Mechanisms of rock failure in compression -- axial splitting and shear failure -- are studied, based on a microscopic consideration. Analytical models are constructed and model experiments on plates of a brittle polymer are performed. It is shown that unstable growth of tension cracks which propagate from the tips of pre-existing cracks and curve towards the maximum compressive direction. is the fundamental mechanism that produces axial split - ting of a uniaxially compressed rock specimen. whereas shear failure of a triaxial l y compressed specimen is a result of sudden growth of tension cracks at tips of a suitably arranged interacting set of microcracks. The simultaneous out-of-plane unstable growth of a suitably oriented row of cracks is analyzed and. on the basis of this model. the variat ions of the "ultimate strength" and the orientation of the overall fault plane with the confining pressure are estimated. The brittle - ductile transition is discussed with the aid of a model which includes both tension crack extension and plastic zone development from the pre-existing cracks.

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