A polycrystalline brittle solid may undergo grain boundary micro-cracking due to residual stresses or applied load. This paper contains some results pertaining to the loss of macroscopic stiffness of such solids when all micro-cracks are open and when some may be closed and be subject to frictional sliding. Two specific models are investigated: first micro-cracking on the grain boundaries of a regular hexagonal array, second, micro-cracks which are randomly located and oriented. It is shown that for many purposes the two models give identical results. The paper concludes with some analysis of the possible toughening due to process zone micro-cracking at the tip of a macroscopic crack. It is found that toughening can only occur if the saturation crack density is very large.

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