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Rock Mech Rock Eng (2014) 47:11651181 DOI 10.1007/s00603-013-0451-2
ORIGINAL PAPER
Factors Affecting Crack Initiation in Low Porosity Crystalline Rocks
Mohsen Nicksiar C. D. Martin
Received: 22 September 2012 / Accepted: 27 June 2013 / Published online: 10 July 2013 Springer-Verlag Wien 2013
Abstract Crack initiation in uniaxial compressive loading of rocks occurs well before the peak strength is reached. The factors that may inuence the onset of cracking and possible initiating mechanisms were explored using a discrete element numerical approach. The numerical approach was based on grain-based model that utilized the Voronoi tessellation scheme to represent low porosity crystalline rocks such as granite. The effect of grain size distribution (sorting coefcient ranging from 1.5 to 1.03), grain size (average grain size ranging from 0.75 to2.25 mm), and the heterogeneities of different mineral grains (quartz, K-feldspar, plagioclase) on the onset of cracking were examined. The modelling revealed that crack initiation appears to be a tensile mechanism in low porosity rocks, and that shear cracking along grain boundaries is only a prominent mechanism near the peak strength. It was also shown that the heterogeneity introduced by the grain size distribution had the most signicant effect on peak strength and crack initiation stress. The peak strength ranges from 140 to 208 MPa as the grain size distribution varies from heterogeneous to uniform, respectively. However, the ratio of crack initiation to peak stress showed only minor variation, as the heterogeneity decreases. The other factors investigated had only minor effects on crack initiation and peak strength, and crack initiation ratio.
Keywords Crack initiation stress Discrete element
methods Grain-based models Mineral-based models
Cracking model
1 Introduction
It is well known that the microstructure of a rock should be taken into account when determining its compressive strength. To minimize its inuence on the test results, the ISRM Suggested Methods for determining the uniaxial compressive strength (UCS) and deformability of rock materials states that the test specimen shall be right cylinders having a height to diameter ratio of 2.53.0 and a diameter preferably of not less than 54 mm. The method then states that the diameter of the specimen should be greater than ten times the largest mineral grain in the rock (Brown 1981). Presumably, the latter requirement ensures that the right-cylinder specimen...