Showing papers on "Shock (mechanics) published in 1981"

Shock compression of aluminum, copper, and tantalum

Abstract: Hugoniot curves for Al (alloy 11000), Cu (type oxygen‐free high‐conductivity), and Ta have been measured in the shock pressure range 30–430 GPa (0.3–4.3 Mbar) with a two‐stage light‐gas gun. Impactor velocities were measured to 0.1% by flash radiography. Shock velocities were measured to 0.5–1.2% with an electronic detection system with subnanosecond time resolution. Our data and those of other workers were fitted to a linear relation between shock and mass velocities. The fractional standard deviations of the data from the fits range from 0.6 to 0.9% for the three metals. Methods of data analysis and error analysis for individual data points and for the least‐squares fitting to the data sets are presented. Bands of uncertainty about the fits, arising from experimental uncertainties in the data, are presented and are used to calculate the systematic error introduced by the method of shock‐impedance matching. The accuracy of the data and of the fits qualifies these metals as equation‐of‐state standards for shock‐wave experiments.

Hydromagnetic shock structure in the presence of cosmic rays

Abstract: The time asymptotic structure of a shock significantly modified by the back-reaction from the diffusive acceleration of cosmic rays is investigated Making a physically plausible assumption about the diffusion, it is shown that for given upstream conditions and shock speed only a finite odd number of shock structures are possible; an explicit method of determining these is given (in many cases the solution is unique) The results of this nonlinear study are contrasted with those of the linear test-particle theory and shown to confirm the possibility of efficient particle acceleration in shocks

Simulation of a perpendicular bow shock

Abstract: Simulations of a high Mach number shock with parameters typical of the earth's bow shock have been performed using a hybrid (particle ions, fluid electrons) code. The simulations reproduce the observed ion reflection and overshoots in the magnetic field and density. These features are shown to be closely associated with ion gyration.

Effects of Laser Induced Shock Waves on Metals

Abstract: A high-energy, pulsed laser beam combined with suitable transparent overlays occn generate pressure pulses of up to 6 to 10 GPa on the surface of a metal. The propogation of these pressure pulses into the metal in the form of a shock wave produces changes in the materials micro structure and properties similar to those produced by shock waves caused in other ways. This paper reviews the mechanism of shock wave formation, calculations for predicting the pressure pulse shape and amplitude, in-depth microstructural changes and the property changes observed in metals. These property changes include increases in hardness, tensile strength and fatigue life. The increases in fatigue life appear to result from significant residual surface stresses introduced by the shock process.

Force Wave Transmission Through the Human Locomotor System

Abstract: A method to measure the capability of the human shock absorber system to attenuate input dynamic loading during the gait is presented. The experiments were carried out with two groups: healthy subjects and subjects with various pathological conditions. The results of the experiments show a considerable difference in the capability of each group's shock absorbers to attenuate force transmitted through the locomotor system. Comparison shows that healthy subjects definitely possess a more efficient shock-absorbing capacity than do those subjects with joint disorders. Presented results show that degenerative changes in joints reduce their shock absorbing capacity, which leads to overloading of the next shock absorber in the locomotor system. So, the development of osteoarthritis may be expected to result from overloading of a shock absorber's functional capacity.

Determination of the Shear Strength of Shock Compressed 6061-T6 Aluminum

Abstract: The strength of 6061-T6 aluminum was assessed over the stress range of 8–40 GPa using velocity interferometry to measure reloading and unloading profiles from the initial shocked state. These results show that the shear stress which can be supported in the shocked state increases by about a factor of five over this range. This observation is in agreement with previous investigations. However, an important new observation is that a substantial increase in shear stress occurs during reloading, resulting in a well-defined elastic precursor. This result indicates a significant departure from the elastic-plastic model and suggests that softening occurs during initial shock compression.

Residual microstructure - mechanical property relationships in shock-loaded metals and alloys.

Abstract: This chapter reviews the results of a large number of shook loading experiments utilizing a common design and a constant shock pulse duration of 2 µs. It is shown that dislocations, dislocation cells, planar dislocation arrays, stacking faults, twins, twin faults, and point defects all contribute in specific systems to residual shock strengthening. Shock-induced microstructures are determined primarily by the stacking fault free energy. High stacking-fault free energy metals and alloys are characterized by dislocation cell structures while low stacking-fault free energy metals and alloys (with the fcc structure) are characterized by planar dislocation arrays, stacking faults and twins in {111} planes. High stacking-fault free energy metals and alloys also twin according to critical shear stress criteria, and the (001) orientation is the initial orientation where twinning occurs. Residual shock microstructures and specific lattice defects induced by the peak shock pressure are shown to be related to resisdual hardness and engineering yield stress. Body-centered cubic metals and alloys are characterized by irregular dislocation arrays as a result of the more numerous slip planes, although twinning also occurs in bcc metals.

Decorative light tubing

Abstract: Decorative light tubing, which has unbased miniature lamps connected in series by soldering or welding the ends of the flexible wire leads together to form parallel strings between a pair of elongated parallel wire conductors, is substantially filled with mineral oil or other clear, viscous dielectric fluid to protect the lamps and wiring against the adverse effects of vibration, shock and moisture while also enhancing the visual effect

Energetic particle spectra in finite shocks: the earth's bow shock

Abstract: Particle acceleration and escape at the earth's bow shock wave are discussed in order to account for reported exponential fast particle spectra. A model is presented of particle acceleration in a finite two-dimensional shock perpendicular to the magnetic field, with particle parallel and perpendicular diffusion coefficients inversely proportional to each other. It is shown that the exponential particle energy per unit charge spectra observed by Ipavich et al. (1979) for the case of a radial solar wind magnetic field may be obtained if the fast particles escape the shock by means of resonant diffusion to unconnected field lines. The calculated e-folding value of the energy/charge ratio is found to be independent of the level of turbulence near the shock and in good agreement with observations. For the case of a nonradial solar wind magnetic field, the model is noted to predict that convection may be the dominant means of escape. It is also pointed out that the parallel and perpendicular diffusion coefficients may be measured indirectly at the bow shock.

The optical continuum of solar and stellar flares

Abstract: A further development of the Kostyuk-Pikelner's model is presented. The response of the chromosphere heated by non-thermal electrons of the power-law energy spectrum has been studied on the basis of the numerical solution of the one-dimensional time-dependent equations of gravitational gas dynamics. The ionization and energy loss for the emissions in the Lyman and Balmer lines have been determined separately for the optically thin and thick Lα-line layers. Due to the initial heating, a higher-pressure region is formed. From this region, disturbances propagate upwards (a shock wave with a velocity of more than 1000 km s-1) and downwards. A temperature jump propagates downwards, and a shock is formed in front of the thermal wave. During a period of several seconds after the beginning of this process, the temperature jump intensifies the downward shock wave and the large radiative loss gives rise to the high density jump (ϱ2/ϱ1 ∼ 100). The numerical solution has been analyzed in detail for the case heating of the ionized and neutral plasma, and a value of this heating is close to the upper limit of the admissible values. In this case, the condensation located between the temperature jump and the shock wave front, may emit in the observed optical continuum.

Monte Carlo Simulation of charged particles upstream of the Earth's bow shock

Abstract: A simple model of the earth's bow shock is presented and compared with ISEE-1 observations of diffuse ions upstream of the shock. Ion behavior is modeled in a test particle simulation using Monte Carlo techniques for a plane parallel shock at which cold solar wind ions are heated sufficiently to allow diffusion back upstream. Fitting of the model to the observations indicates that the diffusion coefficient must increase faster than linearly with velocity, and provides an upper limit to the acceleration efficiency defined as the proportion of incoming solar wind ions accelerated and observed upstream. Results suggest that astrophysical shocks are capable of accelerating ions directly from the thermal plasma without the need for a distinct preenergizing injection stage.

The modulation of low‐energy proton distributions by propagating interplanetary shock waves: A numerical simulation

Abstract: A numerical simulation has been developed to study the formation in the intensities of ≲0.1- to ∼10.0-MeV protons of the long-lived energetic storm particle (ESP) events observed in association with the passage of flare-produced fast mode MHD shocks. Protons are traced numerically backward in time from a given observation point (i.e., particle detector), through their adiabatic scatter-free motion along the assumed laminar spiral interplanetary magnetic field (IMF), and finally through the shock discontinuity to their pre-shock interaction state. An ambient proton can undergo at most one shock encounter, during which, as seen in the shock frame, the proton is accelerated by its effective ‘grad-B drift’ along the induced electric field each time it crosses the shock. The time-reversed calculation reveals which of the observed protons interacted with the shock, where the shock interaction occurred, and by how much each interacting proton's kinetic energy was increased at the shock. Time profiles of the omnidirectional and unidirectional protons fluxes are formed from the trajectories by invoking Liouville's theorem and an ambient proton unidirectional flux j ∼T−γ, where T is the kinetic energy and γ the spectral exponent. Simulation predictions are shown for different observed proton energies under various assumptions concerning the shock speed and strength, the slope of the ambient proton energy spectrum, the variation of the angle β1 between the shock surface and the upstream IMF vector, and the heliocentric radial position r of the observation point. The simulation model correctly recovers the gross features of the intensity variations, flux anisotropies, and energy spectra time evolutions of many observed ESP events. Representative simulation results predict shock-induced enhancements in low-energy proton fluxes that are characterized by (1) long, steady preshock or upstream rises to peaks before the shock arrival and abrupt declines following the shock passage; (2) peak enhancement amplitudes that increase for lower-energy protons, stronger and faster shocks, softer ambient proton spectra, decreased β1 and increased r, (3) large, highly field-aligned preshock flux anisotropies directed away from the sun along the IMF, (4) small postshock flux anisotropies nearly perpendicular to the magnetic field and directed towards the sun along the IMF, and (5) steadily softening preshock energy spectra that are steepest at the peak flux enhancement and slightly softer than the ambient spectra after the shock passage.

A numerical study of two-dimensional shock vortex interaction

Abstract: The numerical analysis of shock vortex interaction in a two-dimensional channel is discussed in this paper. The Euler equations and the two-step MacCormack scheme are employed in this numerical analysis. The comparison of the numerical results with the theoretical analyses and experimental results in the literature has provided unique insights into the dynamics of the transient interaction process and the needs for further theoretical developments. The results are useful in applications to problems in aeroacoustics and the transient aerodynamics of high performance aircraft.

Refractive index change and curvature in shock waves by angled beam refraction

Abstract: Observations of front geometry and refractive index jump across shock waves in rare gases have been made with a new and particularly simple technique. The technique involves determination of the angular deflection of a narrow laser beam intersecting the shock front at a shallow angle. Measured refractive index jumps in rare gases are in excellent agreement with those calculated using Snell’s law and ideal shock theory. The apparent shock curvature is in close accord with deBoer’s theory for loading pressures below 20 Torr, but above this presure there is evidence of an indentation near tube center.

Acceleration of electrons by interplanetary shocks

Abstract: Ion acceleration is a well known phenomenon at both interplanetary shocks, while there have been only a few reports concerning an acceleration of electrons at interplanetary shocks. The considered investigation reports that electron acceleration occurs at many shocks. The instrumentation used is discussed, taking into account the ISEE 3 (International Sun Earth Explorer) spacecraft and the electrostatic analyzer employed for the measurement of electrons. Changes in the electron distribution function caused by shocks are found to be quite varied. Some events show a simple step to a higher flux level, some show fluctuations as the flux rises, and some have one or more pulses at the time of the shock. Attention is given to data obtained on July 26, 1979, December 25, 1978, and November 12, 1978. A calculation of shock geometries is also reported.

Strain‐rate dependence of the effective viscosity under steady‐wave shock compression

Abstract: Relationships among Hugoniot pressure, effective viscosity, and strain rate under conditions of steady‐wave shock compression are predicted based on the assumption of invariance of a shock property equal to the product of the energy dissipated in shock compression and the rise time of the steady wave. Effective viscosity is found to decrease as η∼e−1/2, while strain rate increases as e∼p4h with Hugoniot pressure. These results are consistent with steady‐wave profile measurements on aluminum.

Shock compression measurements of single‐crystal forsterite in the pressure range 15–93 GPa

Abstract: Shock compression experiments have been performed on pure synthetic single-crystal forsterite (Mg2SiO4) in the pressure range 15–93 GPa using a newly installed two-stage light gas gun. The values of the Hugoniot elastic limit (HEL) vary with the shock propagation directions with respect to the crystallographic orientations, reflecting a notable elastic anisotropy in the olivine crystal. The largest value of HEL up to 12 GPa is observed in the shock direction parallel to [010]. In the hydrostatic regime, shock data in the shock velocity-particle velocity (Us-up) plane are segmented into two parts by a transient plateau starting at around Us = 8.4 km/s, indicating a phase transition. For the shock data of the low-pressure olivine phase below about 50 GPa, a linear fit with the equation Us = C0 + sup has yielded parameters of C0 = 6.26 km/s and s = 1.12. The value of C0 is close to the bulk sound velocity of forsterite. Murnaghan-Birch fit of shock data for the low-pressure olivine phase incorporated with the static compression data by Olinger (1977) has resulted in K0 = 132 GPa and K0′ = 3.4, which are in reasonable agreement with the ultrasonic data. The close correspondence of the present shock data to the observation of shock residual effects in olivines is noteworthy. The appearance of melt zone or diaplectic glass observed above about 50 GPa seems to correspond to the onset of the phase transition inferred in the present measurements.

Shock models with phase type survival and shock resistance

Abstract: : New closure theorems for shock models in reliability theory are presented. If the number of shocks to failure and the times between the arrivals of shocks have probability distributions of phase type, then so has the time to failure. PH-distributions are highly versatile and may be used to model many qualitative features of practical interest. They are also well-suited for algorithmic implementation. The computational aspects of our results are discussed in some detail. (Author)

Defect Generation in Shock-Wave Deformation

Abstract: A theoretical framework elucidating the generation of shock-induced defects is presented. Three different mechanisms responsible for the generation of point, line, and planar defects (twins and stacking faults), respectively, are described. Dislocations are homogeneously nucleated at or slightly behind the shock front by the powerful deviatoric stresses generated by the shock pulse; they are accelerated by the residual deviatoric stresses either towards or away from the front. Dislocation dynamical considerations limit their velocity to the velocity of shear waves in the medium. Their self-energy and stiffness are very high at the high velocities; hence, their ability to generate point defects upon intersecting each other is greatly enhanced, because the drag stress produced by the jogs is essentially independent of the velocity.

Shock metamorphism in the Vredefort Collar: Evidence for internal shock sources

Abstract: Shock metamorphic microstructures in the Vredefort collar include planar features, crystallographically controlled cleavage, crystallographically controlled faults, and mosaic extinction. In addition, several recrystallization textures are developed in the quartzites of the collar, and quartz c axis distributions for both primary and recrystallized quartz grains are random. The degree of recrystallization decreases away from the core-collar contact. Two events of shock deformation have been identified in the collar, and using planar feature orientations, shock pressures have been estimated using the technique of Robertson (1975). The first shock (D1) subjected the lowermost Witwatersrand rocks to shock pressures of about 150 kbar and the uppermost beds to pressures of about 60 kbar. Following a period of extensive recrystallization of the quartzites came the second shock event (D2), which was weaker than the first and subjected the lowermost strata in the collar to pressures of between 75 and 100 kbar. The D2 event has been shown to be separated in time from the D1 event. The results are used to show that the shock sources were probably within the earth and that the Vredefort ring structure has formed as a result of endogenous processes rather than hypervelocity meteorite impact.

Adjustable leveling suspension unit for vehicles

Abstract: An adjustable leveling suspension unit for vehicle shock absorbers having an external coil compression spring in cooperation therewith in the general manner of the MacPherson type strut assembly. The adjustable leveling unit includes a seat for the coil spring which is slidable on the shock cylinder, a collar and sleeve combination on the shock cylinder in position to support the seat and to slide the seat to different positions on the shock cylinder when necessary to correct for spring sag and restore the shock absorber to its desired position, and a guide or key to hold the spring seat against rotation while being moved on the shock cylinder.

Shock attenuation system for headgear

Abstract: A shock attenuation system for headgear having an outer protective shell, the system comprising a liner adapted to be secured to the internal surface of the shell, the liner comprising a series of tubes of elastomeric material extending with their axes generally parallel to the internal surface of the shell, and generally parallel to one another. Each tube is open at an end thereof and is elastically deformable under loads applied against the side of the tube, whereby, with the headgear worn by a wearer and upon application of an impact load to the headgear, the tubes in the area of impact deform elastically under the applied load and attenuate the shock upon the wearer from the applied load.

Design of Uniaxial Strain Shock Recovery Experiments

Abstract: We present an elementary introduction to the art and science of uniaxial strain-shock recovery experiments. Subjects discussed include generation of planar stress waves, design of sample recovery experiments, stress-gage instrumentation, and temperature effects. The emphasis of the present paper is practical; we hope to provide the neophyte with the basic information needed to design and interpret well-characterized shock recovery experiments.

Formation of supernova remnants - The pre-blast-wave phase

Abstract: The effect of stellar structure on supernova remnant formation is studied with a series of computer models of a 10/sup 51/ erg explosion in a 15 M/sub sun/ star We find that immediately after the explosion shock wave travels down a steep density gradient, the material in the gradient goes into free expansion, forming a collapsible piston At the outer edge of such a piston are two shock waves: the expanding supernova shock and a reverse shock moving back into the collapsible piston Until the piston is completely collapsed it is Rayleigh-Taylor stable, but after collpse the inner material behaves as a massive piston and the interface is R-T unstable If there is a significant mass in an external density gradient, the material between the supernova shock moving out through the interstellar medium and the reverse shock will be a singificant source of x-rays during the pre--blast-wave phase of remnant formation