Mechanics of short fibre thermoplastic based composites containing UHMWPE interlayers : a preliminary investigation
FEM has been applied to analyse the influence of interlayers on the stress distribution around cylindrical fibres in an elastic matrix. This being part of a research programme aiming to enhance the energy absorption characteristics of composites by means of interlayers from materials exhibiting high ductility under plane strain conditions, e. g. UHMWPE. The theoretical model is based on the Galerkin weighted residual finite element in conjunction with a "penalty approach". The advantage of this method for polymer composites is in its ability to cope very effectively with non-linear systems. Glass microscope slides were initially used to develop a technique for bonding the ductile layers on to glass and polyamides respectively, as well as to provide simple verifications of the applicability of the aforementioned model. Further experiments were carried out on glass fibres coated with UHMWPE from a xylene solution, in order to evaluate the applicability of the above technique and of the theoretical model in actual composite systems. Although great difficulties were encountered in achieving a well bonded uniform coating on the glass fibres the results have confirmed the viability of the approach: The impact strength of compression moulded glass reinforced Nylon plaques, measured with an instrumented falling weight apparatus was increased up to 400% with a corresponding loss in flexural modulus of only 10-15%. The ductile nature of the interfacial failure between fibres and matrix was also confirmed by SEM examination of fractured specimens. Thermal analysis results, especially from DMA indicate that an UHMWPE interlayer substantially increases the tan 8 of the short fibre composite over a wide temperature range, albeit with some reduction in modulus. There was good agreement between the flexural modulus results obtained from DMA tests and those obtained using 3 point bending at room temperature.