Surface Spalling Experiments with Damage Observations

A surface instability apparatus was used to produce spalling in a laboratory setting, and damage in the rock was monitored by acoustic emission (AE) and digital image correlation (DIC) techniques. Lateral displacement served as the feedback signal to control the post-peak response with a closed-loop, servo-hydraulic load frame. A clustering analysis using the concept of a hierarchical dendrogram applied to low signal to noise ratio events provided "super AE" locations that matched the crack trajectories. DIC was used to investigate incremental displacement fields during surface spalling. Real-time images were successfully captured under high stress levels through modification of the device. Displacement fields were computed in the early stage of loading and around peak stress. Young’s modulus was reasonably estimated with the axial strain measured by DIC. Fracture from spalling phenomena was revealed by contours of incremental horizontal displacement around peak stress. A concentration of deformation leading to fracture was identified, as was a region of relaxation behind the damage zone.