Fracture Coalescence Process Between Two Pre-existing Flaws in Granite Based on Coupling Exterior and Interior Observation Techniques

Though the exterior cracking process of flawed rocks has been studied intensively, the exterior and interior coalescence process, especially from the aspect of micro-crack development, has not been systematically studied. In the present study, the acoustic emission (AE) techniques and digital image correlation (DIC) are applied together to monitor the damage process of double-flawed granite samples under uniaxial compression. Multiple AE signal characteristics, including cumulative event, ring count, waveform frequency, initial frequency (IF), and rise angle (RA) are utilized to analyze the evolution of the coalescence properties before and after the initiation of macro-cracks. The experimental result indicates that the exterior and interior fractures in microscopic and macroscopic scales can be identified from the combination of AE parameters and DIC results. In the early stage of loading, most initiated micro-cracks are attributed to tensile mode based on the analysis of IF and RA values, while the strain concentration cannot be conspicuously observed by the DIC technique. Then, the micro-crack mode changes from tensile to shear or mixed mode with the reduction of the average IF value. With the initiation of macro-cracks, the RA value increases accordingly, and the deformation around the crack pattern can be obviously recognized by the DIC strain field. The property of the coalescence strongly depends on the flaw geometries. When increasing the bridging angle β, the crack mode of the coalescence varies from shear to mixed tensile–shear or pure tensile (β ≤ 90°) and to a more complex mode (β > 90°).