基于图像相关分析的砂土模型试验变形场量测

高分辨率数码相机的出现为砂土模型试验变形场非接触量测法的研究开发提供了一条简便经济而且实用的途径,在模型试验观测面上,不再需要布置嵌入式量测标志点或描画网格,直接用数码相机在试验各阶段拍摄照片,然后对照片序列进行图像相关分析,即可得出模型试验的变形场分布。利用图像匹配技术追踪模型上点的位移,借用FEM常用的等参单元的概念进行图像校准和应变计算,采用双线性插值技术可以实现微小变形量测,并提出同时用平移和旋转搜索方法,以适应砂土模型的变形特点。精度检验试验结果表明,一个像素以下的平移量测标准方差达到0.19个像素,数值试验表明,15°旋转刚体变形量测,非旋转搜索和旋转搜索的标准方差分别为0.49和0.08个像素。最后给出一个在砂土地基承载力离心机模型试验中的应用实例,分析结果包含砂土模型位移场、最大剪应变场和体积应变场的分布图。结果表明这一方法使得砂土模型局部化变形、剪切带和渐进破坏过程的量化分析成为可能。

Deformation field measurement for granular soil model using image analysis

Recently digital camera with high resolution provides a simple, economical and practical solution to the deformation measurement for granular soil model, without resource to the embedded target markers or mesh grid drawn on the model. The image matching method was employed to track the displacement of the sand particles on models and the concept of isoparametric element used widely in FEM was introduced to validate the image and calculate the strain of soil. Subpixels precision of displacement measurement was achieved by means of bilinear interpolation, and not only the translating search but also the rotating search strategy were proposed to recognize the deformation accurately induced by rotation of local zone in soil model. Precision validation test shows that the standard deviation of measuring translation displacement was up to 0.19 pixels and numerical test also shows that standard deviation of rotating search was up to 0.08 pixels for measuring displacement of 15 degree rotation. The performance of this technique is illustrated by an example application in a centrifuge test of bearing capacity of sand foundation. The result consists of the deformation filed distribution of displacement, maximum shear strain and volumetric strain.This method can be used to analyze the local deformation, shear band and progressive failure quantitatively for granular soil.