Fracture Mechanics Characteristics Of Granite After Natural Cooling At High Temperature

In recent years,the development and utilization of green energy has become a major issue of human society due to the increasing depletion of traditional fossil fuels and the serious environmental pollution caused by burning fossil fuels.Hot dry rock geothermal resource has been widely concerned because of its huge resources,green clean and renewable.There are abundant hot dry rock geothermal resource in China and they are wroth to extraction.Based on digital image correlation?DIC?method and three-point bending experiment,the fracture characteristics of granite mode I crack after natural cooling at high temperature?100?,200?,900??are studied.The results are as follows:1.The load-displacement curves tend to be gentle with the increase of temperature,indicating that the higher the temperature,the greater the thermal damage of the granite and the transition from brittle failure to ductility.2.The K_?CC of granite is higher after treatment at 100?than that at normal temperature,indicating that the fracture resistance of granite is enhanced after treatment at 100?.After100?,the K_?Cof granite decreases with the increase of temperature and breaks at500?-600?.It is consistent with the micro-mechanism of great changes in the internal structure of granite and micro-cracks caused by thermal cracking.3.The surface displacement field of specimens at normal temperature was measured by DIC method.It shows that the development of crack experiences three stages,including initiation?stable propagation and instability propagation by analyzing the CTOD-CMOD curves.The initiation point was near 80%P_maxax before the peak load,and the instability propagation of the crack was near the peak load.4.The position of neutral layer was found by analyzing the fitting straight lines of the X-direction displacements curves of different sections.It is found that the position of the neutral layer moves towards the compression zone during the process of granite fracture.5.The extension law of granite fracture process zone?FPZ?is obtained by analyzing the X-direction displacement curves of different horizontal sections on the surface of normal temperature samples.It shows that the expansion speed of the tip of FPZ increases with the application of load.It reaches its maximum value within the peak load to 90%P_maxax after peak load,and then it slows down.6.The length of FPZ increases with the increase of the load.It reaches the maximum value near the peak load,and then decreases.The phenomenon indicates that the expansion speed of the macro crack is greater than the expansion of the FPZ after the peak load.It is obtained that the length of the FPZ increases linearly with the increase of temperature by analyzing the length of the FPZ under the peak load state of granite.