Abstract
The mechanical properties and fracture mechanics of a series of short glass fibre reinforced and rubber toughened polypropylene composite grades has been studied. The microstructural characteristics of composite grades were examined and, through appropriate models, related to the observed mechanical properties. The moulded material was modelled as being composed of fibre reinforced plies of varying average fibre orientation. The rubber was distributed uniformly throughout the specimens. The main effect of the rubber was to reduce the stiffness of the matrix and hence also the efficiency of the load transfer at the fibre/matrix interface while at the same time improving the fracture toughness and critical strain energy release rate of the matrix. Automated image analysis has been used to characterise the rubber particles' size, shape and distribution, and glass fibres' length and orientation distributions. The fibre/matrix interface has been studied using a novel single fibre fragmentation technique. Iterative computer simulations have been developed to accurately predict the stress-strain response of the various grades. The fracture mechanics properties of this series of materials are highly strain rate sensitive. At low strain rates the addition of glass fibres reduces the toughness of the material because the fibres act as discontinuities within the matrix, aiding initiation and propagation of a crack. At higher strain rates the fibres toughen the material by increasing the energy dissipation associated with fibre pull-out. These effects result in changes in the fracture surface morphology. Fibres pulled-out at low strain rates had clean surfaces. At higher strain rates the surfaces of pulled-out fibres were coated in an adherent sheath of matrix material. These effects are considered to be a consequence of the viscoelastic nature of the matrix. At low strain rates the matrix deforms plastically. At impact speeds the matrix responds in a predomoninantly brittle manner.