WorldCat Identities

Ayad, Rézak (19..-....).

Overview
Works: 18 works in 22 publications in 2 languages and 26 library holdings
Roles: Thesis advisor, Opponent, Author, Other, Publishing director
Classifications: TA417.7.S5, 620.118
Publication Timeline
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Most widely held works by Rézak Ayad
Éléments finis de plaque et coque en formulation mixte avec projection en cisaillement by Rézak Ayad( )

2 editions published in 1993 in French and held by 3 WorldCat member libraries worldwide

La première partie de la thèse concerne la formulation d'éléments de plaque en flexion-cisaillement transversal (CT). Nous proposons le modèle MiSP (Mixed Shear Projected) : l'approximation des efforts de CT résulte de celle des moments de flexion en satisfaisant explicitement les équations de l'équilibre des moments. Les déformations de CT sont définies en fonction des déformations de bords que nous projetons sur les degrés de liberté nodaux. Nous dévéloppons sur la base de MiSP deux éléments mixtes-hybrides : MiSP3 (triangle à 3 noeuds et 3 ddl/noeud) et MiSP4 (quadrilatère à 4 noeuds et 3 ddl/noeud). Nous proposons également une forme modifiée de MiSP en considérant les efforts de CT constants et indépendants des moments, l'élément quadrilatéral correspondant est MMiSP4, la flexion est représentée par un modèle déplacement. Nous vérifions la convergence des trois éléments développés à l'aide de patch-tests de modes rigides et constants. Leur précision est évaluée à travers une série de cas-tests standards relatifs aux problèmes de plaques biaises et circulaires. La seconde partie concerne la formulation d'éléments de coque de forme quelconque. Nous proposons pour améliorer la membrane deux éléments de type déplacement avec rotation autour de la normale au plan moyen. Ils sont basés sur un concept que nous appelons "Fibre Plane Rotatoire". Ces éléments sont FRT (Fiber Rotation Triangle : 3 noeuds et 3 ddl/noeud) et FRQ (Fiber Rotation Quadrilateral : 4 noeuds et 3 ddl/noeud). Nous retenons l'approche géométrique par facettes planes pour formuler deux éléments de coque triangulaires à 3 noeuds et 6 ddl/noeud : MiSP3-CST et MiSP3-FRT où les éléments de membrane CST (Constant Strain Triangle) et FRT sont superposés à l'élément mixte-hybride de plaqueMiSP3. L'approche isoparamétrique dégénérée est retenue pour formuler deux éléments de coque quadrilatéraux courbes à 4 noeuds et 6 ddl/noeuds : MiSP4-Q4 et MiSP4-FRQ. L'approximation des contraintes de CT est indépendante de celle des contraintes dans la plan tangent à la surface moyenne ; l'élément de flexion - CT MiSP4 est mixte au sens de Hellinger-Reissner avec déformations de CT projetées. Une représentation tridimensionnelle de FRQ est nécessaire pour qu'il puisse intégrer l'approche ci-dessus. Des cas-tests standards ou non standards de coque considérés comme outils de validation d'éléments de coque sont traités
Long-term Immersion in Water of Flax-glass Fibre Hybrid Composites: Effect of Stacking Sequence on the Mechanical and Damping Properties by Khouloud Cheour( )

1 edition published in 2020 in English and held by 2 WorldCat member libraries worldwide

Eléments finis mixtes-hybrides naturels sans facteurs correctifs du cisaillement pour les plaques et les coques composites multicouches by Achraf Tafla( Book )

2 editions published in 2007 in French and held by 2 WorldCat member libraries worldwide

This study deals with the formulation and the evaluation of new hybrid-mixed finite elements for multilayered composite plates and shells. These finite elements are based on the first order theory(Reissner/Mindlin).They present the main advantage of not using transverse shear correction factors. The Mixed with Shear Projection 4-node multilayered model MiSP4/ml is first developed. It is an adaptation to composite plates of the isotropic model MiSP4, which has been proposed by Ayad in 1993. It models composite plates with one or several layers, without using shear correction factors. Two curved isoparametric models with 4 nodes are developed for the analysis of composite shells. The first model is natural mixed-hybrid (NHMiSP4/ml: Natural Hybrid Mixed with Shear Projection 4-node/multilayer), for which both membrane and bending parts are respectively hybrid (pian approach) and hybrid-mixed with substituted shear strains. The second model HMiSP4/Q4/ml remains hybrid-mixed for bending/shear behaviour and develops a displacement membrane formulation. These two shell models takes into account in the theoretical formulation of shell warping. Their adaptation to the multilayered shells follows the same step as for the plate model MiSP4/ml. Shear corrections factors are also avoided. All of developed elements are confronted with some known high-order elements, across testing multilayered structure problems under static and dynamic loadings (free vibrations)
Contribution à la modélisation 3D volumique de la mise en forme des corps plastiques creux by Tewfik Ghomari( Book )

2 editions published in 2007 in French and held by 2 WorldCat member libraries worldwide

The principal objective of this thesis is to develop a software of working of the plastic bodies hollow 3D axisymmetric, with a precise distribution of the thicknesses and a cost of less low calculation. That became possible with the development of a new finite element of axisymmetric solid (baptized SFRQ-Axi) based on the kinematic concept of rotation of a space fiber (Space Fiber Rotation concept). The developed finite element has the advantage of converging quickly for the problems with contact presenting of the flexing areas. The choice of the algorithm of search for contact plays also a part in the fast treatment of the management of the nodes candidates to the contact. A simple algorithm of research local is worked out to test the penetration of the segments “masters” by the “Slavic” nodes. Encountered numerical difficulties to which we had faced, and who are due to the strong not geometrical linearities, material and especially with nonthe linearities of contact. Let us underline also the difficulty in managing the incompressibility of the voluminal or axisymmetric finite elements 3D. Two approaches of implicit calculation and clarifies are treated for an example of feeding-bottle. The results obtained shows the good precision of implicit calculation compared to explicit calculation. The tests of validation of the SFRQ-Axi element with contact on a test of inflection of a circular plate, being rolled up on a rigid torus, shows the good speed of convergence and a better precision of the results thicknesses. The results of the tests of working give also a good distribution thicknesses inside the beach given by the experimental results
Modélisation numérique des structures composites multicouches à l'aide d'une approche discrète au sens de Mindlin. Le modèle DDM (Displacement Discrete Mindlin) by Siham Sakami( Book )

2 editions published in 2008 in French and held by 2 WorldCat member libraries worldwide

Ce travail de thèse consiste le développement d'un élément fini de plaque et de coque courbe géométriquement simple (4 nœuds), basé sur une nouvelle approche variationnelle appelée DDM (Displacement Discrete Mindlin). Il prend en compte l'effet du cisaillement transversal CT à travers l'épaisseur. Le modèle DDM introduit de manière discrète deux hypothèses de Mindlin. La première hypothèse est cinématique, elle consiste à introduire sous la forme d'une intégrale de contour une équation cinématique de la déformation de CT. Elle permet l'élimination du verrouillage en CT sans introduire des fonctions bulles ou sans recourir à l'intégration réduite ou sélective. Il s'agit de l'approche des déformations de CT de substitution, connue sous le nom de « ANS method : Assumed Natural Strains ». La seconde hypothèse fait appel à deux lois de comportement, l'une en flexion et l'autre en CT, et deux équations d'équilibre d'une plaque en flexion/CT. Elle a pour principal avantage une élimination locale des degrés de libertés de rotation, introduits initialement au milieu d'un bord élémentaire par le biais d'une approximation quadratique des rotations de la normale à la surface moyenne
Discrete-Mindlin finite element for nonlinear geometrical analysis of shell structures by Lakhdar Sedira( )

1 edition published in 2015 in English and held by 2 WorldCat member libraries worldwide

Contribution à l'étude de matériaux biocomposites à matrice thermoplastique polyamide-11 et renforcés par des fibres de lin by Clément Gourier( )

1 edition published in 2016 in French and held by 2 WorldCat member libraries worldwide

This thesis has been carried out as part of the project Fiabilin, which includes 15 different academic and industrial partners, with an aim to develop industrial production of polyamide-11/flax biocomposite. The purpose of this work is to determine multi-scale performances of 100% biosourced composite, in order to substitute composite materials containing glass fibers and/or matrix derived from petroleum. First, we highlighted the flax fiber sensibility toward processing cycles (time and temperature), from mechanical and biochemical structure aspects. Then, we revealed the capacity of PA11-flax association to produce competitive mechanical properties compared to others usual composites. Fiber-matrix interface of the biocomposite was studied at micro and macro scales, showing a higher compatibility than some flax-thermoset resin systems. The end-of-life of the biocomposite was considered by recycling with successive grinding and injections. Then stiffness and strength at break of short fiber biocomposites thus obtained are similar to PPgMA-flax composites, whereas a strong increase of the strain at break according to the number of injection cycles was observed. A life cycle analysis of some composites production steps shows lower environmental impacts of PA11-flax when sizing was made through equivalent material stiffness
Biocomposites : élaboration, durabilité, valorisation( Book )

1 edition published in 2017 in English and held by 1 WorldCat member library worldwide

Contribution à la compréhension et à la modélisation du comportement mécanique de matériaux composites à renfort en fibres végétales by Abdelmonem Elouaer( )

1 edition published in 2011 in French and held by 1 WorldCat member library worldwide

L'industrie des matériaux composites ne cesse d'évoluer et de croître en mettant en place de nouveaux matériaux et de nouvelles technologies. En substitution des matériaux d'origine fossile que les matériaux d'origine naturelles (et surtout végétales) commencent à voir le jour. C'est dans ce contexte que notre travail de recherche est proposé. Il s'intéresse à la caractérisation du comportement mécanique d'un composite à matrice Polypropylène, renforcé avec des fibres de Chanvre et du bois de Chanvre (Chènevotte). Les différents moyens et techniques de caractérisation, utilisés par la présente étude, ont montré que ces nouveaux matériaux sont dotés de propriétés, en particulier mécaniques, de haut niveau, qui viennent rivaliser avec celles des autres composites classiques à base de fibres de verre et de carbone.Les essais expérimentaux en statique et de fatigue, ont révélé beaucoup de détails en comparaison avec d'autres matériaux composites. Ces informations ont permis de créer une sorte de base de données qui pourra servir de référence pour d'autres composites de la même famille à base de fibres végétales. Ainsi, des mécanismes d'endommagement ont été mis en évidence grâce aux essais mécaniques (traction monotone, charge-décharge, ...) associés à des observations microscopiques (Microscope Electronique à Balayage), et à des outils de détection du dommage basés sur l'émission acoustique. Par le biais de cette technique, nous avons pu apprécier la qualité et l'importance de l'interface fibre/matrice qui est un paramètre fondamental pour la présente étude et pour la détermination de la loi de comportement du composite.La modélisation micromécanique a été intégrée dans ce travail de thèse, grâce au modèle de Mori-Tanaka. Le comportement des matériaux à l'endommagement n'a pas été pris en considération ; seule l'élasticité a été étudiée. A l'aide de ce modèle, nous avons pu remonter aux propriétés intrinsèques des constituants (le module d'élasticité longitudinale des renforts: Chanvre et Chènevotte)
Modélisation numérique des structures composites multicouches à l'aide d'une approche discrète au sens de Mindlin. Le modèle DDM (Displacement Discrete Mindlin) by Siham Sakami( )

1 edition published in 2008 in French and held by 1 WorldCat member library worldwide

The present work of the thesis deals with the theoretical formulation and the evaluation of a new first order finite element for multilayered/sandwich plates and shells. It's based on a displacement variational model that we consider as discrete, insofar as we introduce kinematic and mechanical hypothesises in a discrete manner. This model, labelled DDM (Discrete Displacement Mindlin), leads to a finite element which is geometrically simple (4 node) and efficient, owing to the linearity of bending curvatures obtained from a quadratic approximation of the normal rotations to the plate mid-surface. The new element takes into account the transverse shear effects along thickness direction and gives thin plate results when the ratio L/h (Length/ thickness) becomes big. It has been successfully validated across some known testing problems, from thin to thick laminated and sandwich
Analyse du comportement en vibration de matériaux composites à fibres végétales. by Khouloud Cheour( )

1 edition published in 2017 in French and held by 1 WorldCat member library worldwide

This PhD research work aimed at analysing the free vibration behaviour of non-hybrid and hybrid flax-glass composites. First, a modal analysis approach was developed to study the mechanical and dynamic behaviour of these materials. Their elastic and damping properties were identified from their natural frequencies and a comparison with the traditional composites was carried out. In the second part, a finite element modelling of the damping of non-hybrid and hybrid composites was implemented by considering the classical laminate theory, taking into account the transverse shear effects. Different topics were studied such as the fibres orientation, the stacking sequence, the reinforcement architecture, the choice of the stacking sequence layers for the hybridisation. This analysis resulted in optimising both mechanical and damping performances of non-hybrid composites and hybrid flax-glass composites. In the last part of this work, the effect of water ageing on the dynamical and mechanical properties of non-hybrid and hybrid glass-flax composites was studied. To this end, these composites were subjected to free vibrations at different ageing durations in order to identify the effect of water ageing on their mechanical and damping properties and their evolution with ageing time. Finally, a cycle of ageing until saturation was reached followed by a drying operation, which was carried out to analyse the reversibility of their properties
Étude expérimentale et modélisation de la durabilité des biocomposites à fibres de lin by Abderrazak Chilali( )

1 edition published in 2017 in French and held by 1 WorldCat member library worldwide

In this thesis work, we study the durability of two twill flax fabrics reinforced thermosetting and thermoplastic composites. Firstly, the diffusion behaviour of these composites is investigated by identifying their 3D Fick's and Langmuir's diffusion parameters using an optimization algorithm. The influence of several geometric parameters and fibre orientation on their 3D moisture diffusion is also studied. Then, we analyse the effect of water ageing on their elastic and failure properties. Finally, a numerical finite element analysis is performed in order to study their diffusive and hydro-mechanical behaviour. The water diffusion parameters of the flax fibre and the used resins are estimated by a numerical inverse analysis exploiting experimental water uptake data. The heterogeneity of the studied composites is considered by modelling the twill weave fabrics undulation of their unit-cell. In particular, the mechanical behaviour of the unaged composites is found to exhibit a Kaiser effect contrary to the aged materials which exhibit a significant Felicity effect synonymous of substantial damage induced by water ageing. Besides, it is found that high mechanical stress concentrations are developed at the fibre-matrix interface, which could cause damage initiation and lead to the final composite failure
Modélisation du comportement mécanique de biocomposites hybrides en présence du vieillissement by Belahcen Djellouli( )

1 edition published in 2021 in French and held by 1 WorldCat member library worldwide

Natural fibre-reinforced composites are progressively becoming an alternative to certain synthetic fibres. The use of vegetal fibres as reinforcement makes it possible to combine respect of the environment with having composite materials presenting attractive specific mechanical properties.However, the sensitivity of vegetal fibres to moisture limits their use in structural applications. Therefore, the hybridization of vegetal fibres with synthetic fibres, such as glass fibre, can be one of the solutions to the variation of mechanical properties of vegetal fibre composites. In this context, we propose in this PhD study to evaluate the durability of flax, glass and flax-glass hybrid composites after their exposure to a humid environment at room temperature.First, the static behaviour of the resin and the hybrid and non-hybrid composites are analysed. Then, we evaluate the water diffusion kinetics in the composites by identifying their 1D and 3D diffusion parameters, using an optimisation approach based on the Fick and Langmuir models. Then, we develop and implement in the ABAQUS FEA software two triangular three-node membrane finite elements to model the hygroscopic behaviour of the studied composites. This modelling allows us, among other things, to estimate the radial and longitudinal diffusive parameters of the flax fibre, which are difficult to determine experimentally.Then, we develop and implement in ABAQUS three-node triangular membrane finite elements to model the hygroelastic behaviour of the studied composites. The objective of this part is to estimate their durability through the calculation of hygroscopic stresses and strains. For this purpose, decoupled and coupled hygroelastic models, based on the Fick' and Langmuir's model, are considered.Finally, we study the impact of moisture on the mechanical behaviour, through the investigation of the progressive loss of elastic properties of these composites
Éléments finis spéciaux pour l'analyse linéaire et non-linéaire géométrique des structures composites à renforts fibreux by Mohamed Amine Tiar( )

1 edition published in 2017 in French and held by 1 WorldCat member library worldwide

La modélisation numérique des structures composites à renfort fibreux de géométrie complexe constitue un axe de recherche majeur afin de prédire correctement leur comportement mécanique. Dans ce contexte, l'étude menée dans ce travail de thèse porte sur le développement de nouveaux éléments finis basés sur une approche numérique multi-échelle, appelée Approche de la Fibre Projetée (AFP). Cette approche a l'avantage de tenir compte de la présence des fibres au sein d'un espace matrice sans les discrétiser, ce qui limite considérablement la taille du système à résoudre. Pour analyser le comportement des structures composites, plusieurs éléments finis 2D et 3D ont été développés et implémentés dans le code ABAQUS via la routine UEL. Plusieurs cas tests de validation sont considérés pour tester la précision et l'efficacité des éléments finis proposés et les résultats obtenus sont globalement en bon accord avec les solutions de référence. De plus, l'intérêt de la nouvelle approche (AFP) est particulièrement mis en exergue en étudiant des structures composites complexe à renfort 3D : une plaque sandwich cousue et une plaque sandwich à âme creuse renforcée par des fibres en forme de « 8 »
Optimisation de fibres de carbone pour leur application à des composites hautes-performances à matrice organique polymérisés par voie radicalaire sous rayonnement. by Arnaud Martin( )

1 edition published in 2014 in French and held by 1 WorldCat member library worldwide

Polymerization under high energy radiation is a promising alternative to autoclave processing for manufacturing high-performance composite materials. Electron beam can initiate free radical polymerization processes of acrylate-based matrix. However, the comparison with state-of-the-art thermally cured composites reveals the lower transverse mechanical properties of radiation-cured composites. The aim of this project was to improve the transverse mechanical properties of these radiation-cured composites by a surface modification of carbon fiber. We have investigated several points related to these issues, and particularly we have inquired about the influence of the chemistry present at the surface of the carbon fiber on the polymerization step initiated under irradiation curing. We tried to have a better understand on their influence on the polymerization and on the curing process based on radiation-induced free radical chemistry. Then, the next step deals with the formulation of a surface modification treatment applied by sizing in order to improve the interface quality with the creation of an interphase and even better a covalent link between the fiber and the matrix. We worked on the compatibility of this formulation with the industrial process and we propose aqueous and non-aqueous-based emulsion processes to apply the surface treatment. Finally, demonstrator materials were manufactured and the mechanical properties in the fiber transverse direction were measured. The obtained results illustrate the efficiency of our surface modification solution on the mechanical performance of acrylate-based radiation cured composites
Contribution à la Modélisation de Composites 2D/3D à l'Aide d'Eléments Finis Spéciaux. by Lakhdar Sedira( )

1 edition published in 2013 in French and held by 1 WorldCat member library worldwide

This doctoral thesis deals with the finite element formulation and evaluation of a modified Mindlin's discrete variational model for static, dynamic, linear and non-linear composite plates and shells analysis. Including additional terms of zigzag type, in order to improve the accuracy of stress, the model has been reformulated to take into account the linear picewise of displacement variation. Consequently, two finite plate and shell elements with four nodes, called DMQP and DMQS (Discrete Quadrilateral Mindlin Plates and Shells respectively), enhanced by quadratic field rotations, have been developed and validated under REFLEX and ABAQUS codes.Both elements including the zigzag effect have been also developed in a second version, and validated through several static and dynamic test problems known from the literature, highlighting the independence towards the transverse shear correction and in particular the stress accuracy with respect to the initial model without the zigzag effect.The satisfactory results of this model found through cases of linear isotropic shell tests, motivated us to extend this approach to the non-linear geometric applications. An isoparametric curve element of shell has been developed for this purpose, where small elastic deformation assumptions of and large displacements and moderate rotations are adopted. It is geometrically simple and has only four nodes at corners and 6 DOF/node. The elementary calculation of the tangent stiffness matrix consists in combining the linear part of the curved shell element (DMQS) with that of the membrane Q4 non-linear part. An Updated Lagrangian Formulation at each Iteration (ULFI) is used with Newton-Raphson resolution Method. Some standard tests of nonlinear geometrical shell structures are presented; they show a very good convergence and global behavior better than such elements
Etude de l'effet du vieillissement hygrothermique sur le comportement mécanique d'éco-composites à renfort hybride by El Hadi Saidane( )

1 edition published in 2015 in French and held by 1 WorldCat member library worldwide

L'objectif de ce travail est d'étudier l'effet de différents types de vieillissement sur le comportement mécanique en traction de matériaux composites à fibres de lin et de verre et hybrides lin-verre. L'effet de plusieurs types de vieillissement a été d'abord analysé dans le cas de matériaux composites stratifiés à fibres de lin. Cette étude a permis de suivre l'absorption d'eau dans ces matériaux, et de déterminer leurs paramètres de diffusion en utilisant les modèles de Fick 1D et 3D. Ces paramètres ont été estimés en utilisant une procédure itérative développée à l'aide du logiciel Matlab. Ensuite, une caractérisation des propriétés élastiques et ultimes, à partir des essais de traction des matériaux composites non vieillis et vieillis a été réalisée. Dans le but de réduire l'absorption d'eau et d'améliorer les propriétés mécaniques des composites à fibres de lin, l'hybridation des renforts sergé lin-verre a été choisie. Après l'analyse du comportement mécanique des différents matériaux, l'identification des mécanismes d'endommagement est ensuite menée en utilisant une méthodologie associant la technique d'émission acoustique (EA) et des observations microscopiques. Les signaux d'EA ont été analysés, en considérant une analyse multivariable avec le Logiciel Noesis. La dernière partie aborde une 1ère étape de la modélisation des propriétés élastiques des composites hybrides lin-verre. A partir d'une décomposition de la cellule de base, l'application de la théorie des stratifiés a permis de déterminer les propriétés élastiques du composite en fonction de ses constituants (fibres et matrice) en tenant compte de la géométrie de l'armure
Evaluation des performances dissipatives des composites non-tissés à fibres végétales pour des applications « automobile » : analyses expérimentale et numérique by Hajer Hadiji( )

1 edition published in 2020 in French and held by 1 WorldCat member library worldwide

This thesis deals with the experimental and numerical analysis of the mechanical and damping behaviour of non-woven composites reinforced with natural fibres. Firstly, the elastic properties of the nonwoven composites reinforced with different types of fibres are determined by means of static tensile tests. The corresponding results highlight the competitiveness of natural fibres compared to those reinforced with synthetic fibres, by combining rigidity with reduced density. Then, we investigate the damping properties through free flexural vibration tests. This analyse highlights the importance of the high damping capability of a non-woven composite. Secondly, we carried out a parametric study on a non-woven composite reinforced with flax fibres, varying the porosity content or the fibre volume fraction. The obtained results showed that these two parameters have a major influence on the absorption of vibrations and that porosity promotes the energy dissipation within the composite. Finally, we propose a 2D and 2.5D finite element models, that takes into account the geometrical characteristics of the fibres, such as their form, length and orientation as well as the random aspect of the porosities. Both proposed numerical models respect well the fibre volume fraction and the porosity content. Based on the experimental results, we can assert that this numerical analysis can estimate, using an inverse approach, the Young's modulus of the fibres thanks to the 2D model and their damping properties, via the 2.5D model
 
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French (19)

English (3)