Results 1 - 10 of 54814
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[en] A new approximate solution for wave propagation in graded-index media is developed whereby the exact wave propagation equation is replaced by any one of a sequence of higher-order (m, n) Padé approximant operators. The resulting formulations may be discretized by common numerical schemes and solvable by existing numerical techniques. The accuracy of this approximate calculation of the wave propagator is demonstrated in comparison with the exact result. We then employ the resulting method to investigate wave propagation in metamaterials with graded-index profiles which change according to a hyperbolic tangent function along the propagation direction
[en] In this paper the transient diffraction of an elastic wave by an extending but branching crack is considered. The incident wave is a plane horizontally-polarized wave. It is assumed that crack division is generated at the instant that the tip of a stationary crack is struck. The original length of the crack is therefore immaterial for small times, and the analytical work can be simplified by considering a semi-infinite crack. Size effects become important for later times. The two new cracks tips are are assumed to propagate at different but constant velocities under different arbitrary angles with the original crack plane. The dependence of the elasto-dynamic stress intensity factors on the crack tip velocities and the angles of branching is examined
[en] The consecutive numerical method is implemented for construction of a depth velocity model by full waveform inversion. The inverse dynamic seismic problem is reduced to finding the minimum point of the objective functional characterizing the mean square deviation of the recorded data from those calculated for the current model of the medium. A distinctive feature of the proposed approach is decomposition of the model space into two components: a smoothly varying propagator (macrovelocity model) and a rapidly spatially varying component called a reflector. The minimum point is calculated in sequence in these two subspaces. This paper reports the obtained data on numerical experiments related to reconstruction of the Marmoussi2 velocity model using a real frequency range and a source–receiver offset.
[en] This report discusses a new model of cosmic ray propagation in the Galaxy. In contrast to the known models based on the principles of Brownian motion, the proposed model agrees with the relativistic principle of speed limitation and takes into account the large-scale turbulence of the interstellar medium, justifying introduction of fractional differential operators.
[en] The plasma surface treatment, using hydrogen gas, of silicon wafers was studied as a pretreatment for silicon direct bonding. Chemical reactions of the hydrogen plasma with the surfaces were used for both surface activation and removal of surface contaminants. Exposure of the silicon wafers to the plasma formed an active oxide layer on the surface. This layer was hydrophilic. The surface roughness and morphology were examined as functions of the plasma exposure time and power. The surface became smoother with shorter plasma exposure time and lower power. In addition, the plasma surface treatment was very efficient in removing the carbon contaminants on the silicon surface. The value of the initial surface energy, as estimated by using the crack propagation method, was 506 mJ/M2, which was up to about three times higher than the value for the conventional direct bonding method using wet chemical treatments
[en] The acoustic emission (AE) testing of large components has to consider the signal distortion by sound propagation. The transfer functions for energy, peak amplitude, signal duration of crack propagation signals in steel plates were determined experimentally as well as the group velocity and the change in the frequency content. The results are represented and discussed with regard to the application of acoustic emission testing of thick-walled components. The potential of AE is demonstrated by some results of actual AE test of nuclear reactor components. (orig.)
[en] Recent results in theoretical fracture mechanics have been utilized to construct a microphysical model of brittle fracture which is computationally straightforward and depends on real physical processes and parameters. The Bedded Crack Model (BCM) was developed for oil shale rubbling calculations and follows the growth of cracks from preexisting microcracks along a set of parallel crack planes, due to both tensile and shear stresses. By orienting these planes parallel to the ground surface, spall can be modeled. The results of BCM calculations with the new SHALE code show that spall phenomena can be modeled in a physically realistic way. Complete separation of spall layers occurs naturally in the calculations, so that free fall and slapdown are well represented without artificial constructs. Accuracy in spall modeling improves accuracy in strength and timing of the downward traveling tensile wave which is so important in predicting the formation of a containment cage. An improved crack model now exists which follows crack growth in three orthogonal sets of crack planes and should prove valuable in modeling the cavity region
[en] The sensitivity of resonant gravitational wave detectors is reviewed. The effect of cosmic rays on a large spherical detector is considered. It is shown that the sensibility to short bursts, to monochromatic and to stochastic GW is not significantly degraded by cosmic rays. For a two-detector experiment, only one detector needs to be installed in an underground laboratory. This supports the idea to install a resonant detector at sea-level near a GW interferometer
[en] An analytical model for predicting the propagation of longitudinal stress waves in cracked prestressed concrete piles is presented. The displacements of the steel and the concrete are considered separately; bond stresses supply the essential coupling of the resulting equations of motion. The theoretical predictions are compared with experiments conducted on scaled-down piles. (orig.)
[de]Ein analytisches Modell zur Berechnung der Fortpflanzung longitudinaler Dehnungswellen in einem vorgespannten, gerissenen Raumpfahl wird erlaeutert. Die Stahl- bzw. Betonverschiebungen werden getrennt betrachtet; die wesentliche Kopplung der resultierenden Bewegungsleichungen erfolgt mittelt eines gegebenen Gesetzes des verschieblichen Verbundes. Anhand von Vergleichen mit den Ergebnissen von Versuchen an Modellpfaehlen wird die Anwendbarkeit der Theorie gezeigt. (orig.)
[en] A method of forming approximate representations for propagators with external field dependence is suggested and discussed in the context of potential scattering. An integro-differential equation in D+1 variables, where D represents the dimensionality of Euclidian space-time, is replaced by a Volterra equation in one variable. Approximate solutions to the latter provide a generalization of the Bloch-Nordsieck representation, containing the effects of all powers of hard-potential interactions, each modified by a characteristic soft-potential dependence
[fr]Une solution approximative pour les propagateurs dans un champ exterieur est suggeree et discutee dans le contexte de la diffusion par un potentiel. Une equation integro-differentielle a D+1 variables, ou D represente la dimension de l'espace-temps Euclidien, est remplacee par une equation de type Volterra a une dimension. Les solutions approximatives de cette derniere fournissent une representation qui generalise celle de Bloch-Nordsieck; elle contient les effets de toutes les puissances des interactions de potentiel dur, chacune de ces puissances etant modifiee par un terme qui depend de facon caracteristique des interactions de potentiel mou