An experimental study of the fragmentation process in rock cutting.

Abstract

Previous studies of rock cutting have established the existence of two modes of rock fragmentation in cutting: (i) a ductile regime, at depth of cut smaller than a critical value, characterized by a constant specific energy (energy per unit volume required for fragmentation); (ii) a brittle regime, at depth of cut larger than a critical value, characterized by a specific energy decreasing with increasing depth of cut. The critical depth of cut is related to the intrinsic length scale deduced from the rock toughness and compressive strength. In regards to the brittle regime, a few general questions arise. How does the specific energy scale with depth of cut? Which parameters control the specific energy? Which mechanical rock properties can we extract from cutting tests? These questions are addressed via a combined experimental study of the particlesize distribution of the excavated material and of the cutting force measurements. The study suggests that the force signals of homogeneous rocks can be characterized through a statistical and spectral analysis, and that the two failure modes present different signatures that can be observed in both statistical and spectral parameters. The results of the particle-size distribution of Tuffeau limestone show a power law dependence of the number of particles and the particle size, suggesting that a fractal model can be applied for some rocks. In particular, the results imply that the energy scales by the volume of the fragments at small depths of cut, but by the total surface area of the fragments at large depths of cut. Also, by assuming a fractal model for the particle-size distribution of fragments, the values of the specific energy at large depths of cut can be predicted.