same material. Both the composites with knitted and non-woven fabric reinforcements exhibit a high specific energy absorption capacity. In particular, the non-woven composite achieves a higher level of energy absorption than that of the knitted composites. In addition, the non-woven cellular textile composite is able to retain the same level of energy absorption capacity under multiple impacts. This can be attributed to material properties of the composites such as its tensile and shear properties. By further varying the fibre volume fraction in the non-woven composite, different fracture modes are observed and different levels energy absorption capacity are obtained. (Abstract shortened by UMI.)...