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For description of pre-fracture zone, the modified Leonov-Panasyuk-Dugdalemodel is proposed. The simple relations have been derived for the followingcritical parameters: shear stresses, pre-fracture zone length, and III mode SIF.Passage to the limit from the sufficient fracture criterion to the necessary onetakes place when the pre-fracture zone length vanishes. The critical stressesobtained under the necessary and sufficient criteria differ substantially. Thecritical SIF obtained under the sufficient criterion are calculated within theframework of the model by the grain diameter, and conventional parameters ofthe diagram of shear stresses versus shear strain.