| In geotechnical engineering such as tunnel and underground space,when fire and explosion occurred,the temperature of surrounding rock strata can reach thousands of degrees Celsius,which will seriously damage the structure and threaten the safety of trapped personnel and rescue personnel(under real-time high temperature).Different disaster relief methods,such as sprinkler or natural fire extinction,have significantly different effects on the stability of the rock mass,ultimately affecting the design and decision of post-disaster recovery and reconstruction programs(after high temperatures).Therefore,it is of great academic value and engineering significance to study the impact compression mechanical properties of rocks under high temperature and different cooling methods for guiding fire rescue and the assessment,repair and reconstruction of disaster-hit buildings.In this paper,the granite of an engineering section in Sichuan-Tibet area is taken as the research object.By using X-ray diffraction(XRD)test,digital image correlation(DIC)techniques,split Hopkinson pressure bar(SHPB)test and screening test,the physical and mechanical characteristics and failure mechanism of rock samples under real-time high temperature,natural cooling and swamp cooling after high temperature were compared and analyzed.Based on the dynamic increase factor,a thermal-mechanical coupling constitutive model was constructed and compared with the constitutive model of rock after high temperature,and the conclusions were drawn as follows:(1)The variation of rock sample physicochemical properties was analyzed.Compared with the natural cooling rock samples,the quartz content of swamp cooling rock samples was lower,while the feldspar content was significantly increased.The changes in the mechanical properties of the rock samples were mainly influenced by the microstructure and degree of mineral particle bonding.The mass loss rate of the rock samples under high temperature and different high temperature cooling methods was proportional to the temperature.With the increase of temperature,the density changing rate of the rock samples after high temperature increased exponentially,while the p-wave velocity decreased quadratically.(2)The characteristics and mechanism of impact compression failure of granite were revealed.During the process of impact compression,the displacement of incident end of the rock sample after high temperature was greater than that of transmission end after high temperature,and the axial displacement was greater than that of radial displacement at yield stage.The surface cracks of rock sample expanded along the axial and oblique direction,and the failure modes were mainly cleavage mode and a combination of cleavage and shear modes.The impact compression failure of granite after high temperature was a result of crack extension and penetration,while the failure of the samples under high temperature was closely related to mineral melting.(3)The mechanical properties of the rock samples,including the stress-strain curve,fractal dimension of fragmentation,and fracture energy consumption,were analyzed.The shape of the impact compression test curves of the natural cooling and swamp cooling rock samples after high temperature was basically the same,and the plasticity of the real-time high temperature rock samples is obviously enhanced;The peak strength of the rock samples had little difference before 400°C under the three working conditions,but afterwards,the optimal performance was observed in the natural cooling samples,followed by the swamp cooling samples,and finally the rock samples under real-time high temperature.Due to the influence of quartz phase transformation,the peak values of specific energy absorption of the rock samples after high temperature both occurred at 600°C,while for samples under real-time high temperature,it occurred between 500°C and 600°C.The fractal dimensions of the samples under real-time high temperature and swamp cooling were both greater than that of the natural cooling samples.(4)A dynamic damage constitutive model of thermal-mechanical coupling for rocks was established.The thermal-mechanical coupling constitutive model based on the dynamic increase factor can describe the mechanical properties of rock samples under impact compression at high temperatures.Compared with the constitutive model for rocks after high temperature,the damage variable of the thermal-mechanical coupling constitutive model is larger,and the evolution process of damage is shorter. |
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