颗粒材料双轴压缩试验的光弹测试

Biaxial Compression Experimental of Granular Media Based on Photo-Elastic Technique

摘要:

通过自主设计的柔性边界试验装置对由两种不同粒径组成的光弹颗粒混合系统进行了应变控制式的双轴压缩试验。结合试验数据，利用Hough变换等数字图像处理方法对每个颗粒进行定位，并通过粒子跟踪测速（particle tracking velocimetry，PTV）技术对各颗粒进行编号，实现了对每个颗粒在时间和空间上的连续定位。通过分析颗粒非仿射（non-affine）位移场发现，在应变为25%时，颗粒材料中心位置处颗粒的相对位移非常小（小于3 mm），试样主要通过固定端和压缩端附近以及试样中部靠近边界处的颗粒移动实现变形，而在固定端和压缩端主要以沿压缩方向的位移为主，试样中部靠近变形处的颗粒主要以垂直于压缩方向的位移为主。颗粒材料的光弹效应同时也清晰揭示了颗粒在压缩过程中局部颗粒几乎没有受力的现象。而颗粒中心处的力链也呈现出相对各向同性的性质，密度也相对较小，正是剪切带所穿越的地方。

Abstract:

The strain-controlled biaxial compression experiments in bi-disperse granular system are conducted by using a homemade photo-elastic apparatus， which can provide soft boundary pressure to the granular sample. The Hough transform is used to locate each particle position based on the experimental images. The particle tracking velocimetry （PTV） is applied to track the particle in the time scale. Hence， each particle is marked in both time and space scales. The affine motion of each particle is taken out based on the shear strain and space position of corresponding particle. The non-affine displacement field shows that the main displacements happen close to the compression sides and very small displacement in the center of the sample. Meanwhile， the photo-elastic responds of the granular systems also reveal the cage effect clearly， and some of the particles do not have any force even the sample has been compressed a lot. The anisotropy of the force chain network can be seen at the center of the sample with relatively low packing fraction， which is just the place where the shear band passes through.