Dynamic fragmentation of laser shock-melted tin: experiment and modelling

Abstract

Dynamic fragmentation of shock-loaded metals is an issue of considerable importance for both basic science and a variety of technological applications, such as pyrotechnics or inertial confinement fusion, the latter involving high energy laser irradiation of thin metallic shells. Whereas spall fracture in solid materials has been extensively studied for many years, little data can be found yet about the evolution of this phenomenon after partial or full melting on compression or on release. Here, we present an investigation of dynamic fragmentation in laser shock-melted tin, from the "micro-spall" process (ejection of a cloud of fine droplets) occurring upon reflection of the compressive pulse from the target free surface, to the late rupture observed in the unspalled melted layer (leading to the formation of larger spherical fragments). Experimental results consist of time-resolved velocity measurements and post-shock observations of recovered targets and fragments. They provide original information regard-T. de. Rességuier (ing the loss of tensile strength associated with melting, the cavitation mechanism likely to occur in the melted metal, the sizes of the subsequent fragments and their ejection velocities. A theoretical description based on an energetic approach adapted to the case of a liquid metal is implemented as a failure criterion in a onedimensional hydrocode including a multi-phase equation of state for tin. The resulting predictions of the micro-spall process are compared with experimental data. In particular, the use of a new experimental technique to quantify the fragment size distributions leads to a much better agreement with theory than previously reported. Finally, a complementary approach focused on cavitation is proposed to evaluate the role of this phenomenon in the fragmentation of the melted metal.

Laser shock experiments to investigate and to model various aspects of the response of metals to shock loading

“…Upon reflection of a triangular compressive pulse (or "unsupported shock wave") from the free surface of a molten sample, cavitation is expected to occur under very low tension, very close to the free surface, and thus to lead to the ejection of a cloud of tiny droplets (referred to as "micro-spall") with a wide range of ejection velocities [10]. High pressure laser shocks have been applied onto tin samples to quantify the progressive loss of tensile strength associated with incipient melting on release [11], to investigate the debris ejection from relatively thick specimens after shock-induced melting [1,12], and to characterize more specifically the micro-spall ejected from thin foils [13]. In parallel, some theoretical issues have been addressed [14,15] and a well known, energy-based fragmentation model from the literature [16] has been adapted to the case of liquid metals then implemented in a hydrocode.…”

Exploration of the fragmentation of laser shock-melted aluminum using x-ray backlighting

“…This process, referred to as microspalling by some authors, 1,2 has been of increasing interest because of its scientific and engineering significance. [3][4][5][6][7][8][9] However, high-quality experimental work on the subject remains limited and experimental data, such as the mass distribution of the fragmentation products in space, particle size distribution, successive evolution of the fragmentation product topology and the thermodynamic states, are quite sparse. These data are essential for developing predictive models of the phenomenon.…”

“…[10][11][12][13][14][15][16] However, the development of high-energy laser facilities has enabled dynamic fragmentation experiments that employ intense laser irradiation, where, in most cases, particle size data of the ejecta have been obtained by means of a new technique involving postshock analysis of targets and recovered fragments. 3,5,[17][18][19] However, to date, the x-ray backlighting technique, which has been widely used to analyze and image plasmas in hydrodynamic experiments, [20][21][22][23][24][25][26] has not been used to study this phenomenon. With the advantages that the motion blur can be neglected owing to the short pulse of the driven laser beams, x-ray backlighting can clearly image high-velocity objects, and these images can be used to derive reliable mass density distribution of the objects.…”

Molecular dynamics studies of the roles of microstructure and thermal effects in spallation of aluminum

“…to ejection of solid scabs from the free surface. In microspallation, a solid-liquid phase transformation occurs by shock compression or on shock release before spall fracture starts (Xiang et al, 2013a,b;Andriot et al, 1984;Signor et al, 2010;Soulard, 2008;De Rességuier et al, 2010;Chen et al, 2012). In such cases, tensile stress emerges in melted materials and leads to the expansion of a cloud of fine liquid debris from the free surface.…”