Based on the realization that Newtonian fluids have the unique property to redirect the forces applied to them in a perpendicular direction, a new apparatus, called the Granular Friction Analyzer (GFA), and the related GFA index, were proposed for characterizing the internal friction and related flow behavior of granular materials under uniaxial compression loading. The calculation of the GFA index is based on the integration of the internal pressure distribution along the cylinder wall, within which the granular material is being uniaxially compressed by a piston. In this paper an optical granular friction analyzer (O-GFA) is presented, where a digital image correlation (DIC) method is utilized to assess the cylinder strains used to calculate the internal pressure distribution. The main advantage of using the DIC method is that the starting point (piston–powder contact point) and the length of the integration considering the edge effects can be defined. By using the DIC full-field, instead of a few points strain measurements, a 2% improvement of the GFA index’s accuracy has been achieved and its robustness with respect to the number of points has been demonstrated. Using the parametric error analysis it has been shown that most of the observed total error (7.5%) arises from the DIC-method-based measurements of the strains, which can be improved by higher-resolution cameras and DIC algorithms for the strain evaluation. Additionally, it was shown that the GFA index can be used for determining the well-known Janssen model parameters. The latter was demonstrated experimentally, by testing three SS 316 L granular material samples with different mean particle sizes. The results confirm that the mean particle size regulates the internal friction of granular materials.

中文翻译：

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基于数字图像相关的颗粒材料内摩擦表征

基于认识到牛顿流体具有将施加在其上的力沿垂直方向重新定向的独特性质，提出了一种称为颗粒摩擦分析仪 (GFA) 和相关 GFA 指数的新设备，用于表征内摩擦和单轴压缩载荷下颗粒材料的相关流动行为。GFA 指数的计算基于沿气缸壁的内部压力分布的积分，其中颗粒材料被活塞单轴压缩。在本文中，提出了一种光学颗粒摩擦分析仪 (O-GFA)，其中利用数字图像相关 (DIC) 方法来评估用于计算内部压力分布的气缸应变。使用 DIC 方法的主要优点是可以定义起点（活塞-粉末接触点）和考虑边缘效应的积分长度。通过使用 DIC 全场，而不是几个点的应变测量，GFA 指数的精度提高了 2%，并证明了它在点数方面的稳健性。使用参数误差分析表明，大部分观察到的总误差 (7.5%) 来自基于 DIC 方法的应变测量，这可以通过更高分辨率的相机和 DIC 算法进行改进以进行应变评估。此外，还表明 GFA 指数可用于确定著名的 Janssen 模型参数。后者被实验证明，通过测试三个不同平均粒径的 SS 316 L 颗粒材料样品。结果证实平均粒径调节颗粒材料的内摩擦。