| It becomes more and more important to study and understand the dynamic mechanical properties of rocks under temperature condition to meet the increasing demands of deep mining, nuclear waste disposal and development and utilization of geothermal. For rocks with different temperatures and pre-stress state, the dynamic experiments were conducted by using an innovative testing system. And the rock dynamic properties of compression, tensile and fracture after thermal treatment were measured using the Split Hopkinson Pressure Bar (SHPB). Some theoretical researches were also carried on in this study. The main contents and conclusive results are as follows:(1) An innovative testing system was constructed for tests of rock with coupling temperature and pre-stress under dynamic loading. Pulse shaping techniques were discussed for rock brittle material, such as, removing wave head, the incident pulse shaping technique and special-shaped striker. The effect of thermal transfer to the experimental results was analyzed when sample touch with elastic bar under temperature loads. Several common problems in SHPB test were discussed, for instance, dynamic force balance, friction effect and thermal transferring.(2) The dynamic properties of sandstone under different temperatures and pre-stress were observed and studied. The dynamic strength relationship of temperature and pre-stress was obtained. Based on present mechanical and damage model, dynamic constitutive model of rocks under coupling with temperature and pre-stress were built. The influence of temperature on pre-stress and energy dissipation of sandstone was analyzed, the results shows that the rate of energy absorption changes with rise of temperature.(3) The mechanical properties of sandstone specimens such as density, longitudinal wave velocity, peak dynamic strength are studied under dynamic compression loading after being heated to different temperatures(25℃～800℃). Though the experimental results are discrete, the general law is obvious. The results show that, with the increasing temperature, the specimen density, the longitudinal wave velocity and the peak strength all gradually decrease, and the decreased magnitude of the longitudinal wave velocity has been increased when the temperature is over200℃. And the temperature changes from400℃to600℃, the peak strength of rock had less decreased magnitude, and decreases sharply when the temperature is over800℃. With the increasing temperature, the size of rock fragments becomes smaller. Through high-speed photography, dynamic failure process of rock has been expressed. It is obtained that radial cracks are distributed round specimens along the loading direction, the failure mode of rock is not representative at initial loading, but the final failure mode of rock is obtained through the stress wave multiple reflection.(4) The difference of rock failure mode, peak strength and peak strain after high temperature under static and dynamic loading are analyzed and compared systematically. The effects of temperature on rock mechanical properties are investigated from microcosmic point of view. The results show that the mechanical properties of rock dynamic and static have difference evidently. With the increasing temperature, the failure mode of rock exhibit splitting failure with brittle fracture under static loading while the rock failure mode is tensile fracture under dynamic loading. The peak strength decreased evidently with the increasing temperature in a nearly linear way under static and dynamic loading. The average peak strength decreased from126.37MPa to64.76MPa in an extent of48.8%under static loading, as well as from176.3MPa to83.1MPa in an extent of52.9%under dynamic loading. The peak strain increased with the increasing temperature under static and dynamic loading. Thermal stress and variation of microstructure induce different the mechanical properties of sandstone due to the effects of temperature, and different methods of internal pore expansion and formation of microcracks due to different loading method, which result in difference of the resistance of deformation.(5) The dynamic tensile properties of Laurentian granite after various high temperatures treatment were studied using Brazilian Disc (BD) and Semi-Circular bending (SCB) testing methods and the feasibility of applying the quasi-static tensile strength formula of Semi-Circular sample for dynamic test was analyzed based on finite element method. The test results show that the physical properties of specimen such as density, thermal expansion coefficient and ultrasonic wave velocity were influenced by temperature. The dynamic tensile strength increases linearly with the rise of loading rate and decreases with increasing treatment temperature. The sample appears cracks with the shape of "X" along the loading direction during dynamic tensile test.(6) The dynamic fracture test of Laurentian granite after thermal treatment were carried on using the semi-circular three point bending specimens with pre-crack. The results shows that the dynamic fracture toughness of granite specimens increases linear with the rate of loading, but the higher treatment temperature, the smaller dynamic fracture toughness. The influence of temperature on inner structure of granite was analyzed using scanning electron microscope (SEM). Beyond250℃, crack propagation induced by thermal effect is more distinct, which is the main reason for the decrease of fracture toughness.
|
PDF Full Text Request