Study On Time-dependent Mechanical Behavior Of Hard Rock During Fracture Under Bending Load

Static and dynamic pressures in the process of bending fracture of mining roof have an effect on coal seam and roadway floor,which can lead to coal seam rock burst,coal and gas outburst,roof caving,side wall,floor heave and impact.The mechanical mechanism of the relationship between mining speed and bending subsidence and fracture of goaf roof can be attributed to the scientific problem of the relationship between rock bending loading rate and bending fracture.In order to study this scientific problem,three-point bending and acoustic emission tests at different loading rates of hard rock in laboratory were used to analyze the characteristics of acoustic emission and rock mechanics parameters in the process of rock fracture,to explore the precursor information of initial damage and main fracture of rock and the time-dependent of rock mechanics behavior(hereinafter referred to as time-dependent).By scanning the rock fracture surface with electron microscope,observing the loading rate effect of Meso-Structural characteristics,conducting damage mechanics analysis,studying the meso-mechanism of rock fracture;establishing a three-point bending numerical calculation model to simulate and analyze the meso-characteristics of the roof rock bending and fracture process by using RFPA2D numerical analysis software;combining with the field mining microseismic monitoring data,analyzing the law of roof fracture.It is expected to deepen the understanding of the mechanism of bending and breaking of mining roof and the principle of hazard prediction.The main research contents and achievements are as follows:(1)A method for identifying the initial damage acoustic emission(Kaiser point)of rock under bending load by acoustic emission counting,energy and spectrum is proposed.Acoustic emission test shows that the faster the loading rate is,the higher the stress condition of initial damage acoustic emission(Kaiser point)is and the higher the accumulated strain energy is.It is concluded that the stress condition of Kaiser point of rock under bending load at different loading rates is not a constant,but a variable related to loading rate,that is,the Kaiser effect of rock under bending load has time-depedent.(2)Some mechanical behaviors of bending rock are time-dependent.Loading rate is positively correlated with the initial stage of bending failure load and bending strength of rock samples.After exceeding a certain extreme value,the failure load and bending strength tend to converge,and the rock failure has time lag.The bending elastic modulus of rock samples is nonlinear negatively correlated with the loading speed,and the faster the loading rate is,the greater the gradient of bending elastic modulus is,and the easier the deformation will occur.The failure load,flexural strength and modulus of elasticity are functions of loading rate and have time effect.It is concluded that due to the different loading rates stipulated in the current rock mechanics test regulations,the test results will produce "regular" errors.(3)The law of rock damage changing with time is obtained by physical and numerical experiments.Based on damage mechanics theory,a constitutive model of bending rock damage is established,which is related to loading rate.(4)The relationship between mining speed and energy released from roof breakage is revealed in mechanism.Loading rate is exponentially positively correlated with peak energy of AE and failure load,and negatively correlated with peak counts of AE in hard rock fracture.The faster loading rate is,the greater the failure load required for rock fracture is,the less the peak AE counts are,and the higher the one-time release energy is.Peak AE in hard rock is a function of loading rate and is time-dependent.It is found that the loading rate has an effect on the fracture mode by scanning the rock fracture surface with electron microscopy.With the increase of loading rate,the proportion of transgranular fracture increases,the energy released from fracture increases,and the fractal dimension of fracture increases.From the microscopic mechanism,this paper reveals the reason why the larger the mining speed is,the greater the energy released from the roof rock breakage.From the mechanism,it reveals the theoretical basis of reducing the energy released from the roof rock by controlling the mining speed.(5)It is found that the time length of AE precursor of rock breaking is exponentially negatively correlated with the loading rate.The faster the loading speed is,the shorter the AE precursor time and the worse the predictability is.The AE precursor information of sandstone is more difficult to monitor than that of granite,and the catastrophic change is relatively strong.This provides a direction for AE,microseismic monitoring and early warning technology of roof breaking impact and its induced dynamic disasters,and provides a theoretical basis for controlling mining speed and improving the predictability of roof breaking.(6)The research results are applied in rock mechanics test,roof breakage monitoring and early warning,mining speed control and other aspects.