Experimental studies on strain distribution and gradients for sand specimens under uniaxial compression based on digital image correlation coarse-fine search method

The distribution and the evolution of three kinds of strain fields are measured for two sand specimens under uniaxial and stress-controlled compression by use of a digital image correlation method with coarse-fine search capability based on the particle swarm optimization algorithm and the Newton-Raphson method. The range of longitudinal strain gradients in a wide non-uniformly deformational zone is determined before strains rise rapidly in a small band with highly localized strain. It is found that the peak values of the positive (or tensile) horizontal linear strain and the negative (or compressive) vertical linear strain fall into the future strain localized band after the loading exceeds a certain value, while the peak of the shear strain can deviate from the band, as can be explained by the differences between linear strain fields and shear strain fields. The former two are inclined and zonal beyond the uniformly deformational stage, while the latter is lumpy. The nearly same range is found for the longitudinal (or vertical) gradients of three kinds of strains, i.e., 0.001~0.002 mm, before the occurrence of the narrow localized band. If the strain gradients are lower than the values mentioned above, rapid strain increases will not occur in a small band within the sand specimen, nor will macroscopic cracks appear. Outside the band, certain plastic strains and gradients exist, as cannot be explained by the nonclassical elastoplastic models.