Experimental study on deformation,permeability and AE characteristics of sandstone under multi-stage cyclic loading with a constant amplitude
In order to explore the influence of multi-stage cyclic loading with a constant amplitude on rock deformation,damage and permeability,a four-stage cyclic loading and unloading triaxial test was carried out on sandstone samples under different confining pressures.The permeability was measured and acoustic emission(AE)signals were monitored in real time during the test.The effects of confining pressure and cyclic loading on the characteristic stresses,permeability,b-value and RA-AF(risetime/amplitude-average frequency)value of AE signals during multi-stage cyclic loading are analyzed.The results show that:(1)Compared with the conventional triaxial compressive test,the volume strain of the sandstone crack under each stage of the multi-stage cyclic loading test is expanded,and the peak stress decreases.With the increase of confining pressure,the difference between the two peak stresses decreases.(2)Under the two loading conditions,the permeability decrease first and then increase.After cyclic loading,the permeability strain-based loss rate(PSL)of the rock sample is smaller,and the macroscopic deformation of the rock is aggravated.When the confining pressure increases,the PSL increases and the macroscopic deformation decreases.Under low confining pressure,the stress-based irrecoverable permeability coefficient(EIP)in each cycle is negative or small,and the permeability increases.Under high confining pressure,EIP value in the fourth cycle is positive,indicating that the permeability decreases,and the formation of seepage channel is affected by confining pressure constraints.(3)Furthermore,the b-value of AE signals under multi-stage cyclic loading fluctuates more than in the conventional triaxial test.Under the two loading modes,the rock samples are dominated by tensile cracks.Under the condition of multi-stage cyclic loading,there are more shear cracks in the rock sample,and with the increase of confining pressure,the proportion of rock tensile cracks gradually decreases,and the proportion of shear cracks increases.
rock mechanicsrock failuremulti-stage cyclic loadingpermeability evolutionacoustic emission
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