A. Dragon

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...

Spallation in laser shock-loaded tin below and just above melting on release

Spall damage in solid materials has been one of the most widely studied shock-induced phenomena for several decades, for both applied and basic scientific motivations. Comparatively, very little data can be found yet about spallation in liquid metals. In a recent paper, we have reported an exploratory investigation of liquid spall in tin samples melted upon laser shocks of very high intensities. Here, we present further experimental results obtained over a lower pressure range, where we focus on the transition from the ductile fracture behavior of solid tin to the cavitating spall expected above melting. This transition is clearly evidenced from both time-resolved free surface velocity measurements and post-test examination of the recovered targets. The drop in tensile strength associated with melting is evaluated from the velocity profiles. Detailed views of the fracture surfaces in the spall craters provide an insight into the cavitation process. Experimental data are compared to preliminary computation...

On the dynamic fragmentation of laser shock-melted tin

Dynamic fragmentation in liquid metals is an issue of increasing interest in shock physics, both for basic and applied scientific motivations. In a recent paper, we have reported an exploratory study of liquid spall in tin samples melted upon laser-driven shocks. The need for further experiments and analyses to answer some questions raised by that work had been pointed out. Thus, we present here some complementary results, including specific recovery tests as well as high speed transverse shadowgraphy to visualize both the expanding cloud of fast droplets and the late motion of large fragments that had been inferred from postshock observations.

Micromechanical modelling of damage evolution in highly filled particulate composites – Induced effects at different scales:

An unconventional multi-scale approach is investigated to model interfacial damage and subsequent effects in highly filled particulate composites such as solid propellants. The basic framework by Nadot et al. (2006 Damage modelling framework for viscoelastic particulate composites via a scale transition approach, Journal of Theoretical and Applied Mechanics 44:553–583) is enhanced to deal with damage state and configuration evolution under any loading. Three criteria are formulated at the microscale. The first one concerns the nucleation of new defects, while the other two (for closure and re-opening) monitor the respective proportions of open and closed defects. The formulation exploits the explicit microstructure representation and the knowledge of the local displacement field as a function of local morphology around the interfaces. The multi-scale model is finally implemented through a numerical solving procedure able to deal with successive and/or simultaneous discrete damage events (nucleation/closure/re-opening of defects) depending on the loading path. Numerical illustrations are given for a three-dimensional microstructure containing 351 particles submitted to complex loading sequence involving extension, shear and contraction stages. The results show the ability of the model to estimate damage characteristics (position, orientation within the material) in addition to induced effects at both microscale and macroscale (induced anisotropy, unilateral effects and influence on local fields).

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.

Ballistic properties of debris produced by laser shock-induced micro-spallation of tin samples

Dynamic fragmentation in the liquid state after melting under shock compression or upon release leads to the ejection of a cloud of droplets. This phenomenon, called micro-spallation, remains essentially unexplored in most metals. We present laser shock experiments performed on tin, to pressures ranging from about 60 to 220 GPa. Experimental diagnostics include skew Photonic Doppler Velocimetry (PDV) measurements of the droplets velocities, transverse observations of the expanding cloud of droplets, and soft recovery of ejecta within a low density gel. Optical microscopy of the gel reveals the presence of droplets which confirm shock-induced melting prior to fragmentation. To quantify size distribution of the debris, 3D X-ray micro-tomography has been performed at the ESRF synchrotron facility in France (similar to US Advanced Photon Source), where sub-micrometer resolution could be achieved. In this paper, the resulting velocity and size distributions are presented and compared with theoretical predictions based on a one-dimensional description accounting for laser shock loading, wave propagation, phase transformations, and fragmentation. Discrepancies between measured and calculated distributions are discussed. Finally, combining size and velocity data provides estimates of the ballistic properties of debris and their kinetic energy, which are key issues for anticipating the damage produced by their impacts on nearby equipments.

Micro-Tomography to Characterize Size Distribution of Fragments Created by Laser Shock-Induced Micro-Spallation of Metallic Sample

Dynamic fragmentation in the liquid state after shock-induced melting, usually referred to as micro-spallation, is an issue of great interest for both basic and applied science. Recent efforts have been devoted to the characterization of the resulting ejecta, which consist in a cloud of fine molten droplets. We present laser shock experiments on tin and aluminium, to pressure ranging from about 50 to 300 GPa, with complementary diagnostics including a Photonic Doppler Velocimeter (PDV) set at a small tilt angle from the normal to the free surface, which enables probing the whole cloud of ejecta1, and a soft recovery device consisting of a low density gel to collect debris. Optical microscopy of these gel collectors reveals the presence of droplets which confirm shock-induced melting prior to fragmentation. To quantify size distribution of the collected debris, 3D micro-tomography has been performed, using the consistent and high-energy X-ray irradiation available at the ESRF synchrotron facility in France (similar to Japan SPRING-8), where sub-micrometer spatial resolution could be achieved. In this paper, the resulting size distributions are presented and compared with theoretical predictions based on a one-dimensional description accounting for laser shock loading, wave propagation, phase transformations, and fragmentation. Discrepancies between measured and calculated size distributions are discussed. Finally, combining size and velocity data provides access to the ballistic properties of debris and their kinetic energy, which are key issues for anticipating the damage produced by their impacts on nearly equipments.

TRANSITION FROM SOLID TO LIQUID SPALL IN TIN UNDER LASER SHOCKS OF INCREASING INTENSITY

When a shock‐loaded target is melted on compression or on release, the tensile stresses generated upon reflection of the pressure pulse from a free surface are induced in a liquid state. Instead of the well‐known spallation process occurring in solid targets, cavitation takes place in the melted material and liquid fragments are ejected from the free surface. Although increasing interest is manifested on the subject, related data are still scarce. In a recent paper, we have reported an exploratory investigation of liquid spall in tin targets submitted to intense laser shocks. Here, we present new results obtained over a lower pressure range, where we focus on the progressive transition from the ductile fracture behaviour of solid tin to the cavitation expected above melting. Both time‐resolved free surface velocity measurements and post‐test examination of the recovered samples clearly indicate this transition. Incipient melting produces a decrease in tensile strength that can be evaluated from the veloci...