A surface instability apparatus (SIA) was used to reproduce 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.
Two types of clustering analysis, spatial clustering through the fractal dimension and hierarchical clustering through a dendrogram construction, were performed to analyze AE locations. Results showed an increasing FD as the damage level developed. The fractal dimension was close to 2.0 at lower load ratios, representing a planar distribution of samples. Then FD increased to a value of 2.4 to 2.5 in the post-peak stage, meaning a more scattered distribution in space. The hierarchical clustering was applied to low signal to noise ratio events provided "super AE" locations, a group of events with similar wave signals. These locations 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 computed by DIC were through the comparison of two digital sub-images (subsets) with a unique white-light speckle pattern, and strain fields can be estimated from the displacement information.
A continuum damage mechanics model was used to describe the damage progression in surface instability test. The continuity parameter serves as a state variable representing the evolution of damage, which is linked to the inelastic total lateral strain computed from the displacement fields using DIC. The critical value of inelastic total lateral strain was selected at the moment of spalling being captured by the digital imaging system. For the two Berea sandstone and one Serena sandstone specimens, consistent values were observed. This observation implies that the critical inelastic lateral strain may provide an indicative metric for determining the moment of surface instability.