Experimental Study On Fracture Mechanical Behavior And Meso-failure Characteristics Of Rocks After Temperature Cycling

The high temperature rock fracture problems involved in geothermal resource development and nuclear waste geological disposal projects.In this paper,granite and sandstone samples are selected as research objects.Experimentally,the variation of mode-I mode-II and mixed-mode(I+II)fracture toughness of granite after different thermal loading paths was studied.SEM,XRD,micro-slice analysis,ultrasonic detection and permeability test were used to study the mesostructure,failure mechanism,fracture morphology and crack propagation of granite before and after thermal damage.The mainly research contents and results are as follows.The effects of different temperatures on the macroscopic fracture behavior and meso-damage characteristics of granite were studied.The results show that the fracture toughness decreases rapidly with increasing temperature at 20?400?,and the rate of decline decreases at 400?600?.At lower temperatures(20?,100?),fine-grained dense granites are mainly transgranular and intergranular fractures.At moderate temperatures(200?,300?),mainly intergranular fracture.When the high temperature is reached(above 400?),it is mainly transgranular,intergranular and their mutual coupling fracture mechanism.At 20? and 100?,a small amount of cracks appear on the quartz mineral particles.A small amount of cracks in feldspar particles at 200?.As the temperature increases,the number and size of cracks on the quartz and feldspar particles increase.The cracks in the mineral particles and the cracks along the particle cracks in all directions did not show an dominant orientation.At the same time,the mechanism of high temperature on the meso-structure cracking of granite.As the temperature increases,the number and size of cracks gradually increase.When the temperature reaches a high temperature above 400?,the crack length increases,and the crack develops,connects and penetrates.The crack width increases,causing the gradual separation between the mineral particles,and the inside of the mineral particles is cut into small pieces.As a result,voids,loose and staggered cracks in the structure are formed,resulting in a fractured structure,resulting in cracking of the granite.The fracture toughness characteristics of granite after heat treatment with different continuous holding times(10 hours,10 days,30 days and 60 days)werestudied.The results show that the fracture toughness of granite has almost no change in the first 10 days.After more than 10 days,the fracture toughness tends to decrease slowly.The quantitative empirical relationship between rock type I fracture toughness and tensile strength after different temperature treatments was studied.The results show that the ratio of mode-I fracture toughness to tensile strength varies with different treatment temperatures.The similar fracture morphology after rock failure is the direct cause of their positive correlation.The fracture behavior and meso-damage characteristics of granite after natural cooling cycle were studied.The results show that as the number of cycles increases,the fracture toughness of granite decreases gradually,and the higher the cycle temperature,the greater the decrease.Granite resistance to fracture is greatly reduced,appearing in the first 5 times of the natural cooling cycle.As the number of cycles increases,the number of cracks of intergranular fracture and the transgranular fracture increases.Crack propagation directions on quartz and feldspar particles and between mineral particles are random.The fracture behavior and meso-damage characteristics of granite after cold water cooling cycle were studied.The results show that with the increase of the number of cycles,the pure mode-I,pure mode-II and mixed-mode(I+II)fracture toughness of granite decrease with the increase of the number of cycles.In the fifth cold water cooling cycle,the fracture toughness reduction was very significant.Under the cold water cooling cycle,there are many occurrences of intergranular fracture mechanism inside the granite.The statistical results of the total cracks in all directions show that the crack propagation has no anisotropic characteristics.Both the cold water cooling cycle and the internal structure of the rock in the natural cooling cycle are deteriorated,and the degree of deterioration in cold water cooling is more serious.Granite generates a temperature gradient during the cooling process to form thermal stress.Temperature differences and the rate of temperature change can significantly cause rock damage.The rate of temperature gradient change is the main cause of the difference in the degree of cracking of rock internal structure.The influence mechanism of temperature factor on crack propagation radius is analyzed.By introducing the FPZ coefficient,the crack propagation radius prediction model based on the maximum circumferential shear stress theory(MMTS)is modified,which effectively improves the accuracy of the MMTS fracture criterion forpredicting thermal damage granite fracture.