Rock Failure Energies and Failure Processes

Carving rock by indenting sharp objects into the surface under an impact force has been a favored method of fragmentation for millenia . However, as rock gets harder the lifetime of these sharp tools becomes short and, in a compromise hardened bits of rounder contour have become more common, particularly when driven by mechanical power. To better understand the behavior of these newer generation tools, and thus to ultimately enhance them and better predict their performance, a study is under way to define the penetration and failure mechanisms of polycrystalline diamond tools into a suite of hard, mainly igneous, rocks. The initial part of the study has shown that a relationship exists between the energy imparted through the tool and the amount of penetration achieved into the rock. The relationship is consistent in the same rock type, whether the energy is imparted dynamically or quasi-statically. The differences from rock type to rock type appear to be useful in quantifying the effect of rock type on cuttability. After testing, mechanisms of rock failure under the tool are then evaluated by considering the roles of overall confinement, development of the crushed zone, and the fracture system around the impact point. Based on this information the paper discusses the potential development of a standard of toughness for rock, and its role in quantifying machine performance.