The Influence of Loading Rate on the Mode III Interlaminar Fracture Toughness of Composite/Steel Bi-material Systems

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 GIIIc 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.