冻融红砂岩损伤演化多尺度分析

以红砂岩为研究对象，进行冻融循环、CT扫描及力学特性试验，采用图像处理技术结合遗传算法寻优模型实现了0、5、10、20、40 次冻融循环后 CT 扫描图像的去噪、增强、分割及三维重构处理，通过对同一对象跨尺度的损伤识别与对比研究，建立了基于细观损伤的弹性模量劣化预测公式，并从材料细观结构的物理本质诠释了冻融红砂岩宏观力学行为。结果表明：基于图像最大熵值的遗传算法能够快速精确地选取阈值进行图像分割，实现对岩石细观结构中基质和缺陷的识别；随着冻融次数增加，岩石孔隙率上升、孔隙分维下降，细观尺度上呈现出孔隙扩展、数量增多，但结构复杂程度下降的演化行为；传统方法以有效承载面积、弹性模量为度量基准定义的宏、细观损伤变量未能全面考虑损伤物理机制和材料内部结构信息，宏细观损伤演化曲线差异较大；基于2种物理机制定义细观损伤变量和考虑岩石天然损伤定义宏观损伤变量，实现了损伤的宏-细观结合。最后通过冻融循环过程中细观结构演化与宏观力学响应之间的关系，提出了弹性模量劣化预测公式，并分析冻融砂岩孔隙大小及孔隙结构形态变化在损伤过程中占据的不同主导作用，根据细观结构的物理机制解释宏观砂岩冻融破坏的力学机制。

Multi-scale analysis of damage evolution of freezing-thawing red sandstones

We take red sandstone as the research object and apply the freeze-thaw cycles, CT scans and mechanical properties experiments. We use image processing technology combined with genetic algorithm optimization model to achieve the denoise, enhancement, segmentation and three-dimensional reconstruction of CT scan images after 0, 5, 10, 20, and 40 freeze-thaw cycles. With the damage identification and comparative study of the same object across scales, we established a prediction formula of elastic modulus deterioration based on mesoscopic damage. Therefore, the macroscopic mechanical behavior of freeze-thaw red sandstones can be interpreted from the physical nature of the material meso-structure. The results show that genetic algorithm based on image maximum entropy can quickly and accurately select the threshold for image segmentation, and achieve the recognition of matrix and defects in rock meso-structure. With the increase of freezing and thawing cycles, the porosity of rock increases, and the fractal dimension of pore decreases. On the meso-scale, the evolution shows that the pores expand and the number increases, but the structural complexity decreases. The macroscopic and mesoscopic damage variables defined by the traditional methods are based on the effective bearing area and elastic modulus, and they fail to fully consider the damage physical mechanism and the internal structure information of the material. The damage evolution curves are different. Based on the two physical mechanisms, we define the meso-damage variable and the macro-damage variable that considers the natural rock damage, which achieves the combination of macroscopic and mesoscopic damages. Finally, according to the relationship between meso-structure evolution and macroscopic mechanical response in the process of freeze-thaw cycles, we propose a prediction formula of elastic modulus degradation, and analyze the different dominant roles of pore size and pore structure morphology through the damage process. We interpret the mechanical mechanism of macroscopic sandstone freeze-thaw damage based on the meso-structure physical mechanisms.