Optical techniques for in situ dynamical investigation of plastic damage

We follow the damage process of high-density polyethylene during tensile tests. We simultaneously track changes in the density and average orientation of cavities using incoherent light transport. At the same time, we measure the true strain with a video-extensometer and the heat with an infrared imager. We see that the damage process has two major separate steps. First, a globally isotropic nucleation and growth of cavities occurs up to a deformation of about 1.1. Then, at higher deformations, cavities stop growing. Instead, they progressively orient and elongate along the tensile axis. The transition between these two damage processes seems to be related to strong physical and geometrical constraints, also probed through a typical thermal signature.