Thermal Shock Fracture Mechanism Of Granite And Its Quantitative Characterization

In key projects such as the development of hot dry rock geothermal resources,deep buried storage of nuclear waste,and thermal rock fragmentation,it is a very common phenomenon for rocks to produce thermal damage under the action of the temperature field.Especially when the temperature changes drastically,the thermal shock damage caused will often bring serious safety hazards to the hot dry rock wellbore and nuclear waste repository.The rational use of this phenomenon can also improve the efficiency of thermal rock fragmentation and the enhancement of permeability of hot dry rock reservoirs.Thermal shock stress is caused by the temperature gradient that changes drastically during thermal shock.Therefore,analyzing the evolution mechanism of thermal shock damage from the perspective of heat transfer and realizing the characterization of thermal shock damage is of great significance to the abovementioned engineering practice.This paper focuses on the influence of thermal shock on the heat conduction law of granite,explores the feasibility of using thermal shock factors to characterize thermal damage,and analyzes the evolution of thermal shock damage of granite under different conditions on this basis.Based on the formation mechanism of thermal stress and the internal energy change law of the rock during thermal shock,this paper proposes the physical quantity of thermal shock factor to characterize thermal shock damage after theoretical reasoning.Then,the surface temperature change of the specimen during the thermal shock process was measured and combined with numerical simulation and CT scanning technology,the thermal shock process was inverted on the basis of the measured data,and the thermal shock factor in the thermal shock process was calculated.The volume expansion ratio,density change ratio,thermal conductivity attenuation,and evolution of the mesopore and fissure structure of granite under different thermal shock conditions were compared and analyzed,and the feasibility of using thermal shock factors to characterize thermal shock damage was verified.The method of constant temperature conduction heating is used to study the heat conduction characteristics of the specimen before and after thermal shock.Finally,use COMSOL Multiphysics 5.6 for numerical simulation to analyze the damage area and damage evolution process of the rock under different conditions in the thermal shock process.The main research results are as follows:(1)The thermal shock factor can be used to characterize the damage caused by thermal shock.The thermal conductivity of granite decreases as the temperature of the cooling medium increases during the thermal shock,and there is an upper threshold for the attenuation of the thermal conductivity.The volume change ratio and density attenuation ratio of granite increase with the increase of the temperature of the cooling medium,and the mass loss ratio changes very little.The number of cracks,the volume,surface area,volume fraction of pores and cracks,and space complexity all increase with the increase of the temperature of the cooling medium on the meso-level of granite.(2)The heating process of granite under constant temperature conduction heating can be divided into three stages: rapid heating,slow heating and stable heating according to the heating rate.After thermal shock,the stable temperature of granite under constant temperature conduction heating is lower than that before thermal shock treatment,and the time to reach the stable stage is shorter.The peak temperature gradient and stable temperature gradient of the granite after thermal shock are higher than before the thermal shock treatment,and the time to reach the peak temperature gradient is longer.The above phenomena all indicate that the thermal conductivity of granite deteriorates after thermal shock.(3)The appearance of thermal shock cracks will cause great differences in the temperature field and temperature gradient field on the upper and lower sides of the crack.The influence of thermal shock cracks on temperature distribution is mainly reflected in the rapid heating stage.With the increase of heating time,the influence of cracks on temperature distribution gradually weakens.The impact of thermal shock cracks on the temperature gradient distribution is mainly reflected in the stable stage.Cyclic thermal shock will cause more serious damage to the granite body,and the degree of damage will gradually increase with the number of cycles of thermal shock.(4)The main reason for the spalling failure of the rock surface during the thermal shock is that the existence of cracks inside the rock affects the distribution of temperature gradients,and finally induces ultra-high thermal shock stress at the crack tip.When the convective heat transfer boundary remains unchanged,as the thermal conductivity increases,the thermal shock stress generated inside the rock decreases,and the thermal shock damage area decreases.(5)The area of the thermal shock damage area under the action of external stress shows a trend of first increasing rapidly and then decreasing slowly with time,and this trend is consistent with the changing law of temperature gradient,indicating that there is a correlation between the two.The thermal shock damage at the tip of the crack is more serious than the middle position of the crack.When the damage area reaches its peak,the shape of the damaged area is like a dumbbell.In the process of reducing the damage area,the shape of the damage area is closely related to the external stress state of the specimen.The change of stress in the vertical direction has a greater influence on the size of the damaged area than the influence caused by the stress in the horizontal direction.Increased stress in the vertical direction will inhibit the development of damage at the tip of the crack,but at the same time it will aggravate the damage at the middle of the crack.The increase of the horizontal stress will suppress the development of the thermal shock damage during the thermal shock as a whole.