多级等幅循环荷载作用下砂岩变形、渗透及声发射特征试验研究

Experimental study on deformation,permeability and AE characteristics of sandstone under multi-stage cyclic loading with a constant amplitude

为探明多级等幅循环荷载对岩石变形、损伤及渗透规律的影响,对砂岩试样开展不同围压下的四级等幅循环加卸载三轴试验,试验过程中实时监测渗透和声发射信号。分析多级等幅循环荷载作用过程中围压、循环荷载对试样特征应力、渗透率及声发射b值和RA-AF值的影响。结果表明:(1)与常规三轴加载相比,多级等幅循环加载试验中每级循环荷载下砂岩裂纹体积应变均扩张,循环加载试验岩样的峰值应力下降,随着围压增大,两者峰值应力差值减小。(2)在2种加载条件中渗透率均先下降后上升,循环加卸载作用后岩样基于应变的渗透率损失率(PSL)更小,表明岩石内部宏观变形加剧,随着围压增大,PSL随之增大,表明宏观变形减弱;低围压下各级循环中基于应力的渗透率损失系数(EIP)为负值或数值较小,表明渗透率增加,而高围压下第4级循环中EIP为正值,表明渗透率减小,渗流通道的形成受围压约束的影响加剧。(3)与常规三轴试验相比,多级等幅循环加载声发射b值起伏变化更大;2种加载方式下岩样均以拉伸裂纹为主,但多级等幅循环加载条件下岩样内部产生的剪切裂纹更多,随着围压增大,岩石拉伸裂纹占比逐渐减小,剪切裂纹占比增大。

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.