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International audience; This paper provides multi-level modeling of microcracked viscoelastic masonries generally present in historical masonries or refractory linings. It is an extension of the Cecchi and Tralli (2012) or Cecchi & Barbieri (2008) and Cecchi & Taliercio (2013) works for the typical Burgers and Modified Maxwell models followed by both undamaged and microcracked masonries. For the sake of simplicity and in order to provide rigorous analytical global estimates, only the mortar is assumed to be viscoelastic and microcracked. Bricks are assumed to be undamaged, elastic or quasi-rigid. The distribution of microcracks is assumed to be isotropic. The effective behavior of the viscoelastic microcracked masonry is provided by two steps. The first one relies on the coupling between the Griffith’s brittle fracture theory and linear mean-field homogenization scheme in order to account for the effect of microcracks on the macroscopic deformation of the mortar and establishes a linear relation between apparent macroscopic stress and strain. This step allows to easily and fast determine the effective creep function of the microcracked mortar without recourse to ’complex’ or heavy numerical inversion of the Laplace–Carson transform. The second step is based on the coupling between asymptotic analysis and homogenization theory applied for a periodic masonry. The proposed models provide analytical solutions - explicit functions of the crack density parameter - for the effective orthotropic behavior of a microcracked viscoelastic periodic masonry cell. This study proves that the Cecchi & Tralli’s and Cecchi & Taliercio’s extension estimates are close and that the later are softer. Such overall properties are used to perform finite element computations on a compressed masonry panel as a first application. These models allow then the prediction of mostly stressed and deformed areas in microcracked masonry structures. This study demonstrates that modeling a mortar (at its undamaged or microcracked state) with this Burgers formulation is only suitable for a masonry with too high values of the Maxwell’s relaxation time otherwise it yields to vanishing effective properties with the increase of time and crack density leading thus to a premature collapse of the masonry. On the other hand, the Modified Maxwell model permits the masonry to preserve a certain resistance for every range value of Maxwell’s relaxation time. These conclusions are valid for masonry either with elastic or quasi-rigid undamaged bricks.

International audience; In order to support the optimization of mortarless brick linings of steel producing furnaces it is proposed in this paper to investigate only the compressibility of dry refractory joints. To this end, optical measurements based on the Digital Image Correlation method were carried out during compression of Magnesia-Carbon brick samples with dry joints. The second main objective is to assess the accuracy of the secant linearization schemes (the classical secant procedure and its modified extension) to reproduce the reference local and global behaviour of refractory mortarless linings accounting for the identified inelastic convex powerlaw behaviour of the Magnesia-Carbon dry joint. The reference nonlinear solution is obtained by means of finite elements method. Under normal compressive loading, unlike for usual (concave) power-law viscoplastic composites for which the secant schemes are known to provide too stiff results, it was found that the modified secant scheme leads to good overall predictions. The classical secant procedure underestimates the reference local and overall behaviour. To improve the latter result, an empirical improved secant formulation was proposed and implemented. It leads to better estimates at local and global levels.

International audience

International audience

National audience; En service, les garnissages réfractaires sont souvent soumis à de sévères sollicitationsthermo-mécaniques. Par conséquent, ces maçonneries peuvent avoir des comportements linéaire ounon-linéaire (viscoélastique ou viscoplastique). Il est alors intéressant de prendre en compte lecomportement en fluage et l’endommagement de ces doublures réfractaires. Durant les dernièresdécennies, les techniques multi-échelles ont démontré leur efficacité dans la prédiction ducomportement global et local de structures maçonnées avec de faibles coûts numériques. Dans cecontexte et comme première étape, nous proposons dans ce travail de modéliser la maçonnerie avecdes briques saines et un mortier viscoélastique suivant le modèle rhéologique de Burgers ou deMaxwell modifié que ce soit à l’état sain ou microfissuré. Le modèle proposé est basé sur le couplageentre la technique d'homogénéisation linéaire et la théorie de Griffith. Ceci permet d’exprimer lafonction de fluage effective du mortier microfissuré. Le comportement macroscopique fonction dutemps de la maçonnerie est ensuite déterminé grâce à la technique d'homogénéisation périodique. Lapertinence du modèle proposé est évaluée par comparaison à une solution numérique en utilisant laméthode des éléments finis.

International audience

International audience; During service, refractory linings are often subjected to severe thermo-mechanical loadings. Accordingly, these masonries may behave linearly or nonlinearly in a viscoelastic and/or viscoplastic manner. It is then interesting to investigate the creep behaviour and cracking of refractory linings. In the last decades, multi-level techniques have demonstrated their efficiency for predicting the global and local behaviour of masonry structures with low numerical cost. In this context and as a first step, we propose in this work to model masonry with safe bricks and a mortar following the Burgers or modified Maxwell rheological models at its safe and microcracked states. The proposed model is based on the coupling between linear homogenization technique and the Griffith theory. This allows the determination of the effective creep function of the microcracked mortar. The time-dependent macroscopic behaviour of the masonry is determined thanks to analytical periodic homogenization technique. The relevance of the proposed model is evaluated by reference to a numerical solution computed by finite elements method based on an incremental scheme. A similar methodology can be then adapted to viscoplastic masonries at safe and micro-cracked states using one of the available linearization schemes. 1. Introduction In the iron and steel industry, the refractory lining of furnaces is often made of masonry in which joints between bricks may be either mortared or dry (i.e. without mortar [1]). In most cases, refractory brick linings are installed with mortar because its use to bond the brickwork provides more resistance to thermal shock and a cushion at the brick joint [7]. The temperature inside these structures can reach 1650 degrees which induces nonlinear mechanical behaviour for the masonry and even leads to the initiation and propagation of cracks in the joints. Note that a thick mortar tends to decrease the stiffness of structure and increases the likelihood of the possible penetration of process materials into the joints, resulting in the deterioration of the lining. So, the use of thin mortar joint is appropriate and necessary in designing the refractory brick lining system. Concerning the creep behaviour of the mortar which can be induced by severe service conditions, various rheological models namely the Maxwell, Kelvin-Voigt, Ross, Burgers and Modified-Maxwell models may be investigated [5]. A number of rheological models are examined to assess their ability to predict the creep of masonry. It was proved that the Modified Maxwell model is the most accurate. According to this result, the Burgers model, namely a Maxwell system connected in series with a Kelvin-Voigt one, and the Modified Maxwell scheme (a parallel combination of the Maxwell model and a spring) models are adopted in this paper to describe the mortar joint's creep. Moreover, in the literature, there are few works investigating the global and local behaviour of viscoelastic masonry. For instance, Cecchi and Tralli [3] used the asymptotic homogenization technique to deduce the behaviour of a safe (without cracks) viscoelastic periodic masonry cell. The effective behaviour of a masonry prism is provided due to periodic homogenization. In the present study, the coupling between the Griffith's theory and the dilute scheme [4] will be applied to provide the effective behaviour of a micro-cracked mortar [6]. In a second step, the expressions proposed in [3] are extended to determine the effective behaviour of a periodic microcracked viscoelastic masonry cell. As a first application, the case of a compressed wall was treated in order to assess the

International audience

International audience; Forming thick, complex shapes with several layers is needed in high technology fields. During forming, defects can occur and have to be taken into account because they can significantly affect the mechanical performance of the part. This experimental study shows that, when working with dry fabric forming, the type and number of defects is a function of the punch geometry, the process parameters, the orientation of the fabric with respect to the punch and the inter-ply friction. Inter-ply friction has a huge effect on the quality of the preform when inter-ply sliding occurs. This inter-ply friction leads to several overhanging yarn shocks that generate high tangential forces, which inhibit the relative sliding of plies. In addition, to reduce the number and amplitude of defects, the layers subjected to severe defects can be placed in the inner position where they are subjected to the compression applied by the upper layers.