Exact boltzmann-gas filled bianchi type V space-times

Bianchi type V cosmological models are studied that contain a relativistic ideal Boltzmann gas. The effect of a cosmological constant upon the space-time geometry is also considered. In both high and low temperature limit the general solution of the Einstein gravitational field equations can be expressed in an exact closed parametric form. At final stages, depending on the presence or absence of the cosmological constant, cosmologies are driven to an isotropic inflationary open de Sitter type Universe or to an isotropic open Friedmann era.link_to_subscribed_fulltex

Surgical Management of Cleft and Lip Palate - an Overview

In 1971, Elber reported the discovery of a crack closure phenomenon that occurs with fatigue. He noted that closure of the crack planes near the crack tip can occur while the applied stress is still tensile [1]. The existence of a closure stress opens the way of defining an effective stress intensity factor, Keff, given by, Keff=(σ−σclosure)(πa)1/2 where σ is the applied tensile stress, σclosure is the crack closure stress, and a is the crack half length. The stress intensity factor is useful in correlating fatigue crack propagation data, especially after overloads [2]. The precision of Keff depends upon how well one can determine σclosure However it is difficult to experimentally determine σclosure, since conventional crack opening determination is imprecise. The purpose of this paper is to present an ultrasonic technique that shows promise as a means to accurately determine when the crack is open

Detection of Acoustic Second Harmonics Using a Laser Interferometer

Non-linear acoustic measurements can provide information on the microstructure or internal state of stress of materials and offer great potential for the nondestructive characterization of materials. But non linear effects are difficult to measure, especially in industrial environments. For instance, changes in acoustic velocity with temperature or with applied stress are very small. The detection of acoustic harmonics, the interaction of multiple acoustic wavefronts, or of acoustic-radiation-induced static strain requires sensitive transducers calibrated for absolute amplitude measurements to allow die determination of the third order elastic constants. Among these techniques, the detection of second harmonics offers potential for industrial applications because it requires no applied stress, no (slow) change in temperature and only one acoustic source.Peer reviewed: YesNRC publication: Ye

An efficient reduction from constrained to unconstrained agreement subtrees

An important aspect of nondestructive evaluation is materials characterization, in particular, detection of changes in the microstructure, affecting the mechanical properties. The goal of this project is to correlate the mechanical properties of high strength alloys with nonlinear acoustical properties of the materials. Many high strength alloys are precipitation hardened, and therefore, their mechanical properties are dependent on microstructural changes during thermal aging. Since the precipitates are formed by diffusion a property sensitive to the precipitation changes is the electrical resistivity. Standard ultrasonic techniques, on the other hand, have been unreliable in microstructural characterization, since in these alloys linear acoustic properties (e.g. sound velocity, attenuation) change by no more than 1%. Literature indicates however that nonlinear acoustical properties change by roughly 50% [1]. From this evidence it appears a nonlinear acoustic technique would be an additional method to examine microstructural changes in precipitation hardened materials

Light-induced size changes in BiFeO₃ crystals

Multifunctional oxides are promising materials because of their fundamental physical properties as well as their potential in applications. Among these materials, multiferroics exhibiting ferroelectricity and magnetism are good candidates for spin electronic applications using the magnetoelectric effect, which couples magnetism and ferroelecticity. Furthermore, because ferroelectrics are insulators with a reasonable bandgap, photons can efficiently interact with electrons leading to photoconduction or photovoltaic effects. However, until now, coupling of light with mechanical degrees of freedom has been elusive, although ferroelasticity is a well-known property of these materials. Here, we report on the observation, for the first time, of a substantial visible-light-induced change in the dimensions of BiFeO3 crystals at room temperature. The relative light-induced photostrictive effect is of the order of 10-5 with response times below 0.1 s. It depends on the polarization of incident light as well as applied magnetic fields. This opens the perspective of combining mechanical, magnetic, electric and optical functionalities in future generations of remote switchable devices