Cracks often initiate from grain boundaries, pores, or pre-existing cracks and then coalesce, a process which can lead to system-size failure. Analysing how fracture networks develop from pre-existing isolated cracks may provide knowledge of the preparation process leading to dynamic rupture. Recent observations suggest that cracks initiate and propagate from the tip of a pre-existing crack (on-fault) into intact rock (off-fault) and may link with another crack, leading to a phenomenon known as coalescence. We reproduced this process experimentally in granite core samples that contain pre-existing notches. Here, we characterize the microphysical process of crack growth in two Westerly granite cylindrical samples (8 mm diameter, 20 mm height) that contains two notches oriented at 300 with respect to the axial direction of the cylinder, and the first principal stress direction. Triaxial compression experiments are performed at room temperature and constant confining pressure of 20 MPa. We image microfracture development using dynamic in-situ X-ray tomography on beamline ID19 at the European Synchrotron Radiation Facility. During compressive loading at increasing steps of differential stress, we acquired a series of tomograms of the samples. We segment the cracks from the host rock using a global thresholding technique, and then quantify the statistical properties of the cracks, including their spatial distribution before the rock approaches failure. Results show that cracks nucleate and then propagate away from the notch in a direction parallel to the maximum principal compressive stress. They grow and interact with each other. As new cracks nucleate, they coalesce and form a shear fault oriented at 300, parallel to the initial notches. Most of the damage that develops during the growth of a fault to system size failure is due to the nucleation and coalescence of tensile cracks that might emanate from the tip of the pre-existing notches. These results show the dominance of dilatant deformation before system size failure, which has implications for understanding the earthquake preparation process in the crust.
Figure 1. a) Volume rendering of Westerly granite with pre-existing notches before failure. Each individual segmented cracks are shown with a different color. The red dashed line shows the volume where cropped volume of cracks is computed. b) Evolution of the damage index as a function of the distance to failure for the two experiments. The yield point, computed from the mechanical data, is indicated by a pink triangle.
