Abstract: 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.

Key words: soil tensile, microstructure, test system, image processing, quantitative feature parameters