ABSTRACT
A scheme of loading with a tool not at the upper bench edge but under it (Fig.2a) has been proposed at the Ins titute for Superhard Materials. In this case the tool itself made of cemented carbide is an unsymmetrical wedge, which indentation into rock allows the formation of distributed normal (qx) and shearing (qy) components of the applied load(Fig.2b). The tool cutting-in is accompanied by the formation of a median crack and a hertzian crack, the latter propogates by the mechanism of mixed fracture , i.e. by the combined mechanism of normal fracture (Kic) and antiplane shear load (Kj|c). Once formed, an initial crack does not propagate. The hertzian crack is directed at an angle 8 5 = 2 0 . . . 25° with respect to the surface, and with a specific Px/Py ratio and the bench block height one should expect the crack turning with respect to its initial direction as well as its spontaneous extension to come to the surface (Fig.2b and 3 ) .
