It has been established that there exist two distinct material removal mechanisms during rock scratching, ductile and brittle, whose occurrence depends on the magnitude of depth of cut. At shallow depth of cut, ductile-mode cutting dominates and the cutting force is associated with uniaxial compressive strength; at larger depth of cut, brittle failure is dominant and the fracture toughness control the force responses. This study aims at assessing the strength within the ductile-mode cutting and the fracture toughness within brittle failure via rock scratching tests. First, we apply Bažant’s Size Effect Law (SEL) to identify the critical ductile-to-brittle transition depth. Then we employ experimental data for ductile mode and brittle mode to determine strength and fracture properties respectively. The good agreement between experimental results and reported values indicates the potential of scratch test as a new technique to characterise rock strength and fracture toughness simultaneously.
The uniaxial compressive strength (UCS) and fracture toughness of rocks are the most common parameters used in civil, mining and petroleum engineering, such as the design of underground structures and stability assessment of rock excavations. The main challenge lies in the fact that conventional measurements of rock strength and fracture properties normally requires intact rock cores of high quality, sophisticated setups or complex sample preparations. In addition, the values determined from experiments are, to some degree, influenced by the size of the specimens. All of these undesirable issues give rise to uncertainty in engineering practice.
Recent advances in understanding the mechanism of rock scratch provide an encouraging alternative to the determination of the rock strength and fracture toughness simultaneously. During rock scratching, there is a critical depth dc beyond which rock failure shifts from ductile to brittle regime (Richard et al. 2012 and He et al. 2015). When the depth of cut is small, the rock failure is a strength-driven process with energy dissipated within the failed material in plastic flow (Figure 1a) and the average cutting force is proportional to the depth of cut (see the black solid line in Figure 1c). On the other hand, in deep cutting, in which the rock failure is dominated by the brittle regime with energy dissipated in creating macroscopic discontinuous cracking surfaces ahead of the cutting tool (Figure 1b), the cutting force deviates from the linear proportionality and exhibits a scaling relationship with the square root of the depth of cut (see the red solid curve in Figure 1c), following the theory of linear elastic fracture mechanics (LEFM). Therefore, a critical threshold of depth of cut dc exists that separates the two distinct failure regimes.