Effects of high temperatures on dynamic rock fracture

The dynamic fracture toughness of Fangshan gabbro and Fangshan marble subje cted to high temperature was measured by means of the split Hopkinson press ure bar (SHPB) system. The specimens for measuring the fracture toughness w ere manufactured according to the requirements for the Short Rod (SR) speci men suggested by ISRM. Two cases were investigated: (1) the SR specimens of the gabbro and marble were fractured at high temperature (100-330 degreesC ), and (2) the specimens of the rocks were first pre-heat-treated at 200 de greesC for the marble and 600 degreesC for the gabbro, and then fractured a t room temperature. The experimental results showed that under dynamic load ing the fracture toughness of both the gabbro and the marble tested in the above-mentioned cases increased with increasing loading rates. The relation ship between the fracture toughness and the loading rates in the two cases is similar to that obtained in the room temperature environment, i.e., with out high temperature. (This is defined as the third case.) It can be conclu ded that temperature variation affects the dynamic fracture toughness of th e two rocks to a limited extent within the temperature ranges tested. This is different from the results obtained under the static loading condition. Furthermore, by means of the scanning electronic microscope (SEM), the vert ical sections of the fracture surfaces for some gabbro specimens were exami ned. In addition, the fractal dimensions of the fracture surfaces of some s pecimens were measured by means of fractal geometry. The results showed tha t under dynamic loading: (1) macro-crack branching near the fracture surfac es was universal; (2) the fractal dimensions increased with increasing load ing rates; (3) in the sections of the specimens tested at high temperature there were many micro-cracks that were probably induced by thermal cracking . On the basis of the above macro- and micro-experimental investigation, an energy analysis of the process of dynamic rock fracture was performed. The results showed that the energy utilisation in dynamic fracture was much lo wer than that in static fracture. (C) 2001 Elsevier Science Ltd. All rights reserved.