1,122 research outputs found

    A planetary ultra hypervelocity impact mechanics and shock wave science facility

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    Using the concept of intercepting orbits from a pair of Space Station serviced free flyers, a class of impact and shock wave experiments pertinent to planetary science can be performed. One proposed free flying vehicle is an impactor dispensor, and the second is the impact laboratory. How collision is achieved by utilizing essentially twice orbital velocity is demonstrated. The impactor dispensor contains a series of small flyer plates or other projectiles which are launched into the trajectory of the impactor laboratory at appropriate positions. The impactor laboratory is a large impact tank similar to those in terrestrial gun laboratories, except that it contains a supply of targets and instrumentation such as high speed cameras, flash X-ray apparatus, and digital recorders. Shock and isentropic pressures of up to 20 Mbar are achievable with such a system which provides 15 km/sec impact velocities for precisely oriented projectiles

    On the nature of pressure‐induced coordination changes in silicate melts and glasses

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    Progressive decreases in the Si‐O‐Si angles between corner‐shared silicate tetrahedra in glasses and melts with increasing pressure can lead to arrangements of oxygen atoms that can be described in terms of edge‐ or face‐shared octahedra. This mechanism of compression can account for the gradual, continuous increases in melt and glass densities from values at low pressure that indicate dominantly tetrahedral coordination of Si to values at several tens of GPa that suggest higher coordination. It also can explain the unquenchable nature of octahedrally coordinated Si in glasses, the absence of spectroscopically detectable octahedrally coordinated Si in glasses until they are highly compressed, the gradual and reversible transformation from tetrahedral to octahedral coordination in glasses once the transformation is detectable spectroscopically, and the fact that this transformation takes place in glass at room temperature. It may also have relevance to pressure‐induced transformations from crystalline to glassy phases, the difficulty in retrieving some metastable high pressure crystalline phases at low pressure, and the observed differences between the pressures required for phase transformations in shock wave experiments on glasses and crystals

    The relation between the shock-induced free-surface velocity and the postshock specific volume of solids

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    The release of solids from a state of shock compression at a free surface is examined. For isentropic release, the postshock specific volume V[prime]0 is shown to be constrained by V[prime]0? (Ufs–Up)2/P1+V1, where (P1,V1) is the pressure-volume Hugoniot state of shock compression and Ufs and Up are the free-surface and shock particle velocities, respectively. When a sudden phase change occurs during the release process, this lower bound is increased, subject to simplifying assumptions about the phase transition

    Superheating systematics of crystalline solids

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    Systematics of superheating (theta= T/Tm–1) of crystalline solids as a function of heating rate (Q) are established as beta= A(Q)(theta+ 1)theta2, where the normalized energy barrier for homogeneous nucleation is beta= 16pigammasl3/(3kTmDeltaHm2), T is temperature, Tm melting temperature, A a Q-dependent parameter, gammasl interfacial energy, DeltaHm heat of fusion, and k Boltzmann's constant. For all elements and compounds investigated, beta varies between 0.2 and 8.2. At 1 and 10^12 K/s, A = 60 and 31, theta= 0.05–0.35 and 0.06–0.45, respectively. Significant superheating is achievable via ultrafast heating. We demonstrate that the degree of superheating achieved in shock-wave loading and intense laser irradiation as well as in molecular dynamics simulations (Q~10^12 K/s) agrees with the theta–beta–Q systematics

    Multiwavelength optical pyrometer for shock compression experiments

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    A system for measurement of the spectral radiance of materials shocked to high pressures (~100 GPa) by impact using a light gas gun is described. Thermal radiation from the sample is sampled at six wavelength bands in the visible spectrum, and each signal is separately detected by solid-state photodiodes, and recorded with a time resolution of ~10 ns. Interpretation of the records in terms of temperature of transparent sample materials is discussed. Results of a series of exploratory experiments with metals are also given. Shock temperatures in the range 4000–8000 K have been reliably measured. Spectral radiance and temperatures have been determined with uncertainties of 2%

    Shock wave propagation in porous ice

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    We present data on shock wave propagation in porous ice under conditions applicable to the outer solar system. The equation of state of porous ice under low temperature and low pressure conditions agrees well with measurements under terrestrial conditions implying that data on terrestrial snow may be applicable to the outer solar system. We also observe rarefaction waves from small regions of increased porosity and calculate release wave velocities

    Depth of Cracking beneath Impact Craters: New Constraint for Impact Velocity

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    Both small-scale impact craters in the laboratory and less than 5 km in diameter bowl-shaped craters on the Earth are strength (of rock) controlled. In the strength regime, crater volumes are nearly proportional to impactor kinetic energy. The depth of the cracked rock zone beneath such craters depends on both impactor energy and velocity. Thus determination of the maximum zone of cracking constrains impact velocity. We show this dependency for small-scale laboratory craters where the cracked zone is delineated via ultrasonic methods. The 1 km-deep cracked zone beneath Meteor Crater is found to be consistent with the crater scaling of Schmidt (1) and previous shock attenuation calculations
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