Increase In Shock Velocity Research Articles

Shock Wave–Induced Dynamic Mechanical Behavior of Calcium Silicate Aluminate Hydrate at the Molecular Scale

Calcium silicate aluminate hydrate (C-A-S-H) is the main hydration product of cement mixed with industrial wastes. The purpose of this study is to understand the dynamic mechanical behavior and structural transformations of molecular-scale C-A-S-H induced by shock waves. Three C-A-S-H models with Al/Si ratios of 0.0, 0.1, and 0.2 are constructed and reactive molecular dynamics simulations are used to perform shock compressions with different shock velocities from 0.1 km/s to 3.6 km/s. The distributions of particle velocity, pressure, and density along the shock direction are calculated using the binning analysis method, allowing Hugoniot pressure-specific volume curves to be derived. The results reveal that shock waves may induce elastic, elastic-plastic, or shock Hugoniot responses in molecular-scale C-A-S-H, depending on the Al/Si ratio and the shock velocity. Below the Hugoniot elastic limit (HEL), higher Al/Si ratios cause the elastic wave to propagate farther due to the cross-linking effect of aluminate units. Above the HEL, higher Al/Si ratios give rise to a distinct two-wave structure characteristic comprising a plastic front and an elastic precursor. This characteristic becomes less pronounced as the shock velocity increases. Analysis of the molecular structural transformations of C-A-S-H revealed that the main atomic deformation behavior below the HEL involves a reduction of interatomic distances; above the HEL the main response is a densification of water molecules followed by a general collapse of the layered structure as the shock velocity increases.

Molecular dynamics study of shock-induced deformation phenomena and spallation failure in Ni-based single crystal superalloys

Shock-induced dynamic mechanical behavior and spallation failure of Ni-based single crystal superalloys are studied via non-equilibrium molecular dynamic simulations. The results show that the spallation failure of Ni-based single crystal superalloys exhibits two modes of classical spallation and micro-spallation. At low shock velocity, the deformation mechanism is dominated by slip and drag of dislocations. As the shock velocity increases, the atomic structure undergoes complex phase transitions from FCC to BCC or disordered structures, and the spallation strength gradually decreases. The decreasing tendency of the spallation strength corresponds to the characteristics of the deformation process. Moreover, the spallation strength and spallation threshold velocity drop significantly with increasing initial temperature, which is due to the softening of Ni-based single crystal superalloys at high temperatures. In addition, the plastic mechanism dominated by shear stress leads to void collapse along the vertical direction at lower shock velocities, while the internal jetting mechanism, controlled by normal stress, induces void collapse in the horizontal direction under higher shock velocities. The existence of voids significantly lowers the spallation strength, and the amplitude reduction of the strength is directly proportional to the radius of the voids.

Phase transition in shock compressed high-entropy alloy FeNiCrCoCu

High-entropy alloys (HEAs) show promising prospects to be extensively applied as functional and structural applications. Nevertheless, due to technical limitations in real-timely detecting microstructural evolution at the atomic level, an in-depth understanding with regard to dynamic deformation mechanisms is still limited. In present work, nonequilibrium molecular dynamics simulations were performed to investigate the shock-induced phase transition for the equiatomic FeNiCrCoCu HEA in terms of the crystallographic direction and shock velocity. The face-centered cubic to body-centered cubic phase transition due to uniaxial compression and lattice rotation was demonstrated to be prone to appearing for shock along the [001] orientation. This behavior was conducive to activating dislocation nucleation to release shear stress. More importantly, a shift from a dislocation-dominated deformation to a phase-transition-dominated one with the increase in shock velocity was corroborated to facilitate the swift stress relaxation at higher strains, contributing to the attenuation of the shock wave and thereby weakening the shock damage. These outcomes render valuable insights into understanding the dynamic deformation behavior of the FeNiCrCoCu HEA.

Shock-induced amorphization in medium entropy alloy CoCrNi

We perform molecular dynamics simulations to investigate shock-induced amorphization in CoCrNi, a medium entropy alloy (MEA) and its mean-field variant without lattice distortion. We show that a critical velocity exists above which amorphization occurs. At a low shock velocity of 800 m/s, dislocation slip and twins dominate and amorphization does not happen, but as the shock velocity increases, the deformation mechanism transitions from slip and twinning to solid-state amorphization. Under ultra-high shock velocities, extensive amorphization occurs, following the precursor of shock wave, eliminating anisotropy in spall strength. Compared to the mean-field model, lattice distortion in the MEA causes substantially more amorphization, resulting in a lower spall strength, since voids nucleate and grow preferentially in the amorphous regions.

Initial dynamic response and reaction mechanism of cyclotrimethylenetrinitramine under shock loading

At present, the relative safety of energetic materials exposed to extreme environments is concerned widely. Understanding the initial decomposition mechanism of energetic materials under impact loading is the basis for exploring new energetic materials with high energy and low sensitivity. In this paper, we study the initial dynamic response and reaction mechanism of perfect cyclotrimethylenetrinitramine (RDX) crystal and RDX crystal with a molecular vacancy defect under shock loading by using the multiscale shock technique (MSST) combined with reactive force field (ReaxFF) molecular dynamics method. The RDX perfect supercell and supercell containing a molecular vacancy are constructed to simulate the shock process by using the generalized gradient approximation method in density functional theory and Perdew-Burke-Ernzerhof functional. Before loading the shock wave, one NVE ensemble and Berendsen thermostat are used to control the RDX equilibrium process. A multi-scale impact compression is loaded along the crystal <i>A</i> direction. The initial temperature is 300 K and the initial pressure is set to be an atmospheric pressure. The radial distribution functions between main atoms are calculated, and the influences of shock velocity and molecular vacancy defect on shock loading process are analyzed. The evolution of N—NO<sub>2</sub> bond and C—N bond with time in RDX perfect crystals and vacancy crystals under shock velocity of 11 km/s are given. As a result, the possible initial decomposition path of perfect RDX crystal and vacancy RDX crystal are the first fracture of N—NO<sub>2</sub> bond, followed by the cleavage of C—N bond at small shock velocity. The initial reaction of the RDX crystal with a molecule vacancy is earlier than that of the perfect crystal, which indicates that the vacancy crystal is more sensitive to shock and more prone to decomposition. Furthermore, the fracture of C—H bond is possible after the initial cleavage of N—NO<sub>2</sub> bond and C—N bond, and then the H atom is transferred to oxygen atom in nitro group, forming HONO. As the shock velocity increases, the number of broken chemical bonds in the two kinds of RDX crystals increases, and the reaction becomes strong. The presence of molecular vacancy defect enhances the activity of N—NO<sub>2</sub> bond and makes it easier to break, thus accelerating the initial reaction of the vacancy crystal. The shock velocity and the particle velocity of the RDX crystal are consistent with previous experimental results and theoretical data, which shows the validity of our calculation results.

Exact Solution for an Unsteady Isothermal Flow Behind a Cylindrical Shock Wave in a Rotating Perfect Gas with an Axial Magnetic Field and Variable Density

An exact solution of the problem on the propagation of a cylindrical shock wave in a rotating perfect gas with an axial magnetic field in the case of an isothermal flow is obtained. The initial density, magnetic field strength, and the initial angular velocity in the ambient medium are assumed to vary according to the power law. An exact similarity solution obtained by the McVittie method for an isothermal flow in a rotating gas is first reported. Similarity transformations are used to transform a system of partial differential equations into a system of ordinary differential equations, and then the product solution of McVittie is used to obtain the exact solution. The effects of the values of the gas specific heat ratio, rotational parameter, and of the strength of the initial magnetic field are discussed. It is shown that the shock velocity increases and the shock strength decreases with increase in the values of these parameters. The effect of variation in the value of the initial density index is also studied. The obtained solutions show that the radial fluid velocity, density, pressure, and the magnetic field strength tend to zero as the axis of symmetry is approached.

Shock propagation in regular wetted arrays of fibers

We present results of shock propagation in a wetted foam modeled as a regular staggered row lattice of parallel cylinders in two-dimensions immersed in a background of different density. Both media are perfect mono-atomic gases. We show that shock velocity increases due to the heterogeneity of wetted foams in comparison to the velocity of shock in a homogeneous medium with the same average density. The post-shock medium is characterized by turbulence and multiple vortices. Destructive or constructive interactions between vortices appear in the downstream fluid depending on the fiber alignment. We show that the shock velocity increase is related to the kinetic energy stored in the downstream fluid in the turbulence and the vortices. A turbulent model is used to analytically relate the turbulent kinetic energy to the shock velocity increase.

Initial Decomposition of the Condensed-Phase β-HMX under Shock Waves: Molecular Dynamics Simulations

We have performed quantum-based multiscale simulations to study the initial chemical processes of condensed-phase octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX) under shock wave loading. A self-consistent charge density-functional tight-binding (SCC-DFTB) method was employed. The results show that the initial decomposition of shocked HMX is triggered by the N-NO(2) bond breaking under the low velocity impact (8 km/s). As the shock velocity increases (11 km/s), the homolytic cleavage of the N-NO(2) bond is suppressed under high pressure, the C-H bond dissociation becomes the primary pathway for HMX decomposition in its early stages. It is accompanied by a five-membered ring formation and hydrogen transfer from the CH(2) group to the -NO(2) group. Our simulations suggest that the initial chemical processes of shocked HMX are dependent on the impact velocity, which gain new insights into the initial decomposition mechanism of HMX upon shock loading at the atomistic level, and have important implications for understanding and development of energetic materials.

Shock velocity increase due to a heterogeneity produced by a two-gas layer

Shock tube experiments are performed in order to study shock propagation along a two-gas layer in a confined geometry and to compare it to the case of a homogeneous density equivalent mixture. The analysis of the homogeneous case gives values for the adiabatic coefficient and density of the mixture of both gases, while the comparison between heterogeneous and homogeneous media with the same averaged density shows modifications of the shock front shape and velocity. In the two-gas layer, the shock propagates faster than in the homogeneous medium. The shock front is curved with a triple point which appears close to the shock-tube wall, in the slow medium, while it stays planar during its whole propagation in the homogeneous mixture. A correlation is found between the angle of curvature and the shock velocity increase. It is confirmed by two-dimensional Eulerian numerical calculations. Experiments and calculations exhibit very good agreement on all the measurements when molecular diffusion is taken into account in the numerical calculations. A sustained irregular refraction pattern of the shock front at the diffuse interface of both gases is obtained experimentally and confirmed by the calculations.

Multiple spherically converging shock waves in liquid deuterium

To achieve ignition, inertial confinement fusion target designs use a sequence of shocks to compress the target before it implodes. To minimize the entropy acquired by the fuel, the strength and timing of these shocks will be precisely set during a series of tuning experiments that adjust the laser pulse to achieve optimal conditions. We report measurements of the velocity and timing of multiple, converging shock waves inside spherical targets filled with liquid (cryogenic) deuterium. These experiments produced the highest reported shock velocity observed in liquid deuterium (Us = 135 km/s at ∼25 Mb) and observed an increase in shock velocity due to spherical convergence. These direct-drive experiments are best simulated when hydrodynamic codes use a nonlocal model for the transport of absorbed laser energy from the coronal plasma to the ablation surface.

Formation of a detonation-like condensation wave

Amplification of a shock wave and its transition to a detonation-like regime due to the energy release in the course of condensation of a strongly supersaturated carbon vapor are observed experimentally. The carbon vapor behind the shock front is formed as a result of the thermal dissociation, C3O2 → C + 2CO, of the unstable carbon suboxide in a mixture of 10% C3O2 and 90% Ar. The rapid condensation of the vapor into nanoparticles gives rise to a temperature increase by over 500 K from 1600–2200 to 2200–2800 K, a pressure increase from 4 to 6 bar, and the resulting shock-velocity increase by 130–170 m/s. An analysis of the kinetics of the heat release in these mixtures shows that the temperature increase due to the particle formation is observed during a few microseconds if the initial temperature exceeds 1800 K. Calculations of the Hugoniot relations for the initial and final mixtures indicate that the supercompressed detonation is observed in the studied flow regimes. It is shown that the conditions for self-sustained detonation can be realized by increasing the C3O2 content in the mixture.

Shock velocity in weakly ionized nitrogen, air, and argon

The goal of this research was to determine the principal mechanism(s) for the shock velocity increase in weakly ionized gases. This paper reports experimental data on the propagation of spark-generated shock waves (1<Mach<3) into weakly ionized nitrogen, air, and argon glow discharges (1<p<20Torr). In order to distinguish between effects due solely to the presence of electrons and effects due to heating of the background gas via elastic collisions with electrons, the weakly ionized discharge was pulsed on/off. Laser deflection methods determined the shock velocity, and the electron number density was collected using a microwave hairpin resonator. In the afterglow of nitrogen, air, and argon discharges, the shock velocity first decreased, not at the characteristic time for electrons to diffuse to the walls, but rather at the characteristic time for the centerline gas temperature to equilibrate with the wall temperature. These data support the conclusion that the principal mechanism for the increase in shock velocity in weakly ionized gases is thermal heating of the neutral gas species via elastic collisions with electrons.

Shock-induced reaction in a flake nickel + spherical aluminum powder mixture

Mixtures of flake-shaped nickel and spherical aluminum powders have been subjected to shock-loading up to 6 GPa to investigate the occurrence of shock-induced chemical reactions. The high-pressure Hugoniot state is determined through time-resolved measurements of the input stress and shock transit time through the specimen. An increase in shock velocity is observed above stress levels of 3.5 GPa, suggesting the initiation of an ultrafast shock-induced chemical reaction and formation of Ni-Al compound(s) occurring in the time scale of the high-pressure shock state.

Local acoustic shock velocity and shock structure recovery measurements in glow discharges

Simultaneous local acoustic shock wave velocity measurements show that the upstream velocity exceeds the downstream velocity inside a low pressure positive column nitrogen plasma in contrast to the opposite behavior in neutral gas. The shock velocity increase is also dependent on the direction of the applied electric field.

Strong spherical shock in a self-gravitating gas

The Chisnell-Chester-Whitham method has been used to investigate the propagation of strong diverging spherical shock waves through a self-gravitating gas having an initial density distribution ϱ0=βr-w, where r is the distance from the origin,β andw are constants. Analytical expressions for shock velocity and shock strength have been obtained. It is observed that the casew =A + 2, whereA = 8γ[2γ/(γ - 1) + + √2γ/(γ - 1)2 + √2γ(γ - 1)]-1 and γ is the adiabatic index of the gas, corresponds to the shock moving with constant strength. An increase inw from 0 toA + 2 reduces the shock velocity, whereas, for further increase in w fromA + 2 to 3, the shock velocity increases. Finally, the expressions for the pressure, the density and the particle velocity immediately behind the shock have been obtained.

Energy considerations of plane and cylindrical hydromagnetic variable energy blast waves

Numerical results describing the propagation of plane and cylindrical blast waves with time dependent energy inputs in an ideal gas in presence of an axial magnetic field have been obtained by modified similarity method. These results are in agreement with Patrick's [6] experimental observations of non-existence of ordinary shock waves. For strong shocks the hottest region is located near the piston, whereas for weaker shocks the front becomes the hottest. The presence of magnetic field, however, reduces the temperature in the shocked region, except near the piston where it increases. An increase in magnetic field leads to an increase in shock velocity. Predicted variations of the percentages of magnetic, internal and kinetic energies with shock strength support the experimental observations of Kolb [3]. With increase inβ, equipartition between kinetic and internal energies is seen to shift towards higher shock strength.