Applying accurate normal load to a specimen in direct shear tests under constant normal stiffness (CNS) is of importance for the quality of the resulting data, which in turn influences the conclusions. However, deficiencies in the test system give rise to a normal stiffness, here designated as system normal stiffness, which results in deviations between the intended and actual applied normal loads. Aiming to reduce these deviations, this paper presents the effective normal stiffness approach applicable to closed-loop control systems. Validation through direct shear tests indicates a clear influence of the system normal stiffness on the applied normal load (13% for the test system used in this work). The ability of the approach to compensate for this influence is confirmed herein. Moreover, it is demonstrated that the differences between the measured and the nominal normal displacements are established by the normal load increment divided by the system normal stiffness. This further demonstrates the existence of the system normal stiffness. To employ the effective normal stiffness approach, the intended normal stiffness (user defined) and the system normal stiffness must be known. The latter is determined from a calibration curve based on normal loading tests using a stiff test dummy. Finally, a procedure is presented to estimate errors originating from the application of an approximate representation of the system normal stiffness. The approach is shown to effectively reduce the deviations between intended normal loads and the actual applied normal loads.

中文翻译：

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CNS 直剪试验中系统法向刚度影响的补偿方法

在恒定法向刚度 (CNS) 下的直接剪切试验中，向试样施加准确的法向载荷对于所得数据的质量非常重要，这反过来又会影响结论。然而，测试系统中的缺陷会导致法向刚度，这里称为系统法向刚度，这导致预期和实际施加的法向载荷之间存在偏差。为了减少这些偏差，本文提出了适用于闭环控制系统的有效法向刚度方法。通过直接剪切测试的验证表明系统法向刚度对施加的法向载荷有明显影响（本工作中使用的测试系统为 13%）。补偿这种影响的方法的能力在此得到证实。而且，结果表明，实测法向位移和标称法向位移之间的差异是由法向载荷增量除以​​系统法向刚度确定的。这进一步证明了系统法向刚度的存在。要采用有效法向刚度方法，必须知道预期法向刚度（用户定义）和系统法向刚度。后者是根据使用刚性测试假人基于正常负载测试的校准曲线确定的。最后，提出了一个程序来估计源自应用系统法向刚度的近似表示的误差。该方法被证明可以有效地减少预期法向载荷与实际施加的法向载荷之间的偏差。