Experimental Study On Mechanical Properties Of Brittle Fracture Of Sandstone Under Thermal-Hydraulic-Mechanical Action Under Loading And Unloading

The research on physical and mechanical properties and brittle fracture of rock under the combined action of multiple fields is a frontier topic in rock mechanics and engineering.In this paper,the mechanical characteristics of unloading brittle fracture of sandstone under three fields will be studied by laboratory test,and the brittle fracture characteristics of rock will be analyzed from multiple angles by acoustic emission test and nuclear magnetic resonance test.The main conclusions are as follows:1.The uniaxial compression test results of sandstone show that the strength and strain of sandstone increase slightly with the increase of temperature,the peak strength increases by 3-8 MPa,the saturation coefficient increases,and the strength of sandstone decreases by about 60%.With the increase of saturation coefficient,the fracture Angle of sandstone decreases gradually,and the fracture characteristics change from tensile failure to shear failure deformation,and shear failure is more likely to occur,thus reducing brittleness.With the increase of temperature,the fracture characteristics of sandstone have little change,indicating that the saturation coefficient is the controlling factor of the failure mode of sandstone.2.The results of high temperature triaxial unloading permeability test show that the maximum strength and elastic modulus of sandstone are between 25℃ and 90℃with the increase of temperature,and the mechanical properties of sandstone are strengthened with the increase of temperature.When the pore pressure is 0 MPa～12MPa,with the increase of pore pressure,the peak strength of sandstone decreases,the peak strain decreases,and the rock mass will fracture when the deformation is small,and the brittleness is enhanced.The high temperature causes the rock samples to change from shear failure to tensile fracture,and the fracture form is mainly tension-shear composite.With the increase of pore pressure,the fracture Angle of the rock decreases,the brittleness of the sandstone increases,and there are more cracks at both ends,which are like fish scales.The crack at both ends is intensified by pore water pressure.The influence of pore water pressure on rock failure characteristics is greater than that of temperature.3.In the process of high temperature triaxial unloading permeability test,the permeability is tested,and the results show that with the increase of axial stress,the change of permeability generally decreases first and then increases.The temperaturerising inhibits the permeability of sandstone,and restricts the effect of pore osmotic pressure in rock samples.After reaching the peak strength,the influence of temperature on permeability decreases,and the main controlling factor is the fracture.With the increase of pore water pressure,the permeability to sandstone generally increases.The NMR results of the triaxial unloading test show that the pore diameter of the fractured sandstone increases with the increase of pore water pressure.With the increase of temperature,the small pore volume decreases,while the mesopore and macropore volume have little change.4.With the increase of saturated water coefficient,the AE activities of rock samples during the uniaxial compression test significantly increase during the compaction and elastic deformation periods,and the AE energy value of rock samples decreases at the time of fracture,indicating that the increase of saturated water coefficient leads to the decrease of fracture degree of rock samples and the weakening of brittleness of rock samples.During the triaxial unloading test,with the increase of pore water pressure,the cumulative number of acoustic emission rings increases,and the brittleness of rock is enhanced.5.With the increase of temperature,the dissipation energy of sandstone decreases with the increase rate of strain,and the dissipation energy released by sandstone increases at the time of fracture with the increase of temperature,and the brittleness of sandstone increases;With the increase of saturation coefficient,the total energy absorbed by sandstone decreases with the strain growth rate,the total energy absorbed by sandstone decreases,and the dissipated energy of sandstone also decreases,indicating that the brittleness of sandstone decreases.With the increase of pore water pressure,the turning point of the growth curve of dissipation energy curve is advanced,the growth slope of dissipation energy is larger in the fracture stage of rock samples,and the fracture of sandstone is more sudden,indicating that with the increase of pore water pressure,the brittleness of sandstone increases.6.A new rock brittleness index BHW based on temperature,saturation coefficient and pore water pressure is proposed.The analysis of loading and unloading brittleness of sandstone under heat-water-force shows that the brittleness of sandstone tends to increase with the increase of temperature between 25 ℃ and 90 ℃;With the increase of saturation coefficient,the brittleness of sandstone decreases slightly;With the increase of pore water pressure,the brittleness of sandstone increases.Rock brittleness index BHW has a good discrimination degree for rock brittleness characteristics.