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

By means of the Scanning Electron Microscope (SEM), an examination was perf ormed of the fracture surfaces (including their vertical sections) of both Fangshan gabbro and Fangshan marble specimens fractured at the loading rate s 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 specime ns were clear and the cracks increased with increasing loading rates. Howev er, such branching cracks were rarely seen near the static fracture surface s. In addition. with the aid of the Split Hopkinson Pressure Bar (SHPB) tes ting system and a high-speed framing camera, the energy partitioning in the dynamic fracture process of a short rod (SR) rock specimen was analysed qu antitatively. The total energy W-L absorbed by an SR specimen in the dynami c 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 co uld be quantitatively calculated through stress wave measurement and high-s peed 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 lo ading rate; (2) the energy W-FD for dynamic rock fracture was markedly grea ter than that for static rock fracture, and the W-FD increased with an incr ease in the impact speed of the striker bar or the loading rate; and (3) th e value W-L/W-B (W-B is the energy input into the loading system) in the ca se of dynamic fracture is much lower than that in the case of static fractu re. In addition, the ratio decreases with an increase in the loading rate o r the impact speed of the striker bar. This means that the energy utilisati on decreases when the loading rate or the impact speed of the striker bar r ises. Finally, some application problems are discussed in the paper. (C) 20 00 Elsevier Science Ltd. All rights reserved.