Interfacial strength in thermoplastic composites - at last an industry friendly measurement method?

Many elegant techniques have been developed for the quantification of composite micro-mechanical parameters in recent years. Unfortunately, most of these techniques have found little enthusiastic support in the industrial product development environment, where they are viewed as time consuming, complex, inefficient, labour intensive, and in many cases unproven or inapplicable in 'real' systems. Despite this reaction, there is a real need for a 'user-friendly' micro-mechanics to aid the composites industry to move to the next level of development. A method for deriving values for τ (the interfacial shear strength) and ηo (a fibre orientation factor) from a simple combination of the composite tensile stress-strain curve and the fibre length distribution has been available for some time. Despite the recent wealth of activity in the development of micro-mechanical test techniques, there has been little follow-up on this older technique. In this paper we explore this analysis by its application to injection moulded glass-fibre-reinforced thermoplastic composites produced using three matrices (polypropylene, polyamide 6,6 and polybutyleneterephthalate) and containing different levels of glass-fibre. We furthermore show how the analysis can be extended to obtain another important micro-mechanics parameter, σuf, the fibre stress at composite failure. Values of τ and ηo obtained using this improved version of the original model are presented and discussed.