Study On Fracture Characteristics Of Lu Gray Granite After High Temperature Water Cooling

One of the difficult problems to be solved in the mining process of high-temperature rock mass geothermal resources is the deep drilling construction problem.During the construction process,the injection of drilling fluid and mud will cool the high-temperature surrounding rock,and the high-temperature surrounding rock will be fractured by thermal shock.On the one hand,this phenomenon is beneficial to the rock breaking,but on the other hand,it is unfavorable to the stability of the high temperature surrounding rock,which easily causes the well wall to be unstable.Therefore,the fracture characteristics of rock after high temperature water cooling have important research value.In this paper,the fracture characteristics of type I cracks of Lu gray granite were studied by SC3PB method.The evolution law of rock fracture process was analyzed based on digital image correlation method?DIC?.The specific research results are as follows:?1?The fracture toughness K_ICC of Lu gray granite decreases with the increase of temperature,and the fracture resistance of granite decreases gradually,but K_ICC is higher than normal temperature at 100?.The K_ICC at normal temperature is 0.90-1.40 MN/m^1.5,the K_ICC at100?is 1.30-1.50 MN/m^1.5,and the K_ICC at 900?is reduced to 0.13-0.10 MN/m^1.5.?2?In the range of test temperature points,the peak load and fracture toughness of granite are the highest at 100?,and the CMOD_C and CTOD_C values are also higher than normal temperature and 200?.It shows that the granite has the strongest fracture resistance at 100?and the best mechanical properties.The reason is that the granite crystal aggregates and squeezes at 100?,which leads to the increase of cohesive force between the particles and the rock is less likely to break.The K_IC,CMOD_C,and CTOD_C of the granite change drastically at 300?and 600?because the granite begins to lose structural water at 300?and the phase transition occurs at 573?.?3?The tail end and tip position of the fracture process zone were determined in two ways.Method 1:The section of the pre-fabricated crack tip is the end of the process zone,and the section of the horizontal strain threshold is used as the tip of the process zone.Method 2:Before the peak load,the position of the pre-fabricated crack tip is taken as the end of the process zone.After the peak load,the position at which the critical open displacement is reached is the end of the process zone,and the position of the turning point of the open displacement curve is taken as the tip of the process zone.?4?As the temperature increases,the load level of the Lu gray granite entering the process area is continuously reduced.Strain is used as a parameter to measure whether or not to enter the process area.The horizontal strain threshold value of the initial entry into the fracture process zone at normal temperature is 0.11754%,and the horizontal strain threshold value of the initial entry into the fracture process zone at 900?is 1.06492%.As the temperature increases,the strain value entering the process zone gradually increases.?5?During the entire three-point bending loading process,the length of the process zone reaches a maximum between the peak load and the 90%load after the peak.The length of the process zone at normal temperature⁵00?fluctuates greatly with the load,and the length of the process zone from 600?to 900?fluctuates gently before the peak load.?6?With the increase of temperature,the overall trend of the length of the process zone corresponding to the peak load time gradually increases,but the length of the process zone corresponding to the peak load at 100?decreases relative to the normal temperature.?7?The load-displacement curve,the load-COD variation law and the increase of the critical strain entering the process zone indicate that the brittleness of the Lu gray granite is weakened and the plasticity is enhanced with the increase of temperature.The fracture characteristics of granite change drastically between 500?and 600?,which is an important turning point for the transition from brittle to plastic.Therefore,in the numerical simulation of granite fracture,it is recommended to use the K_ICC fracture criterion at normal temperature⁵00?,and the COD fracture criterion can be used from 600?to 900?.