Effects of Grain Size on the Initiation and Propagation Thresholds of Stress-induced Brittle Fractures |
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| Authors: | E Eberhardt B Stimpson D Stead |
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| Institution: | (1) Engineering Geology, Swiss Federal Institute of Technology (ETH), Zürich, Switzerland , CH;(2) Department of Civil and Geological Engineering, University of Manitoba, Winnipeg, Canada , CA;(3) Camborne School of Mines, University of Exeter, U.K., GB |
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| Abstract: | Summary The microstructure of rock is known to influence its strength and deformation characteristics. This paper presents the results
of a laboratory investigation into the effects of grain size on the initiation and propagation thresholds of stress-induced
brittle fracturing in crystalline rocks with similar mineralogical compositions, but with three different grain sizes. Strain
gauge and acoustic emission measurements were used to aid in the identification and characterization of the different stages
of crack development in uniaxial compression. Results indicate that grain size had only a minor effect 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 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 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 weakness
coalesced at lower stresses. |
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