Effects of grain size on the initiation and propagation thresholds of stress-induced brittle fractures

The microstructure of rock is known to influence its strength and deformati on characteristics. This paper presents the results of a laboratory investi gation into the effects of grain size on the initiation and propagation thr esholds of stress-induced brittle fracturing in crystalline rocks with simi lar mineralogical compositions, but with three different grain sizes. Strai n gauge and acoustic emission measurements were used to aid in the identifi cation and characterization of the different stages of crack development in uniaxial compression. Results indicate that grain size had only a minor ef fect on the stress at which new cracks initiated. Crack initiation threshol ds were found to be more dependent on the strength of the constituent miner als. Grain size did have a significant effect, however, in controlling the behaviour of the cracks once they began to propagate. The evidence suggests that longer grain boundaries and larger intergranular cracks, resulting fr om increased grain size, provide longer paths of weakness for growing crack s to propagate along. This promoted degradation of material strength once t he longer cracks began to coalesce and interact. Thus, rock strength was fo und to decrease with increasing grain size, not by inducing crack initiatio n at lower stresses, but through a process where longer cracks propagating along longer planes of weakness coalesced at lower stresses.