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An explicit failure model for ceramics undergoing a solid–solid phase transition under shock compression is developed and tested on silicon carbide and aluminum nitride. This model enhances the applicability of our failure model recently developed for boron carbide. Smoothed particle hydrodynamics simulations of ceramics under shock loading are performed to optimize the model parameters using the velocity profiles obtained in available shock-wave experiments. It is demonstrated that the inclusion of a phase transition with hysteresis is essential for agreement between simulations and experiments. Evolution of damage spreading in samples with propagation of the failure wave front is discussed. We show that it changes from a homogeneous damage pattern to regular structures of failure bands, where growth is guided by distributions of equivalent stress and shear strength of material within the band tips. [ABSTRACT FROM AUTHOR]

Richtmyer–Meshkov instability developed at a solid–vacuum interface after reflection of a shock wave is studied using the smoothed particle hydrodynamics (SPH) method. SPH simulations are performed for aluminum, copper, and tantalum samples with free surfaces having machined grooves of sinusoidal shape. The obtained simulation results agree well with the experimental data for different loading regimes. Our simulations demonstrate three regimes of material response to shock loading, where conditions depend on the yield strength for a given strain rate. First, at weak elastic shocks, the grooved surface experiences shear oscillations only. Then, a more intense shock loading produces plastic strain resulting in a plastic spike with the limited run from the surface. It is found that after the arrest of the plastic motion, the formed spike oscillates with the same period as in the elastic regime. Finally, the heavy load produces the unlimited growth of plastic or liquid jet, which leads to its fragmentation at later times. The transition from limited to unlimited jet growth depends on the geometry of the corrugated surface. We estimate the critical amplitude of corrugations required for unlimited spike growth. The used simulation techniques can provide the more accurate mechanical properties of materials to achieve a better agreement. [ABSTRACT FROM AUTHOR]

Clusters of atoms in dense gold vapor are studied via atomistic simulation with the classical molecular dynamics method. For this purpose, we develop a new embedded atom model potential applicable to the lightest gold clusters and to the bulk gold. Simulation provides the equilibrium vapor phases at several subcritical temperatures, in which the clusters comprising up to 26 atoms are detected and analyzed. The cluster size distributions are found to match both the two-parameter model and the classical nucleation theory with the Tolman correction. For the gold liquid–vapor interface, the ratio of the Tolman length to the radius of a molecular cell in the liquid amounts to ∼0.16, almost exactly the value at which both models are identical. It is demonstrated that the lightest clusters have the chain-like structure, which is close to the freely jointed chain. Thus, the smallest clusters can be treated as the quasi-fractals with the fractal dimensionality close to two. Our analysis indicates that the cluster structural transition from the solid-like to chain-like geometry occurs in a wide temperature range around 2500 K. [ABSTRACT FROM AUTHOR]

Predictive simulation of the long-term response of multilayer targets with ceramics layers to shock compression demands appropriate material models. Because ceramics are complex brittle materials, which tend to lose their strength under heavy loads, such simulation requires the failure models well-proven for a wide range of strains and strain rates. Standard plate impact experiments provide the main data utilized for developing and validating the mechanical models of material response to shock compression. However, apart from the fact that such experimental data are inherently one-dimensional, they can be insufficient to verify the failure model at relatively low strain rates typical for long unloading waves. Here, we present the experimental results for explosive compression of a spherical multilayer shell initiated by a single detonator. The explosive-coated shell consists of the nested spherical layers: the outer made of boron carbide and the inner of lead. X-ray images showing the evolution of those layers after detonation are then compared with simulation results. Propagation of the compression wave through the layers resulting in ceramics damage is analyzed in detail. We demonstrate that the failure model of boron carbide should be adjusted for compressions below 10 GPa to achieve a good agreement with our experimental images. Such an improved failure model provides the predictive simulation of long-term dynamics of targets after unloading, and it has almost no effect on wave profiles after plate impact. [ABSTRACT FROM AUTHOR]

Ceramic materials have a long-term industrial demand due to their high mechanical hardness and chemical and temperature resistance. They are brittle and tend to lose strength under heavy loads which complicates the development of a comprehensive material model for simulation of engineering prototypes containing ceramic parts. We developed an improved failure model of ceramics based on the well-known Johnson–Holmquist approach. This model redefines the damage rate equation using a consistent definition of the total plastic strain in the failed material. It reduces the number of free model parameters and enables the plastic strain to be explicitly accumulated during the failure process. The corresponding non-iterative algorithm utilizing this explicit failure model is developed. It is successfully validated by simulation of the wave profiles obtained in plate-impact experiments with boron carbide using the contact smoothed particle hydrodynamic method. [ABSTRACT FROM AUTHOR]

Fundamental physical phenomena in metals irradiated by ultrashort laser pulses with absorbed fluences higher than few tens of mJ/cm2 are investigated. For those fluences, laser-produced electron distribution function relaxes to equilibrium Fermi distribution with electron temperature T e within a short time of 10-100 fs. Because the electron subsystem has T e highly exceeding much the ion subsystem temperature T i the well-known twotemperature hydrodynamic model (2T-HD) is used to evaluate heat propagation associated with hot conductive electron diffusion and electron-ion energy exchange. The model coefficients of electron heat conductivity κ ( ϱ, T e, T i) and electron-ion coupling parameter α ( ϱ, T e) together with 2T equation of state E ( ϱ, T e, T i) and P ( ϱ, T e, T i) are calculated. Modeling with 2T-HD code shows transition of electron heat wave from supersonic to subsonic regime of prop-agation. At the moment of transition the heat wave emits a compression wave moving into the bulk of met al. Nonlinear evolution of the compression wave after its separation from the subsonic heat wave till spallation of rear-side layer of a film is traced in both 2T-HD modeling and molecular dynamics (MD) simulation. For fluences above some threshold the nucleation of voids in frontal surface layer is initiated by strong tensile wave following the compression wave. If the absorbed fluence is ∼30 % above the ablation threshold than void nucleation develops quickly to heavily foam the molten met al. Long-term evolution of the metal foam including foam breaking and freezing is simulated. It is shown that surface nano-structures observed in experiments are produced by very fast cooling of surface molten layer followed by recrystallization of supercooled liquid in disintegrating foam having complex geometry. Characteristic lengths of such surface nanostructures, including frozen pikes and bubbles, are of the order of thickness of molten layer formed right after laser irradiation. (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [ABSTRACT FROM AUTHOR]

• Experiments show similar ablation craters in Ru caused by different fs light sources. • Absorbed energy localizes near Ru surface due to Auger electrons. • Energy localization causes thermo-mechanical ablation via stress confinement. • Damage mechanism at grazing incidence conditions does not depend on photon energy. We performed combined experimental and computational research on damage processes in ruthenium thin films induced by femtosecond lasers with various photon energies. We present an experiment with an optical laser at normal incidence conditions and compare it with previously reported experiments at grazing incidence conditions with XUV and hard X-ray photons, covering a large range of photon energies. Analysis of ablation craters in Ru shows very similar crater morphology and depth of about 10–20 nm for all considered irradiation conditions. Simulations of light-matter interactions are performed with our combined Monte Carlo and two-temperature hydrodynamics approach. The simulation results show that the primal cause of eventual ablation is Auger decay of core-shell holes created after absorption of XUV and hard X-ray photons in the vicinity of ruthenium surface. They lead to the creation of many low-energy electrons which consequently release the absorbed energy near the surface, resembling the optical irradiation case. Similar absorbed energy distributions in the top part of ruthenium induce a similar thermo-mechanical response and, therefore, similar ablation process. Our results suggest that such mechanism is universal in a wide range of photon energies at grazing incidence conditions, when the photon absorption depth is smaller than the photoelectrons range. [ABSTRACT FROM AUTHOR]