Papers by Vincent Picandet
La rupture d'une chaîne axiale endommageable est étudiée en comparant la réponse du système discr... more La rupture d'une chaîne axiale endommageable est étudiée en comparant la réponse du système discret considéré avec celui d'un milieu continu non local équivalent. L'approche de la Mécanique de l'Endommagement Discret (MED) est basée sur un modèle microstructuré composé d'une série périodique de ressorts élastiques endommageables (MED d'un système axial). Un tel système discret endommageable est mathématiquement équivalent à la formulation en différences finies du comportement d'un Milieu Continu Endommageable (MCE). Le modèle du MCE non local considéré dans cette étude est principalement construit à partir d'une procédure de continualisation appliquée à un schéma en différences fines centrées basé sur des conditions aux limites de type statique, relevant d'une loi cohésive entre effort normal et déplacement. Une comparaison des résultats issus de la méthode discrète avec l'approche continue montre l'efficacité de cette nouvelle modélisation non locale construite à partir d'arguments micromécaniques, en particulier pour analyser les effets d'échelle.
Dans la littérature, la plupart des mélanges chaux/chanvre étudiés montrent un comportement fragi... more Dans la littérature, la plupart des mélanges chaux/chanvre étudiés montrent un comportement fragile et une très faible résistance mécanique. Les formulations sont généralement riches en liant et légèrement compactées. Jusqu'à présent, ce matériau n'est pas considéré comme un matériau porteur et est principalement utilisé comme isolant de remplissage, combiné avec des composants de structure en bois, en béton ou en maçonnerie. Le présent travail est une étude expérimentale du comportement du béton de chanvre à la compression et au cisaillement, afin d'évaluer la capacité porteuse et au contreventement de ce matériau bio-sourcé, tout en assurant de bonnes qualités d'isolation thermique. Deux séries de tests sont effectuées. Le premier est un test de compression uniaxiale dans chaque direction pour caractériser l'anisotropie mécanique du matériau. Le second permet de caractériser le comportement au cisaillement de différentes configurations de la composition. Les expériences réalisées montrent une ductilité élevée de ce matériau en cisaillement, ce qui confirme les résultats de la littérature sur les structures de paroi. Ces résultats sont très prometteurs, notamment pour des applications parasismiques.
International Journal for Numerical and Analytical Methods in Geomechanics, 2015
ABSTRACT The failure of a discrete elastic-damage axial system is investigated using both a discr... more ABSTRACT The failure of a discrete elastic-damage axial system is investigated using both a discrete and an equivalent continuum approach. The Discrete Damage Mechanics approach is based on a microstructured model composed of a series of periodic elastic-damage springs (axial Discrete Damage Mechanics lattice system). Such a discrete damage system can be associated with the finite difference formulation of a Continuum Damage Mechanics evolution problem. Several analytical and numerical results are presented for the tensile failure of this axial damage chain under its own weight.The nonlocal Continuum Damage Mechanics models examined in this paper are mainly built from a continualization procedure applied to centered or uncentered finite difference schemes. The asymptotic expansion of the first-order upward difference equations leads to a first-order nonlocal model, whereas the asymptotic expansion of the centered finite difference equations leads to a second-order nonlocal Eringen's approach. To complete this study, a phenomenological nonlocal gradient approach is also examined and compared with the first continualization methods.A comparison of the discrete and the continuous problems for the chains shows the effectiveness of the new micromechanics-based nonlocal Continuum Damage modeling, especially for capturing scale effects. For both continualized approaches, the length scale of the nonlocal models depends only on the cell size, while for the so-called phenomenological approach, the length scale may depend on the loading parameter. This apparent load-dependent length scale, already discussed in the literature with numerical arguments, is found to be sensitive to the postulated structure of the nonlocal model calibrated according to a lattice approach. Copyright © 2015 John Wiley & Sons, Ltd.
The goals of this research are to study the relationship between cracking and permeability of con... more The goals of this research are to study the relationship between cracking and permeability of concrete and to compare the experimental results against theoretical models. Three concrete mixes are used; one of ordinary concrete, and two of high performance concrete, with similar composition save that one is reinforced with steel fibre. Two discs, 50 mm thick slices, are cut from the mid section of 110 x 220 mm cylindrical specimens and are dried prior to being subjected to permeability tests. The discs are diametrically loaded, as for a normal splitting test, until obtaining a predetermined lateral displacement, normal to the loading direction, also called the crack opening displacement (COD). A special removable device, equipped with two transducers, monitors the COD on both sides of the discs. When the predetermined COD level is reached, the discs are unloaded, and a gas permeability test is performed. After the last loading phase, gas and water permeability tests are both performe...
International Journal of Damage Mechanics, 2014
Slump has often been correlated with the yield stress of concrete as defined by the Bingham model... more Slump has often been correlated with the yield stress of concrete as defined by the Bingham model. The discussion is still open as to what the yield stress value actually is and how to measure the yield stress of a suspension in general and for a cementitious material in particular. A plate rheometer is a recent development in the measurement of yield stress of suspensions that allows shear rates far below most rotational rheometers. This paper will present the rheometer and the modifications made so that it can be used with suspensions such as welan gum and bentonite in aqueous solution, as well as cement paste. A careful analysis of the yield stress measurements will be discussed.
Le comportement au flambement d'un poteau élastoplastique avec une loi globale moment-courbure bi... more Le comportement au flambement d'un poteau élastoplastique avec une loi globale moment-courbure bilinéaire, est étudié de façon analytique pour une charge axiale centrée. Une telle étude peut trouver des applications en génie civil et peut concerner le calcul des structures en béton armé, acier, bois ou matériaux composites. Il est montré analytiquement que la solution théorique de radoucissement inélastique peut être obtenue si une condition non locale d'écrouissage est introduite, conformément aux études déjà publiées dans le cas de flexion de poutres avec écrouissage négatif (ou poutres composées d'un matériau quasi fragile [1] ). Une loi moment-courbure de plasticité au gradient est donc introduite, de manière à contrôler la phase post-localisation dans la plage de comportement post-flambement du poteau. L'influence de la variable d'écrouissage et celle de la longueur interne, inhérente à l'hypothèse de non-localité considérée, sur la propagation de la zone plastifiée et sur la flèche de ce système élastoplastique illustrent les principaux résultats.
The buckling and the post-buckling behaviours of a perfect axially loaded column are analytically... more The buckling and the post-buckling behaviours of a perfect axially loaded column are analytically investigated through a global bilinear moment-curvature elastoplastic constitutive law. Three plasticity cases are studied, namely the linear hardening plasticity law, the perfect elastoplastic case and the softening case. The applications of such a study can be found in various structural engineering problems, including reinforced concrete, steel, timber or composite structures. It is analytically shown that for all kinds of elastoplastic behaviors, the plasticity phenomena lead to a global softening branch in the load-deflection diagram. The propagation of the plasticity zone during the post-buckling process is analytically characterized in case of linear hardening or softening plasticity laws. However, it is shown that the unphysical elastic unloading solution necessarily occurs in presence of local softening moment-curvature constitutive law. A nonlocal plasticity moment-curvature softening law is then used to control the localization branch in the post-buckling stage. This nonlocal plasticity law includes the explicit and the implicit gradient plasticity law. Higher-order plasticity boundary conditions are derived from an extended variational principle. Some parametric studies finally illustrate the main findings of this paper, including the plasticity modulus effect on the post-buckling behaviour of these plasticity structural systems.
Le béton de chanvre est un matériau de construction qui présente l'avantage d'être performant pou... more Le béton de chanvre est un matériau de construction qui présente l'avantage d'être performant pour ses propriétés thermiques lors de son utilisation, et sur le plan environnemental dans la globalité de son cycle de vie. Ses propriétés mécaniques, quant à elles, diffèrent à bien des égards des matériaux traditionnels de construction. Cette étude présente l'effet des inclusions végétales sur les modules d'élasticité de composites chaux-chanvre. Des approches théoriques et numériques sont confrontées aux résultats expérimentaux obtenus pour des pâtes de chaux avec des concentrations volumiques de chènevotte allant de 0 à 35%. Les modules d'élasticité sont déterminés par des essais de compression : les allongements axial et circonférentiel sont mesurés par un outil spécialement développé pour ce type de matériau. Sur le plan numérique, les simulations effectuées par éléments finis en 3D s'intéressent au comportement mécanique d'une matrice comportant
European Journal of Mechanics - A/Solids, 2015
In this paper, we revisit the capability of numerical approaches such as finite difference method... more In this paper, we revisit the capability of numerical approaches such as finite difference methods and finite element methods, in approximating exact one-dimensional continuous eigenvalue problems (such as lateral vibrations of a string, the axial or the torsional vibrations of a bar, and the buckling of elastic columns). The numerical methods analysed in this paper are converted into difference equations. Following a continualization procedure or the method of differential approximation, the difference operators are then expanded in differential operators via Taylor expansion or Pad e approximants. Analogies between the finite numerical approaches and some equivalent enriched continuum are shown, using this continualization procedure. The finite difference methods (first-order or higher-order finite difference methods) are shown to behave as integral-based nonlocal media (or stress gradient media), while the finite element method is found to behave as gradient elasticity media (or strain gradient media). The length scale identification of each equivalent enriched continuum strongly depends on the order of the numerical method considered. For the finite difference methods, the length scale identification of the equivalent nonlocal medium depends on the static versus dynamic analysis, whereas this length scale appears to be independent of inertia effects for the finite element method. Some comparisons between the exact discrete eigenvalue problems and the approximated continuous ones show the efficiency of the continualization procedure. An equivalent enriched Rayleigh quotient can be defined for each numerical method: the integral-based nonlocal method gives a lower bound solution to the exact eigenvalue multiplier, whereas the gradient elasticity method furnishes an upper bound solution.
Applied Mathematical Modelling, 2014
This paper deals with a phenomenological model based on a nonlocal Bingham constitutive law. This... more This paper deals with a phenomenological model based on a nonlocal Bingham constitutive law. This nonlocal viscoplastic law accounts for possible microstructured effects of a heterogeneous fluid. Thermodynamic arguments are presented for the justification of this two-scale nonlocal shear rate law, which can be specialized either as a purely nonlocal or a gradient-type Bingham law. Plane shearing and uniaxial flow assumptions between parallel walls are basic and tractable hypotheses for a first investigation of nonlocal and gradient shear rate law. Analytical solutions are presented for the plane Poiseuille stationary flow inside a straight smooth channel of infinite width. Two higher boundary conditions at the walls are assumed for this socalled micromorphic medium, a static higher-order boundary condition and a kinematic one. A boundary layer phenomenon can be observed for such nonlocal problems. Parametric studies show the key role of the characteristic lengths of the nonlocal model on the shear flow phenomena. Consequently, the heterogeneous nature of microstructured yield-stress fluids may significantly affect velocity profiles, maximum velocity and total flow rate, which have been recently reported in literature. developed by Picard et al. [15] and Bocquet and Colin [16].
Cement and Concrete Research, 2015
Cellulose ether (CE) admixtures are used to improve the water retaining properties of cement-base... more Cellulose ether (CE) admixtures are used to improve the water retaining properties of cement-based 15 materials in order to ensure their homogeneity during casting operations. Water permeation through 16 concrete is usually quantified by permeability. In this study, an oedometric cell has been adapted to 17
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Papers by Vincent Picandet