[en] Theoretical spall strength, defined as the stress needed to separate a material along a plane surface instantaneously, is one order of magnitude larger then the measured spell strength at strain rates up to 10⁶ s^-1. The discrepancy is explained by material initial flaws and cavities which grow and coalesce under stress and weaken the material. Measurements of spall strength of materials shocked by a high power laser shows a rapid increase in the spall strength with the strain rate at strain rates of about 10⁷ s^-1. This indicates that the initial flaws does not have time to coalesce and the interatomic forces become dominant. In order to break the material more cavities must be created. This cavities are characterized by the interatomic forces and are created statistically: material under tensile stress is in a metastable condition and due to thermal fluctuations cavities are formed. Cavities larger than a certain critical size grow due to the stress. They grow until the material disintegrates at the spall plane. The theoretical results predict the increase in spall strength at high strain rates, as observed experimentally. (authors)

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[en] In this work we study the influence of the Quark Gluon Plasma - Hadron Gas phase transition, on the transverse momentum of the D-meson which is produced in very high energy heavy ions collisions. We assume that the plasma is unstable with respect to the break-up into droplets as the system expands, and model the phase transition with a cascade simulation. The main mechanism of the model is the process of multiple hadronization to which the c-quark is subject (while it moves in the mixed phase): the repeated conversion of quark to meson and back as the boundary are crossed. These passages through droplets can degrade appreciably the transverse momentum of the c-quark. We find that the D-meson which did pass through one or more droplets, has a transverse momentum which is close to the thermal momentum, and hence the effect is more important in low transition-temperatures. The transverse momentum of the c-quark is little affected by the pions radial flow, or by the globs transverse expansion. (author) 89 refs

[en] This work deals with the problem of output robust stabilization of uncertain discrete systems with matched uncertainties. In the continuous case, the problem of the existence of an output min-max controller for systems with matched uncertainties was treated extensively and has been fully resolved. It was shown that an output min-max controller exists if and only if there exists a feedback gain matrix that makes the nominal system strictly positive real. It was found that if the system is minimum phase and has a high frequency gain with a full column rank, then there exists an output controller that makes the system. These results have motivated this work that presents a comprehensive solution to the problem of the existence of an output discrete min-max controller (static and dynamic) for multiple input multiple output systems which can be stabilized and are detectable. The question of the existence of an output feedback gain, K, that assures that for the resulting closed loop there exists a static output LMMC is investigated. For square systems an extremely simple criterion is derived. For non square systems, the condition involves searching for the matrix W that assures that asymptotically stabilized the system. Conditions for the existence of a static output RMMC are also investigated. 44 refs

[en] We have chosen to search for additional differences between lattice and continuum percolation. Accordingly, we have performed a wide range of simulations to an extent not carried out previously and have investigated three basic phenomena relating to continuum penetrating objects: percolation thresholds, geometrical critical properties, and critical behavior of electrical transport properties. For percolation thresholds, we have characterized the general behavior of the percolation threshold for various continuum cases which in general is very different from the threshold behavior in lattices. We have found the critical number of bonds per site, B_c, for hyper spheres and hypercube in various dimensions, as well as the for randomly aligned boxes in three dimensions. We have shown that the dependence of the percolation threshold on aspect ratio follows the criterion of constant total excluded volume. However, in the transition from boxes to cubes, the total excluded volume does not remain constant, but goes down with increasing aspect ratio. Furthermore, the total excluded volume decreases with increasing dimension for hyper spheres and hypercube, but differently whereas for hypercube this decrease converges to an asymptotic values greater than one, for hypersphere this value descends to a valueless than than unity. (author) 156 refs

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[en] Although the number of victims in the nuclear industry has been extremely low as compared with the number of victims in other spheres of human activity society has been willing to put up with a high number of accidents resulting in few victims per accident but refuses to accept an extremely rare accident resulting in a high number of victims. The U.S. nuclear industry is spending almost 2000 dollars for each reduction of a man x rem unit and this investment raises the ''man-life value'' in the nuclear industry to 10 million dollars as compared with 10,000 to 20,000 dollars spent in other activities (roentgen, early cancer detection, etc.). To reduce the exaggerated burden placed on the nuclear industry the safety expenditures should be spread over a maximum possible range of human activities. (B.G.)

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[en] Results of NMR measurements in the 3d - electron transition metals nickel and cobalt are presented. Measurements were performed in the paramagnetic and ferromagnetic states, as close as possible, experimentally to the Curie transition temperature Tsub(c). It is shown that NMR is a powerful tool to study the behavior of magnetic metals. The developed experimental technique enables measurements in an extended temperature range in metals with strongly temperature dependent parameters. (B.G.)