Effects of rate on crack growth in a rubber-modified epoxy

The rate-dependent fracture behavior of a 10-phr rubber-modified epoxy was investigated using double-cantilever-beam tests at various crosshead speeds . Dramatic rate effects were observed in the R-curve behavior and in the re lationship between the applied energy-release rate and the crack velocity. Furthermore, a transition between fracture with toughening mechanisms opera ting (kinetic crack growth) and brittle behavior (dynamic crack growth) was observed. This transition depended on the crack velocity and applied energ y-release rate. Such behavior is expected to depend on how the intrinsic to ughness and/or the extrinsic toughening mechanisms are influenced by strain rate. It was shown that the size of the process zone was only weakly depen dent on the crack velocity until the onset of dynamic fracture. Furthermore , the extent of void growth was virtually independent of the crack velocity in the kinetic regime. These results appear to rule out the notion that cr ack-tip shielding is significantly affected by rate effects in this rubber- modified epoxy. Rather, the rate effects may arise from a rate-dependent in trinsic toughness. It was observed that the intrinsic toughness decreased s ignificantly with increasing crack velocity. The crack instability was show n to be associated with an abrupt cessation of the development of the proce ss zone, with both cavitation and void growth being totally suppressed. (C) 2000 Acta Metallurgica me. Published by Elsevier Science Ltd. All rights r eserved.