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DISCRETE HONEYCOMBS, RATIONAL EDGES AND EDGE STATES
No full textInternational audienceConsider the tight binding model of graphene, sharply terminated along an edge l parallel to a direction of translational symmetry of the underlying period lattice. We classify such edges l into those of "zigzag type" and those of "armchair type", generalizing the classical zigzag and armchair edges. We prove that zero energy/flat band edge states arise for edges of zigzag type, but never for those of armchair type. We exhibit explicit formulas for flat band edge states when they exist. We produce strong evidence for the existence of dispersive (non flat) edge state curves of nonzero energy for most l
Heterogeneous Combustion of AP / HTPB / RDX with Detailed Kinetics
No full textInternational audienceIn this work, the effect of hexogen (RDX) inclusion in conventional ammonium perchlorate (AP)/ hydroxy 1terminated polybutadiene (HTPB) composite propellants is investigated. To this end, a detailed gas-phase kinetic mechanism for AP/HTPB/RDX combustion is assembled and tested. The combustion of a single RDX particle surrounded by a layer of homogenized AP/HTPB binder is simulated with this kinetic model for various pressures and RDX particle sizes. A phenomenological description of the heterogeneous flame structure forming above the propellant is proposed. Two combustion modes are identified: hot and mild. Critical conditions for the combustion mode change are determined in terms of ambient pressure and RDX particle size
A Dominating Tree Based Leader Election Algorithm for Smart Cities IoT Infrastructure
No full textInternational audienceIn wireless sensor and IoT networks dedicated to smart-cities, a leader node performs critical tasks such as generating encryption/decryption keys. In this paper, the leader is the node situated at the extreme left of the network. It is the node which starts the algorithm of searching the boundary nodes. These nodes will be used to monitor any sensitive, dangerous or inaccessible site. For this type of application, the used algorithm must be robust and fault-tolerant because it is difficult or even impossible to intervene in the presence of node failures. If this node is the leader, such a situation can be catastrophic. In this article, we present a new algorithm called DoTRo, which is based on a tree routing protocol. It starts with local leaders which will launch the flooding process to determine a spanning tree. During this process, their values will be forwarded. If two spanning trees meet, the tree that routes the best value continues its process while the other tree stops. The remaining tree root will be the leader. This algorithm is low energy consuming with reduction rates that can exceed 85% with respect to the classical minium finding algorithm. It is efficient and fault-tolerant since it works even in the presence of node failures and communication disconnectivity. Additionally, the energy consumption is well balanced between nodes. Finally, the complexity and the proof of convergence of the proposed algorithm is presented
An optimal control-based numerical method for scalar transmission problems with sign-changing coefficients
No full textInternational audienceIn this work, we present a new numerical method for solving the scalar transmission problem with sign-changing coefficients. In electromagnetism, such a transmission problem can occur if the domain of interest is made of a classical dielectric material and a metal or a metamaterial, with for instance an electric permittivity that is strictly negative in the metal or metamaterial. The method is based on an optimal control reformulation of the problem. Contrary to other existing approaches, the convergence of this method is proved without any restrictive condition. In particular, no condition is imposed on the a priori regularity of the solution to the problem, and no condition is imposed on the meshes, other than that they fit with the interface between the two media. Our results are illustrated by some (2D) numerical experiments
Additive Manufacturing and Composite Materials for Marine Energy: Case of Tidal Turbine
No full textInternational audienceThe global trend in additive manufacturing is the technology of three-dimensional (3D) printing with a high potential to avoid some of the weaknesses of conventional fabrication techniques. This new technology has been used to manufacture small tidal and wind turbines. In isolated areas, small turbines can be manufactured and assembled on-site for green energy production. The purpose of this document is to evaluate the thermomechanical behavior of a printed tidal turbine using Digimat-AM (Additive Manufacturing) with fused filament fabrication method. The finite element computes the mechanical deflection, temperature, residual stresses, and warpage fields of the printed part. The composites used during printing are thermoplastic polymers (acrylonitrile butadiene styrene, polyamide 6 [PA6], polyamide 12 [PA12], and polyetherimide [PEI]) reinforced with carbon and glass fillers in the form of fibers and beads (CF/GF and CB/GB). Through the simulation, one could show that the blade printed with PEI-CB/CF has excellent mechanical performance of low mechanical deflection and warpage, compared to PA6-CB/CF. In addition, the fiber-shaped fillers are better than the bead-shaped ones for the 3D printing process. In general, this study has shown the potential and feasibility of 3D printing as an excellent opportunity in the fabrication of small blades in the future, but more studies are required to understand this potential
Compressive fatigue of carbon fibers/epoxy laminate: an approach combining residual strength and self-heating
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Key role of boundary conditions for the 2D modeling of crack propagation in linear elastic Compact Tension tests
No full textInternational audienceIn fracture mechanics, the use of experimental tests are fundamental to characterize the material properties in terms of crack initiation and propagation behavior. When modeled in boundary value problems, simplifications need to be made. Notably, the loading has to be reduced to a set of boundary conditions and the choice between plane stress and plane strain has to be done in the 2D case. Here we focus on the Compact Tension (CT) test which is a fracture setup commonly used to measure the fracture toughness at crack propagation onset and we question the possibility to use it to study crack propagation. For this, the tests are monitored by digital image correlation and compared to finite element method simulations. Three ways to guide the choice between plane stress and plane strain hypotheses are proposed. They lead to the same conclusion that the plane stress conditions are the most relevant for the geometry of the samples used here. The key role of boundary conditions is highlighted by testing several models, with imposed force or displacement boundary conditions, against the experimental data. Imposed force boundary conditions on the pin are shown to be able to reproduce the experiments before crack propagation and to be insensitive to the way this force is applied, in line with Saint Venant principle. The results with imposed displacement are in contrary very sensitive to their distribution along the pin. While the stage before propagation is accurately predicted by imposed forces, we show that for the propagation phase, Saint Venant is put in default and accurate results can only be obtained by imposing the displacement fields issued from the digital image correlation. These results can be extended to other fracture experiments, involving pin loading, like the Compact Tension Shear (CTS) or the (Tappered) Double Cantilever Beam ((T)DCB) tests
Concentration of transformation-induced plasticity in pseudoelastic NiTi shape memory alloys: Insight from austenite–martensite interface instability
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Ultra-fast and sustainable formal [3 + 3] cycloadditions enabled by mixed variable optimization on an automated micromole scale flow platform
No full textInternational audienceHerein, we report a simple and comprehensive mixed variable optimization strategy which enabled the development of ultra-fast formal [3 + 3]-cycloadditions of 1,3-dicarbonyl equivalents with enals in flow. Our approach, which includes sequential sampling, statistical filtering, and black box optimization, is coupled with an automated micromole scale flow platform to perform complex optimizations with limited chemical expense and minimal human intervention. The efficiency of the sampling, filtering and optimization (SFO) strategy was demonstrated with the development of ultra-fast, sustainable and mild formal cycloadditions [3 + 3] that usually require in batch prolonged reaction times and/or high temperatures. Successful scaling experiments demonstrated the transferability from the micromole scale flow platform to a standard flow chemistry reactor