Quantification of dynamic damage and breakage in granite under confined indentation

Understanding of indentation rock damage and breakage under confining stress is necessary for underground engineering applications, as both loading rate and in-situ stress can affect the failure behaviour and mechanism. A triaxial apparatus capable of applying both static lateral confinement and dynamic loading is utilised to experimentally investigate the influence of both loading rate and confinement on rock indentation failure. The force-displacement relationships and energy evolution were determined for different dynamic loading and confinement conditions, while digital reconstruction of indented rock samples was achieved for crater depth and damage volume measurements. The failure mechanisms at microscale are studied using the optical microscopy and Synchrotron-based X-ray Computed Tomography (CT). The relationship between the incident energy and indentation force is correlated with the crater depth and volume of rock removed. The crater depth increases linearly with increasing force and incident energy, while the volume increases exponentially. The biaxial confinement controls the propagation of radial cracks, which propagate in the direction of major confining stress, while impact velocity dominates the crater depth and lateral crack formation beneath the crater. The formation of chipping during indentation is reflected on both the force-displacement relationships and X-ray CT measurements. Different minerals within the rock also affect the propagation and coalescence of cracks, harder mineral such as quartz has better resistance to crushing, whereas biotite deflects and terminates cracks.