DYNAMIC FRAGMENTATION OF LASER SHOCK-MELTED METALS: SOME EXPERIMENTAL ADVANCES

While spall damage in solid materials has been one of the most widely studied shock-driven phenomena, very little data can be found yet about spallation in liquid metals. In recent papers, some of present authors have reported on exploratory investigations of liquid spall sometimes called "microspall" - in tin samples melted upon laser shocks of high intensities. Modelling approaches have also been discussed and compared with experimental observations. In particular, theoretical fragment size distributions have been compared with experimental distributions inferred from image analyses on fragment-impacted recovery polymer shields. While the curves representing the number per unit area vs. size of fragments present a similar aspect, the numbers predicted by an energy-based model are significantly greater than those observed experimentally. This is due predominantly to the characterisation technique, limited to surface examination, while a number of fragments penetrate the polymer shield and lay below its surface. This is why a new fragment recovery technique is being tested in this paper. This technique, based on a highly transparent low density gel, allows soft recovery and easier observation (compared e.g. with foams or aerogels) of the fragments size, shape and penetration depth, with improved spatial resolution. Coupled with shadowgraphy, scanning electron micrography and optical micrography, this recovery technique progress allows for an advanced insight into the phenomenology of microspall related fragmentation. To show it, laser shocks upon a tin target and an aluminium target are investigated, leading to different scenarios regarding melting conditions and fragmentation processes.