Abstract
The nucleation of a microcrack in the grain junction of grains due to induced athermal grain boundary sliding under the action of stresses from the accumulation of virtual dislocations of planar shear mesodefect, stresses from junction disclinations and external stress is considered. It is shown that the existing criteria do not allow us to take into account the influence of the stress field of the junction disclination on the crack nucleation. A new energy criterion based on the concept of spontaneous crack nucleation of the minimum possible length for a crystalline body is proposed. Using this criterion, an analysis of the conditions necessary for the crack nucleation has been carried out. The dependences of the critical value of an external load required for crack nucleation on the strength of junction disclinations at different values of the parameters of the fragmented structure of material (the size of the boundaries of fragments and the strength of rotational-shear mesodefects) and the threshold stress of athermal sliding were obtained. It is shown that the presence of a sufficiently high-strength negative disclination at the grain junction can significantly facilitate the nucleation of a crack.
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