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首页> 外文期刊>Pure and Applied Geophysics >Fracture in Westerly Granite under AE Feedback and Constant Strain Rate Loading: Nucleation, Quasi-static Propagation, and the Transition to Unstable Fracture Propagation
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Fracture in Westerly Granite under AE Feedback and Constant Strain Rate Loading: Nucleation, Quasi-static Propagation, and the Transition to Unstable Fracture Propagation

机译：AE反馈和恒定应变速率载荷下的西部花岗岩破裂：成核，准静态传播以及向不稳定破裂传播的转变

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摘要

New observations of fracture nucleation are presented from three triaxial compression experiments on intact samples of Westerly granite, using Acoustic Emission (AE) monitoring. By conducting the tests under different loading conditions, the fracture process is demonstrated for quasi-static fracture (under AE Feedback load), a slowly developing unstable fracture (loaded at a `slow' constant strain rate of 2.5 × 10−6 /s) and an unstable fracture that develops near instantaneously (loaded at a `fast' constant strain rate of 5 × 10−5 /s). By recording a continuous ultrasonic waveform during the critical period of fracture, the entire AE catalogue can be captured and the exact time of fracture defined. Under constant strain loading, three stages are observed: (1) An initial nucleation or stable growth phase at a rate of ~ 1.3 mm/s, (2) a sudden increase to a constant or slowly accelerating propagation speed of ~ 18 mm/s, and (3) unstable, accelerating propagation. In the ~ 100 ms before rupture, the high level of AE activity (as seen on the continuous record) prevented the location of discrete AE events. A lower bound estimate of the average propagation velocity (using the time-to-rupture and the existing fracture length) suggests values of a few m/s. However from a low gain acoustic record, we infer that in the final few ms, the fracture propagation speed increased to 175 m/s. These results demonstrate similarities between fracture nucleation in intact rock and the nucleation of dynamic instabilities in stick slip experiments. It is suggested that the ability to constrain the size of an evolving fracture provides a crucial tool in further understanding the controls on fracture nucleation.
机译：通过使用声发射（AE）监测对Westerly花岗岩的完整样品进行的三个三轴压缩实验，提出了新的断裂成核观察结果。通过在不同的载荷条件下进行测试，证明了准静态断裂（在AE反馈载荷下），缓慢发展的不稳定断裂（以2.5×10−6的“恒定”恒定应变率加载）的断裂过程。 > / s）和几乎瞬间发展的不稳定裂缝（以“快速”恒定应变率5×10−5 / s加载）。通过在断裂的关键时期记录连续的超声波波形，可以捕获整个AE目录并定义确切的断裂时间。在恒定的应变载荷下，观察到三个阶段：（1）初始成核或稳定的生长阶段，速率为〜1.3 mm / s，（2）突然增加到恒定或缓慢加速的〜18 mm / s的传播速度（3）不稳定的加速传播。在破裂前约100毫秒内，高水平的AE活动（如在连续记录中所见）阻止了离散AE事件的定位。对平均传播速度的下限估计（使用断裂时间和现有的断裂长度）表明值为几m / s。但是，从低增益声学记录中，我们可以推断出，在最后几毫秒内，裂缝的传播速度提高到175 m / s。这些结果证明了完整岩石中的断裂成核与粘滑实验中的动态不稳定性成核之间的相似性。有人认为，限制不断演变的裂缝尺寸的能力为进一步了解裂缝成核控制提供了至关重要的工具。

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