Shock Wave

The threats of bomb attacks by criminal organizations and accidental events involving chemical explosives are a danger to the people and buildings. Due the severity of these issues and the need of data required for a safety design, more research is required about explosions and shock waves. This paper presents an assessment of blast wave overpressures using a computational fluid dynamics software. Analyses of phenomena as reflection of shock waves and channeling effects were done and a comparison between numerical results and analytical predictions has been executed, based on the simulation on several models. The results suggest that the common analytical predictions aren’t accurate enough for an overpressure analysis in small stand-off distances and that poorly designed buildings may increase the shock wave overpressures due multiple blast wave reflections, increasing the destructive potential of the explosions.

Dynamic crack propagation in a unidirectional carbon/epoxy composite is studied through finite element analyses of asymmetric impact (shear loading) of a rod against a rectangular plate. A finite deformation anisotropic visco-plastic model is used to describe the constitutive response of the composite. Crack propagation is simulated by embedding zero thickness interface element along the crack path. An irreversible mixed-mode cohesive law is used to describe the evolution of interface tractions as a function of displacement jumps. Contact and friction behind the crack tip are accounted for in the simulations. The failure of the first interface element at the pre-notch tip models onset of crack extension. Crack propagation is modeled through consecutive failure of interface elements. The dynamic crack propagation phenomenon is studied in terms of crack initiation time, crack speed, mode I and mode II displacement jumps and tractions associated with the failure of interface elements, effective plastic strain at the crack tip and path independent integral J′. Analyses are carried out at impact velocities of 5, 10, 20, 30 and 40 m/s, assuming the crack wake is frictionless. Moreover, analyses at impact velocities of 30 and 40 m/s are also carried out with a friction coefficient of 0.5, 1, 5 and 10 along the crack surfaces. The analyses show that steady-state intersonic crack propagation in fiber reinforced composite materials occurs when the impact velocity exceeds a given threshold. A steady-state crack speed of 3.9 times the shear wave speed and 83% of the longitudinal wave speed is predicted in the cases in which the impact velocity is above 10 m/s. Detailed discussion is given on the features of sub-sonic and intersonic crack propagation. It is shown that friction effects, behind the crack tip, do not have a significant effect on maximum crack speed; however, they do on characteristics of the shock wave trailing the crack tip. The analyses also show that the contour integral J′, computed at contours near the crack tip, is indeed path independent and can serve as a parameter for characterizing intersonic crack propagation.

. Formation mechanisms of planar deformation features in naturally shocked quartz. Phys. Earth Planet. Inter., 74: 219-240. Shock waves induce peculiar defects in quartz: shock mosaics, high-pressure polymorphs (coesite and stishovite), diaplectic glass and 'planar deformation features' (PDFs). Together, these features are the indices of 'shock metamorphism'. PDFs appear under the optical microscope as straight and narrow defects parallel to the (lOln} rhombohedral planes with n = 3 and 2 as most frequent values, although n = 1, 4 and~(i.e. basal plane) are also found. We report in the first part of this paper a detailed investigation by transmission electron microscopy of the fine structure of PDFs in shocked quartz grains from a variety of sites (Slate Islands, La Malbaie and Manson in North America; the Vredefort complex in South Africa; the Ries Crater in Germany and the Cretaceous-Tertiary boundary layer at Raton Basin, Colorado). We distinguish four PDF fine structures: (1) bands of dislocations; (2) lamellae of mixtures in various proportions of amorphous silica and small crystallites; (3) thin Brazil twin lamellae; (4) short, parallel lamellae forming serrated ladder structures. The possible formation mechanisms of these PDFs are discussed in the second part of the paper. It is suggested that the dislocation bands are probably not original PDF structures, but stem from a later recrystallization stage. The Brazil twins have been produced by relatively large deviatoric stresses which accompanied the shock wave. The other PDF structures, which consist of partial amorphization in the (lOin) planes, must result from elastic instabilities of the shear modulus of the quartz structure at high pressure in these planes. Theoretical estimates of the elastic stiffness coefficients indicate that amorphization should start at approximately 10 GPa. The growth of the straight and narrow amorphous lamellae would be driven by the front of the shock wave. Correspondence to: 0. Goltrant, Laboratoire de Structure et world (Bohor et a!., 1987). At some sites, coesite Propriétés de l'Etat Solide (URA 234), University of Lille, and stishovite have also been detected (Kieffer et 59655 Villeneuve d'Ascq-Cedex, France.

We extend the reactive force field ReaxFF to describe the high energy nitramine RDX and use it with molecular dynamics (MD) to study its shock-induced chemistry. We studied shock propagation via nonequilibrium MD simulations at various collision velocities. We find that for high impact velocities ( > 6 km=s) the RDX molecules decompose and react to form a variety of small molecules in very short time scales ( < 3 ps). These products are consistent with those found experimentally at longer times. For lower velocities only NO 2 is formed, also in agreement with experiments.

Over the last century, seismic instruments have recorded, with increasing frequency, the ground motion produced by meteorically generated shock waves striking the Earth's surface. In this review, the history of meteorrelated seismic signals is discussed, along with documented waveform characteristics, source mechanisms, air-ground coupling phenomena, and kinematic methods of determining meteor trajectories and event locations. Uncertainties in the mechanics of air-ground coupling, however, have left methods of measuring meteor source energy underdeveloped. To date, coupling of acoustic waves directly with the Earth's surface represents the bulk of the observed meteor-related seismic signals, while precursory and impact-related seismic waves remain an observational rarity. With proliferation of infrasound and seismic monitoring systems, new opportunities exist to explore the relationship between Earth's atmosphere and surface. Continued study of meteor seismology will lead to new methods to constrain energies, sizes, and fluxes for moderately (cm to m) sized meteoroids on Earth and potentially on Mars.

Neurological complications of ankylosing spondylitis (AS) are reported in 2.1% of patients. Cauda equina syndrome (CES) is rare and occurs at the ankylosing stage. MRI and CT of the lumbar spine show a cauda equina deformation with dural ectasia and bony erosion. We report ...

High pressure atomizing nozzle includes a high pressure gas manifold having a divergent expansion chamber between a gas inlet and arcuate manifold segment to minimize standing shock wave patterns in the manifold and thereby improve filling of the manifold with high pressure gas for improved melt atomization. The atomizing nozzle is especially useful in atomizing rare earth-transition metal alloys to form fine powder particles wherein a majority of the powder particles exhibit particle sizes having near-optimum magnetic properties.

In the present study systematic photographic investigations were performed of detonation interactions with foams and wire meshes lining the channel walls. An initial cellular detonation wave was propagating along a damping section (acoustic absorbing walls) which removed the transverse waves associated with its cellular structure. In some cases the wave had failed and a fast deflagration wave (a shock followed by a decoupled flame) was obtained and propagated at about half the C-J detonation speed. The events were studied photographically using a high speed framing camera and smoked foils.

Crow 1 discussed the distortion of sonic booms in the presence of atmospheric turbulence. He showed that the scattering from a weak shock is given by a surface integral over a paraboloid of dependence. The paraboloid of dependence describes the two-dimensional surface that contains the scatterers that contribute to the scattered sound at a point behind the shock at a given time. In Crow's analysis the sonic boom is treated as a plane wave. Lipkens et al. 2,3 used electrical sparks that propagate through turbulence created by a plane jet to simulate sonic boom propagation through a turbulent atmosphere. Experiments were done with plane and spherical waves. An extension of Crow's scattering surface for spherical wave propagation is presented. For spherical waves the two-dimensional scattering surface is an ellipsoid of dependence.