Three-dimensional evaluation of strain field during neck propagation process of semi-crystalline polymer by time-series net DIC

Three-dimensional displacement fields in the neck propagation process in uniaxial tensile test of High Density Polyethylene (HDPE) and Polyethylene (PP) were evaluated by newly proposed Digital Image Correlation (DIC) method, called time-series netDIC. This method uses four images of specimen surface taken at two different times by two cameras for the identification of the three-dimensional displacements of measurement points. The displacement was obtained by minimizing the objective function, which is a summation of correlative coefficients of brightness distributions between two times and two cameras. Two cameras and a mirror were used to evaluate the three-dimensional displacement fields on both surfaces of plate-type specimen. Obtained results provided the quantitative information about degree of strain localization at the necked region of the specimen. Transverse strains in thickness and width directions were different for PP while those were almost same for HDPE. Furthermore, large positive volumetric strain was confirmed at necked region in inelastic deformation stage.