三轴循环加卸载下砂岩声发射分形特征试验

Experiment of Fractal Characteristics of Acoustic Emission of Sandstone Under Triaxial Cyclic Loading and Unloading

为探究砂岩在三轴循环加卸载条件下的声发射特性及其分形特征,对砂岩进行不同围压等级下常规三轴试验和三轴循环加卸载声发射试验,通过分析砂岩在三轴循环加卸载条件下峰前与峰后全过程的声发射特性及其分形特征,探究岩石渐进破坏过程中试样裂纹开展和损伤演化规律。采用MTS815 Flex Test GT岩石力学测试系统与PCI–Ⅱ型声发射3维定位实时监测系统,开展了在10、20和30 MPa不同围压下的三轴循环加卸载声发射试验,并研究了峰后残余阶段岩石试样的损伤演化规律。通过常规三轴和循环加卸载试验得到了砂岩变形破坏全过程的应力应变曲线、声发射振铃计数和撞击数等数据,并据此探讨了砂岩强度与变形、声发射与应力之间的关系,并运用声发射撞击率时间序列计算关联维数,分析不同循环阶段相空间维数对关联维数的影响。结果表明:1)围压对岩石破坏有抑制作用,围压的增大会提高岩石的强度。循环加卸载过程对砂岩有强化作用,三轴循环加卸载过程中岩石的峰值强度较常规三轴试验高,卸载模量的变化能较好地表征岩石内部结构演化,岩石内部的孔隙被压密会导致卸载模量上升,岩石内部损伤扩展则会导致卸载模量下降。2)不同围压下,岩石循环加卸载阶段的声发射振铃计数率与撞击率具有相同的变化规律。峰前阶段,声发射的产生基本与应力加载同步,卸载段几乎不产生声发射现象。峰后残余阶段,岩石承载力降低;峰后破坏阶段,应力降低后声发射振铃计数率与撞击率仍然在增加,显示出相对于应力加载的“滞后”现象;峰后卸载初期,声发射振铃计数率与撞击率急剧降低,但卸载末期次生裂隙的产生也会造成一定声发射现象。3)声发射撞击率具有分形特征,峰前循环加卸载过程中关联维数呈降低趋势,且从相空间维数对关联维数的影响可以看出,随着循环次数的增加,不同循环阶段内的声发射撞击率时间序列逐渐由随机序列转化为有序序列,表明宏观裂隙逐渐形成,岩石内部裂纹逐渐趋于有序;峰后残余阶段由于次生裂隙的开展与卸载末期的声发射现象,关联维数有所上升。

To explore the acoustic emission(AE) characteristics and corresponding fractal characteristics of sandstone under triaxial cyclic loading and unloading conditions, a series of conventional triaxial tests, as well as triaxial cyclic loading and unloading AE tests, were conducted on sandstone under different confining pressures. By analyzing the AE characteristics and fractal characteristics before and after the peak stress under the conditions of triaxial cyclic loading and unloading, the crack development and damage evolution during the progressive failure process of rocks were explored. Using the MTS815 Flex Test GT rock mechanics testing system and the PCI–Ⅱ type AE three-dimensional positioning realtime monitoring system, the three-axis cyclic loading and unloading acoustic emission tests under three groups of confining pressures 10, 20, and30 MPa were carried out, and the damage evolution at the residual stage was also studied. Through conventional triaxial and cyclic loading and unloading tests, the stress-strain curves, AE ringing count, and hit number of the sandstone during the whole process of deformation and failure were obtained. The relationship between strength and deformation, as well as the relationship between AE and stress, was investigated, and the AE hit rate time series was used to calculate the correlation dimension. The influence of the phase space dimension on the correlation dimension at different cycle stages was analyzed. The test results showed that: 1) The confining pressure had an inhibitory effect on rock failure, and confining pressure increase could increase the strength of the rock. The cyclic loading and unloading process had a strengthening effect on the sandstone, and the peak strength in the triaxial cyclic loading and unloading process was higher than that in the conventional triaxial test. The change of the unloading modulus could better characterize the evolution of the internal structure of the rock. The compaction of inner pores could cause the unloading modulus to increase, and the internal damage expansion would cause the unloading modulus to decrease. 2) The acoustic emission ringing count rate and the hit rate under different confining pressures during the cyclic loading and unloading stages had the same changing law.The AE at the pre-peak stage was basically synchronous with stress loading, and almost no acoustic emission phenomenon occured in the unloading process;the rock bearing capacity at the residual stage after the peak decreased, and AE ringing count rate and hit rate were still increasing after the peak, showing a “lag” comparing with stress loading. The AE ringing count rate and hit rate decreased sharply at the beginning of unloading, but the generation of secondary fractures at the end of unloading would also cause a certain AE phenomenon. 3) The hit rate of AE had fractal characteristics. The correlation dimension decreased in the pre-peak cycle loading and unloading process;under the influence of phase space dimension on correlation dimension, the AE hit rate time series at different cycle stages gradually changed from random sequence to ordered sequence with the increase of cycle times, indicating that macro fractures gradually formed and cracks inside the rock tended to be orderly;in the post-peak residual stage, due to the development of secondary cracks and the acoustic emission phenomenon at the end of unloading,the correlation dimension increased.