The influence of strain rate on the mode III interlaminar fracture of composite materials

The Mode III interlaminar fracture toughness, GIIIc, of composite materials based on both thermoplastic and thermosetting-matrices have been investigated using the edge crack torsion (ECT) test geometry. Tests were undertaken at room temperature and over a range of crosshead displacement rates to study the influence of strain rate on the interlaminar fracture properties of these materials. Further information concerning the crack tip loading conditions was obtained by undertaking a finite element analysis of the ECT specimen geometry. The experimental results show that the value of GIIIc depends on initial crack length, increasing steadily with increasing crack length for both types of material. It has been shown that the interlaminar fracture toughness of the glass fiber/epoxy-based system was superior to that offered by its thermoplastic counterpart, an effect that may be due to the fact that the glass fiber-reinforced polypropylene composite was slow-cooled from its processing temperature. The interlaminar fracture toughness of both types of composite remained roughly constant over the range of crosshead displacement rates considered here suggesting that they do not exhibit any rate-sensitive fracture behavior. The finite element analysis of the ECT specimens showed that the specimen is subjected to pure Mode III loading over the central part of the test specimen whereas regions of locally-high Mode II loading were observed over the region in which the load was applied. The Mode III strain energy release rate profile does not depend on specimen thickness or the displacement of the ECT test geometry.