| Summary: | Knowing correct values of the rock mechanical properties is crucial for many engineering applications in subsurface. Rocks may show two failure modes during cutting: ductile and brittle. In the ductile mode, rock deforms plastically, and the debris is powdered ahead of the cutting face. On the other hand, chips are the major cutting characteristics for the brittle failure during rock cutting. The critical depth of cut represents the transition point between these two models, so knowing this value helps better predict the failure mechanism of rock. In this paper, a new method is introduced based on measuring the roughness of the groove for determining the transition point of failure modes for every rock sample after the scratch test. The graph depicting the average change in the surface roughness (<inline-formula><math display="inline"><semantics><mrow><msub><mi>R</mi><mi>t</mi></msub></mrow></semantics></math></inline-formula>) versus the scratched surface roughness (<inline-formula><math display="inline"><semantics><mrow><mi mathvariant="sans-serif">Δ</mi><mi>R</mi></mrow></semantics></math></inline-formula>) can be used to identify the rock failure mode and determine the transition point for the cutting process. The value of this slope increases until the depth of cut reaches the transition point, and then the slope reaches a constant value. The main purpose of this paper is to estimate the critical depth of cut of different rock specimens employing the new surface roughness model.
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