在压缩位移控制加载条件下（加载速率为5 mm/min），针对不同含水率（12.7%~16.5%）砂样，利用自主开发的基于粒子群优化的数字图像相关方法，开展了最大剪切应变场的观测研究，获取了子区尺寸对砂样不同位置最大剪切应变的影响规律。研究发现，随着砂样含水率的增加，变形更加均匀，最大剪切应变高值区分布变得宽广，反映了剪切带的数目增加，微裂纹出现之前的最大剪切应变提高；随着纵向应变的增加，在应变高值区，最大剪切应变以非线性方式迅速增加，但在应变低值区，一般以线性方式增加。当剪切带出现以后，特别是在变形后期，子区尺寸对最大剪切应变有明显的影响。最大剪切应变随子区尺寸变化的演变规律与子区所处位置密切相关。

Experimental research on the maximum shear strain fields of sand specimens with different water contents (12.7%-16.5%) under displacement-controlled loading condition (loading rate is 5 mm/min) is carried out with a digital image correlation method based on the particle swarm optimization algorithm. Also, the influence of the size of subset on the maximum shear strains at different positions of sand specimen is studied. It is found that with the increasing of the water content of sand specimen, deformation becomes even uniform, region of maximum shear strain becomes wider, which reflects an increase in the number of shear bands, and a raise in the maximum shear strain prior to microcrack initiating. With increasing the longitudinal strain, the maximum shear strain increases rapidly in a nonlinear form at region with higher strain, while it generally increases linearly at region with lower strain. After shear bands are generated, especially in a later deformational stage, the size of subset significantly affects the maximum shear strain. The evolution of the maximum shear strain with the size of subset is closely related to the position of subset.