Fragmentation of rocks under ultrasonic loading

A total of 25 granites, 20 greenstones, and three norite nominal 1-in. cores were cut into approximately 10-in. lengths which were glued to the amplifying horn of a 10-kHz transducers. Each specimen was subjected to a maximum load of a few hundred watts of ultrasound. Tests were terminated at fracture, at pronounced detuning accompanied by abrupt changes in the strain and load power signals, or after approximately five minutes if the specimen did not fail. The generation of ultrasonic resonances in the rocks was shown to lead to local stress exceeding their tensile strength and hence has caused failure at kerfing energies which are significantly lower than by several other techniques for fracturing rock. Cyclic loading under high-frequency ultrasound has produced significant weakening of rock whereby the compressive strength has been decreased by up to 70%. Fractures occur preferentially along ore veins at low input power. In samples of softer rock, large temperature increases are induced while harder rock undergoes small or no temperature change.<>