Spalling-caused rock microfracture and energy partitioning

Tensile-wave-caused spalling is a common phenomenon in brittle materials like rock and concrete. In this study rock spalling tests were conducted using a split Hopkinson pressure bar (SHPB). During the tests a high-speed camera recorded the process of spalling, and the stress waves in the incident bar were measured; after the tests, the fracture surfaces of selected fragments were continuously examined along a diameter of rock specimen by scanning electronic microscope (SEM). By analysing the videos recorded and the stress waves measured during the tests, the energy components were determined. The results show that: (1) rock specimens were all spalled into 2 to 8 cylindrical fragments. (2) Spalled fragments of each rock specimen flew at different velocities dependent on the spalling sequence; the measured maximum velocity of spalled fragment was 26.6 m/s. (3) The translational kinetic energy of spalled fragments varied from 37 % to 70 % of the input energy (the energy of the incident compressive wave), with an average of 49 %. (4) The measured rotational kinetic energy of spalled fragments varied from 0 % to 1.5 % of the input energy, meaning that the rotational energy is ignorable, compared with the translational kinetic energy. (5) The energy of reflected wave varied from 28 % to 59 % of the input energy, with an average of 46 %. (6) The energy used to create fracture surface area varied from 2 % to 17 % of the input energy, with an average of 5 %. (7) On average, the smooth surface, rough surface, and tearing surface occupied approximately 89 %, 7 %, and 4 % of the totally scanned surface area at a magnification of 1000x, respectively. (8) The average nominal specific fracture energy of the gabbro was 268.4 J/m², and the nominal specific fracture energy of the black schist was 244.8 J/m².