The rate of Maximum Stress Drop (RMSD), which is drived from the complete stress-strain curve and indicates the severity of unstable brittle fracture, is function of confining pressure. It becomes maximum when the confining pressure corresponds to about half the overburden pressure. RMSD also depends on the fracturing mode, being relatively larger when it is of single-shear type, which is the dominant mode when the rock fabric shows anisotropy.
Based on a model study to simulate the post-failure behavior, an index named “brittleness” was derived which is defined as the ratio of the stored strain energy to the one required to create the fracture surfaces. It is shown that the relationship between brittleness and confining pressure is similar to the one between RMSD and confining pressure, implying that brittleness is an index to effectively measure the severity of unstable fracturing. Also, the magnitude of the brittleness is larger when simple shear-type is assumed as the fracturing mode. Finally, the prediction of the magnitude of the confining pressure at the point of brittleductile transition coincided with the measured one.
Concerning to the effects of geological processes on the mechanical properties, the following was noted. Firstly, the magnitude of the confining pressure at the brittle-ductile transition point is approximately equal to the overburden pressure corresponding to maximum depth of burial. Secondly, the value of the confining pressure, which corresponds to the bending point of the peak strength criterion, correlates with the overburden pressure at the site where the samples were collected from.
