基于亚像素角点检测的数字图像测量系统能够记录三轴试样表面方块角点的位移，从而获得试样表面每一时刻的局部应变及应变场。分析了试样不同位置的局部径向应变在剪切过程中的变化，获得了以不同初始成样含水率制备的松砂和密砂试样在不同特征时刻的应力与应变的特征值。通过轴向应变场的变化分析了从应变局部化出现到剪切带发育、形成的这一完整的渐进破坏过程，总结了剪切带形成时的局部最大轴向应变特征值，并定性地分析了剪切带内、外土体在渐进破坏过程中不同的轴向应变增长率。试验结果表明：在应变场中试样应变局部化明显，并可以依此确定应变局部化的出现、剪切带的形成；对于密砂及初始含水率为0%制备的松砂试样，应力在应变局部化出现之初即达到峰值，剪切带形成时应力已经开始下降，进入应变软化阶段；以初始含水率为6%和12%制备的松砂试样在达到应力峰值时剪切带已经形成；剪切带内土体的局部轴向应变增长幅度比剪切带外的土体大得多。试样整体轴向应变的增大主要是由剪切带内土体剪切破坏产生的较大轴向应变所致。

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.