Gilles Roy

Experimental investigation of liquid spall in laser shock-loaded tin

When a metal is shocked above its melting pressure or melted on release, the tensile stresses generated upon reflection of the compressive pulse from a free surface are induced into a liquid state. Instead of the well-known spallation process observed in solid targets, cavitation is expected in the melted material, and liquid fragments are ejected from the free surface. Their size, velocity, and temperature distributions are issues of increasing interest, as well as their impact on other nearby materials, but data are limited on the subject. Here, we present an experimental study performed on tin samples subjected to high pressure laser shocks (ranging from about 50to200GPa) of short duration (∼5ns). The results include post-test observations of the ejecta recovered after impact on a polycarbonate shield and time-resolved measurements of the free surface velocity through the shield. For shock pressures below some 80GPa, the velocity profiles are compared to the predictions of one-dimensional simulations i...

DEBRIS CLOUD EJECTION FROM SHOCK‐LOADED TIN MELTED ON RELEASE OR ON COMPRESSION

A triangular shock‐wave of sufficient intensity propagating in a metal sample may induce melting. When it reaches the free surface, tensile stresses are generated in the liquid state and lead to the creation of an expanding cloud of liquid debris. This phenomenon called micro‐spalling consists in a dynamic fragmentation process in the melted material. Plate impact experiments, associated to the so‐called Asay window technique, have been performed on tin to investigate this phenomenon.

FRAGMENT‐SIZE PREDICTION DURING DYNAMIC FRAGMENTATION OF SHOCK‐MELTED TIN: RECOVERY EXPERIMENTS AND MODELING ISSUES.

We are interested in dynamic fragmentation of shock‐melted metals. The present work is devoted to laser‐shock experiments in tin samples including fragments recovery and post‐test evaluation of the fragment‐size distribution. These results are compared with theoretical predictions from hydrocode simulations coupled with a modified formulation of a fragmentation model from the literature.

Interferometric windows characterization up to 450 K for shock wave experiments: Hugoniot curves and refractive index

Conventional shock wave experiments need interferometric windows in order to determine the equation of state of a large variety of materials. Lithium fluoride (LiF) and sapphire are extensively used for that purpose because their optical transparencies enable the optical diagnostics at interfaces under a given range of shock pressure. In order to simulate and analyse the experiments it is necessary to gather a correct knowledge of the optical and mechanical properties for these windows. Therefore, our window supplies are systematically characterized and an experimental campaign under shock loading is conducted. Our preliminary work on LiF windows at 532 nm is in good agreement with literature data at room temperature and the new characterization at 450 K enables a better interpretation of our preheated target experiments and confirms the predominant effect of density on optical properties under pressure and temperature. The present work demonstrates that the initial density determination is a key point and that the uncertainties need to be improved. For that purpose, complementary experiments are conducted on LiF windows with simplified target designs and enriched diagnostics, coupling VISAR (532 nm) and PDV (1550 nm) diagnostics. In the future, a similar campaign will be conducted on sapphire windows.Conventional shock wave experiments need interferometric windows in order to determine the equation of state of a large variety of materials. Lithium fluoride (LiF) and sapphire are extensively used for that purpose because their optical transparencies enable the optical diagnostics at interfaces under a given range of shock pressure. In order to simulate and analyse the experiments it is necessary to gather a correct knowledge of the optical and mechanical properties for these windows. Therefore, our window supplies are systematically characterized and an experimental campaign under shock loading is conducted. Our preliminary work on LiF windows at 532 nm is in good agreement with literature data at room temperature and the new characterization at 450 K enables a better interpretation of our preheated target experiments and confirms the predominant effect of density on optical properties under pressure and temperature. The present work demonstrates that the initial density determination is a key point an...

Iron Damage and Spalling Behavior below and above Shock Induced α ⇔ ε Phase Transition

The study of dynamic damage and fracture of iron has been undertaken below and above phase transition by series of time resolved experiments using both light gas launcher and powder gun. Shock wave tests were conducted by symmetrical impacts of high purity iron. To reveal the material behavior we have done shock experiments where the target is covered with a window in order to limit release amplitude and to avoid specimen fragmentation. Metallurgical analysis of soft recovered samples yields information about damage and fracture processes related to thermo‐mechanical loading paths. Tests conducted without window allow studying effects of both phase change and release transition. Optical and SEM characterizations lead us to observe several modes of damage: brittle, ductile diffuse with void growth and heavily localized smooth one. These figures are related with: rarefaction shock waves or interfaces between transformed and not transformed iron. Simulations are performed with the 1D to compare experimental ...

Study of Spalling for High Purity Iron below and above Shock Induced α ⇔ ε Phase Transition

The study of the dynamic fracture of iron has been undertaken below and above phase transition. Light gas launcher and powder gun combined with Doppler Laser Interferometry (DLI) measurements have been used to evaluate the loading and spall fracture values. Symmetrical impacts have been conducted without windows below and above phase transformation to study the effects of both phase change kinetics and release transition on spallation response of iron. Shots performed below and above phase transition pressure show noticeable differences on pull back signals. Samples were soft recovered for SEM examination. Spall plane surfaces reveal several aspects linked to changes in material behavior. Simulations are conducted with a 2D hydrocode to compare experimental data with numerical results.