Effect of cooling methods on mechanical behaviors and thermal damage distributions of granite: Experiments and simulations

This work investigated the fundamental reason for the deterioration of mechanical properties of granite after two different heat treatments (slow heating followed by water or air cooling). The responses of mechanical and acoustic properties of the granite after two different heat treatments were obtained by a servo testing machine, a Digital Image Correlation technology (DIC), and an acoustic emission (AE) system. Meanwhile, the heat transfer laws and thermal damage distributions of granite under different cooling methods were discussed by using COMSOL software. The results show that the tensile strength of the water-cooled specimen is lower than that of the air-cooled. The AE counts of the water-cooled specimen are more intensive than that of the air-cooled during the compaction stage. With increasing initial temperature, the accumulated AE counts of the air-cooled specimen increase, but the accumulated AE counts of the water-cooled specimen first increase and then decrease. Meanwhile, the cooling methods have a great influence on the distribution of tensile stresses and thermal damage of the specimens. The distribution of tensile stress and thermal damage of air-cooled specimens is random, which is mainly controlled by the heterogeneity of the thermal expansion coefficient. However, the tensile stresses and thermal damage of water-cooled specimens are distributed on the surface of the specimen, which is mainly controlled by the temperature gradient. Meanwhile, water cooling has a more severe thermal damage on specimens than air cooling. The difference in the convective heat transfer coefficient between rocks and cooling mediums determine heat transfer process. Results indicate that the selection of work fluid with a higher convective heat transfer coefficient is of great significance for improving the efficiency of geothermal energy extraction.