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.