The drive to mechanize the excavation of hard rock in civil or mining engineering projects has brought to the forefront the need to quantify the mechanics of tool-rock interaction. A related issue is the interpretation of the scratch test. This experimental technique has garnered interest lately, because it appears to offer a simple and economical means to measure strength and fracture properties of quasi-brittle materials such as rock, ceramics, and concrete. However, there remain questions on which parameters can be determined in these experiments and how to interpret them from the test data. Therefore, whether predicting the cutting force (mean and fluctuations) to design mechanical excavation equipment and establish the excavation schedule and costs, or interpreting the measured force in controlled scratch experiments, the same fundamental question arises: what is the dependence of the cutting force (i) on the depth of cut and the cutter geometry, and (ii) on the strength and fracture properties of the rock. By recognizing that different regimes of failure take place in rock cutting and therefore that different parameters affect the cutting force depending on the prevailing regime, the research has the potential to develop universal scaling laws for rock cutting. Furthermore, this research has also the potential of informing our understanding of the failure of engineered materials with a grain structure, such as concrete or ceramics. Failure of structures made of these quasi-brittle materials is characterized by scale effects and possible transition between shear- and tensile-dominant failure mechanisms. However, engineering design practice is still rooted in the strength-based approach, despite its restricted range of applicability when dealing with quasi-brittle materials. If successful, the research will result in a simple experimental means for characterization of fracture properties, thus promoting fracture mechanics -- an essential concept for such materials -- to engineering practice. The findings of this research will be incorporated into classroom teaching at both undergraduate and graduate levels, in an effort to equip the next generation of engineers with fundamental knowledge of rock mechanics, fracture mechanics and experimental mechanics.
The project addresses the fundamental question of energy scaling in rock cutting, i.e., the dependence of the specific energy - the energy spent per volume of rock removed, on the depth of cut and on the rock properties. Understanding how the specific energy scales (or alternatively how the cutting force scales) is critical for designing mechanical excavators for hard rocks, as well as for interpreting the scratch test - a technique to assess the properties of quasi-brittle materials. The research is grounded on empirical evidence that suggests the existence of three asymptotic regimes in rock cutting. With increasing depth of cut (larger than the rock mean grain size), different modes of failure can indeed be observed: namely, (i) a ductile regime at shallow depth of cut, with the rock intensely sheared in front of the cutter; (ii) a fragmentation regime with the rock breaking into fragments that are distributed according to a power law over a significant range of sizes, and (iii) brittle regime with macroscopic cracks initiating from the tool tip and propagating unstably ahead of the cutter. The research builds on three respective empirical scaling laws for these regimes, which are informed by the available experimental data and insights from numerical simulation and scaling analysis, for the case where the cutter is at least as wide as the rock slab thickness. The main objective of this research is to provide theoretical underpinnings to these empirical scaling laws using theoretical and computational models, as well as to obtain experimental results for the brittle regime, for which the empirical evidence is weakest.
