| Rock breakage has always been a hot problem in the field of rock mechanics,especially for granite which is a typical rock with high strength and high abrasion resistance,whether the rock can be broken efficiently is directly related to the quality,safety,benefit and other issues of the project.In recent years,more and more attentions have been paid to the use of rock thermal damage properties to assist mechanically-induced rock breaking.For example,in tunnel engineering,the tunnel face can be heated rapidly by using flame ejector,laser irradiation and microwave radiation to reduce its strength,and then mechanical tunneling is carried out.If the high temperature granite after heat treatment can be rapidly cooled by ultra-low temperature medium——“liquid nitrogen（LN2）”,the huge thermal stress produced in the rock mass can further promote its damage and cracking.In addition,the hot dry rock（HDR）reservoir rock itself has a high temperature,and the use of ultra-low temperature LN2 as fracturing fluid for reservoir volume reconstruction is expected to construct a complex fracture network in the formation,thus improving the efficiency of water-rock heat exchange.Therefore,it is of great significance to deeply investigate the physico-mechanical properties and fracture characteristics evolutions of granite after the thermal treatment of“heating-LN2 cooling”for the development of rock breaking technology and its application in engineering.In this paper,a series of researches were carried out by means of modern analysis and testing methods,conventional physical and mechanical tests and numerical simulation.The main contents and conclusions are as follows:（1）The microstructures and physico-mechanical properties of granite rocks at room temperature（25oC）and heated to different temperatures（100oC,200oC,300oC,400oC,500oC,600oC,800oC and 1000oC）were studied by inverted optical microscope,RMT-301 rock composite testing machine and ultra-low temperature LN2 cooling system.The experimental results show that,overall,the volume,porosity,peak strain of rock samples increase,while the density,P-wave velocity,uniaxial compressive strength（UCS）,elastic modulus decrease with the increase of heating temperature.The P-wave velocity of granite after the thermal treatment of“heating-LN2 cooling”decrease linearly with temperature,while there is a threshold temperature for the variations of volume,density,porosity,UCS,elastic modulus and peak strain,and the threshold temperature is between 500oC～600oC.When the heating temperature is below the threshold temperature,the volume,density,porosity are almost constant,and there is no obvious regular change of UCS,elastic modulus and peak strain;when the heating temperature exceeds the threshold temperature,they all change rapidly with the increase of temperature.When the temperature of granite increases from 500oC to 600oC,the microfractures inside it suddenly increase,with the temperature further increasing to800oC,some minerals have been melted.（2）The uniaxial loading device suitable for working in ultra-low temperature environment was developed,and the mechanical properties of granite after the combined effect of LN2 cooling and uniaxial force were studied.The mechanical properties of granite samples after coupling conditions were compared with those samples after LN2 cooling treatment only.The experimental results indicate that the damage degree of LN2-cooled granite samples with uniaxial load is lower than that of LN2-cooled samples without uniaxial load.（3）The different cyclic thermal treatments of“heating-LN2 cooling”were performed on granite samples,the cyclic temperatures are 200oC,300oC,400oC,respectively and 1,3,6,12,20 cycles under each cyclic temperature.Evolutions of thermal conductivity,P-wave velocity,UCS,elastic modulus and peak strain with cyclic temperature and cyclic treatment times were obtained.The results indicate that,for different cyclic temperatures,the changes of the above parameters mainly occur in the first 6 cyclic thermal treatments of“heating-LN2 cooling”,and after 12 cycles,all the parameters are almost maintained in a constant state.By using nanoindentation technology,the micromechanical properties of granite after cyclic thermal treatment of“400oC-LN2 cooling”were analyzed.The results show that the micromechanical properties of granite do not show obvious regular changes with the increase of numbers of cyclic thermal treatment of“400oC-LN2 cooling”.It is inferred that the deterioration of macroscopic mechanical properties of granite mainly comes from the development of microcracks instead of the damage of mineral crystals themselves.（4）A series of three-point bending（TPB）tests were performed on semi-circular bend（SCB）granite specimens after the thermal treatment of“heating-LN2 cooling”.The acoustic emission（AE）technique was employed to monitor the AE characteristics during the whole loading process.Based on which,the load-time-AE curves were obtained.As for the thermal treatment of“heating-LN2 cooling”,with the increase of heating temperature,granite changes from typical brittle fracture mode to plastic fracture mode,the earlier the acoustic emission signal appears,the more evenly distributed throughout the whole loading stage and the ratio of the crack initiation strength to peak load of granite rock samples under TPB tests decreases gradually.（5）The mode-I fracture surfaces of the SCB granite samples after different thermal treatments of“heating-LN2 cooling”were scanned by a non-contact 3D laser scanner,and the fracture surface point cloud was extracted and reconstructed.The fractal theory was used to quantitatively describe the roughness of the mode-I fracture surfaces of SCB granite samples after different thermal treatments.The results show that the roughness of mode I fracture surface increases with heating temperature.（6）The thermal cracking behavior induced by different thermal treatments of“heating-LN2cooling”in the SCB granite sample containing an artificial pre-crack and its influence on the mode-I fracture path were simulated by FDEM-TM.The simulation results show that the microfractures of granite induced by“heating-LN2 cooling”are mainly tension cracks accompanied with a small amount of shear cracks,and mainly intergranular cracks.As the heating temperature increases,the microcracks increases gradually,and the interference to the expansion path of artificial pre-crack increases gradually,which makes the expansion trajectory become more and more complex and tortuous,leading to the increase of mode-I fracture surface roughness. |
