Study On The Dynamic Mechanical Properties And Fracture Characteristics Of Thermal Shock Treated Sandstone

In deep rock engineering,rock mass may be subject to thermal stress concentration caused by sudden temperature change,which is called thermal shock.Thermal stress concentration can cause damage and cracking in the rock,thus reducing the strength of the rock.In addition,the deep rock mass not only bears high static in-situ stress,but also is affected by dynamic disturbance caused by blasting load,excavation unloading and earthquake.The superposition effect of the above factors can have an adverse impact on the stability of deep rock engineering structure.In view of the complex stress conditions and dynamically changing temperature environment faced by deep rock engineering,the dynamic mechanical properties and fracture process of sandstone under thermal shock are systematically studied.In this paper,sandstone specimens are heated at four different high temperature levels(250℃,450℃,650℃ and 850℃),and then cooled by three cooling methods(stove cooling,air cooling and freezer cooling),so as to provide different cooling rates for the thermal shock process.The physical properties(density,P-wave velocity and porosity)of sandstone specimens cooled by different high-temperature heating and cooling methods are tested;The dynamic and static coupling loading test of sandstone after thermal shock is carried out by using the improved separated Hopkinson compression bar system.The influence of thermal shock on the dynamic mechanical properties and failure mode of sandstone specimens is analyzed.During the test,the whole failure process of sandstone specimens is recorded by high-speed camera.The typical thermal shock crack morphology in sandstone specimen is observed by scanning electron microscope(SEM)to judge the damage caused by thermal shock to sandstone specimen.According to the research content of this paper,the effects of thermal shock on the mechanical properties and fracture characteristics of sandstone specimens are as follows:(1)With the increase of heating level(the same cooling rate)or cooling rate(the same heating level),the dry density,P-wave velocity and dynamic compressive strength of sandstone specimens gradually decrease,while the porosity of sandstone specimens gradually increases.(2)With the increase of heating level(the same cooling rate)or cooling rate(the same heating level),the energy absorbed by sandstone specimens decreases gradually,while the reflected energy increases gradually.(3)The failure mode of sandstone samples changes with the increase of temperature,and the proportion of shear failure increase with the increase of temperature.In particular,there are many small fracture paths in sandstone samples cooled by air with temperature grade of 850 ℃ and sandstone samples cooled by freezer with temperature grade of 650 ℃ and above.Higher cooling rates also have a significant impact on the failure mode of sandstone for each heating level.(4)According to the observation results of SEM,with the increase of heating level or cooling rate,the length microcracks in sandstone specimens gradually increase.(5)Two typical fracture modes(orthogonal and polygonal fracture)reflecting the action of thermal shock were observed in air-cooled and freezer-cooled sandstone specimens by SEM,and the threshold of thermal shock was discussed according to the observation results.