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We explored the microstructure-related scale effect on the tensile failure of coal. Disc specimens with different diameters (25 mm, 38 mm, 50 mm, and 75 mm) were processed for the Brazil split test. The influence of microstructures on fracture initiation and propagation was detected and analysed by the combination of X-ray computed tomography scanning, digital image correlation approach, and acoustic emission monitoring. The investigation indicated that tensile strength decreased with increasing specimen diameter, and the relationship can be described by an exponential equation. Four tensile failure modes were observed in specimens with different diameters, namely central, non-central, central edge, and central multiple pattern. Larger specimens exhibited more complicated failure patterns (central edge and central multiple failure patterns) and a greater percentage of shear failure fractures. Strain concentration and strain reversal were observed, respectively, in mineral—inclusion-rich regions and pre-existing discontinuities regions. This possibly contributed to fracture initiation, propagation, and coalescence. Fractures tended to grow along with the interface of mineral inclusion and coal matrix, and they could have propagated into and connect the pre-existing discontinuities. The greater volume of microstructure in larger specimens resulted in fractures that were more complex and may have increased the number of shear failure cracks and led to failure modes that were more complicated. [ABSTRACT FROM AUTHOR]

The behavior of transversely isotropic rock material was studied under Brazilian test conditions for nine different rocks (two sandstones, one shale, two slates, one schist, and three gneiss). Both the variation of the strength and the final fracture patterns induced by testing were examined as a function of the inclination angle of the weak planes. The combination of the observations for both parameters illustrated clearly the complexity of the failure of such rocks, which was summarized by assuming four different trends. The four trends for the variation of the strength as a function of the inclination angles range from little or no variation (trend 1, i.e., isotropic behavior for strength) to a sharp decrease of the strength from very small angles onwards, followed by a leveling off (trend 4). Trend 2 is characterized by a constant value between 0° and about 45°, followed by a linear decrease, while trend 3 corresponds to a decrease of the strength over the entire interval, but a rather systematic decrease, approximating a linear variation. Different variations were observed among these four trends in terms of the relative lengths of the respective fractures along the weak planes and in any other direction. For the rocks investigated, there is a cross-over from dominant fractures in directions other than the weak planes to dominant fractures along the weak planes. For the four trends, there are systematic changes in the position of this cross-over point, i.e., on average, at about an inclination angle of 75° for trend 1 and at about 15° for trend 4. For inclination angles larger than the cross-over point, the rock specimens failed primarily by splitting between both loading lines. [ABSTRACT FROM AUTHOR]