Role of grain boundaries and micro-defects on the mechanical response of a crystalline rock at multiscale

The paper examines the cracks initiation, propagation and coalescence until failure in a crystalline rock under stress states involving rock volumes varying in size, from nano (Berkovich indentation in Continuous Stiffness Measurement mode) to micro (Vickers indentation, scratching) and macro (Rockwell indentation, Rock Impact Hardness Number) scales. The experimental work was performed on a calcitic marble characterized by two different textures (xenoblastic and granoblastic). The grain boundaries microcracks were deeply studied with Focused Ion Beam-Scanning Electron Microscopy technique, innovative approach in Rock Mechanics. The experimental results show that, at the three investigation scales, the microcracks give rise to the rock failure with fractures propagation mechanisms varying with the microstructure. In the granoblastic texture, fracturing is ruled by grain boundaries microcracks, which often appear almost open. Conversely, the fracture behavior of the xenoblastic marble seems dominated by the grains cleavage, even at the macroscale.