Abstract Servo-controlled direct shear tests of rough (RMS = 500 μm) and smooth (RMS = 7 μm) surfaces of Gabbro are performed under a constant normal stress of 5 MPa and a constant shear displacement rate of 0.01 mm/s. The tested interfaces are 10 cm long and 8 cm wide. Two servo-controlled shear displacement modes are studied: 1) Piston Displacement Control [PDC], and 2) Block Displacement Control [BDC]. In PDC mode the displacement output from the shear piston is used as the feedback signal for servo control whereas in BDC mode the displacement output from two horizontally oriented LVDT's positioned directly on the sheared interface are used for control. We find that when the surfaces are tested under BDC mode they exhibit a distinct peak shear stress followed by a stress drop to residual shear strength, whereas a continuous transition to “steady state” sliding is exhibited by the same surfaces when tested under PDC mode. This leads to the important observation that while peak shear strength is similar in both control modes the residual shear strength in PDC mode is distinctly higher. All tested interfaces exhibit stick-slip oscillations, the magnitude and frequency of which are strongly related to the control mode. We show that stick slip amplitude and frequency are artificially higher in BDC mode because of servo-control effects and that the correct stick slip behaviour can only be obtained in PDC mode. We find that the displacement control mode in direct shear tests drastically affects the obtained results and therefore investigators must consider the consequences before specifying which servo-control mode to use in direct shear tests.

中文翻译：

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位移控制方式在岩石节理直剪试验中的意义

摘要 在 5 MPa 的恒定法向应力和 0.01 mm/s 的恒定剪切位移速率下，对辉长岩的粗糙 (RMS = 500 μm) 和光滑 (RMS = 7 μm) 表面进行伺服控制直剪试验。测试接口长10厘米，宽8厘米。研究了两种伺服控制的剪切位移模式：1) 活塞位移控制 [PDC]，和 2) 块位移控制 [BDC]。在 PDC 模式下，剪切活塞的位移输出用作伺服控制的反馈信号，而在 BDC 模式下，直接位于剪切界面上的两个水平定向 LVDT 的位移输出用于控制。我们发现当在 BDC 模式下测试表面时，它们表现出明显的峰值剪切应力，然后应力下降到残余剪切强度，而在 PDC 模式下测试时，相同的表面表现出向“稳态”滑动的连续过渡。这导致了一个重要的观察结果，即虽然两种控制模式中的峰值剪切强度相似，但 PDC 模式中的残余剪切强度明显更高。所有测试的接口都表现出粘滑振荡，其幅度和频率与控制模式密切相关。我们表明，由于伺服控制效应，BDC 模式下粘滑幅度和频率人为较高，并且只有在 PDC 模式下才能获得正确的粘滑行为。我们发现直接剪切测试中的位移控制模式会极大地影响获得的结果，因此研究人员必须在指定直接剪切测试中使用哪种伺服控制模式之前考虑后果。