Kinematic study of necking in a semi-crystalline polymer through 3D Digital Image Correlation

This study is devoted to the necking phenomenon and to the kinematic aspects associated with this plastic instability. The material studied is a High Density PolyEthylene deformed in tension. Full-field 3D-DIC (Digital Image Correlation) strain measurements were carried out on one face of a tensile specimen. On the opposite face, the longitudinal strain was measured in the central section using video-traction®, an extensometer based on marker tracking. This last measurement was used with feedback to control the strain rate in the specimen center. Specific data processing was run on the 3D-DIC measurements to obtain strain and strain rate maps up to true strain levels greater than 1.8. Even with measurements made on a relatively small strain rate range (smaller than 1 decade), we highlighted how much the necking phenomenon depends on the strain rate. In particular, we found that, around the yield point, the strain localization is more pronounced with higher strain rates. By analyzing the evolution of the strain rates in the necking shoulder regions, we carried out direct and original measurements of the strain level corresponding to the onset of necking stabilization (Natural Draw Ratio). This was possible because our original experimental protocol enabled us to obtain such strain maps even when the true strain values were significantly greater than 1. From an experimental point of view, we found that a video-extensometer based on marker tracking cannot efficiently measure the strain rate in the specimen center at large strain levels when the markers are greatly very deformed. In addition, a wide range of new experimental results (2D and 1D strain-rate and velocity data) is presented in this paper and we consider that this data should be particularly useful for the validation of numerical simulations of necking in polymers.