基于亚塑性理论天然砂土多尺度演化的渐进破坏机理研究

Sand is continuous on macroscopic and discrete on meso-microscopic level. Failure in sand is often characterized by instability in form of localized bifurcation with progressive nature. Owing to the complex multi-scale nature of granular materials, deformation and failure analyses are usually performed separately. The engineering application of traditional elastoplastic theory is difficult because of the complicated formulation and numerous model parameters. This project focuses on the multi-scale evolutionary mechanism of instability for natural sand and investigates both the uniform deformation before a persistent shear band and the localized deformation after it. The fabric evolution image will be obtained with laboratory test. The quantitative fabric evolution for the origin of particle alignment, contact and porosity is obtained by a novel technique for the three fabric tensors proposed by the applicant. The quantitative technique based on the image analysis of the morphology and stereology. To clarify the meso-microscopic evolution of force chain and shear band, we carry out the simulation of progressive failure with discrete element method with the samples of different particle shape, size and arrangement to collect the fabric evolution. By introducing the porosity fabric as the state variable and two origin fabric tensor to the nonlinear tensor equations of hypoplasticity directly, we establish the constitutive model with multi-origin and multi-scale mechanism and implement this model into a finite element code. In this fund, based on quantitative analysis of the novel multi-origin fabric and used a multi-fabric evolution as a link, we try to interpret the progressive failure of the multi-scale mechanism of natural sand with a hypoplastic constitutive model and to provide the scientific evidence for engineering applications.

砂土宏观连续、细微观离散、失稳和破坏都是渐进的过程，复杂的多尺度机理造成其稳定和变形分析长期单独进行。传统弹塑性理论复杂及其模型参数多造成其渐进破坏的工程应用非常困难。项目针对天然砂失稳的多尺度演化机理，聚焦渐进破坏的两个特殊点，即剪切带贯通前的均匀变形及之后的局部化变形，用室内试验采集变形及其组构动态演化图像，用申请者新定义的三个成因组构及其基于形态学、体视学图像定量检测技术，分析颗粒排列、接触法向量和孔隙等成因组构演化规律；用离散元模拟不同颗粒形状、尺寸及排列试样的渐进破坏过程，分析组构演化规律，阐明其细微观演化与力链、剪切带的关系；基于亚塑性理论，将孔隙组构引入状态变量，另两个成因组构张量直接引入其非线性张量方程，构建多成因、多尺度机理的亚塑性模型并编制其有限元程序；以新组构定量分析为基础，以多成因组构演化为纽带，用新理论诠释天然砂多尺度渐进破坏机理，为其工程应用提供科学依据。

砂土为颗粒离散材料，其力学特性具有宏观连续、细微观离散、失稳和破坏都是渐进的过程，复杂的多尺度机理造成其稳定和变形分析长期单独进行。传统弹塑性理论复杂及其模型参数多造成其渐进破坏的工程应用非常困难。为此，本项目针对天然砂失稳的多尺度演化机理，聚焦渐进破坏的两个特殊点，即剪切带贯通前的均匀变形及之后的局部化变形，用室内试验采集变形及其组构动态演化图像，用申请者新定义的三个成因组构及其基于形态学、体视学图像定量检测技术，分析颗粒排列、接触法向量和孔隙等成因组构演化规律；用离散元模拟不同颗粒形状、尺寸及排列试样的渐进破坏过程，分析组构演化规律，阐明其细微观演化与力链、剪切带的关系；基于亚塑性理论，将孔隙组构引入状态变量，另两个成因组构张量直接引入其非线性张量方程，构建多成因、多尺度机理的亚塑性模型并编制其有限元程序；有限元结合离散元的双轴试验的模拟结果显示：离散元能够较好获取不同形状颗粒材料的接触组构在渐进破坏条件下的演化规律，将该演化规律结合到有限元单元中，有限元程序能够较好模拟颗粒材料组构变化造成的剪切带的变化，结合微极理论，亚塑性模型能够较好模拟颗粒形状变化对剪切带产生、发展、厚度和角度的影响，较好模拟了砂土整个渐进破坏过程。同时研究结果显示，相对弹塑性而言，亚塑性本构形式简单、模型参数少、有限元实现容易等优势明显，有利于岩土基础理论的工程实际应用。本项目以新组构定量分析为基础，以多成因组构演化为纽带，用新理论诠释天然砂多尺度渐进破坏机理，为颗粒材料的渐进破坏机理研究提供参考，同时也为新模型和多尺度的工程应用提供科学依据。