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Douville M.-A.,Mines Douai | Douville M.-A.,University Claude Bernard Lyon 1 | Le Grognec P.,Mines Douai
International Journal of Solids and Structures | Year: 2013

Sandwich structures are widely used in many industrial applications, due to the attractive combination of a lightweight and strong mechanical properties. This compromise is realized thanks to the presence of different parts in the composite material, namely the skins and possibly core reinforcements or thin-walled core structure which are both thin/slender and stiff relative to the other parts, namely the homogeneous core material, if any. The buckling phenomenon thus becomes mainly responsible for the final collapse of such sandwiches. In this paper, classical sandwich beam-columns (with homogeneous core materials) are considered and elastic buckling analyses are performed in order to derive the critical values and the associated bifurcation modes under various loadings (compression and pure bending). The two faces are represented by Euler-Bernoulli beams, whereas the core material is considered as a 2D continuous solid. A set of partial differential equations is first obtained from a general bifurcation analysis, using the above assumptions. Original closed-form analytical solutions of the critical loading and mode of a sandwich beam-column are then derived for various loading conditions. Finally, the proposed analytical formulae are validated using 2D linearized buckling finite element computations, and parametric analyses are performed. © 2013 Elsevier Ltd. All rights reserved.

Traore M.,Lille University of Science and Technology | Chammas A.,Mines Douai | Duviella E.,Mines Douai
Reliability Engineering and System Safety | Year: 2015

In this paper, we are concerned by the improvement of the safety, availability and reliability of dynamical systems' components subjected to slow degradations (slow drifts). We propose an architecture for efficient Predictive Maintenance (PM) according to the real time estimate of the future state of the components. The architecture is built on supervision and prognosis tools. The prognosis method is based on an appropriated supervision technique that consists in drift tracking of the dynamical systems using AUDyC (AUto-adaptive and Dynamical Clustering), that is an auto-adaptive dynamical classifier. Thus, due to the complexity and the dynamical of the considered systems, the Failure Mode Effect and Criticity Analysis (FMECA) is used to identify the key components of the systems. A component is defined as an element of the system that can be impacted by only one failure. A failure of a key component causes a long downtime of the system. From the FMECA, a Fault Tree Analysis (FTA) of the system are built to determine the propagation laws of a failure on the system by using a deductive method. The proposed architecture is implemented for the PM of a thermoregulator. The application on this real system highlights the interests and the performances of the proposed architecture. © 2014 Elsevier Ltd. All rights reserved.

Hybrid dynamic systems (HDS) combine both discrete and continuous dynamics. Discretely controlled continuous systems (DCCS) is an important class of HDS in which the system switches between several discrete modes in response to discrete control events issued by a discrete controller. Their continuous dynamics depend on the discrete mode in which the system is. Wind turbine converters are an example of DCCS. Faults in converters may impact significantly the availability and the production performance of wind turbines. These faults can occur as a gradual abnormal change in the values of parameters describing the system continuous dynamics in a discrete mode. In this case, they entail a drift in the system operating conditions until the failure takes over completely. Detecting this drift in early stage allows reducing the power production losses as well as the wind turbine unavailability and maintenance costs. However, this drift can be observed only when the system is in the discrete modes where the continuous dynamics described by the affected parameters are active. Consequently, this paper proposes an approach based on the use of hybrid dynamic classifier able to monitor a drift in normal operating conditions of the converter in discrete modes where the continuous dynamics are impacted by a parametric fault. This allows keeping the useful patterns representative of the drift and therefore to detect it in its early stage. © 2015 Elsevier B.V.

Cosson B.,Mines Douai
Key Engineering Materials | Year: 2015

Thermoplastic composites for structural applications are under growing development from the aerospace (carbon fibers with PEI, PPS or PEEK matrices mainly) to the automotive industry (glass and carbon fibers with PP, PA). The plastic deformation they can provide and the assembly facilities through welding techniques are well appreciated. Among the available welding technics, laser offers the possibility to assemble materials in a precise and localized manner and can be easily automated. However, due to the presence of continuous fibers at a high fiber volume fraction, propagation of the laser energy through the composite that present local variation of fiber volume fraction is not as straight forward as in a homogeneous material. Modelling of the laser welding of a thermoplastic/continuous glass fiber is considered here. The study takes into account the microstructure of the composite in order to evaluate changes in local energy absorption and diffusion directly linked with the thickness. Modelling of the welding process is developed from the representation of the moving laser beam. The beam propagation through the composite thickness is considered thanks to the ray tracing method. The proposed method is able to optimise the welding process in function of the microstructure and the material properties of the welded parts. © (2015) Trans Tech Publications, Switzerland.

Sad Saoud K.,Mines Douai | Le Grognec P.,Mines Douai
Computational Mechanics | Year: 2016

Sandwich constructions have been widely used during the last few decades in various practical applications, especially thanks to the attractive compromise between a lightweight and high mechanical properties. Nevertheless, despite the advances achieved to date, buckling still remains a major failure mode for sandwich materials which often fatally leads to collapse. Recently, one of the authors derived closed-form analytical solutions for the buckling analysis of sandwich beam-columns under compression or pure bending. These solutions are based on a specific hybrid formulation where the faces are represented by Euler–Bernoulli beams and the core layer is described as a 2D continuous medium. When considering more complex loadings or non-trivial boundary conditions, closed-form solutions are no more available and one must resort to numerical models. Instead of using a 2D computationally expensive model, the present paper aims at developing an original enriched beam finite element. It is based on the previous analytical formulation, insofar as the skin layers are modeled by Timoshenko beams whereas the displacement fields in the core layer are described by means of hyperbolic functions, in accordance with the modal displacement fields obtained analytically. By using this 1D finite element, linearized buckling analyses are performed for various loading cases, whose results are confronted to either analytical or numerical reference solutions, for validation purposes. © 2016, Springer-Verlag Berlin Heidelberg.

This paper deals with the thorough characterization of a new experimental test bench designed to study the devolatilization and oxidation of pulverized fuel particles in a wide range of operating conditions. This lab-scale facility is composed of a fuel feeding system, the functioning of which has been optimized by computational fluid dynamics. It allows delivering a constant and time-independent mass flow rate of fuel particles which are pneumatically transported to the central injector of a hybrid McKenna burner using a carrier gas stream that can be inert or oxidant depending on the targeted application. A premixed propane/air laminar flat flame stabilized on the porous part of the burner is used to generate the hot gases insuring the heating of the central coal/carrier-gas jet with a thermal gradient similar to those found in industrial combustors (>105 K/s). In the present work, results issued from numerical simulations performed a priori to characterize the velocity and temperature fields in the reaction chamber have been analyzed and confronted with experimental measurements carried out by coupling particle image velocimetry, thermocouple and two-color pyrometry measurements so as to validate the order of magnitude of the heating rate delivered by such a new test bench. Finally, the main features of the flat flame reactor we developed have been discussed with respect to those of another laboratory-scale system designed to study coal devolatilization at a high heating rate. © 2016 AIP Publishing LLC.

Thiery V.,Mines Douai
Microscopy and Microanalysis | Year: 2014

Mimetite, Pb5(AsO4)3Cl, is a ubiquitous mineral that is of interest in various fields such as ore mineralogy, environmental studies, and minerals engineering. Mimetite generally forms hexagonal prisms and barrel-shaped crystals, but its fibrous form is quite an uncommon one. Here we attempt to present its worldwide occurrences on the basis of a literature review. We then present a study on fibrous mimetite from one of its historical locations based on the petrographic microscope, SEM-SE and EDX, and Raman spectroscopy. © Microscopy Society of America 2014.

Habchi C.,Lebanese University | Harion J.-L.,Mines Douai
Applied Thermal Engineering | Year: 2015

Abstract Streamwise vortices can be readily generated to enhance the mass and momentum transfer in continuous static mixers. The topology of these vortices is related to the shape and dimensions of the vortex generator (VG). It is essential to design the VG in such a way that the mass and momentum transfer are obtained for relatively small power dissipation. In the present paper, a streamwise and angular optimization study is performed for the passive scalar mixing enhancement downstream from rectangular wings inserted in a laminar circular pipe flow. The rectangular wing generates streamwise vortices which size is of the same order of the pipe radius. The optimal distance between two successive VGs is determined by analyzing the streamwise intensity of segregation and the pressure drop. The angular positioning of the vortices is also determined via local analysis of the scalar intensity of segregation. Such a study may be applied for other configurations of VGs where streamwise vorticity is used to enhance the mixing process in circular pipe flows. © 2015 Elsevier Ltd.

Sad Saoud K.,Mines Douai | Le Grognec P.,Mines Douai
Thin-Walled Structures | Year: 2014

Sandwich structures are increasingly employed in many practical applications thanks to their interesting compromise between lightweight and high mechanical properties. However, due to some specific geometric and material features, such structures are subject to global as well as local buckling phenomena, which lead to collapse in most cases. The buckling analysis of sandwich panels is therefore an important issue for their mechanical design. In this respect, this paper is devoted to the theoretical study of the elastic local/global buckling of rectangular sandwich plates under uniaxial or biaxial compression(-tension). Only classical sandwich materials are considered with homogeneous and isotropic core/skin layers. In the present formulation, a Love-Kirchhoff plate model is used to represent the thin skins, whereas the relatively thick core is modeled as a 3D continuous solid. Furthermore, the proposed approach is based on the elastic bifurcation theory in a general 3D framework, and leads to closed-form analytical expressions of the critical loadings and the corresponding bifurcation modes. The accuracy of the derived formulae is checked for both local and global modes by comparison with the results of finite element computations. Parametric analyses are finally performed, investigating primarily the influence of the aspect ratio of the plate and the ratio of the compressive (or tensile) loadings between both directions on the first buckling mode type and the associated minimum critical value. © 2014 Elsevier Ltd.

Le Grognec P.,Mines Douai | Sad Saoud K.,Mines Douai
International Journal of Non-Linear Mechanics | Year: 2015

Sandwich structures are widely used in many industrial applications thanks to their interesting compromise between lightweight and high mechanical properties. This compromise is realized thanks to the presence of different parts in the composite material, namely the skins which are particularly thin and stiff relative to the homogeneous core material and possibly core reinforcements. Owing to these geometric and material features, sandwich structures are subject to global but also local buckling phenomena which are mainly responsible for their collapse. The buckling analysis of sandwich materials is therefore an important issue for their mechanical design. In this respect, this paper is devoted to the theoretical study of the local/global buckling and post-buckling behavior of sandwich columns under axial compression. Only symmetric sandwich materials are considered with homogeneous and isotropic core/skin layers. First, the buckling problem is analytically addressed, by solving the so-called bifurcation equation in a 3D framework. The bifurcation analysis is performed using an hybrid model (the two faces are represented by Euler-Bernoulli beams, whereas the core material is considered as a 2D continuous solid), considering both an elastic and elastoplastic core material. Closed-form expressions are derived for the critical loadings and the associated bifurcation modes. Then, the post-buckling response is numerically investigated using a 2D finite element bespoke program, including finite plasticity, arc-length methods and branch-switching procedures. The numerical computations enable us to validate the previous analytical solutions and describe several kinds of post-critical responses up to advanced states, depending on geometric and material parameters. In most cases, secondary bifurcations occur during the post-critical stage. These secondary modes are mainly due to the modal interaction phenomenon and give rise to unstable post-buckled solutions which lead to final collapse. © 2015 Elsevier Ltd. All rights reserved.

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