A new model for interface failure in fibre-reinforced polymers

The shear failure process in polymers is critically examined, and it is shown that it involves tensile failures in planes at 45 degrees to the principal shear planes. (Evidence here comes from shear tests on fibre-reinforced polymers, and the polymers themselves.) It is then shown that fourth-order constraints inhibit polymer failure in the region of the fibre/polymer interface. This explains why the interface can sometimes be nearly ten times stronger than the polymer. On the basis of these principles, together with experimental observations, it is suggested that fibre/polymer interfacial failure is progressive, and in the case of fibre pull-outs, starts near the fibre entry point and proceeds most of the way to the embedded end. It then arrests while the end debonds and the remaining ligaments joining the fibre and polymer stretch. Finally, it continues progressively to the embedded end of the fibre. The failure process is entirely tensile, and is probably governed by surface roughness on a nanometre scale. While the importance of maximizing chemical interactions between the fibre and the polymer is confirmed, geometrical considerations are also of primary importance. This suggests that other types of adhesive bonds which involve shear could be improved markedly by appropriate surface grooving to generate centro-symmetric regions.