Acoustic, Electromagnetic, and Photon Emission from Dynamically Fracturing Granite

Abstract

Laboratory samples of Westerly granite were impacted by a falling striker, and the time series of acoustic and electromagnetic emission (AE and EME) from fracturing rock were recorded with the time resolution of 10 ns. In order to determine precisely the actual instants of beginning and termination of fracturing, the fractoluminescence (FL) signal related to chemical bond ruptures on the sample surface was recorded in parallel to both AE and EME signals. Energy distributions in all three series were characterized by the function N(E > E′), where N is the number of emitted signals characterized with the energy release E exceeding a threshold value E′. The fracture dynamics was studied using both the scaling analysis and the non-extensive Tsallis statistics. The energy distributions were approximated with both the empirical Gutenberg–Richter-type relation N(E > E′) ∝ E′−b, and the analytical function Nq(E > E′) that includes the Tsallis’ parameter of non-extensivity, q, and the released energy density. A comparative consideration of information taken from the data of EME, AE and FL emission techniques was carried out in the context of the physical sense of the parameters b and q. The AE time series reflected adequately the cracking process both at microscopic and laboratory scale levels, while the photon emission did not show a direct response on the split-off formation. Time resolution of the EME method was found to be insufficient for studying the dynamic fracture of the given kind.