Dynamic criterion of fracture of brittle materials

On the basis of an analysis of experimental data, we suggest a new model of brittle fracture. According to this model, cracks begin to propagate with a certain stepwise increasing rate. The existing criteria of brittle fracture are based on the explicit or implicit assumption of the stationary or quasistationary beginning of crack propagation (the first stage of fracture) and, therefore, fail to give an adequate description of the conditions of brittle fracture. By applying an approach suggested somewhat earlier by the author, we estimate the conditions of the dynamic start of a crack. In this case, we formulate a variational principle and consider several examples.