Study On Fracture Model And Fracture Forecast Of Lu Gray Granite After High Temperature

In the exploitation of geothermal resources,the temperature of geothermal reservoir decreases gradually with the exploitation of geothermal energy.There are two ways included about the cut down of temperature:slow reduction and water cooling.When cooled the mechanical property of rock changes,which will affect the development of rock mass engineering at high temperature.Therefore,it is of great significance to study characteristics of fracture failure about rock after natural cooling and water cooling at high temperatureIn this paper,the Brazilian splitting and three points bending experiments were carried out on the samples of Lu gray granite after experiencing different temperatures(room temperature-900?)and cooling methods(water cooling and natural cooling).The effects of temperature and cooling mode on the mechanical properties and fracture characteristics of granite were studied.Based on the experimental results,which established fracture model of granite after high temperature,the type I fracture failure of granite after high temperature was predicted.The achievements are as follows(1)The particle observation experiment of granite at room temperature was carried out using LEICA DM4M microscope.The experiment found that the average particle size of Lu gray granite mineral particles is about 3mm(2)The tensile strength of granite was measured at different temperature segments by Brazilian splitting experiment.The tensile strength of granite shows a decreasing trend with the increase of temperature,decreases rapidly about 500? to 600?,and declines slowly after 600?.The tensile strength of natural cooling samples at different temperature segments is greater than that of water cooling samples.Through digital speckle technique,it was observed that along the line direction of two loading points,the failure front of the splitting test sample appeared the large strain area that throughout all samples,where the macroscopic failure crack of the sample was born.The shape was similar.Deterioration of mechanical property of granite under water cooling(thermal shock)is higher than that under natural cooling(thermal stress).The tensile strength and fracture toughness of the granite after water cooling at the same temperature is smaller compared with that after natural cooling.(3)Based on the experimental results,bilinear softening curves of Lu gray granite were obtained.The curve reflects the relation between the cohesive stress and the crack opening displacement in the process of granite fracture.The abscissa wq of turning point of the softening curve is between 0.006?0.009mm,which corresponds to the critical state of instability failure.The critical crack opening displacement w is between 0.05?0.07mm,which corresponds to the end of micro crack area.(4)Based on the model of boundary effect,the coefficient of dispersion of granite ? at normal temperature was determined to be around 2.0.By comparing the results of three point bending tests of different rocks by other scholars,the rationality of the number of Lu gray granite was verified.The method of determining ? that through the tensile strength and fracture toughness of granite greatly reduces the workload of the experiment.The model of boundary effect of Lu gray granite was established after high temperature according to the material parameters measured in the experiment.The virtual crack expansion ?afic of granite increased at 100? and the whole decreased after 100?though appeared fluctuations.The discreteness of ?afic affected by thermal shock on samples of water cooling was bigger than that of natural cooling.(5)In view of the influence of temperature on the mechanical property of Lu gray granite,the author mentioned that the temperature coefficient aT of granite after high temperature should be increased in the formula of virtual fracture growth.The temperature coefficient aT increases first and then decreases as the temperature increases.The range of granite under natural cooling aT is between 0.9 and 1.6,while that of Lu gray granite under water cooling is between 0.9 and 2.1.Based on the model of modified boundary effect,the minimum size Wmin of granite at various temperature section satisfying linear elastic fracture mechanics was determined.When the size of samples is larger than Wmin,the fracture failure of rock satisfies the fracture toughness criterion and adopt the fracture toughness criterion to evaluate and analyze the fracture of rock.