Characterisation of microcracks in the Bohus granite, western Sweden, caused by uniaxial cyclic loading

Microcracking is highly dependent on the mineralogy, fabric, and microstructure of a given rock type. In this study, we investigate how crack initiation and propagation occurs during uniaxial cyclic loading. The mechanical test was performed on drill cores from an isotropic granite. This granite has a well-developed existing microcrack pattern. A sample that was intact after 35,000 loading cycles of a maximum loading of 122 MPa was used for the microcrack analyses. In order to characterise the microcracks, the tested material was vacuum impregnated with epoxy resin containing fluorescent dye. Using fluorescent and polarised microscopy together with digital image analysis, the length and orientation of different crack types were measured. The results demonstrate that cyclic loading caused new cracks to form and caused extension of existing microcracks. The orientation of the major mode of the existing microcracks correlates with the orientation of a well-developed horizontal joint set. The new cracks occur mainly within feldspar and are oriented parallel to the drill-core axis. However, if the feldspars are altered, cracks are less abundant, because of the disturbance of the crystal lattice. Micas and opaque minerals can improve crack propagation because of the difference in mechanical strength and E-modulus compared to the main rock-forming minerals.