Energy release rate for non smooth cracks in planar elasticity

This paper is devoted to the characterization of the energy release rate of a crack which is merely closed, connected, and with density $1/2$ at the tip. First, the blow-up limit of the displacement is analyzed, and the convergence to the corresponding positively $1/2$-homogenous function in the cracked plane is established. Then, the energy release rate is obtained as the derivative of the elastic energy with respect to an infinitesimal additional crack increment

The partition bundle of type A_{N-1} (2, 0) theory

Six-dimensional (2, 0) theory can be defined on a large class of six-manifolds endowed with some additional topological and geometric data (i.e. an orientation, a spin structure, a conformal structure, and an R-symmetry bundle with connection). We discuss the nature of the object that generalizes the partition function of a more conventional quantum theory. This object takes its values in a certain complex vector space, which fits together into the total space of a complex vector bundle (the `partition bundle') as the data on the six-manifold is varied in its infinite-dimensional parameter space. In this context, an important role is played by the middle-dimensional intermediate Jacobian of the six-manifold endowed with some additional data (i.e. a symplectic structure, a quadratic form, and a complex structure). We define a certain hermitian vector bundle over this finite-dimensional parameter space. The partition bundle is then given by the pullback of the latter bundle by the map from the parameter space related to the six-manifold to the parameter space related to the intermediate Jacobian.Comment: 15 pages. Minor changes, added reference

An Analytical Model for Repositioning of 6 D.O.F Fixturing System

Lien vers la version éditeur: http://journals.cambridge.org/action/displayAbstract?fromPage=online&aid=8749247&fulltextType=RA&fileId=S2257777712000164Dimensional errors of the parts from a part family cause the initial misplacement of the workpiece on the fixture affecting the final product quality. Even if the part is positioned correctly, the external machining forces and clamping load cause the part to deviate from its position. This deviation depends on the external load and the fixture stiffness. In this article, a comprehensive analytical model of a 3-2-1 fixturing system is proposed, consisting of a kinematic and a mechanical part. The kinematic model relocates the initially misplaced workpiece in the machine reference through the axial advancements of six locators taking all the fixturing elements to be rigid. The repositioned part then shifts again from the corrected position due to the deformation of fixturing elements under clamping and machining forces. The mechanical model calculates this displacement of the part considering the locators and clamps to be elastic. The rigid cuboid baseplate, used to precisely re-locate the workpiece, is also considered elastic at the interface with the locators. Using small displacement hypothesis with zero friction at the contact points, Lagrangian formulation enables us to calculate the rigid body displacement of the workpiece, deformation of each locator, as well as the stiffness matrix and mechanical behavior of the fixturing system. This displacement of the workpiece is then finally compensated by the advancement of the six axial locators calculated through the kinematic model

Can molecular dynamics simulations help in discriminating correct from erroneous protein 3D models?

<p>Abstract</p> <p>Background</p> <p>Recent approaches for predicting the three-dimensional (3D) structure of proteins such as <it>de novo </it>or fold recognition methods mostly rely on simplified energy potential functions and a reduced representation of the polypeptide chain. These simplifications facilitate the exploration of the protein conformational space but do not permit to capture entirely the subtle relationship that exists between the amino acid sequence and its native structure. It has been proposed that physics-based energy functions together with techniques for sampling the conformational space, e.g., Monte Carlo or molecular dynamics (MD) simulations, are better suited to the task of modelling proteins at higher resolutions than those of models obtained with the former type of methods. In this study we monitor different protein structural properties along MD trajectories to discriminate correct from erroneous models. These models are based on the sequence-structure alignments provided by our fold recognition method, FROST. We define correct models as being built from alignments of sequences with structures similar to their native structures and erroneous models from alignments of sequences with structures unrelated to their native structures.</p> <p>Results</p> <p>For three test sequences whose native structures belong to the all-<it>α</it>, all-<it>β </it>and <it>αβ </it>classes we built a set of models intended to cover the whole spectrum: from a perfect model, i.e., the native structure, to a very poor model, i.e., a random alignment of the test sequence with a structure belonging to another structural class, including several intermediate models based on fold recognition alignments. We submitted these models to 11 ns of MD simulations at three different temperatures. We monitored along the corresponding trajectories the mean of the Root-Mean-Square deviations (RMSd) with respect to the initial conformation, the RMSd fluctuations, the number of conformation clusters, the evolution of secondary structures and the surface area of residues. None of these criteria alone is 100% efficient in discriminating correct from erroneous models. The mean RMSd, RMSd fluctuations, secondary structure and clustering of conformations show some false positives whereas the residue surface area criterion shows false negatives. However if we consider these criteria in combination it is straightforward to discriminate the two types of models.</p> <p>Conclusion</p> <p>The ability of discriminating correct from erroneous models allows us to improve the specificity and sensitivity of our fold recognition method for a number of ambiguous cases.</p

Adaptive multi-carrier spread-spectrum with dynamic time-frequency codes for UWB applications

International audienceIn this paper, we propose a spread spectrum multi-carrier multiple-access (SS-MC-MA) waveform for high data rate UWB applications, taking into consideration the European UWB context. This new UWB scheme respects the parameters of the multiband orthogonal frequency division multiplexing (MB-OFDM) technique which is one of the candidates for wireless personal area networks (WPAN) standardization. We optimize the spreading code length and the number of codes in our proposed scheme in order to maximize the system range for a given target throughput. Furthermore, we dynamically distribute the time-frequency codes that provide frequency hopping between users in order to improve our system range. We show that our adaptive system transmits information at much higher attenuation levels and with larger throughput than the ones of the MB-OFDM proposal. Hence, we conclude that our proposed system can be advantageously exploited for UWB applications

Efficient allocation algorithms for multicarrier spread-spectrum schemes in UWB applications

International audienceIn this paper, we propose a multicarrier spread spectrum (MC-SS) waveform for high data rate UWB applications, taking into account the American and the European UWB contexts. This new waveform for UWB is presented as an evolution of the well known Multiband OFDM Alliance (MBOA) solution and does not increase the system complexity significantly. First, we optimize the number of spreading codes to maximize the system range for a fixed QPSK constellation. Secondly, we use variable constellation orders and we propose a low-complexity resource allocation algorithm that maximizes the system throughput. We show that our adaptive MC-SS system transmits information at much higher attenuation levels and with higher throughput compared to the MBOA solution, and can be advantageously exploited for UWB applications

Adaptive multi-carrier spread-spectrum with dynamic time-frequency codes for UWB applications

International audienceIn this paper, we propose a spread spectrum multi-carrier multiple-access (SS-MC-MA) waveform for high data rate UWB applications, taking into consideration the European UWB context. This new UWB scheme respects the parameters of the multiband orthogonal frequency division multiplexing (MB-OFDM) technique which is one of the candidates for wireless personal area networks (WPAN) standardization. We optimize the spreading code length and the number of codes in our proposed scheme in order to maximize the system range for a given target throughput. Furthermore, we dynamically distribute the time-frequency codes that provide frequency hopping between users in order to improve our system range. We show that our adaptive system transmits information at much higher attenuation levels and with larger throughput than the ones of the MB-OFDM proposal. Hence, we conclude that our proposed system can be advantageously exploited for UWB applications

Resource allocation for multicarrier CDMA systems in ultra-wideband communications

International audienceUltra-wideband (UWB) is a fast emerging technology that has attracted considerable interest in short range, high data rate wireless personal area networks (WPAN) applications. One of the main candidates for WPAN standardization is the multiband orthogonal frequency division multiplexing (MB-OFDM), supported by the Multiband OFDM Alliance (MBOA). In this paper, we propose a new low-complexity resource allocation algorithm applied to a spread spectrum multicarrier multiple-access (SS-MC-MA) waveform, which is new for high data rate UWB applications. The proposed scheme aims at maximizing the system's throughput while taking into consideration the WPAN environment and respecting the OFDM parameters of the MBOA solution. The adaptive allocation algorithm applied to OFDM and SS-MC-MA leads to roughly double the throughput compared to the MBOA solution at low attenuation levels. Furthermore, at high attenuation levels, SS-MC-MA outperforms the adaptive OFDM. Hence, we conclude that the proposed adaptive SS-MC-MA can especially be advantageously exploited for high attenuation UWB applications