Crack Evolution in Damage Stress Thresholds in Different Minerals of Granite Rock

Crack evolution in a rock depends on the mineralogy, microstructure and fabric of specific rock type. This study aims to investigate how mineralogy and grain shape affect the microcrack initiation and propagation of granite rock, which contains plagioclase, quartz, k-feldspar, biotite and amphibole, during uniaxial compression loading. Physico-mechanical properties and microcrack features such as linear microcrack density (LMD) and microcrack type were investigated. By acoustic emission (AE) measurements, damage stress thresholds were identified. Then, crack characteristics of a fresh sample were compared with the samples that were loaded until damage threshold stresses. The results demonstrate that at an early stage of loading, pre-existing microcracks growth and LMD of intergranular crack increase. In the elastic phase, all microcracks types increase at the same rate. When the loading reaches to the strength limit of the sample, the total LMD reduced, because cracks start to coalesce and form new and large transgranular microcrack. Investigating crack propagation in mineral shows that at first, microcrack generates in biotite and at last in quartz. Plagioclase has the highest LMD and microcracks usually formed within the cleavage plane, but alteration and inclusions of tiny minerals can drastically change the LMD and orientation of microcracks. Biotite can terminate or let the microcrack to pass through the crystal based on the orientation of the microcrack plane and cleavage microcrack within the mineral. Furthermore, crystals with an aspect ratio higher than two have higher LMD. By getting close to the uniaxial compression strength, more microcracks appear close to the grain boundary which increases the circularity of the grain.