Stéphane André

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.

In situ mechanical characterization of polymers with the association of three optical techniques

A combination of three optical techniques is presented for in situ monitoring of macroscopic and microscopic variables characterizing the deformation of polymers. A video extensometer allows for the monitoring of true stress and strain. An infrared imager along with an appropriate mathematical algorithm allows for the determination of the energies that are converted into heat during the whole test. This makes possible a quantification of thermomechanical couplings revealing structural effects at microscopic scales. Finally, incoherent steady light transport is used to produce images of backscattered intensities. A physical model enables the authors to follow damaging processes occurring at microscopic scales.

Plastic instability studied experimentally on a semi-crystalline polymer through thermomechanical heat source identification: the flow stress concept revisited

Micromechanical deformation phenomena such as those leading to macroscopic viscoelastic and plastic behavior must be studied from a thermodynamic viewpoint, as they induce complex and partly irreversible heat effects. Calorimetric measurements of the intrinsic volumetric thermomechanical heat sources (THS) activated in the material bulk during mechanical loads can produce valuable information with respect to that aim. They can be based on infrared imaging if submitted to inverse algorithms that allow a correct reconstruction of THS to be produced. Here, an inverse method relying on a diffusion-advection heat transfer model is applied to experimental temperature maps recorded during tensile tests. These are made on a semi-crystalline polymer that shows a strong development of plastic instabilities. Along with simultaneous kinematic observables produced with a digital image correlation system, the competition between advection and diffusion phenomena may be clearly established. 1-D profiles of the reconstructed THS and measured strain rates illustrate clearly that thermomechanical effects associated with necking onset and propagation follow the kinematic variable in a rather direct manner. Finally, we show for tensile experiments that THS estimations lead to analyze plasticity as a rheological behavior controlled by the flow stress, responsible of necking development and propagation.