Experimental Research On Characteristic Stress Acoustic Emission Characteristic Of Red Sandstone Under Uniaxial Compression

The essence of rock deformation and destruction process is the evolution of internal micro-cracks.Acoustic emission(AE)is the accompanying phenomenon of rock deformation and destruction process,and its activity characteristics are closest to the mechanism of micro-fracturing evolution activities.There is often an AE relative quiescence before rock failure,and its mechanism is not yet clear.In this paper,AE signals were acquired by uniaxial compression test of red sandstone based on the "Study on the relationship between the composition of mineral particles size and the relative quiescence of rock acoustic emission" of the National Natural Science Foundation of China(No.51664018).According to the law of crack propagation,the failure process of a rock is composed of a crack closure stage,an elastic deformation stage,a crack stable growth stage,a crack unstable growth stage(collectively referred to as different deformation stages).The mechanical and AE characteristics in different deformation stages are firstly analyzed.Then the mechanical and AE characteristics during the relative quiescence are also researched.Finally the relationship between the characteristic stress and the starting point stress of AE quiescence and the relationship of AE parameter(include AE energy fractal dimension D,frequency band energy,RA value and r value)between AE quiescence and different deformation were explored.The chief conclusion of this dissertation include:(1)The interval length of stress percentage is different in different deformation stages.The interval length of stress percentage is similar of different specimens in elastic deformation stage.The interval length of stress percentage in crack stable growth stage is the longest among in different stages.And the mechanical properties is the most different among different specimens in crack unstable growth stage.The difference in mechanical properties is greatly affected by cracks.(2)The number of low dominant-frequency AE increases with increasing stress.The AE dominant-frequency band becomes wider,and there are more AE of high dominant-frequency and low dominant-frequency in the post-peak deformation stage.The fractal dimension of AE energy is volatility in characteristic stress.D value is the most violently in the crack closure stress,more violently in crack initiation stress,the least violently in the crack damage stress.D is obvious upward trend in the damage intensity point,and it increase obviously from the crack closure stress to the crack damage stress.With stress increasing,the frequency band energy percentage variance of the characteristic stress,the starting point and the rising point of quiescence gradually decreases,and the oscillation is small that the variance value is approximately zero after the crack damage stress.(3)The relative quiescence of AE event rate can be quantified by using Z test.The size of Z value indicates the level of relative quiescence,and the length of the Z value greater than 0 represents the length of the relative quiescence.Slow increase in stress and faster increase in strain during the stage of the relative quiescence,which appears to be about 90%of red sandstone peak stress under uniaxial compression.For the specimens with relative quiescence,the stress at the start point of the relative quiescence can be inferred from the fitting formula of the stress at the start point of the relative quiescence and the crack closure stress,crack initiation stress and crack damage stress.The stress at the start point of the relative quiescence can be used as early warning value to predict rock failure.(4)Before the peak stress,there is a phenomenon that the RA value(the ratio of rise time and amplitude)of the specimens with relative quiescence decrease.While in the specimens without the relative quiescence,the RA value continues to rising to the peak stress.There is a relative quiescence of AE before the peak stress if the rock is dominated by shear fractures,and no relative quiescence when the rock is dominated by tensile fracture before peak stress.The change rate of the r value(the ratio of accumulated event rate and accumulated energy rate)decreases during the relative quiescence.The change rate of the r value of specimen without relative quiescence is not changed and there is no characteristic of changing in the change rate of r value.