Effects of loading rate on rock fracture: fracture characteristics and energy partitioning

By means of the Scanning Electron Microscope (SEM), an examination was performed of the fracture surfaces (including their vertical sections) of both Fangshan gabbro and Fangshan marble specimens fractured at the loading rates k = 10(-2)similar to 10(6) MPa m(1/2) s(-1). The results showed that one or more branching cracks near the fracture surfaces of dynamic rock specimens were clear and the cracks increased with increasing loading rates. However, such branching cracks were rarely seen near the static fracture surfaces. In addition. with the aid of the Split Hopkinson Pressure Bar (SHPB) testing system and a high-speed framing camera, the energy partitioning in the dynamic fracture process of a short rod (SR) rock specimen was analysed quantitatively. The total energy W-L absorbed by an SR specimen in the dynamic fracture process mainly consisted of the fracture and damage energy W-FD and the kinetic energy W-K of flying fragments. The energies W-L and W-K could be quantitatively calculated through stress wave measurement and high-speed photography in the SHPB testing system. Thus, the fracture and damage energy W-FD could be obtained. The results showed that: (1) the energy W-K increased with an increase in the impact speed of the striker bar or the loading rate; (2) the energy W-FD for dynamic rock fracture was markedly greater than that for static rock fracture, and the W-FD increased with an increase in the impact speed of the striker bar or the loading rate; and (3) the value W-L/W-B (W-B is the energy input into the loading system) in the case of dynamic fracture is much lower than that in the case of static fracture. In addition, the ratio decreases with an increase in the loading rate or the impact speed of the striker bar. This means that the energy utilisation decreases when the loading rate or the impact speed of the striker bar rises. Finally, some application problems are discussed in the paper. (C) 2000 Elsevier Science Ltd. All rights reserved.