Printed in Austria E¨ects of Grain Size on the Initiation and Propagation Thresholds of Stress-induced Brittle Fractures By

The microstructure of rock is known to in¯uence its strength and deformation characteristics. This paper presents the results of a laboratory investigation into the e¨ects of grain size on the initiation and propagation thresholds of stress-induced brittle fracturing in crystalline rocks with similar mineralogical composi-tions, but with three di¨erent grain sizes. Strain gauge and acoustic emission measurements were used to aid in the identi®cation and characterization of the di¨erent stages of crack development in uniaxial compression. Results indicate that grain size had only a minor e¨ect on the stress at which new cracks initiated. Crack initiation thresholds were found to be more dependent on the strength of the constituent minerals. Grain size did have a signi®cant e¨ect, however, in con-trolling the behaviour of the cracks once they began to propagate. The evidence suggests that longer grain boundaries and larger intergranular cracks, resulting from increased grain size, provide longer paths of weakness for growing cracks to propagate along. This promoted degradation of material strength once the longer cracks began to coalesce and interact. Thus, rock strength was found to decrease with increasing grain size, not by inducing crack initiation at lower stresses, but through a process where longer cracks propagating along longer planes of weak-ness coalesced at lower stresses. 1