Penetration of rock by conical indenters

Abstract

Experimental results for the penetration of conical bits into rock have been obtained and compared to theoretical results derived from an existing bit penetration model. The model is similar to the one established by Paul and Sikarskie for wedge penetration, and was developed independently by Miller and Sikarskie, and the author. The tests were made in Swedish Bohus Granite using seven indenters with apex angles in the range 60–150°. For the smaller apex angles chip failure was predominant whereas for the greater apex angles essentially crushing occurred in agreement with the predictions of the theory. Force-penetration curves and crater vol were measured, and a force-penetration parameter characterizing the chipping envelope and the ratio of volume to work were evaluated. The dependence of the force-penetration parameter on apex angle is similar to that predicted by the theory, but it is found that the best agreement between theory and experiment is obtained when lower values than the real values are assigned to the compressive strength, the internal angle of friction, and the angle of friction representing the interaction between the bit and the solid rock. This means that the predicted forces at chip failure are too high, which is believed to depend on the use of a global rather than a local failure criterion. Using those values of the mentioned parameters, which give the best fit for the chipping envelope, good consistency between theory and experiment is obtained for the crater volume to work ratio.