Showing papers on "Shock (mechanics) published in 2004"

Turbulent amplification of magnetic field and diffusive shock acceleration of cosmic rays

Abstract: The diffusive shock acceleration of cosmic rays by supernova remnants depends upon the generation of magnetic fluctuations by cosmic rays upstream of the shock. Strongly driven, non-resonant, nearly purely growing modes grow more rapidly than the resonant Alfven waves usually considered. Non-linear simulation shows that the magnetic field can be amplified from its seed value by orders of magnitude. The consequences for the maximum attainable cosmic ray energy in supernova remnants are explored.

Line-imaging velocimeter for shock diagnostics at the OMEGA laser facility

Abstract: A line-imaging velocity interferometer has been implemented at the OMEGA laser facility of the Laboratory for Laser Energetics, University of Rochester. This instrument is the primary diagnostic for a variety of experiments involving laser-driven shock-wave propagation, including high-pressure equation of state experiments, materials characterization experiments, shock characterization for Rayleigh–Taylor experiments, and shock timing experiments for inertial confinement fusion research. Using a laser probe beam to illuminate a target, the instrument measures shock breakout times at temporal resolutions as low as 20 ps, and spatial resolution ∼4 μm. For velocity measurements the detection limit is <0.1 km/s, and velocities of interfaces, free surfaces, and shock fronts traveling through transparent media can be measured with accuracies ∼1% over the range from 4 km/s to greater than 50 km/s. Quantitative measurements of the optical reflectance of ionizing shock fronts can also be obtained simultaneously with the velocity measurements.

Coseismic and postseismic velocity changes measured by repeating earthquakes

Abstract: [1] Repeating earthquakes that rupture approximately the same fault patch and have nearly identical waveforms are a useful tool for measuring temporal changes in wave propagation in the Earth's crust. Since source and path effects are common to all earthquakes in a repeating earthquake sequence (multiplet), differences in their waveforms can be attributed to changes in the characteristics of the medium. We have identified over 20 multiplets containing between 5 and 40 repeating events in the aftershock zones of the 1989 Loma Prieta and 1984 Morgan Hill, California, earthquakes. Postmain shock events reveal delays of phases in the early S wave coda of as much as 0.2 s relative to premain shock events. The delay amounts to a path-averaged coseismic velocity decrease of about 1.5% for P waves and 3.5% for S waves. Since most of the multiplets are aftershocks and follow Omori's law, we have excellent temporal sampling in the immediate postmain shock period. We find that the amplitude of the velocity decrease decays logarithmically in time following the main shock. In some cases it returns to the premain shock values, while in others it does not. Similar results are obtained for the Morgan Hill main shock. Because the fractional change in S wave velocity is greater than the fractional change in P wave velocity, it suggests that the opening or connection of fluid-filled fractures is the underlying cause. The magnitude of the velocity change implies that low effective pressures are present in the source region of the velocity change. Our results suggest that the changes are predominantly near the stations and shallow, but we cannot exclude the possibility that changes occur at greater depth as well. If the variations are shallow, we may be detecting the lingering effects of nonlinearity during main shock strong ground motion. If the variations are deep, it suggests that pore pressures at seismogenic depths are high, which would likely play a key role in the earthquake process.

Shock-induced star formation in a model of the Mice

Abstract: Star formation plays an important role in the fate of interacting galaxies. To date, most galactic simulations including star formation have used a density-dependent star formation rule designed to approximate a Schmidt law. Here, I present a new star formation rule which is governed by the local rate of energy dissipation in shocks. The new and old rules are compared using self-consistent simulations of NGC 4676; shock-induced star formation provides a better match to the observations of this system.

Dynamic Response of a Clamped Circular Sandwich Plate Subject to Shock Loading

Abstract: An analytical model is developed for the deformation response of clamped circular sandwich plates subjected to shock loading in air and in water. The deformation history is divided into three sequential stages and analytical expressions are derived for the deflection, degree of core compression, and for the overall structural response time. An explicit finite element method is employed to assess the accuracy of the analytical formulas for the simplified case where the effects of fluid-structure interaction are neglected. The sandwich panel response has only a low sensitivity to the magnitude of the core compressive strength and to the degree of strain hardening in the face-sheets. The finite element results confirm the accuracy of the analytical predictions for the rigid ideally plastic sandwich plates. The analytical formulas are employed to determine optimal geometries of the sandwich plates that maximize the shock resistance of the plates for a given mass. The optimization reveals that sandwich plates have a superior shock resistance relative to monolithic plates of the same mass. @DOI: 10.1115/1.1778416#

Selected problems in collisionless-shock physics

Abstract: The physics of collisionless shocks is a very broad topic, which has been well studied for many decades. However, there are a number of important issues which remain unresolved. Moreover, there have been new findings, which cast doubt on well-established ideas. The purpose of this review is to address a subset of unresolved problems in collisionless shock physics from a theoretical and/or numerical modeling point of view. The topics which are addressed are: the nonstationarity of the shock front, the heating and dynamics of electrons through the shock layer, particle diffusion in turbulent electric and magnetic fields, particle acceleration, and the interaction of pickup ions with collisionless shocks.

Models for reliability prediction of fine-pitch BGAs and CSPs in shock and drop-impact

Abstract: Drop-induced failures are most dominant in portable electronic products. In this study, explicit finite element models have been used to study the transient dynamics of printed circuit boards during drop from 6ft. Methodologies for modeling components using smeared property formulations have been investigated. Reduced integration element formulations examined include - shell and solid elements. Model predictions have been validated with experimental data. Results show that models with smeared properties can predict transient-dynamic response of board assemblies in drop-impact, fairly accurately. High-speed data acquisition system has been used to capture in-situ strain, continuity and acceleration data in excess of 1 million samples per second. Ultra high-speed video at 40,000 fps per second has been used to capture the deformation kinematics. Component types examined include - plastic ball-grid arrays, tape-array BGA, QFN, and C2BGA. Model predictions have been correlated with experimental data. Impact of experimental error sources on model, correlation with experiments also has been investigated.

Microscale Laser Shock Peening of Thin Films, Part 1: Experiment, Modeling and Simulation

Abstract: Microscale Laser Shock Peening (LSP), also known as Laser Shock Processing, is a technique that can be potentially applied to manipulate residual stress distributions in metal film structures and thus improve the fatigue performances of micro-devices made of such films. In this study, microscale LSP of copper films on single crystal silicon substrate is investigated. Before and after-process curvature measurement verifies that sizable compressive residual stress can be induced in copper thin films using microscale LSP. Improved modeling work of shock pressure is summarized and the computed shock pressure is used as loading in 3D stress/strain analysis of the layered film structure. Simulation shows that the stress/strain distribution in the metal film is close to equi-biaxial and is coupled into the silicon substrate.

Temperature-stress fields and related phenomena induced by a high current pulsed electron beam

Abstract: Physical models and numerical simulations are applied to describe the thermal–dynamical processes of the high current pulsed electron beam (HCPEB) treatment. The simulation of the temperature distributions reveals an ultrahigh heating/cooling rate in the order of 10 8 –10 9 K/s, as well as rapid melting and re-solidification within microseconds in time and micrometers in depth. It is also pointed out that the melting starts at a sublayer about 1–2 μm in depth, which constitutes the crater formation mechanism. A temperature-induced dynamic thermal stress fields can then generate three principal stress, the quasi-static stress, the thermoelastic stress and the shock stress, the latter two being stress waves. The thermoelastic stress wave has small amplitudes less than 0.1 MPa. The shock stress wave however is a typical nonlinear wave, several hundreds of MPa in amplitudes, much stronger than the thermoelastic stress wave, and has a strong impact on materials structure and properties far beyond the heat-affected zone. The maximum compressive quasi-static stress in the surface layer reaches several hundreds of MPa, which easily induces surface deformation in metallic materials.

Early afterglow emission from a reverse shock as a diagnostic tool for gamma-ray burst outflows

Abstract: The gamma-ray burst‐afterglow transition is one of the most interesting and least studied gamma-ray burst phases During this phase, the relativistic ejecta begins interacting with the surrounding matter A strong short-lived reverse shock propagates into the ejecta (provided that it is baryonic) while the forward shock begins to shape the surrounding matter into a Blandford‐McKee profile We suggest a parametrization of the early afterglow light curve and we calculate (analytically and numerically) the observed parameters that result from a reverse shock emission (in an interstellar medium environment) We present a new fingerprint of the reverse shock emission that is added to the well-known t −2 optical decay Observation of this signature would indicate that the reverse shock dominates the emission during the early afterglow The existence of a reverse shock will in turn imply that the relativistic ejecta contains a significant baryonic component This signature would also imply that the surrounding medium is an interstellar medium We further show the following (i) The reverse shock optical flash depends strongly on initial conditions of the relativistic ejecta (ii) Previous calculations have generally overestimated the strength of this optical flash (iii) If the reverse shock dominates the optical flash, then detailed observations of the early afterglow light curve would possibly enable us to determine the initial physical conditions within the relativistic ejecta and specifically to estimate its Lorentz factor and its width Ke yw ords: hydrodynamics ‐ shock waves ‐ gamma-rays: bursts

Stability of Large-Amplitude Viscous Shock Profiles of Hyperbolic-Parabolic Systems

Abstract: We establish nonlinear L1∩H3→Lp orbital stability, 2≦p≤∞, with sharp rates of decay, of large-amplitude Lax-type shock profiles for a class of symmetric hyperbolic-parabolic systems including compressible gas dynamics and magnetohydrodynamics (MHD) under the necessary conditions of strong spectral stability, i.e., a stable point spectrum of the linearized operator about the wave, transversality of the profile, and hyperbolic stability of the associated ideal shock. This yields in particular, together with the spectral stability results of [50], the nonlinear stability of arbitrarily large-amplitude shock profiles of isentropic Navier–Stokes equations for a gamma-law gas as γ→1: the first complete large-amplitude stability result for a shock profile of a system with real (i.e., partial) viscosity. A corresponding small-amplitude result was established in [53, 54] for general systems of Kawashima class by a combination of ‘‘Kawashima-type’’ energy estimates and pointwise Green function bounds, where the small-amplitude assumption was used only to close the energy estimates. Here, under the mild additional assumption that hyperbolic characteristic speeds (relative to the shock) are not only nonzero but of a common sign, we close the estimates instead by use of a Goodman-type weighted norm [25, 26] designed to control estimates in the crucial hyperbolic modes.

Constant-stress Hugoniostat method for following the dynamical evolution of shocked matter

Abstract: We present an alternative equilibrium molecular dynamics method---the uniaxial constant-stress Hugoniostat---for following the dynamical evolution of condensed matter subjected to shock waves. It is a natural extension of the recently developed uniaxial constant-volume Hugoniostat [Maillet et al., Phys. Rev. E 63, 016121 (2001)]. Integral feedback is employed to reach the Hugoniot (final) state of the shock process by controlling both the normal component of the stress tensor and internal energy. The finite strain rate imposed on the system is closely related to that inherent in the front of a shock wave. The method can easily identify phase transitions along the Hugoniot shock states, even those that exhibit multiple wave structures. As an example of the method, we have simulated the Hugoniot of a Lennard-Jones crystal shocked along the $⟨110⟩$ direction. The results agree well with multi-million-atom nonequilibrium molecular-dynamics simulations.

Observation of laser driven supercritical radiative shock precursors.

Abstract: We present a supercritical radiative shock experiment performed with the LULI nanosecond laser facility. Using targets filled with xenon gas at low pressure, the propagation of a strong shock with a radiative precursor is evidenced. The main measured shock quantities (electronic density and propagation velocity) are shown to be in good agreement with theory and numerical simulations.

Early Afterglow Emission from a Reverse Shock as a Diagnostic Tool for GRB Outflows

Abstract: The Gamma-Ray burst (GRB) - afterglow transition is one of the most interesting and least studied GRB phases. During this phase the relativistic ejecta begins interacting with the surrounding matter. A strong short lived reverse shock propagates into the ejecta (provided that it is baryonic) while the forward shock begins to shape the surrounding matter into a Blandford-McKee profile. We suggest a parametrization of the early afterglow light curve and we calculate (analytically and numerically) the observed parameters that results from a reverse shock emission (in an interstellar medium [ISM] environment). We present a new fingerprint of the reverse shock emission that is added to the well known $t^{-2}$ optical decay. Observation of this signature would indicate that the reverse shock dominates the emission during the early afterglow. The existence of a reverse shock will in turn imply that the relativistic ejecta contains a significant baryonic component. This signature would also imply that the surrounding medium is an ISM. We further show that: (i) The reverse shock optical flash depends strongly on initial conditions of the relativistic ejecta. (ii) Previous calculations have generally overestimated the strength of this optical flash. (iii) If the reverse shock dominates the optical flash then detailed observations of the early afterglow light curve would possibly enable us to determine the initial physical conditions within the relativistic ejecta and specifically to estimate its Lorentz factor and its width.

Large-Scale Hybrid Simulations of Particle Acceleration at a Parallel Shock

Abstract: Using one-dimensional hybrid simulations with very large spatial domains, we study the acceleration of protons by a parallel collisionless shock to energies higher than have been obtained previously with self-consistent plasma simulations of this type. Energy spectra and energetic particle fluxes are determined for four simulations with different-sized spatial domains. We find that the density of energetic particles upstream decays with distance from the shock and approaches a constant. The energetic particles also excite magnetic fluctuations. We find that the variance of these transverse fluctuations decreases with distance from the shock into the upstream region. This implies that the mean free path, λ, of the energetic particles increases with distance from the shock. Since our simulations are spatially limited, the downstream energy spectra are expected to deviate from a power law and become exponential at a characteristic energy Ec. However, we find that because λ increases with distance upstream, Ec is smaller than expected from a simple application of the diffusive theory (which assumes a constant mean free path and a free-escape boundary). Our results are qualitatively consistent with diffusive theories of the coupling of the particles and self-generated waves. However, in contrast to these theories, the hybrid-simulated energetic particle flux approaches a constant at some point far upstream, rather than vanishing, as assumed in the diffusion theory.

Laser shock peening on fatigue crack growth behaviour of aluminium alloy

Abstract: The effect of laser shock peening (LPS) in the fatigue crack growth behaviour of a 2024-T3 aluminium alloy with various notch geometries was investigated. LPS was performed under a 'confined ablation mode' using an Nd: glass laser at a laser power density of 5 GW cm -2 . A black paint coating layer and water layer was used as a sacrificial and plasma confinement layer, respectively. The shock wave propagates into the material, causing the surface layer to deform plastically, and thereby, develop a residual compressive stress at the surface. The residual compressive stress as a function of depth was measured by X-ray diffraction technique. The fatigue crack initiation life and fatigue crack growth rates of an Al alloy with different preexisting notch configurations were characterized and compared with those of the unpeened material. The results clearly show that LSP is an effective surface treatment technique for suppressing the fatigue crack growth of Al alloys with various preexisting notch configurations.

Analysis of two scenarios for the early optical emission of the gamma‐ray burst afterglows 990123 and 021211

Abstract: The optical light curves of gamma-ray burst (GRB) afterglows 990123 and 021211 exhibit a steep decay at 100‐600 s after the burst, the decay becoming slower after about 10 min. We investigate two scenarios for the fast decaying early optical emission of these GRB afterglows. In the reverse‐forward shock scenario, this emission arises in the reverse shock crossing the GRB ejecta, the mitigation of the light-curve decay occurring when the forward shock emission overtakes that from the reverse shock. Both a homogeneous and wind-like circumburst medium are considered. In the wind-bubble scenario, the steeply decaying, early optical emission arises from the forward shock interacting with a r −2 bubble, with a negligible contribution from the reverse shock, the slower decay starting when the blast wave reaches the bubble termination shock and enters a homogeneous region of the circumburst medium. We determine the shock microphysical parameters, ejecta kinetic energy and circumburst density, which accommodate the radio and optical measurements of the GRB afterglows 990123 and 021211. We find that, for a homogeneous medium, the radio and optical emissions of the afterglow 990123 can be accommodated by the reverse‐forward shock scenario if the microphysical parameters behind the two shocks differ substantially. A wind-like circumburst medium also allows the reverse‐forward shock scenario to account for the radio and optical properties of the afterglows 990123 and 021211, but the required wind densities are at least 10 times smaller than those of Galactic Wolf‐Rayet stars. The wind-bubble scenario requires av ariation of the microphysical parameters when the afterglow fireball reaches the wind termination shock, which seems a contrived feature. Ke yw ords: hydrodynamics ‐ plasmas ‐ radiation mechanisms: non-thermal ‐ shock waves ‐ ISM: jets and outflows ‐ gamma-rays: bursts.

A New Field Line Advection Model for Solar Particle Acceleration

Abstract: The diffusive acceleration of solar protons at a shock wave driven by a realistic coronal mass ejection is modeled using a new field line advection model for particle acceleration coupled with a global MHD code. The new model described in this Letter includes effects important for the particle acceleration and transport, by means of diffusive shock acceleration, and employs Lagrangian meshes. We performed a frequent dynamical coupling between two numerical codes in order to account for the time-dependent history of an evolving shock wave driven by a solar eruption. The numerical results discussed here demonstrate that this mechanism can account for the production of high-energy solar protons observed during the early stages of gradual events.

Pore Collapse and Hot Spots in HMX

Abstract: Hot spots are critical for initation of explosives because reaction rates are very temperature sensitive. For a plastic‐bonded explosive, shock desensitization experiments imply that hot spots generated by pore collapse dominate shock initiation. Here, for the collapse of a single pore driven by a shock, the dependence of the temperature distribution on numerical resolution and dissipative mechanism is investigated. An inert material (with the constitutive properties of HMX) is used to better focus on the mechanics of pore collapse. Two important findings result from this study. First, insufficient resolution can significantly overpredict the hot‐spot mass. Second, up to moderate piston velocities (< 1 km/s), shock dissipation alone does not generate sufficient hot‐spot mass for initiation. Two other dissipative mechanisms investigated are plastic work and viscous heating. In the cases studied, the integrated temperature distribution has a power‐law tail with exponent related to a parameter with dimensions of viscosity. The parameter of either dissipative mechanism can be fit to obtain the hot‐spot mass needed for initiation of any single experiment. However, the dissipative mechanisms scale differently with shock strength and pore size. Consequently, to predict initiation behavior over a range of stimuli and as the micro‐structure properties of a PBX are varied, sufficient numerical resolution and the correct physical dissipative mechanism are essential.

Characterization of Plastic Deformation Induced by Microscale Laser Shock Peening

Abstract: Electron backscatter diffraction (EBSD) is used to investigate crystal lattice rotation caused by plastic deformation during high-strain rate laser shock peening in single crystal aluminum and copper sample on (I 10) and (001) surfaces New experimental methodologies are employed which enable measurement of the in-plane lattice rotation under approximate plane-strain conditions Crystal lattice rotation on and below the microscale laser shock peened sample surface was measured and compared with the simulation result obtained from FEM analysis, which account for single crystal plasticity The lattice rotation measurements directly complement measurements of residual strain/stress with X-ray micro-diffraction using synchrotron light source and it also gives an indication of the extent of the plastic deformation induced by the microscale laser shock peening

Dislocation structure behind a shock front in fcc perfect crystals: Atomistic simulation results

Abstract: Large-scale molecular dynamics simulations are used to investigate the dislocation structure behind a shock front in perfect fcc crystals. Shock compression in both the 〈100〉 and 〈111〉 directions induces dislocation loop formation via a sequential emission of partial dislocations, but in the 〈100〉 case, this process is arrested after the first partial, resulting in stacking-fault loops. The large mobility of the bounding partial dislocations results in a plastic wave that is always overdriven in the 〈100〉 direction; the leading edges of the partials are traveling with the plastic front, as in the models of Smith and Hornbogen. In contrast, both partials are emitted in 〈111〉 shock compression, resulting in perfect dislocation loops bounded only by thin stacking fault ribbons due to the split partial dislocations. These loops grow more slowly than the plastic shock velocity, so new loops are periodically nucleated at the plastic front, as suggested by Meyers.