Compression and extension triaxial tests on sand, studied with X-ray microtomography and digital image correlation

In the usual execution of triaxial tests, the longitudinal and volumetric strains are computed cumulatively from displacements measured outside the sample, and thus interpretation is based on the implicit assumption that samples deform homogeneously. This evident misconception often leads to inconsistent sometimes surprising results, mostly near failure where intensive strain localization usually takes place. Aiming to observe the distribution of strains, a series of triaxial compression and extension tests have been carried out at the Laboratoire 3SR of Grenoble. The tests have been performed on small samples of a uniform sand, tracking the particles movement with x-ray tomography and digital image correlation. This paper illustrates this experimental technique and the methodology adopted for image processing with reference to selected experimental results. The results show that strains tend to concentrate in smaller portions of the sample, starting from early loading steps and evolve in largely different ways for compression and extension tests depending on the applied confining stress.