Analysis of intersonic crack growth in unidirectional fiber-reinforced composites

Recent experiments on dynamic fracture of unidirectional fiber-reinforced g raphite/epoxy composite materials showed that, in Mode I, the crack tip vel ocity could never exceed the shear wave speed, while the crack tip velocity in Mode II not only exceeded the shear wave speed but also approached a st able velocity at which the crack grew for a substantial period of time in e xperiments. The experimentally obtained fringe patterns also dearly showed the existence of shear shock waves when the crack tip velocity exceeded the shear wave speed. In the present study, we have obtained the asymptotic fi elds near an intersonically propagating crack tip. It is shown that Mode-I intersonic crack propagation is impossible because the crack tip energy rel ease rate supplied by the elastic asymptotic field is negative and unbounde d, which is physically unacceptable since a propagating crack tip cannot ra diate out energy. For Mode II, however, it is established that there exists a single crack tip velocity (higher than the shear wave speed) that gives a finite and positive crack tip energy release rate. At all other intersoni c crack tip speeds the energy release rate supplied by the elastic asymptot ic field is identically zero. This critical crack tip velocity agrees well with the stable crack tip velocity observed in experiments. The synthetical ly obtained fringe patterns based on the asymptotic field also agree with e xperimentally obtained fringe patterns, particularly on the existence of th e shock waves. (C) 1999 Elsevier Science Ltd. All rights reserved.