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

Abstract

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. Previous studies by others concentrated on the deformation mechanisms of cellular composites with anisotropic knitted reinforcement. The present study is concerned with isotropic materials. The major deformation mode of flat-topped cellular samples made of PET/PP non-woven composites and pure PP polymer is identified as the internal convolution of the conical cell wall during plastic collapse of the cell. Theoretical analysis for the large deformation is carried out to predict the energy-absorption capacity of non-woven cellular composites and pure PP. Good agreement has been found between the predicted and the experimental results, implying that the model can provide a basis for the effective design of similar material systems. Previous studies demonstrated that the membrane deformation of knitted cellular composites dominates the contribution of energy absorption when the cell collapse is large. Hence, in-plane deformation of the knitted composites is investigated in this study. Tensile properties of the composites are related with the knitting directions and individual components. The deformation mechanisms of the flat composites are identified during tensile loading by in-situ SEM observation. Effects of fabric structure, fibre volume fraction, pre-stretching directions and levels of the knitted fabrics prior to composite fabrication and plasma treatment on fibre surface are studied. Raman microscopy is employed to determine the fibre strain distribution in a single loop in the knitted composites, made from ultra-high molecular weight polyethylene (UHMWPE) fibres embedded in low density polyethylene (LDPE) matrix. The fibre itself acts as a sensor embedded in the composite and therefore Raman microscopy can be used to follow the Raman band shift of the single fibre due to the applied strain. It has been found that the fibre strains of the deformed loop are very low even when the composite is under large effective strains. Theoretical analysis is concentrated on the fibre strain on single knitted loop during the large deformation. This phenomenon can be explained in terms of the shape change of the fabric loop and fibre sliding, fibre orientation to the stretching direction, and probably the fibre stress relaxation during the loop mapping process. In summary the fabrication aspects and the performance of cellular textile composites with various material systems have been investigated in detail. The deformation mechanisms of the isotropic non-woven cellular composite material have been identified and a prediction model has been proposed to provide fundamental information on composite design. The in-plane deformation mechanisms of the composites that contribute to the energy absorption have been studied in multi-scales. The study of non-woven composites provides an alternative material selection to achieve further substantial saving on material cost and fabrication time. More importantly, the outcome of this thesis may contribute significantly to development of high energy absorbing cellular textile composites with great potential in making protection devices, such as sport helmets, car crashing elements or other light weight devices where high specific energy absorption capacity is of great concern.