Experimental Study On Shear Failure Characteristics Of Red Sandstone After High Temperature Treatment

In high temperature rock engineering,high temperature thermal damage has become an important factor affecting the shear properties of rock.How the shear failure characteristics of rocks change under high temperature is an urgent problem to be solved.In this paper,the laboratory tests of variable angle shear of red sandstone treated at 25 °C ～ 800 °C were carried out.The effects of high temperature on variable angle shear mechanics and damage characteristics of red sandstone were analyzed from macro,micro and micro perspectives by using acoustic emission(AET),digital speckle pattern(DIC)and scanning electron microscope(SEM).Finally,based on the damage characteristics of rock,the high temperature-shear damage constitutive equation of rock is established.The main conclusions are as follows:(1)High temperature changes the physical properties,internal water content and pore structure of red sandstone,and then leads to the damage of mechanical properties.The rock shows different mechanical responses.The applicability of Mohr-Coulomb strength criterion will not be affected after high temperature treatment of red sandstone within 800 °C,but the shear strength parameters will change significantly,that is,the cohesion and internal friction angle will change with the change of heat treatment temperature.(2)Under variable angle shear test,high proportion of shear stress and high treatment temperature will enhance the plasticity of red sandstone.At the same temperature,with the increase of shear angle,that is,the proportion of compressive stress applied to the specimen decreases(the proportion of shear stress increases),the peak shear strain,shear modulus and shear strength will decrease.(3)For the same shear angle,the peak shear strain first decreased with the increase of treatment temperature,and began to increase after 400 °C.The shear modulus does not change significantly before 600 °C and decreases rapidly after 600 °C.The shear strength gradually increases before 600 °C and then decreases after 600 °C.With the increase of shear angle(the proportion of compressive stress decreases),the effect of temperature on shear strength decreases gradually.(4)High temperature treatment makes the crack initiation position of red sandstone change from the upper-middle(asymmetric)to the upperlower ends(symmetrical)under the preset angle shear.In addition,when the temperature exceeds 600 °C,red sandstone is more sensitive to compression,and the surface spalling is more serious under compressionshear load.(5)With the increase of treatment temperature,the uniaxial compressive strength(UCS)is strongly positively correlated with the fractal dimension of the fracture surface,and both are significantly affected by the change of the number of transgranular and intergranular cracks in red sandstone.(6)In the test temperature range(25 °C ～ 800 °C),the proportion of shear cracks in the process of variable angle shear failure of red sandstone increases with the increase of treatment temperature,but it is still dominated by tensile cracks.(7)Under the influence of shear angle,the peak shear strain damage variable of red sandstone shows different laws with temperature.When the shear angle is small(50° and 55°),it is positive damage before 600 ℃ and then negative damage.When the shear angle is large(60 °and 65°),the peak shear strain damage variable is always positive damage.