Investigation On Deterioration Of Mechanical Properties And Fracture Mechanisms Of Granite Under Open Fire

To understand the problems of fire rescue and post-disaster reconstruction in rock engineering that involve rapid heating of rocks,in this paper,the physical and mechanical properties of granite under open fire were studied.The correlation between a series of physical and mechanical parameters and open-fire heating time or strain rate was analyzed.The evolution of the microstructure of granite after open-fire damage was observed,and the relationship between the degradation of the macro-mechanical properties and the damage of the micro-structure was established.Finally,the temperature field and stress field of granite during the heating process of open fire were simulated and calculated by the method of non-steady-state thermal analysis theory,and the internal mechanism of rock damage caused by the open fire was clarified.The research results of this paper can provide a theoretical basis and experimental support for roadway fire rescue and post-disaster reconstruction problems.The main research contents and results are as follows:(1)The surface color of the granite was significantly affected by the open-fire heating,the surface of the granite gradually becomes lighter and slightly gray as the heating duration increases.The P-wave velocity of granite was obviously affected by the open-fire heating and cooling treatments.Under two different cooling treatments,the P-wave velocity decreased rapidly during the period of 0 to 10 min and slowly decrease after 20 min.Under the same fire duration,the degradation of the P-wave velocity of the granite samples after water cooling was more obvious than that after air cooling.(2)The granite with different open-fire heating durations and different cooling treatments was subjected to a static tensile test,and the influence of open-fire heating duration and cooling treatment method on the static tensile properties of granite samples was analyzed.The results show that the nominal tensile strength of the granite was obviously affected by the open-fire heating.With the increase in the duration of the open fire,the nominal tensile strength of the granite samples under the two different cooling methods showed a downward trend.Under the same open-fire duration,the nominal tensile strength of the sample after water cooling was lower than that of the sample after air cooling.(3)The split Hopkinson bar(SHPB)system was used to carry out dynamic tensile tests and dynamic compression tests.The dynamic compressive strength of granite increased with the increase of strain rate,but decreased with the increase of open-fire duration.The nominal dynamic tensile strength of granite was obviously affected by the open fire,and the nominal dynamic tensile strength decreases rapidly within 0-10 minutes and slowly decreases after 20 minutes.Under the same open-fire duration,the dynamic mechanical properties of water-cooled granite were more degraded than air-cooled granite.(4)The field emission scanning electron microscope was used to observe the microstructure of the rock after damage by the open fire.Due to open fire heating and water-cooling treatment,the microstructure of the granite was destroyed.With the increase of the duration of the open fire,the cracks and holes inside the granite were further enlarged,and the watercooling treatment will further aggravate this damage.(5)Numerical simulation results showed that a large temperature gradient was generated inside the sample within 0-10 minutes of heating by an open fire.The thermal stress caused by the temperature gradient will promote the initiation and propagation of cracks inside the rock,which is an important reason for the thermal damage of the rock caused by the openfire heating.