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.

In this article, the Mode III interlaminar fracture properties of a glass/ epoxy-steel bi-material system are investigated as a function of crosshead displacement rates using the edge crack torsion (ECT) test geometry. For purposes of comparison, tests were also undertaken on the plain glass/epoxy composite. For a given crack length, it has been shown that the interlaminar fracture toughness of the bimaterial system was inferior to that offered by the plain composite, an effect that is attributed to the reduced toughness of the interface between the two materials. The Mode III fracture toughness of both the plain composite and the bi-material samples exhibited a crack length dependency, with the measured value of G IIIc increasing with crack length. Verification of the trends in the experimental data was achieved by conducting a finite element analysis of the ECT specimen and good agreement was achieved. Furthermore, the interlaminar fracture toughness of the plain composite remained roughly constant over the range of crosshead displacement rates considered here, suggesting that it does not exhibit a rate-sensitive fracture behavior. In contrast, the bi-material samples exhibited very low values of Mode III fracture toughness at impact rates, associated with extensive debonding of the steel from the composite material.

The Mode III interlaminar fracture properties, GIIIc, of an adhesively-bonded glass/epoxy composite are investigated over a wide range of crosshead displacement rates, using the edge crack torsion (ECT) test geometry. The ECT test fixture has been modified to conduct impact testing on these bonded materials. Tests on all of the samples highlighted a significant crack length dependency with the value of GIIIc increasing rapidly with increasing crack length. Tests were also undertaken on the plain glass reinforced epoxy over an equally wide range of crosshead displacement rates. For a given crack length, the interlaminar fracture toughness of the adhesively-bonded system was superior to that offered by the plain composite, an effect that is attributed to the presence of significant crack-tip blunting within the adhesive layer. The interlaminar fracture toughness of the composite and the adhesively-bonded system remained roughly constant over the range of crosshead displacement rates considered here, suggesting that these systems do not exhibit any significant rate-sensitive fracture behavior. Finally, the crack tip loading conditions are verified by conducting an FEA analysis of the ECT specimen. Here, it was shown that Mode III loading predominates at the center of the test specimen, whereas regions of Mode II loading were observed close to the test supports.