Direct measurement of the real contact area of rock joints under normal loading is crucial for comprehending the subsurface geological processes. However, measuring this phenomenon quantitatively at site-scale or laboratory-scale is challenging. Here, we investigate the evolution mechanism of the real contact area in rock joints by conducting closure tests on artificial and saw-cut sandstone joints under normal stresses up to 50 MPa. Geometrical shapes of contact patches are quantified by the pressure-sensitive film using the adaptive threshold method. An extensive range of contact stress within contact patches is innovatively measured by integrating the results from multi-type pressure-sensitive films. Experimental results demonstrate that the real contact area increases with the increasing normal stress hyperbolically. Such a nonlinear contact evolution behavior can be attributed to the coalescence of adjacent contact patches. The fractal dimension of composite surface governs the geometrical shapes of contact patches and the distribution of contact stress. The relationship between patch areas and bearing loads follows the Hertzian theory when the patches are small, while it gradually becomes linear with the increasing patch size. A power model with exponential cut-off is proposed to predict the size distribution of contact patches. This work can provide new insights for estimating the patch-dependent seismic nucleation length and slip stability of subsurface joints.

中文翻译：

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法向载荷下岩石节理的分形接触和凹凸聚结：来自压敏膜测量的见解


在正常载荷下直接测量岩石节理的实际接触面积对于理解地下地质过程至关重要。然而，在现场规模或实验室规模上定量测量这种现象是具有挑战性的。在这里，我们通过在高达 50 MPa 的法向应力下对人工和锯切砂岩节理进行闭合测试，研究岩石节理中真实接触面积的演化机制。使用自适应阈值方法通过压敏膜量化接触贴片的几何形状。通过整合多类型压敏薄膜的结果，创新性地测量了接触贴片内的广泛接触应力。实验结果表明，实际接触面积随着法向应力的增加而双曲线增加。这种非线性接触演变行为可以归因于相邻接触面的合并。复合表面的分形维数控制着接触面的几何形状和接触应力的分布。当补丁较小时，补丁面积和轴承载荷之间的关系遵循赫兹理论，而随着补丁尺寸的增加，它逐渐变为线性关系。提出了一个具有指数截止的幂模型来预测接触贴片的大小分布。这项工作可以为估计地下节理的斑块依赖性地震成核长度和滑移稳定性提供新的见解。