INJECTION-MOLDING OF GLASS-FIBER-REINFORCED PHENOLIC COMPOSITES .1. STUDY OF THE CRITICAL FIBER LENGTH AND THE INTERFACIAL SHEAR-STRENGTH

One of the most important factors affecting the mechanical performance of a fibre composite is the characteristics of the bonding between th e fibre and the matrix, which can be expressed by the critical fibre l ength or interfacial shear strength. Two methods were used to study th e critical fibre length and the interfacial shear strength of glass fi bre reinforced phenolic composites, multiple matrix cracking of unidir ectional continuous phenolic composites and fitting of the equation fo r the theory of discontinuous fibre composites to the tensile stress-s train response of injection moulded short fibre phenolic composites. G ood correlation between the results obtained from the two methods was obtained. It was found that the critical fibre length for glass fibre reinforced phenolic composites is relatively large at 5-8 mm and that the interfacial shear strength at the glass fibre-matrix interface is low with a value of 2-4 MPa. It is suggested that the bonding between glass fibre and the phenolic matrix is only frictional.