An investigation of the failure mechanisms of thermoplastic composites at various rates of strain and temperatures

Master Thesis


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University of Cape Town

The focus of this project is on the deformation behaviour of thermoplastic composites. The materials used were polypropylene and polyamide resins with glass fibres and talc as fillers. These materials were provided by PLASTAMID (pty) ltd. The injection moulded specimens of polypropylene, polyamide 6-6, 30% talc filled polypropylene, 30% short glass fibre reinforced polypropylene and 30% short glass fibre reinforced polyamide 6-6 were tested in tension, flexure and impact (lzod, Charpy and drop-weight). Two different injection-moulding machines were used for specimen manufacture. These were an automated injection moulding machine simulating good control of processing conditions and a simple hand operated injection moulding machine simulating different processing and cooling (crystallisation) conditions. The mechanical tests were performed at different rates of strain and temperatures. The mechanical results show higher ductility of the unfilled polypropylene material. The strength and the elastic modulus of the materials are different for different materials. Addition of short glass fibres enhances the mechanical properties of polypropylene and polyamide 6-6. Talc fillers reduce the tensile, Izod and Charpy impact properties of polypropylene while the flexural and dropweight impact strength is increased. Optical and scanning electron microscopy were used to observe the microstructural features and deformation behaviour such as matrix plastic deformation, matrix crazing and tearing, fibre-matrix debonding, fibre fracture, fibre orientation and crack propagation. These deformation behaviours are influenced by the test conditions such as strain rate, temperature and the type of the test conducted. The deformation behaviour is also dependent on the constituents of the material. The mechanical test response together with the macro- and microscopic features observed on the fracture surfaces of tested specimens are evidence of the various mechanisms of failure that take place in different thermoplastic composites. The understanding of the mechanical response and the failure mechanisms of thermoplastic matrix composites is important in the design and processing stages.

Includes bibliographical references .