Rock fragmentation by indentation and disc cutting some theoretical and experimental studies

Abstract: The main objective of the thesis is to increase the understanding of failure mechanisms of rock drilling and rock cutting methods in general, and rock boring by disc cutters in particular. The thesis consists of five separate papers. Paper A presents the theoretical elastic stress field of indexed indenters and some laboratory observations. A fracture mechanics calculation of crack length from disc cutters is described for the first time. Paper B deals with the behavior of crushed material beneath a penetrating tool. A laboratory investigation of the strength of crushed rock is made and the reconsolidation of crushed rock is evaluated. Paper C reports on in-situ indentation test in a scanning electron microscope. The continuous development of cracks is studied. Paper D presents a new theoretical model for side chipping, assuming tensile failure of the rock. The model is tested in the laboratory by full scale disc cutting tests. Paper E deals with energy-absorbing processes in disc cutting. A classification of energy forms is submitted and first order energy forms measured in a full scale disc cutting experiment. The thesis outlines the essential features of the elastic stress field of indexed indenters. Laboratory investigations have verified the existence of subsurface cracks predicted by stress fields. The evolution of such cracks is qualitatively understood. The tensile side chipping model was tested with satisfactory results. In disc cutting practice the process of groove formation should be separated from side chipping. This will result in less formation of unnecessary long cracks, lower energy consumption, better utilization of available torque and thrust, and decreasing vibrations. Future development of cutters should take advantage of long subsurface cracks, possibly resulting in a dramatic increase in capacity.