Research On The Mechanical Characteristics And Micromechanism Of Granite Under Temperature Loads

Mechanical characteristics and micromechanism of granite under temperature loads were systematically researched by using advanced experimental equipments and micro measurement technology. The thermo-visco-elasto-plastic constitutive equation and failure criterion of granite were discussed from the viewpoint of energy. All research findings provide valuable basic references for solving problems of rock thermodynamics in reposition of nuclear waste, practical gasification of coal and oil shale, deep resources mining, exploitation of geothermal resources and safety drilling and comprehensive utilization of coal seam gas. The major contents and innovations of the research are listed as following.Experiments on granite under uniaxial compression at-high-temperature (25~850℃)and after-high-temperature(25~1200℃)were carried on individually using MTS810 with high temperature device and MTS815. The result shows that granite brittleness weakens, ductility increases, peak strength decreases and post-peak behavior enhances significantly with rise of temperature. Mechanical properties of granite weaken continuously at-high-temperature, but show mutation state after-high-temperature, which is closely related to the alteration of rock crystal form and brittle-ductile transition.The influence of temperature on mechanical characteristics and behaviors of granite was analyzed by using experiments as scanning electron microscope, mercury injection apparatus, X-ray diffraction and accoustic emission. Micromechanism of brittle-ductile transition of granite under high temperature was discussed. SEM result shows that about 800℃the fracture surface possesses the mixed-rupture characteristics of transcrystalline cracks, cleavage step, slip band and superficial dimple. X-ray diffraction shows crystal form of some crystals of granite has transformed. AE strength decreases, but the duration increases about 800℃. The residual plastic deformation of granite still releases serried acoustic emission especially after peak strength, which is uniformly with the variation of macromechanical characteristics of granite. With rise of temperature, failure form of granite changes from abruptly brittle fracture to gradually semi-brittle shear fracture.Rock porosity increases with rise of temperature and the threshold temperature is about 800℃, but fractal dimension of pore distribution decreases. Mercury injection experiment result shows that at high temperature thermal damage of rock transforms from irregular crack structure to homogeneous pore structure gradually. The weakening of rock heterogeneity is the basic reason for the decrease of fractal dimension of pore distribution.The differential thermo-visco-elasto-plastic constitutive equation of rock which considers the influence of temperature on elasticity, viscosity and damage has been esTablished based on the Nishihara rheological model by introducing thermal expand coefficient, viscosity attenuation coefficient and damage variable. Creep equation and unloading equation of rock considering temperature effect have also been derived. Theoretic expression of complementary energy discharge rate of visco-elastic damage considering thermal-mechanical coupling has been derived based on damage theory and irreversible thermodynamics. Energy criterion of thermal-mechanical coupling damage fracture of granite has also been esTablished. The result shows that complementary energy discharge rate of thermal-mechanical coupling damage increases with rise of temperature and time. The energy criterion esTablished in this paper is more comprehensive and accepTable for dealing with rock thermal-mechanical coupling damage than the failure criterion esTablished by traditionally single damage parameter.