High speed impact test on rock specimens with a compressive stress pulse

In exploration for a methodology to analyze fast deformation of a thick specimen where a state of equilibrium cannot be assumed, high-speed impact tests on pumice-stone and granite sample specimens are conducted with Hopkinson bar test apparatus. A compressive stress pulse from the input bar is applied to the specimens ranging from 8 mm to 30 mm in thickness. The stresses on the front (input-bar) side and the rear (output-bar) side of the specimen are evaluated separately based on the strain gauge signals from the input and output bars. Optical emission from the specimen is captured with a photomultiplier. Analyses on the time variation of the strain gauge and photomultiplier signals lead to the following observations. (a) The front-side stress plays an important role in the deformation and fracture, (b) the transition from the elastic to plastic regime can be estimated from oscillatory features observed in the front-side stress and other quantities derived from the strain gauge signals, (c) the optical emission is highly correlated with the front side stress and step-wise energy loss in the specimen, and (d) the main frequency components of the stress wave associated with the energy-loss are found in the same frequency band as conventional acoustic emissions observed in rock fractures. A question remains, and provides an interesting subject for future study, regarding the step-wise energy loss observed in the strain gauge signal unaccompanied by the optical emission. In view of a recent fracture mechanical theory that states the evolution of fracture is accompanied by a reduction in frequency on the emission spectrum, it is possible that these energy loss is associated with emission in frequency ranges outside the sensitivity of the photomultiplier.