土体拉伸破坏过程微细结构变化测试系统研发与应用

针对目前土体拉伸测试水平的不足，研发了一套土体拉伸破坏过程微细结构变化测试系统，主要由拉伸加载装置、图像采集装置、图像处理程序3部分组成。拉伸加载装置为测试系统提供均匀、稳定的外部荷载和观测面；图像采集装置可利用跟踪平台连续拍摄拉伸过程中不同应力状态下的微细观结构图像，并利用基于数字散斑相关法的相对位移场计算，通过预拉伸可确定断裂带演化区域，即观测区域；图像处理程序可对所拍摄图像进行增强、融合、拼接、分割处理，并对图像中孔隙与颗粒的量化特征参数进行提取。基于所研发的测试系统开展了黏性土拉伸破坏的微细观试验，研究结果表明：黏性土在整个拉伸破坏过程中，颗粒结构调整在先，孔隙演化在后，孔隙的形成与贯通将最终导致试样的拉伸破坏；孔隙度与孔隙分布分维随拉伸变形量的增加而增长，颗粒分布分维随拉伸变形量的增加而降低，但均表现有相似的阶段性，据此可将整个拉伸破坏过程划分为孔隙的萌生、发展、贯通3个阶段。

Development and application of microstructure change test system for soil tensile failure process

To improve the level of soil tensile testing, a set of microstructure change test system for soil tensile failure process is developed. The system consists of soil tensile loading device, image acquisition device and image processing program. Tension loading device provides uniform and stable external force and observation surface. Image acquisition device can continuously take the microscopic structure images under different stress states during the entire stretching process by the use of tracking platform, and determine the evolution area of tensile fracture zone (i.e., the observation area) using relative displacement field calculation based on the digital speckle correlation method. Image processing program includes enhancement, fusion, splicing and segmentation of the captured images, as well as the extraction of quantitative feature parameters of soil particles and pores. Based on the developed test system, the tensile failure test of clay is carried out. The results show that the particle structure changes first, and then the pores evolves during the whole process of tensile failure. The formation and penetration of the pores eventually lead to tensile failure of the sample. The porosity and fractal dimension distribution of pores increase with the amount of tensile deformation, and the fractal dimension distribution of particles decreases with the increase of tensile deformation, both can be divided into different stages. To be specific, the entire process of tensile failure can be divided into three stages: initiation, development and breakthrough of the fracture zone.