The digital image measurement method based on sub-pixel accurate corner locator is able to record the displacement of corner of squares on the surface of samples to acquire the partial strain and strain field on the surface of samples at any time. With the change of partial lateral strain of different parts of samples during shearing

the feature values of stress and strain of loose and dense sand samples with different initial water contents at different characteristic times are analyzed. With the axial strain fields

the whole process of progressive failure

including the beginning and growth of strain localization then the final forming of shear band

is analyzed. The characteristic values of maximal partial axial strain when shear band formed are summarized. Different growth rates of axial strain inside and outside the shear band in the process of progressive failure are analyzed qualitatively. The experimental results show that the strain localization is apparent in strain field

by which the characteristic points of the beginning of strain localization and the forming of shear band are determined; for loose sand samples with an initial water content of 0% and dense sand samples

the stress reaches peak just after strain localization appears

stress has decreased when shear band forms

entering the stage of strain softening; for loose sand samples with 6% and 12% of initial water contents

shear band has formed when stress reaches peak; the increase of partial lateral strain inside the shear band is much more larger than that outside shear band. The increase of overall axial strain is mainly due to relatively large axial strain resulted from shear failure inside the shear band.