Thermal effect on rock strength: strengthening-weakening transition explored by grain-based model

Rock strength typically decreases when the rock is subjected to temperatures higher than 400 degrees C. Our previous experimental study on Sichuan Marble showed that the rock was strengthened rather than weakened in the temperature range 25-200 degrees C. To numerically model the thermal strengthening/weakening behavior of rock, a grain-based model integrating the thermal effects on grains and grain boundaries in an applicable temperature range 25-400 degrees C (thermo-GBM) is proposed. A Voronoi-tessellated thermo-GBM (GBM-1) is calibrated and verified by our experimental triaxial compressive tests equipped with real-time heating from 25 to 200 degrees C. The same tests are simulated on another thermo-GBM with realistic rock microstructure (GBM-2) to investigate the effects of microstructural heterogeneity. The results suggest that increasing microstructural heterogeneity leads to a higher extent of thermal strengthening. Such an effect is amplified by the application of confining pressure, but it is less dependent on the treated temperature. Then, both uniaxial compression strength (UCS) and confined strength tests are simulated on GBM-1 in a higher temperature range 250-400 degrees C to examine the strengthening phenomenon. Thermal strengthening takes place in all the confined test groups but not in the UCS test group. The thermal strength is enhanced by 8.1% at 250 degrees C as compared with that at room temperature, and decreases to the non-heated value at 350 degrees C. The results suggest that thermal expansion leading to a more compacted structure is the governing mechanism in strengthening effect, meanwhile the degradation of grain-boundary properties is a major competing factor to weaken the rock.