13th International Conference on Fracture June 16–21, 2013, Beijing, China -3- maximum simplification of the problem and using the features of its stress-strain state, we assume that the cracks propagate along the slip lines, and their formation is due to stored elastic energy, but not all, but only the shear energy, the energy of hydrostatic compression being not taken into account So, the model is based on the following provisions: I. Suppose we have an infinite plane with a circular cavity in a uniform compression, stress-strain state is plane. II. As the material strength is exhausted, fracture occurs instantaneously with the formation of a symmetric system of shear cracks. III. Cracks occur along the slip lines, forming a regular pattern like Fig. 1-3. IV. One can neglect the irreversible (inelastic, heat, etc.) losses (plate behaves quasi-brittle), and possible dynamics (waves). V. Energy balance equation expresses the equality of the energy of formation of new surfaces (cracks) to the elastic energy of shear, released from the ring (layer), which was cut through with those cracks. VI. The minimum-energy-consuming fracture scheme occurs, i.e., the scheme with such a number n and length L of the cracks, that the energy needed for its creation, is minimal. Fig. 3 [24, p. 326] The proposed scheme can essentially be considered as a version of that classical approach by Griffith [23], adopted for cases where the problem symmetry requires the hypothesis on occurrence of one crack to be replaced with the assumption of arising a symmetrical system with n cracks. 3. Crack creation energy To formalize the above speculations we write according to the condition «V» the basic equation of energy balance
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