Research On Damage And Fracture Characteristics Of Beishan Granite After Heat Treatment

In practical engineering fields such as geological disposal of high-level radioactive waste and development of geothermal energy,rocks are often exposed to high temperature environment,which will cause damage to rocks,and then affect the physical and mechanical properties of rocks.Therefore,in-depth study of rock damage during heat treatment and the influence of different heat treatment conditions on rock mechanical properties can provide technical guidance and scientific basis for practical engineering.The Beishan granite is heated and cooled by controllable temperature chamber,and the treated rock is tested by acoustic wave,and the relationship between treatment temperature and rock damage is explored.Based on the statistical strength theory of rock failure,the damage evolution equation and damage constitutive model of Beishan granite are established.Two cooling paths,natural cooling and rapid cooling,were set up.The effects of different heat treatment temperatures and cooling paths on the physical properties and fracture characteristics of Beishan granite were investigated through digital image correlation technology and three-point bending test.The main work and research results are as follows:(1)The wave velocity of Beishan granite decreases with the increase of heat treatment temperature,and when the heat treatment temperature reaches 600?,the wave velocity decreases sharply,the second peak of frequency domain signal appears,and the coda of time-frequency signal hardly develops.It shows that 600? is the accelerating point for the increase of thermal damage and thermal crack of Beishan granite.It is found that a large amount of heat damage and thermal crack occurs in the granite at 600? by scanning granite slices with polarizing microscope.A modified expression of damage factor based on P-wave velocity is proposed to describe the thermal damage degree of Beishan granite after treatment at different temperatures.(2)Based on rock damage theory and statistical strength theory,the modified damage factor thermal damage variable based on P-wave velocity is selected.Drucker-Prager strength criterion and Mohr-Coulomb strength criterion are used as criteria for rock failure respectively.The damage evolution equation and constitutive equation of granite considering temperature effect are established.Considering the geometric characteristics of the damage threshold and peak points of stress-strain curves,the parameter expressions of the constitutive model are determined.It is found that the above two strength criteria are equivalent in the damage evolution constitutive model of Beishan granite after heat treatment.The constitutive equation is verified by uniaxial compression tests of Beishan granite after different temperatures treatment.(3)Under three-point bending condition,the peak load of Beishan granite increases first and then decreases with the increase of heat treatment temperature under two heat treatment paths.The peak load reaches its maximum at 100?.Under the same treatment temperature,the difference of peak load of granite under path 1 and path 2 is small.When the treatment temperature is above 400?,the peak load of granite under path 2 is obviously smaller than that under path 1 at the same treatment temperature.(4)The fracture toughness of Beishan granite increases after heat treatment at 100?,and then decreases with the increase of treatment temperature.The fracture toughness of the two cooling paths has little difference when the treatment temperature is within 400?.The fracture toughness of granite under Path 2 is obviously less than that under Path 1 when the treatment temperature reaches 500? and 600?.When the treatment temperature is less than 400?,the fracture process zone length and critical opening displacement of granite under the two paths increase slowly with the increase of treatment temperature,and when the treatment temperature is the same,the difference between the fracture process zone length and critical opening displacement corresponding to the two paths is small;when the treatment temperature is higher than 400?,the fracture process zone length and critical opening displacement of granite under the two paths increase rapidly with the treatment temperature.At the same treatment temperature,the fracture process zone length and critical opening displacement of granite under Path 1 are significantly smaller than Path 2.