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Shock Experiments (shock + experiment)

Distribution by Scientific Domains

Physics and Astronomy	80%

Selected Abstracts

Pressure Ionization and Transitions in Dense Hydrogen

CONTRIBUTIONS TO PLASMA PHYSICS, Issue 3-4 2005
W. Ebeling
Abstract Shock experiments with fluid hydrogen have shown that a transition from insulating behavior to metal-like conductivity occurs at pressures beyond 100 GPa. This requires the development of new methods to describe the transition region of dense plasmas. The traditional approach due to Saha is based on the assumption of chemical equilibrium between charged and neutral components. This is equivalent to minimizing the free energy with respect to the composition. Here we improve an expression for the free energy developed recently to determine Hugoniot curves and isentropes in dense hydrogen and deuterium plasma in the regions of partial dissociation and partial ionization. We show that at high pressures the influence of the excluded volume occupied by neutral species is crucial for the transition to full ionization. We present curves for several thermodynamic functions for the region 5000 K < T < 20000 K and 0.6 g/cm3 < , < 1 g/cm3. The influence of the effective radii of the neutral species is crucial in the transition region. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Experimental shock decomposition of siderite and the origin of magnetite in Martian meteorite ALH 84001

METEORITICS & PLANETARY SCIENCE, Issue 6 2007
M. S. BELL
Naturally occurring siderite was first characterized by a variety of techniques to be sure that the starting material did not contain detectable magnetite. Samples were shocked in tungsten-alloy holders (W = 90%, Ni = 6%, Cu = 4%) to further ensure that any iron phases in the shock products were contributed by the siderite rather than the sample holder. Each sample was shocked to a specific pressure between 30 to 49 GPa. Transformation of siderite to magnetite as characterized by TEM was found in the 49 GPa shock experiment. Compositions of most magnetites are >50% Fe+2 in the octahedral site of the inverse spinel structure. Magnetites produced in shock experiments display the same range of sizes (,50,100 nm), compositions (100% magnetite to 80% magnetite-20% magnesioferrite), and morphologies (equant, elongated, euhedral to subhedral) as magnetites synthesized by Golden et al. (2001) and as the magnetites in Martian meteorite Allan Hills (ALH) 84001. Fritz et al. (2005) previously concluded that ALH 84001 experienced ,32 GPa pressure and a resultant thermal pulse of ,100,110°C. However, ALH 84001 contains evidence of local temperature excursions high enough to melt feldspar, pyroxene, and a silica-rich phase. This 49 GPa experiment demonstrates that magnetite can be produced by the shock decomposition of siderite as a result of local heating to > 470°C. Therefore, magnetite in the rims of carbonates in Martian meteorite ALH 84001 could be a product of shock devolatilization of siderite as well. [source]

Mechanical and microstructural investigations into the crack arrest behaviour of a modern 2¼Cr-1 Mo pressure vessel steel

FATIGUE & FRACTURE OF ENGINEERING MATERIALS AND STRUCTURES, Issue 2 2001
E. Bouyne
Tests were performed on a 2¼ Cr,1 Mo steel to measure the fracture toughness at initiation, KIc and at arrest, KIa,. The results were compared with those obtained on another pressure vessel steel (A508) of similar strength. Two techniques were used to measure KIa,: (i) isothermal compact crack arrest (CCA) tests, and (ii) specially designed thermal shock experiments using an externally notched ring. These specimens were cooled to ,196 °C and then heated by induction in the centre of the ring to produce very steep thermal gradients. This caused crack initiation from the notch. The crack propagates very rapidly (,500 m s,1,) and stopped when it reached the warmer region of the specimen. The specimens were analysed using an elastic,plastic finite element method to determine KIa values. These tests reveal a greater temperature shift (,100 °C) between KIc and KIa in 2¼ Cr,1 Mo steel than in A508 steel. Detailed metallographical examinations of the micromechanisms of crack propagation and arrest in the 2¼ Cr,1 Mo steel showed that this involves the nucleation of a three-dimensional network of cleavage microcracks which change their direction at bainitic packet boundaries. The remaining uncracked ligaments between the cleavage microcracks break by ductile rupture mechanism [source]

Experimental shock decomposition of siderite and the origin of magnetite in Martian meteorite ALH 84001

Selective release of D and 13C from insoluble organic matter of the Murchison meteorite by impact shock

METEORITICS & PLANETARY SCIENCE, Issue 3 2007
Koichi Mimura
We also performed shock experiments on type III kerogen and compared the results of these experiments with the experimental results regarding IOM. The shock selectively released D and 13C from the IOM, while it preferably released H and 12C from the kerogen. The release of these elements from IOM cannot be explained in terms of the isotope effect, whereas their release from kerogen can be explained by this effect. The selective release of heavier isotopes from IOM would be due to its structure, in which D and 13C-enriched parts are present as an inhomogeneity and are weakly attached to the main network. Shock gave rise to a high release of D even at a lower degree of dehydrogenation compared with the stepwise heating of IOM. This effective release of D is probably an inherent result of shock, in which a dynamic high-pressure and high-temperature condition prevails. Thus, shock would effectively control the hydrogen isotope behavior of extraterrestrial organic matter during the evolution of the solar nebula. [source]