Comparison of Visual and Acoustic Emission Observations in a Four Point Bending Experiment on Barre Granite

We present an experimental study in which a pre-notched specimen of Barre Granite was subjected to four point bending under crack mouth opening displacement control. The experimental observations consisted of load–displacement measurements, acoustic emissions, and photography on a macroscopic (~cm) as well as microscopic (~μm) scale. These observations were compared and analysed to better understand process zone development and crack propagation. Load–displacement data showed that the load reaches its maximum at crack initiation, and the machine input work is constant while the crack propagates. AE moment magnitudes between M_w = −6 to −10 were observed, and focal mechanisms consisted of both shear and tensile components. During process zone development, AE formed a large cloud of events located near the notch tip and then tended to occur away from the notch tip as the crack propagated. Image analysis at the microscopic scale showed that microcracks formed and coalesced during process zone development; specifically, the microcracks initiated in tension and then propagated as a series of en-echelon cracks. In general, the synthesis of the three observations showed that a wider bulb of activity at lower energy tended to occur during process zone development, while crack propagation tended to be more spatially concentrated and contained higher energy.