页岩声发射RA值及其分形特征的试验研究

对页岩进行常规三轴压缩条件下的声发射试验,得到其声发射RA值时间序列,并利用MATLAB编程计算其关联维数,得到分形维数与时间的耦合关系,采用分形理论对其进行分析,以研究页岩在常规三轴压缩破坏过程中的微观机制及其分形特征。结果表明:1)页岩在常规三轴压缩破坏过程中的裂纹为张剪复合裂纹,并以剪切裂纹为主,张裂纹主要出现在峰值强度前后,并且随着围压的升高,张裂纹数量明显减少,这说明围压能够抑制张裂纹的产生。2)页岩声发射RA值在时域上具有自相似特征,具有分形特征,其分形维数演化模式为:上升—突降—波动。3)声发射RA值的分形维数在初期上升,表明岩石内部先是微小裂纹无序扩展,并融合贯通成破裂面,此时分形维数突降,表明裂纹沿破裂面有序地扩展,岩石产生宏观裂纹,预示着岩石即将发生破坏,因此可以将分形维数的首次突降作为岩体失稳破坏的前兆。

Laboratory study on RA value fractal feature of shale acoustic emission under conventional triaxial compression

Shale acoustic emission test is conducted under the condition of conventional triaxial compression, and the time series of acoustic emission RA value is obtained. In order to study the micro mechanism and fractal feature in the process of shale destruction under the condition of conventional triaxial compression, the coupling relationship between fractal dimension and time is obtained and analyzed with using the calculation of correlation dimension of time series on MATLAB software. Results show:1) Crack in the process of shale destruction is the combination of tensile crack and shear crack, and shear crack is the main crack. Moreover, tensile cracks mainly appear at the peak strength, and the number of tensile crack is significantly reduced with the increase of confining pressure. 2) The RA value of shale acoustic emission has self-similarity characteristics and fractal feature in the time domain. The evolutionary pattern of fractal dimension is:rise-sudden drop-fluctuation. 3) The fractal dimension rises in the first stage, indicating that in rock interior tiny cracks first disorderly extend until fracture surfaces form. Then the fracture dimension suddenly decreases, suggesting the fracture surfaces extend and macro cracks form, and the rock is about to break. Therefore, the first sudden decrease of fractal dimension can be taken as the premonition of rock destruction.