A non-destructive imaging-based displacement measurement technique, Digital Image Correlation (DIC), was used to evaluate local displacements on surfaces of sand specimens throughout biaxial and triaxial tests. The DIC method yielded thousands of displacement data points across an imaged surface of a specimen, providing the opportunity for analysis of local, micro-level displacement mechanisms that trigger the onset of persistent shear bands. The displacement patterns evidenced both in biaxial and triaxial tests suggested clearly that shear banding naturally initiates along conjugate planes. However, when test boundary conditions did not constrain shear band growth, the formation of a single, persistent shear band was favored; otherwise, conjugate shear was sustained. Shear band formation commenced just before peak stress, near the edges of the specimen, where shear deformation was least constrained. Shear band formation was not instantaneous, but required a finite amount of shear translation to fully initiate. Displacement gradients across shear bands were decidedly parabolic, and displacement fields along the length of a shear band exhibited a notable, non-rotational periodicity, perhaps associated with "force chain" buildup and collapse.
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