Determination of strains in cyclic triaxial tests using digital image correlation

Digital imaging and the Vic-3D program were used to measure deformations during a displacement controlled cyclic triaxial test of a clay specimen confined by vacuum. Seven stages of testing, each of 50 cycles, were completed. Imaging and LVDT measurements were compared. Additionally, measurements from specimen and membrane markings were compared.; Measurements from an LVDT and the Vic-3D image correlation program were compared using a direct comparison of strains with estimated error bands and a statistical test of coincidence of resilient modulus regression curves. The Vic-3D program successfully replicated LVDT results. Both total and plastic strains exhibited similar trends and magnitudes. Resilient modulus curves were statistically coincident at the 99% confidence level.; Digital image correlation provides the advantages of field measurements, allowing the identification and measurement of the area with the most uniform strain field. The dummy variable model and the F test for coincidence offer a quantitative method for comparing any number of regression curves.; Two sample t-tests compared side by side subsets of different marking techniques in the top, middle, and bottom thirds of the specimen. Vertical deformations based on specimen markings were statistically equal to those based on membrane markings for the top and middle thirds of the specimen at overall strain values of less than 10%. Axial strains and radial deformations based on the two marking techniques were not statistical equal. There were indications that the background applied to the specimen affected pore water drainage. Further inquiry is needed to establish the ideal marking technique.; The research raised the question of whether the precision of the imaging measurements and the statistical analysis exceeds the precision of the test method. For example, in one stage, all plastic deformations were within 0.018 mm (0.0007 in.) but were not statistically equal.; Vic-3D offered insight into the effects of data smoothing on strain calculations. Using a high degree of smoothing, strain data was shown to parallel findings of previous researchers. At lesser levels of smoothing, the same data set contradicted previously reported results. More study is needed to establish the proper degree of smoothing for various measurement applications.