Image-based CHARacterisation of extreme dynamic solid-state flows – IChar

Super-plasticity is a state where a material exhibits an extremely large (>400%) neck-free elongation prior to failure. Such an extreme solid-state flow is highly beneficial for forming applications but also dramatic if defects coupled with super-plastic mechanisms trigger brutal plastic instabilities. When metals are deformed at very high-strain rate, e.g. during shock or dynamic manufacturing processes such as electromagnetic welding or forming, thermo-mechanical conditions may lead to extreme micro-structure refinement promoting suitable conditions to trigger super-plasticity mechanisms. At that point, the process is self-catalytic, highly coupled and non-monotonic, since adiabatic conditions promote simultaneously intense temperature rise and competing restoration mechanisms; grain-growth. Until now, no scientific consensus exists yet regarding the exact grain-scale mechanisms at work, their chronology, the role of the temperature and the local material response evolution under such a complex high strain-rate loading scenario. The project aims at developing an innovative full-field characterization technique able to probe, only from images, the evolution of the strain, the stress, the temperature and the micro-structure during severe dynamic loading. To reach that goal, we will develop two unique tools: 1) High-Resolution Ultra-High-Speed Micro Digital Image Correlation, to capture images of the deforming material with spatial and temporal resolution down to 10µm and 250ns respectively and 2) Dynamic Data-Driven-Identification technique, to identify from experimental acceleration fields and conservation law, full-fields of stress tensor. Expected original data will help-us to develop more robust and versatile physically-based models to e.g. better simulate and develop innovative low energy consumption high-speed material processing techniques, keys for the industry of tomorrow, and better safety criteria predicting dynamic plastic instabilities.