The motion of a brittle crack

The acceleration phase and the subsequent motion of a brittle crack of finite length extending in an infinite sheet loaded in the extensional mode are determined. The energy release is assumed to depend on crack length and on crack-tip velocity but not on higher time-derivatives of crack length. This energy release should equal the fracture energy, which, for a given material is supposed to be dependent only on the crack-tip velocity. For polymethylmethacrylate this velocity dependence has been determined from recent experiments on the long strip configuration. The results obtained show good agreement with earlier experimental investigations of crack motion in large sheets of polymethyl-methacrylate. Since the validity of existing exact analytical solutions of the accelerating crack problem seems to be severely restricted from a practical point of view, the approximate method in the present paper could offer a promising alternative approach.