This study performed mechanical tests and monitored acoustic emissions (AE) in shale samples with six bedding layer orientations (β = 0°, 30°, 60°, 90°, 120°, and 150°) to investigate the progressive damage mechanisms under direct shear. The results revealed that the peak shear load (Pcr), crack initiation threshold (Pci), crack damage threshold (Pcd), and cumulative AE count exhibited an approximate M-shaped trend as the bedding angle increased. The Pci, Pcd, and Pcr values were minimal for shale specimens with β = 0°, Pcd and Pcr were maximal at β = 150° (followed by β = 60°), and Pci reached the maximum at β = 60°. Thus, shale exhibits complex and asymmetric mechanical behavior under direct shear, a phenomenon seldom documented. The three-dimensional spatiotemporal evolution of the AE, evolution of b-values, peak frequency distribution, and the rise angle-average frequency (RA-AF) indicated that the microscale mechanism governing the asymmetric progressive failure of anisotropic shale under direct shear involved significant asymmetry in the formation type and scales of cracks. The AE characteristics of anisotropic shale were analyzed using multifractal theory. The width of the multifractal spectrum, Δθ, accurately reflected the anisotropic characteristics of the AE time series. Moreover, the variation in the fractal dimension, Δf, indicated that the different probabilities of microcracks with high AE energy are the fundamental cause of the shale's asymmetric failure.

中文翻译：

---

直剪作用下各向异性页岩的不对称破坏机制


本研究对具有六个层理层方向 （β = 0°、30°、60°、90°、120° 和 150°） 的页岩样品进行了力学测试并监测了声发射 （AE），以研究直接剪切下的渐进损伤机制。结果表明，随着层理角的增加，峰值剪切载荷 （Pcr） 、裂纹萌生阈值 （Pci） 、裂纹损伤阈值 （Pcd） 和累积声发射计数呈近似 M 形趋势。β = 0° 的页岩样品的 Pci、Pcd 和 Pcr 值最小，Pcd 和 Pcr 在 β = 150° 时最大（其次是 β = 60°），Pci 在 β = 60° 时达到最大值。因此，页岩在直接剪切下表现出复杂和不对称的力学行为，这种现象很少被记录下来。声发射的三维时空演化、b 值演化、峰值频率分布和上升角平均频率 （RA-AF） 表明，控制各向异性页岩在直接剪切作用下不对称进行破坏的微观机制涉及裂缝形成类型和尺度的显著不对称性。采用多分形理论分析了各向异性页岩的声发射特征。多重分形光谱的宽度 Δθ 准确地反映了 AE 时间序列的各向异性特性。此外，分形维数 Δf 的变化表明，具有高声发射能量的微裂纹的不同概率是页岩不对称失效的根本原因。