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International audience; The quantification of the plantar arch load and its surface distribution is essential to improve locomotion in many sports as well as rehabilitation protocols for patients. This paper has a threefold objective. First, it focuses on the design, fabrication and characterization of dynamic force sensors, based on both a single and multilayer of Velostat piezoresistive polymer. A matrix of 9 x 3 flexible pressure sensors has been developed to analyze and recognize plantar movements. Second, two models linking the plantar pressure to the resistance of the piezoresistive material, with and without taking into account the viscoelastic nature of the piezoresistive material, are proposed. Lastly, an inversion of the models is employed in order to estimate plantar pressure as a function of the variation in material resistance. An evaluation of the plantar pressure for various movements is also proposed.

This work deals with the study of the delayed effects of moisture content on wood cracking. It is based on two main axes, water diffusion and bending creep of and unnotched and notched beams. The study of diffusion is based on an innovative experimental approach, and analytical solutions of Fick’s diffusion law. It is carried out on three tropical hardwoods, Pterocarpus soyauxii Taub (Padouk), Aucoumea klaineana Pierre (Okoumé) and Milicia excelsa (Iroko) and two softwoods, Abies alba Mill (White Fir) and Pseudotsuga menziesii (Douglas Fir) subjected to relative humidity (RH) steps at a constant temperature of 20°C. The delayed behaviour was studied on Okoumé, Padouk and White Fir only, by means of 3-point bending tests carried out at 20°C and controlled RH on defect-free wood beams oriented in fibre direction, of dimensions 160x6x12 mm (LxRxT). Deflection and transverse expansion were measured with variable linear displacement transducers. In the case of notched beams, optical monitoring of cracking parameters was performed using a USB microscope. The analysis of tests was carried out by applying the straight beam theory and assuming the response of the material to be homogeneous and linear with respect to stress, resulting in expressions of compliance. For the unnotched beams, two series of tests were carried out: 14 days at 10% load, 65% RH with increments of 45 and 75% RH; 5 days at 35% load, 75% RH with increments of 45% RH. The known characteristics of the mechanosorptive effect were confirmed: deflection increase at first wetting and at drying, decrease at subsequent wettings. The analysis of the mechanosorptive trajectories suggested the existence of a creep limit and confirmed the existence of the stationary or pseudo creep/recovery response. For notched beams, creep tests were also carried out in two stages: at 80% load, then adjusting the load to have an initial strain rate of 0.30%, during 6-7 days at 45 and 75% RH. It is found that mechanosorption triggers crack propagation and that the coupling between cracking and creep accelerates strain to failure. The analysis of energy release rate in mode I of the 3 tested species is also presented and compared to the literature data.; Ce travail concerne l’étude des effets différés et de la teneur en eau sur la fissuration des bois tropicaux et tempérés. Il s’articule sur deux grands axes, diffusion de l’eau et fluage des poutres entaillées ou non. L’étude la diffusion est basée sur une approche expérimentale innovante, et analytique de la loi de diffusion de Fick. Elle est réalisée sur trois feuillus tropicaux, Pterocarpus soyauxii Taub (Padouk), Aucoumea klaineana Pierre (Okoumé) et Milicia excelsa (Iroko) et deux résineux, Abies alba Mill (Sapin blanc) et Pseudotsuga menziesii (Douglas) soumis à des paliers d’humidité relative (HR) à température constante de 20°C. Le comportement différé en flexion 3 points a été étudié sur l’Okoumé, le Padouk et le Sapin blanc uniquement, à 20°C et HR contrôlée sur des poutres et orientées dans la direction des fibres, de dimension 160x6x12 mm (LxRxT). La flèche et l’expansion transversale étaient mesurées avec des transducteurs à déplacement linéaire variable. Dans le cas des poutres entaillées, un suivi optique des paramètres de fissuration était réalisé avec un microscope. L’analyse de ces essais s’est faite en se basant sur la théorie des poutres rectilignes et en supposant la réponse du matériau homogène et linéaire par rapport à la contrainte, aboutissant à des expressions de la complaisance. Pour les poutres non entaillées, deux séries de tests ont été réalisées : 14 jours à 10% de charge, 65% HR avec paliers de 45 et 75% HR; et 5 jours à 35% de charge, 75% HR avec palier de 45% HR. On retrouve les caractéristiques connues de l’effet mécanosorptif : augmentation de la flèche à la première humidification et au séchage, diminution aux humidifications ultérieures. L’analyse des trajectoires mécanosorptives a suggéré l’existence d’une limite de fluage et confirmé celle de la réponse stationnaire ou pseudo fluage/recouvrance. Pour les poutres entaillées, les essais de fluage ont été réalisés aussi en deux temps : à 80% de charge, puis en ajustant la charge pour avoir un taux de déformation initiale de 0,30%, durant 6-7 jours à 45% et 75% HR. On trouve que la mécanosorption accentue la propagation des fissures et que le couplage entre fissuration et fluage accélère la déformation jusqu’à la rupture. L’analyse du taux de restitution d’énergie en mode I des 3 essences testées est également présentée et comparé aux rares résultats de la littérature.

The aim of this study was to characterise the viscoelastic properties of different types of human tumour tissue that frequently metastasize to the bone using AFM and to correlate them with tissue composition.

International audience; Using literature data we have studied the rheological behaviour of concentrated maltodextrins and starches. We show that much of their rheology, like zero shear viscosity and shear thinning behaviour, appears to be governed by the ratio of the glass transition temperature and actual temperature, T-g/T, as the scaling parameter. Via this scaling, we can apply time-temperature-solvent superposition principle, which is also validated for linear dynamic viscoelastic experiments at different temperatures, compositions, and moisture content. Furthermore, we show that the dynamic viscoelastic experiments follow the Marin-Graessley model, indicating that concentrated maltodextrins and starches behave as transient (entangled) networks.

SUMMARY Long term deformations strongly depend on the earth model and its rheological parameters, and in particular its viscosity. We give the general theory and the numerical scheme to compute them for any spherically non-rotating isotropic earth model with linear rheology, either elastic or viscoelastic. Although the Laplace transform (LT) is classically used to compute viscoelastic deformation, we choose here instead, to implement the integration with the Fourier transform (FT) in order to take advantage of the fast FT algorithm and avoid some of the LT mathematical difficulties. We describe the methodology to calculate deformations induced by several geophysical signals regardless of whether they are periodic or not, especially by choosing an adapted time sampling for the FT. As examples, we investigate the sensitivity of the displacements due to long period solid Earth tides, glacial isostatic adjustment and present-day ice melting, to anelastic parameters of the mantle. We find that the effects of anelasticity are important for long period deformation and relatively low values of viscosities for both Maxwell and Burgers models. We show that slight modifications in the rheological models could significantly change the amplitude of deformation but also affect the spatial and temporal pattern of the signal to a lesser extent. Especially, we highlight the importance of the mantle anelasticity in the low degrees deformation due to present-day ice melting and encourage its inclusion in future models.

Pharmaceutical tablets can be produced on different kinds of presses that may have very different compaction kinematics. Strain rate sensitivity (SRS) is thus an important property for the powders used to produce pharmaceutical tablets. Viscoelasticity is one of the aspects of the SRS and can be sometimes difficult to characterize. In this work, impulse excitation technique was used as an easy-to-implement method for characterizing viscoelasticity using the fact that this property induces damping which can be detected on resonance spectra as peak enlargements. A damping ratio, related to the first flexural vibration mode, was determined on impulse excitation frequency spectra using the half-power bandwidth method on tablets made with different products. This method made it possible to obtain reproducible results for the damping ratio. As viscoelasticity is not the only phenomena that can promote damping, tests were made in order to assess the influence of other parameters: viscoplasticity, porosity and tablet dimensions. Results indicated that the influence of these phenomena could be considered as negligible. Finally, the damping ratios determined were in good accordance with the known viscoelastic behavior of the studied products. This made it possible to confirm that impact resonance is an easy and quick way to characterize the viscoelastic nature of pharmaceutical tablets.

The effect of physical aging on the viscoelastic (VE) behavior of epoxy resin is investigated experimentally performing strain-controlled tests at various temperatures on specimens aged at different temperatures (TA) for different times (tA). The aging effect is analyzed using as a framework Schapery's type of thermo-aging-rheologically simple (T-A-R simple) VE model that contains aging-state and test-temperature dependent shift factor. Experiments show that in first approximation, the shift factor can be presented as the product of aging related shift factor aA and temperature related factor aT. It is found that for short aging times the change rate of the aging shift factor with tA does not depend on TA, whereas for long tA at high TA the rate increases. Shift factors alone are not able to explain differences in relaxation curves for almost “fully” aged specimens aged at different high TA, It is shown that a T-A-R complex VE model with two additional aging-dependent functions can describe the observed discrepancies.

Foams are concentrated dispersions of gas bubbles in a continuous medium which arewidely encountered in many applications, from food industry to cosmetics, where theirliquid phase is often a fluid with complex rheology. Nevertheless, such systems are intrinsically unstable, as their structure can be altered over time by a competition of differentmechanisms, including pressure-driven gas diffusion between the bubbles. Even thoughthis coarsening process has been widely studied in aqueous foams, its dependence on themechanical properties of the liquid phase and its volume fraction still lacks a thorough understanding. In this thesis, we first experimentally probe how an increasing liquid fractioninfluences the coarsening process of aqueous bubble monolayers confined between two glass plates. The gradual decrease and eventual vanishing of the thin film area between adjacent bubbles results in a reduction of the global coarsening rate over time, which we compare with existing models. We then investigate the effect of a viscoelastic foam continuous phase on the coarsening process, by replacing the aqueous phase with concentrated oil-in-water emulsions. We show that an increase of the emulsion elasticity results in an heterogeneous bubble growth which deeply modifies the foam structure, with the appearance of unrelaxed bubble patterns. This leads to a delay in the shrinkage and disappearance of smaller bubbles which reduces the bubble growth rate. Further investigations on the coarsening dynamics in such systems show that the motion of the bubbles during coarsening is also reduced upon increase of emulsion elasticity. Finally, we show how aerated emulsions can be generated in a single step by simultaneously mixing the oil, gas and aqueous phases, ensuring a good scale separation between gas bubbles and oil drops. Moreover, we show how a standard light scattering technique such as laser diffraction granulometry can be used for a fast measure of the bubble size also in double dispersions of fluids having different refractive index. Our results on the phase separation of foamed emulsions provide new insights on their destabilising mechanism which can help controlling the stability of such complex systems for the design of new materials with atypical cellular morphology., Les mousses sont des dispersions concentrées de bulles de gaz dans un milieu continu,largement rencontrées dans de nombreuses applications, de l'industrie alimentaire à la cosmétique, où la phase liquide de la mousse est souvent un fluide à rhéologie complexe.Néanmoins, ces systèmes sont par nature instables: leur structure peut être modifiée aucours du temps par différents mécanismes, dont la diffusion de gaz entre les bulles due àleur différence de pression. Même si ce mûrissement a été largement étudié dans les mousses aqueuses, sa dépendance aux propriétés mécaniques de la phase liquide et sa fraction volumique manquent encore d'une compréhension approfondie. Dans cette thèse, nous étudions expérimentalement comment l'augmentation de la fraction liquide influence le mûrissement de monocouches de bulles aqueuses confinées entre deux plaques en verre. La diminution progressive et la disparition éventuelle des films minces entre les bulles entraîne une réduction du taux de mûrissement au cours du temps, que nous comparons avec les modèles existants. Nous étudions ensuite l'effet d'une phase continue viscoélastique sur le mûrissement des mousses, en remplaçant la phase aqueuse par des émulsions concentrées d'huile dans l'eau. Nous montrons que l'augmentation de l'élasticité de l'émulsion se traduit par une hétérogénéité de croissance des bulles qui modifie profondément la structure de lamousse, avec l'apparition de motifs de bulles non relaxés. Cela entraîne un retard dans lerétrécissement et la disparition des petites bulles, ce qui réduit le taux de croissance desbulles. D'autres résultats sur la dynamique de mûrissement dans ces mousses d'émulsions montrent que le mouvement des bulles pendant le grossissement est également réduit lors de l'augmentation de l'élasticité de l'émulsion. Enfin, nous montrons comment des émulsionsaérées peuvent être générées en une seule étape en mélangeant simultanément l'huile, l'airet la phase aqueuse, assurant une bonne séparation d'échelle entre les bulles de gaz et lesgouttes d'huile. En outre, nous montrons comment une technique standard de diffusion dela lumière telle que la granulométrie par diffraction laser peut être utilisée pour une mesurerapide de la taille des bulles dans des doubles dispersions de fluides ayant un indice deréfraction différent. Nos résultats sur la séparation de phase des mousses d'émulsions fournissent de nouvelles informations sur leur mécanisme de déstabilisation qui peuvent aider à contrôler la stabilité de ces systèmes complexes pour la création de nouveaux matériaux à morphologie cellulaire atypique.

International audience; This editorial note is dedicated to the Virtual Special Issue about the “Advances in Rheological Behavior of Nanofluids and Ionanofluids”. After a brief presentation of the objectives of this VSI, the contributions published in the Journal of Molecular Liquids are introduced.

A multi-scale modeling strategy is proposed that takes into account the temperature and cure dependence of the viscoelastic behavior of the constituent materials of a composite material and the effects of stress relaxation on thermal expansion and chemical shrinkage. A homogenization strategy is presented, which, starting from the constituent behaviors, yields the homogenized viscoelastic behavior of the composite with time-dependent expansion coefficients. This time-dependence is a direct consequence of the viscoelastic behavior of the constituents, as their expansion coefficients are supposed to be time-independent. In the simulation of a heating process, the model predicts sign changes in the thermal strain rate due to residual stress relaxation close to the glass transition temperature, which cannot be obtained with classical thermo-elastic homogenization methods.

International audience; The properties of artificial food bolus are studied by dynamic oscillatory and capillary rheometry as functions of bolus water content (WC), in the usual range of saliva hydration, for four cereal products: sponge cake, extruded flat bread and their counterpart enriched in legume proteins. All boluses followed the same rheological behavior characterized by (1) solid -like in the linear viscoelastic domain and (2) Herschel-Bulkley model for large shear strain. Hence, four characteristic rheological properties are determined: modulus at viscoelastic plateau, characteristic stress at transition to flow, yield stress and consistency in the flow regime. The decrease of these properties with WC was fitted by an exponential decay function, from which was extracted a coefficient alpha (5 = alpha < 30), defined as a coefficient of interaction of the food with water. The values of alpha are of the same order of magnitude as the plasticization coefficient of starch by water. They were larger for the extruded pea based (EFP, alpha >= 15), and were lower for the sponge cake (SC, alpha < 15). The variations for the different rheological properties are discussed in terms of matter state, envisioning bolus as a suspension of soft swellable particles. The comparison of these values with those encountered for real boluses from similar foods suggests that these results contribute to define a coefficient of interaction of food with saliva.

The diagnosis of various pathologies, including some cancers, has long been based on palpation, since a tumor is significantly stiffer than healthy tissue. More advanced and quantitative methods have been developed over the last decades, especially the Magnetic Resonance Elastography (MRE). This method uses magnetic resonance imaging (MRI) on mechanically stressed tissue to estimate the mechanical properties of soft tissue. The reconstruction of the elasticity modulus of the tissue in MRE is often based on a model with simple behavior laws of the imaged tissue (linear elasticity, isotropy, homogeneity), which could be extended to more complex cases thanks to modeling by the Finite Element Method (FEM) in particular. FEM has been used in MRE to simulate the propagation of mechanical waves in a tissue, to better understand the effects of different parameters on MRE measurements, to reconstruct stiffness, to evaluate novel MRE inversion methods, etc. However, in inhomogeneous cases with complex interfaces, FEM modeling can be particularly costly as it requires an explicit mesh of these interfaces. In this work, we explored, using an explicit dynamic FE solver developed in this thesis, the behavior of mechanical waves propagating in a linear, isotropic, viscoelastic and inhomogeneous medium, in order to better control the parameters of a MRE experiment. In the solver, the eXtended Finite Element Method (XFEM) has been implemented; this formulation of FEM originates from fracture mechanics and has been widely used over the last two decades to model discontinuities such as cracks and inclusions. Using the developed numerical tool, we studied the behavior of waves in the vicinity of an interface (oblique or complex), in particular the phenomena of wave reflection and wave conversion. It has been shown that the use of the XFEM method significantly reduces the computation time while ensuring an equivalent accuracy, in terms of displacement fields and reconstructed stiffness. Furthermore, we have addressed, by simulation, the criterion of establishing the steady state of the waves which must be reached to ensure correct recording by MRI. Today, this criterion is still empirical and can influence the quality of MRE data. We have thus proposed a metric to quantify the achievement of the steady state of wave propagation in a tissue with or without inclusion. Finally, these methods were implemented on a practical case of breast MRE. Based on breast models, we evaluated the influence of experimental parameters (size and position of the inclusion, polarization of the excitation, boundary conditions, etc.) on the achievement of steady state., Le diagnostic de différentes pathologies, dont certains cancers, a longtemps été basé sur la palpation, puisqu'une tumeur est sensiblement plus rigide qu'un tissu sain. Des méthodes plus avancées et quantitatives ont été développées au cours des dernières décennies, notamment l'Elastographie par Résonance Magnétique (ERM). Cette méthode utilise l'imagerie par résonance magnétique (IRM) sur un tissu sollicité mécaniquement pour estimer les propriétés mécaniques des tissus mous. La reconstruction du module d'élasticité du tissu en ERM est souvent basée sur un modèle avec des lois de comportement simples du tissu imagé (élasticité linéaire, isotropie, homogénéité), qui pourrait être étendu à des cas plus complexes grâce à la modélisation par la Méthode des Eléments Finis (MEF) notamment. La MEF a été utilisée en ERM pour simuler la propagation des ondes mécaniques dans un tissu, pour mieux comprendre les effets de différents paramètres sur les mesures d’ERM, reconstruire la rigidité, évaluer de nouvelles méthodes d’inversion d’ERM, etc. Cependant, dans des cas inhomogènes avec interfaces complexes, la modélisation par MEF peut être particulièrement coûteuse car elle nécessite un maillage explicite de ces interfaces. Dans ce travail, nous avons exploré, à l'aide d'un solveur EF en dynamique explicite développé dans cette thèse, le comportement d'ondes mécaniques se propageant dans un milieu linéaire, isotrope, viscoélastique et inhomogène, en vue d'une meilleure maîtrise des paramètres d'une expérimentation d'ERM. Dans le solveur a été implémentée la méthode des éléments finis étendue (XFEM); cette formulation de la MEF provient de la mécanique des fractures et a été largement utilisée au cours des deux dernières décennies pour modéliser des discontinuités comme les fissures et les inclusions. A l'aide de l'outil numérique développé, nous avons étudié le comportement des ondes au voisinage d'une interface (oblique ou complexe), en particulier les phénomènes de réflexion et de conversion des ondes. Nous avons pu montrer que l'utilisation de la méthode XFEM réduit sensiblement les temps de calcul tout en assurant une précision équivalente, en termes de champs de déplacement et de rigidité reconstruite. Par ailleurs, nous avons abordé, par simulation, le critère d'établissement du régime stationnaire des ondes qui doit être atteint pour assurer l'enregistrement correct par IRM. Or aujourd'hui ce critère est empirique et peut influencer la qualité des données ERM. Nous avons ainsi proposé une métrique pour quantifier l'atteinte du régime stationnaire de propagation des ondes dans un tissu avec ou sans inclusion. Enfin, ces méthodes ont été mises en oeuvre sur un cas pratique d'ERM du sein. Avec les modèles du sein, nous avons évalué l'influence des paramètres expérimentaux (taille et position de l'inclusion, polarisation de l'excitation, conditions aux limites, etc.) sur l'atteinte du régime stationnaire.

Particle suspensions in non-Newtonian liquid matrices are frequently encountered in nature and industrial applications. We here study the Taylor–Couette flow (TCF) of semidilute spherical particle suspensions (volume fraction $\leq 0.1$ ) in viscoelastic, constant-viscosity liquids (Boger fluids). We describe the influence of particle load on various flow transitions encountered in TCF of such fluids, and on the nature of these transitions. Particle addition is found to delay the onset of first- and second-order transitions, thus stabilising laminar flows. It also renders them hysteretic, suggesting an effect on the nature of bifurcations. The transition to elasto-inertial turbulence (EIT) is shown to be delayed by the presence of particles, and the features of EIT altered, with preserved spatio-temporal large scales. These results imply that particle loading and viscoelasticity, which are known to destabilise the flow when considered separately, can on the other hand compete with one another and ultimately stabilise the flow when considered together.

This thesis is devoted to the study of the rheology of phosphate glasses out of thermodynamic equilibrium. A method of dynamic measurement of rheological properties has been exploited, the RFDA (Resonance Frequency and Damping Analysis), in order to analyse the behavior of these properties in the region and above glass transition in time and temperature. These analyses allow to identify the different mechanisms involved, their kinetics and their characteristics and how their evolutions impact the macroscopic behavior of glass. The study of the viscoelastic behavior of inorganic glasses is based on the generalized Maxwell model. We have also investigated the application of the TNM model for the fitting and prediction of the relaxation behavior of glasses by RFDA. We have highlighted how a mathematical model is able to determine the changes in the experimental mechanical properties and to predict their evolutions for a wide variety of experimental conditions. We have demonstrated that the parameters, found from the changes of the elastic modulus following a complex thermal profile by RFDA, fit notably the experimental results of DSC and Dilatometry and are able to predict them., Ce travail de thèse est dédié à l'étude de la rhéologie des verres de phosphates hors équilibre thermodynamique. Une méthode de mesure dynamique de propriétés rhéologiques a été exploitée, la RFDA (Resonance Frequency and Damping Analysis), afin d'analyser le comportement des propriétés du verre dans le domaine de la transition vitreuse et au-delà en temps et en température. Ces analyses permettent d’identifier les différents mécanismes mis en jeu, leurs cinétiques et leurs caractéristiques et comment leurs évolutions impactent le comportement macroscopique du verre. L'étude du comportement viscoélastique des verres inorganiques s’appuie sur le modèle de Maxwell généralisé. Nous nous sommes également intéressés à l'application du modèle de TNM pour l'ajustement et la prédiction du comportement de relaxation des verres via la RFDA. Nous avons mis en lumière comment un modèle mathématique est capable de déterminer l'évolution des propriétés mécaniques expérimentales et de prédire leurs évolutions pour une grande variété de conditions expérimentales. Nous avons démontré que les paramètres, trouvés à partir des changements du module élastique, suivant un profil thermique complexe par le RFDA, s'ajustent notamment aux résultats expérimentaux de la DSC et de la dilatométrie et sont capables de les prédire.

Cells are filled with macromolecules and polymer networks that set scale-dependent viscous and elastic properties to the cytoplasm. Although the role of these parameters in molecular diffusion, reaction kinetics and cellular biochemistry is being increasingly recognized, their contributions to the motion and positioning of larger organelles, such as mitotic spindles for cell division remain unknown. Here, using magnetic tweezers to displace and rotate mitotic spindles in living embryos, we uncovered that the cytoplasm can impart viscoelastic reactive forces that move spindles, or passive objects with similar size, back to their original position. These forces are independent of cytoskeletal force generators, yet reach hundreds of piconewtons and scale with cytoplasm crowding. Spindle motion shears and fluidizes the cytoplasm, dissipating elastic energy and limiting spindle recoils with functional implications for asymmetric and oriented divisions. These findings suggest that bulk cytoplasm material properties may constitute important control elements for the regulation of division positioning and cellular organization.Significance StatementThe regulation of mitotic spindle positioning is a key process for tissue architecture, embryo development and stem cells. To date, most models have assumed that spindles are positioned by forces exerted by polar cytoskeleton networks, like microtubule asters or acto-myosin bundles. Here, using in situ magnetic tweezers to apply calibrated forces and torques to mitotic spindles in live dividing sea urchin cells, we found that the viscoelastic properties of the cytoplasm medium in which spindles are embedded can hold spindles in place, and move them back if their original position is perturbed. These viscoelastic forces are large and may significantly participate in the force balance that position and orient mitotic spindles in many cell types.

The mortar component plays an essential role in forming different asphalt mixture distresses. The fine aggregate size affects the viscoelastic behavior of mortar. In this study, three cutting sizes of 3.35, 2.36, and 1.7 mm, on the aggregate grading curve were considered to construct the mortar scales, namely mortar (I), (II), and (III), respectively. With a unique loading mode, a frequency sweep test was performed on four different length scales, including bitumen, asphalt mastic, mortar, and asphalt mixture, to make a direct comparison through a multi-scale approach. Also, a stress sweep test was conducted to find the mortar scale's linear viscoelastic domain. The results indicate that different mortar types have variable linear viscoelastic domains. The master curves of dynamic modulus (E*) and phase angle (δ) show that adding filler to bitumen has more effect on the E* than the δ. Although scaling up from the mastic to mortar (II) causes a significant difference between the viscoelastic parameters, the results remained approximately the same. The main gap between the mixture and mentioned sub-scales is filled with mortar (I). While the Cole-Cole diagrams demonstrate that the bitumen to mortar (II) scales have the same behavior, the closest scale to the mixture is the mortar (I). Therefore, using a cutting size of 3.35 mm is recommended to fabricate mortar (for asphalt mixture with a nominal maximum aggregate size of 19 mm) instead of 2.36 mm, which was suggested in previous studies.

International audience

This work aims at understanding the influence of the substrate temperature (Ts) on the viscoelastic properties of propanethiol plasma polymer films (PPFs). By means of state-of-the-art AFM characterization-based techniques including peak force quantitative nanomechanical mapping (PFQNM), nano dynamic mechanical analysis (nDMA) and "scratch" experiments, it has been demonstrated that the mechanical behaviour of PPFs is dramatically affected by the thermal conditions of the substrate. Indeed, the material behaves from a high viscous liquid (i.e. viscosity ∼ 106 Pa s) to a viscoelastic solid (loss modulus ∼ 1.17 GPa, storage modulus ∼ 1.61 GPa) and finally to an elastic solid (loss modulus ∼ 1.95 GPa, storage modulus ∼ 8.51 GPa) when increasing Ts from 10 to 45 °C. This behaviour is ascribed to an increase in the surface glass transition temperature of the polymeric network. The latter has been correlated with the chemical composition through the presence of unbound molecules acting as plasticizers and the cross-linking density of the layers. In a second step, this knowledge is exploited for the fabrication of a nanopattern by generating surface instabilities in the propanethiol PPF/Al bilayer system.

The perturbation technique based on the retardation-motion expansion is a simple method to obtain flow solutions at low Weissenberg number. In this context, this perturbation analysis is used to develop simple expressions for the motion of fibers suspended in viscoelastic fluids. In particular, the suspending fluid is characterized by a second-order fluid, Giesekus and PPT (Phan–Thien–Tanner) models, and their derivatives, such as the upper and lower convected Maxwell models. The first-order perturbation results in a similar effective velocity gradient that is exploited to express the translation and rotational motion of a single fiber and the associated extra stress tensor. In terms of a parameter related to the various viscoelastic fluid models, it is found that a fiber aligns along the vorticity direction when subjected to a shear flow. However, when a lower convected Maxwell model is considered, the elongated particle orients in the flow direction, as basically predicted by the Jeffery solution for a Newtonian suspending fluid. Furthermore, the conservation equation for particle concentration leads to particle migration in a pressure-driven flow channel and good agreement is observed with experimental data.

6th International Conference on Accelerated Pavement Testing , NANTES, FRANCE, 27-/09/2021 - 29/09/2021; The concept of truck platooning is to take advantage of the connectivity technologies and automated driving support systems to link trucks in close formation (convoy) in order to increase transport efficiency, reduce fuel consumption and gas emissions while improving road safety. However, closely guided trucks following each other could have a different impact on road structures than the usual truck traffic. Notably, different studies with traffic simulators highlight the significant effect of the wander pattern of traffic on the damage observed on test road structures. On the other hand, the positioning systems could offer the opportunity to choose an optimized wander pattern of the trucks within the platoons, especially designed to mitigate the overall damage. In this context, the study reported in this paper addresses, through numerical simulations, the multi-loading effects of truck platoons on road structures for different configurations of wander patterns. The aggressiveness of these patterns is evaluated on the basis of the viscoelastic response of the pavement structures (strain field) computed using software ViscoRoute\textcopyright 2.0. Finally, recommendations on the choice of wander pattern configurations to be tested on APT facilities are proposed.

International audience; In this paper, an original probabilistic micromechanics damage framework involving multi-deformation mechanisms, based on the modified Mori-Tanaka and Transformation Field Analysis (MT-TFA) techniques, is developed to predict monotonic and oligocyclic stress-strain responses in short fiber-reinforced polyamide composites. The proposed model allows simulating actual injection-induced fiber arrangement, which is characterized by arbitrary fractions of randomly oriented fibers distributed in the laminate plane. Furthermore, the modified MT-TFA approach employs a phenomenological model consisting of four Kelvin-Voigt branches and a viscoplastic branch, formulated under the thermodynamics framework, to describe the rate-dependent viscoelastic-viscoplastic deformation and the ductile damage of the polymer matrix phase. In addition, the Weibull probabilistic density function is utilized to simulate initiation and coalescence of the void-type discrete damage in the vicinity of the fiber/matrix interphase, induced by the fiber/matrix debonding as observed experimentally. The parameters of the developed model are calibrated against the experimental response of glass/polyamide (PA66/GF35) composites via uniaxial loading/unloading tests, by taking into account the actual fiber orientation density function (ODF). The reliability and efficiency of the modified Mori-Tanaka and TFA scheme are assessed vis-à-vis the separate and hold-out experimental data subjected to uniaxial and oligocyclic loading at various loading rates. Progressive matrix and interphase damage are compared in support of the modified MT-TFA technique’s capabilities to capture the experimentally observed damage mechanisms. To accurately capture the experimental response, the progressive degradation of the load transfer between the fiber and matrix phases is introduced through a reduction of the active fiber length. The latter is introduced by considering the effect of the interphase void-damage content. The new mean-field formulation provides accurate predictions of the overall response under complex loading paths. It can be combined with other techniques in our future work, such as cycle-jump, towards simulating high-cycle fatigue damage in short-fiber composite structures.

International audience; Characterizing the mechanical properties of tissues is key for the understanding of fundamental biological processes such as morphogenesis or tumor progression. In particular, the intercellular adhesion forces, mediated by transmembrane proteins like cadherins, are expected to control the topology and viscoelastic behavior of tissues under mechanical stress. In order to understand the influence of adhesion in tissues, we use biomimetic emulsions in which droplets mimic cells and adhere to each other through specific bonds. Here, we tune both the binding energy of the adhesive inter-droplets contacts as well as the fraction of contacts that are adhesive, thereby defining a so-called adhesiveness. Our experimental results show that adhesion prevents the emergence of local order in emulsions even at high packing fractions by preventing energetically costly droplet rearrangements. By studying the deformation of droplets within packings with different average adhesiveness values, we reveal the existence of a threshold value of adhesiveness above which all droplets in a packing are deformed as adhesive ones irrespective of their local adhesive properties. We show that this critical adhesiveness coincides with the threshold for percolation of adhesive structures throughout the tissue. From a biological point of view, this indicates that only a fraction of adhesive cells would be sufficient to tune the global mechanical properties of a tissue, which would be critical during processes such as morphogenesis.

International audience; The main challenge with Capacitive Micromachined Ultrasonic Transducer (CMUT) probes packaging is to preserve their intrinsic performances. Indeed, careful attention should be paid to the encapsulation of CMUTs and the impact of a viscoelastic acoustic loading (e.g., polymer) on the array performance. Our group developed a Boundary Element Matrix (BEM) dedicated to the acoustic coupling computation between a viscoelastic medium and a CMUT. As a complementary work, the present study aims to set up an experimental protocol to extract viscoelastic properties of different media and use them as model data inputs. This task is performed using Quartz Crystal Microbalance (QCM) with electrical admittance measurements performed in air and in each viscoelastic medium. A complex viscosity value was obtained for vegetable oils and the tested polymer. Then, using a CMUT array, electrical impedance measurements were performed in each medium. Using the QCM extracted media properties, a very good agreement was obtained between CMUT modeling and measurements where viscoelasticity is proven to reduce inter-cell coupling effects. Full electrode loading of a highly viscous polymer overdamp the QCM resonance and highlight its first order equation limits.

International audience

The relationship between interfacial and bulk emulsion viscoelasticity is investigated. Water-in-dodecane reverse Pickering emulsions, with a dispersed phase volume fraction of 0.66, stabilized with partially hydrophobic silica particles are addressed. Interfacial and bulk viscoelastic properties are probed via 2D and 3D oscillatory shear rheology. The silica particles concentration and NaCl content have been varied. In the whole range of concentrations studied the elastic modulus (G′) remains always greater than the viscous modulus for 2D and 3D rheology. The viscoelastic behavior can be related to the silica concentration both at the interface and in the emulsion. Power law dependences towards the silica content are recorded for the interfacial and the bulk emulsion elastic modulus. When increasing the NaCl concentration, the viscoelastic properties both at the interface and in the emulsion become stronger. The amount of silica and NaCl produces similar effects on both the interface and bulk viscoelastic properties. The increase of their concentration induces a rigidification of the dodecane/water interface since G P ′ (interface) increases. Such interfacial hardening produces a rigidification of the droplets ( G P ′ (Emulsion) is enhanced). Correlations between the interfacial and bulk emulsion viscoelasticity are also extracted from the experimental data. This is the first time that these kinds of relationship are reported. These results highlight that everything that affects the interfacial viscoelasticity is reflected on the bulk scale of emulsions.

The development of new interfacial rheological setup (IRS) for characterizing the interfacial viscoelastic properties of polymer systems is a subject of growing interest and constitutes a well-known challenge of high scientific and industrial application value. Recently, biconical and double-wall Ring (DWR) devices that can easily be attached to standard rheometers have been marketed for this purpose, but measurements must be made below 70 °C to ensure a stable homogeneous temperature at the interface. Meanwhile each device has its own limitation: the bicone has high inertia and a relatively low Boussinesq number, giving it a low signal-to-noise ratio, while the DWR is too fragile to probe the interfaces of high viscous systems in the molten state. Currently, to predict the dynamic interfacial properties of molten polymer systems, the interfacial rheology characterization is based mainly on indirect methods such as numerical modeling. In this study, a novel high temperature resistant interfacial rheology cell has been developed. This new setup allows direct interfacial rheology measurements up to 200 °C with temperature gradients of 1 °C at the polymer-polymer interface. To validate this new IRS device, the surface/interfacial properties of different model fluids having different well-known structure and viscoelastic characteristics have been investigated. To enable a more sensitive measurement of interfacial rheological properties, lightweight titanium based biconical geometry was newly designed. The effect of the molecular weight and the temperature was highlighted. Finally, the interfacial rheology testing of molten semicrystalline polymer systems has been achieved for the first time. The measured apparent interfacial shear properties in both oscillatory and steady flow modes were carefully corrected, considering the contribution of the bulk-subphases during processing of the numerical data.

International audience; We report a new technique for torsional testing of materials under giga-pascal pressures, which uses a shearing module in a large volume Paris-Edinburgh press in combination with highresolution fast radiographic X-ray imaging. The measurement of the relative amplitude and phase lag between the cyclic displacement in the sample and a standard material (Al2O3) provides the effective shear modulus and attenuation factor for the sample. The system can operate in the 0.001 to 0.01 Hz frequency range, and up to 5 GPa and 2000 K; although hightemperature measurements may be affected by grain growth and plastic strain. Preliminary experimental results on San Carlos olivine are in quantitative agreement with previously reported Q-1 factors at lower pressure. This cyclic torsional loading method opens new directions to quantify the viscoelastic properties of minerals/rocks at seismic frequencies and in pressure-temperature conditions relevant to the Earth's mantle, for a better interpretation of seismological data.

International audience; Performances of rotational shear rheometers are sometimes limited to measuring low viscosity at high temperatures of water-based polymer solutions. These limitations are typically due to the instrument resolution, sample inertia, and volumetric effects. Moreover, such measurements are not possible for temperatures exceeding 80 °C because of evaporation phenomena leading to a distortion of the value. The working principle of rheometers suitable for measuring viscosity above the boiling temperature reduces their sensitivity and limits their use to high-viscosity fluids. Acrylamide-based polymers are viscoelastic complex fluids exhibiting non-Newtonian behavior. Their viscosifying properties are strongly related to their charge density, molar mass, temperature, and salinity. The prediction of their rheological properties at high temperatures is challenging and is often extrapolated with an empiric law, such as Arrhenius equation. To the best of our knowledge, no commercially available rheometers are capable of measuring low viscosity of water-soluble complex fluids at high temperatures. In this work, we investigate a home-made fully automated capillary rheometer that has been developed to give an accurate measurement of viscosity and intrinsic viscosity of polymer solutions. This device is an affordable cost portable apparatus compared with a commercialized rheometer specifically designed for a wide range of viscosities and temperatures for various applications. The intrinsic viscosity has been measured on two acrylamide-based polymers of different chemical compositions using the capillary rheometer at high temperatures. This device has also been explored for measurement of a water-soluble polymer solution viscosity commonly used in enhanced oil recovery applications to limit chemical degradation

Viscoelastic liquid transfer from one surface to another is a process that finds applications in many technologies, primarily in printing. Here, cylindrical shaped capillary bridges pinned between two parallel disks are considered. Specifically, the effects of polymer mass fraction, solution viscosity, disk diameter, initial aspect ratio, final aspect ratio, stretching velocity and filling fraction (alike contact angle) are experimentally investigated in uniaxial extensional flow. Both Newtonian and viscoelastic polymer solutions are prepared using polyethylene glycol (PEG) and polyethylene oxide (PEO), with a wide variety of mass fractions. The results show that the increase in polymer mass fraction and solvent viscosity reduces the liquid transfer to the top surface. Moreover, the increase in the initial and final stretching height of the capillary bridge also decreases the liquid transfer, for both Newtonian and viscoelastic solutions. Finally, the shape of the capillary bridge is varied by changing the liquid volume. Now, Newtonian and viscoelastic solutions exhibit opposite behaviors for the liquid transfer. These findings are discussed in terms of interfacial shape instability and gravitational drainage., 11 pages, 5 figures

International audience; The present study experimentally investigates the mechanical response of T700/M21 Carbon Fibre Reinforced Polymers subjected to coupled shear and transverse tension at intermediate strain rates ranging from 10−3 to 15 s−1. Off-axis tension specimens with transverse and oblique tabs are used for the various tests. Moreover, the specimen aspect ratio is quite low, since dynamic tests are performed in this study. The axial stress–strain curves are successfully simulated using finite element simulations with a viscoelastic model. Based on these numerical simulations, specific stress correction factors are computed to take the effect of end constraints into account for such low aspect ratio specimens. Finally, an increase in the material strength with the strain rate is observed.

Viscoelastic granular materials are present in several disciplines. One example is asphalt mixture employed in road construction. In the last three decades, discrete element modeling has been positioned as a valid tool for the analysis of this multiphase material at the grain-scale. All this despite the simplification of the shape of the particles used in these studies. In this work, it is proposed a simplified procedure for the generation of viscoelastic granular samples composed of irregular polyhedra. The numerical aggregates were generated by a Poisson-Voronoi tessellation based on the particle size distribution (PSD) and statistic data of aggregates, without using complex imaging technics. This procedure set the porosity of the packing, while controlling the PSD. Using this procedure implies a significant computational-time reduction by skipping several preparation stages for polyhedral samples, such as deposition by gravity and compaction. This approach can be used for the study granular materials as inclusion in a solid matrix as concrete or asphalt mixtures, particle breaking, and fatigue damage of viscoelastic materials.

International audience; This paper presents the results of a European benchmark on multiscale creep modeling for cementitious materials, performed in the framework of the COST Action TU1404 "Towards the next generation of standards for service life of cement-based materials and structures". Three micromechanical models from three research groups have been adopted for modeling the basic non-aging creep of cementitious materials. The benchmark is based on the hypothesis that creep results only from calcium-silicate-hydrates (C-S-H) and that viscoelastic behavior of all microstructural constituents are maturity-and composition-independent. By comparing model results among themselves as well as by comparing model predictions with experimental results obtained from three different laboratories, we demonstrate that creep upscaling from micrometer-sized C-S-H to centimeter-sized macroscopic samples of cement paste, mortar, and concrete, and from minutes-long creep tests at early ages to days-long creep tests at very mature ages is indeed possible. The benchmark also highlights the importance of considering micro-anisotropy of C-S-H in terms of nonspherical shapes for C-S-H hydrates.

International audience; Objective: The noninvasive measurement of venous wall deformation induced by changes in transmural pressure could allow for the assessment of viscoelasticity and differentiating normal from diseased veins.Methods: In 57 patients with limbs in the C1s (telangiectasia or reticular veins and symptoms), C3 (edema), or C5 (healed venous ulcer) CEAP (clinical, etiologic, anatomic, pathophysiologic) category of chronic venous disease and 54 matched healthy controls, we measured the changes in the cross-sectional area of the small saphenous vein and a deep calf vein in the supine and standing positions and under compression with an ultrasound probe using ultrasonography.Results: The small saphenous vein, but not the deep calf vein, cross-sectional area was smaller in the limbs of the controls than in the limbs with C3 or C5 disease but was not different from that in C1s limbs. When changing from the supine to the standing position, a greater force was required to collapse the leg veins. Their cross-sectional area increased in most subjects but decreased in 31.5% of them as for the small saphenous veins and 40.5% for the deep calf vein. The small saphenous vein area vs compression force function followed a hysteresis loop, demonstrating viscoelastic features. Its area, which represents the viscosity component, was greater (P < .001) in the pooled C3 and C5 limbs (median, 2.40 N⋅mm2; lower quartile [Q1] to upper quartile [Q3], 1.65-3.88 N⋅mm2) than in the controls (median, 1.24 N⋅mm2; Q1-Q3, 0.64-2.14 N⋅mm2) and C1s limbs (median, 1.15 N⋅mm2; Q1-Q3, 0.71-2.97 N⋅mm2). The area increased (P < .0001) in the standing position in all groups.Conclusions: Postural changes in the cross-sectional area of the leg veins were highly diverse among patients with chronic venous disease and among healthy subjects and appear unsuitable for pathophysiologic characterization. In contrast, small saphenous vein viscoelasticity increased consistently in the standing position and the viscosity was greater in limbs with C3 and C5 CEAP disease than in controls.

Starting from the exact solution of the heterogeneous linear viscoelastic Eshelby ellipsoidal inclusion problem obtained in the time domain for an ellipsoidal inclusion embedded in an isotropic matrix (Berbenni et al., 2015), a direct time-incremental homogenization Mori–Tanaka scheme for two-phase non linear elasto-viscoplastic materials is proposed. The extension to non-linear behavior is based on a “modified secant” formulation to obtain the linear comparison composite (LCC) properties. In contrast with more classic homogenization method based on the first moment of stresses (i.e. classic secant formulation), the present approach is based on the second-order moment of stresses making use of the Hill’s lemma. Hence, a modified secant viscoplastic compliance is considered for the matrix phase. The effective behavior as well as the evolution laws of the averaged strains and stresses per phase are directly solved in the time domain, without the need of Laplace-Carson transforms. The estimates provided by the present homogenization model are compared to full-field calculations from the literature for two-phase non linear particulate composites constituted of an isotropic elasto-viscoplastic matrix phase and isotropic elastic particles, and, for radial and non-radial loadings.

International audience; This work presents an experimental analysis of the friction response of an ice-rubber inter- face over five decades of sliding velocity and temperature down to -20°C, combining in-situ contact visualisation and simultaneous force measurements. Viscoelastic properties of the rubber were varied in terms of glassy temperature transition and elastic modulus. Based on the in-situ contact area measurements, the adhesive and viscoelastic contributions were identified. Even though a bell-shape friction-velocity curve was observed, the classical WLF transform did not allow a description of the friction behaviour. A simple analytical model accounting for the thermal dissipation induced by friction was thus proposed and a dimen- sionless master curve was obtained with the sliding velocity, regardless of the temperature and the material properties. From this master curve, a predictive friction model was pro- posed, in which both friction contributions, adhesion-viscoelasticity and thermal dissipation, were multiplicative rather than simply additive.

International audience; This paper presents an uncertainty propagation analysis of viscoelastic sandwich laminated structures using a stochastic collocation method based on Smolyak’s sparse grids approach. Shear deformation in the viscoelastic layers is accurately captured by means of layer-wise plate finite elements. Modal properties are determined by solving nonlinear eigenvalue problems due to the frequency-dependent nature of the considered viscoelastic materials. Variations in material properties and layer thicknesses are both treated in a probabilistic way. The variabilities of natural frequencies and modal loss factors of two different viscoelastic laminated plate structures (plane and cylindrical) are studied. Results show that the present method converges quickly with small sample sizes when compared to the classical Monte-Carlo method. The influence of the variability of several parameters on viscoelastic structure modal properties is commented.

International audience; Unmanned aerial vehicles (UAVs) or drones are attracting increasing interest in the aviation industry, both for military and civilian applications. The materials used so far in the manufacture of UAVs are wood, plastic, aluminum and carbon fiber. In this regard, a new family of high-density polyethylene (HDPE) nanocomposites reinforced with polymethylsilsesquioxane nanoparticles (PMSQ), with mechanical performances significantly superior to those of pure HPDE, has been prepared by a fusion-combination process. Their viscoelastic properties were determined by oscillatory shear tests and their viscoelastic behavior characterized by the Lodge integral model. Then, the Lagrangian formulation and the membrane theory assumption were used in the explicit implementation of the dynamic finite element formulation. For the forming phase, we considered the thermodynamic approach to express the external work in terms of closed volume. In terms of von Mises stress distribution and thickness in the blade, the results indicate that HDPE-PMSQ behaves like virgin HDPE. Furthermore, its materials, for all intents and purposes, require the same amount of energy to form as HDPE.

Hypothesis The distribution of particles in Pickering emulsions can be estimated through a percolation-type approach coupled to the evolution of their rheological features with the dispersed phase volume fraction ϕ. Experiments The rheological behavior of water-in-dodecane Pickering emulsions stabilized with hydrophobic silica nanoparticles is addressed. The emulsions viscosity and elastic modulus are investigated at ϕ varying from 0.1 to 0.75. Various rheological models are adjusted to the experimental data. Findings The comparison of the elastic modulus evolution of the Pickering emulsions with those of emulsions stabilized with surfactants confirms a major contribution of the particles to the rheological behavior of Pickering emulsions and supports the existence of a three-dimensional network between the droplets. The applied percolation approach allows to quantitively estimate a nanoparticles viscoelastic link between the droplets and opposes the classic vision of interfacial monolayers stabilizing the Pickering emulsions. This network of interconnected particles and droplets contributes significantly to the viscosity as well as the elastic modulus of these emulsions. To our knowledge, the applied percolation-based model is the only one capable of providing a structural explanation while describing the abrupt viscosity and elastic modulus growth of Pickering emulsions across the range of ϕ.

From paints to food products, solvent evaporation is ubiquitous and critically impacts product rheological properties. It affects Newtonian fluids by concentrating any non-volatile components and viscoelastic materials, which hardens up. In both of these cases, solvent evaporation leads to a change in the sample volume, which makes any rheological measurements particularly challenging with traditional shear geometries. Here we show that the rheological properties of a sample experiencing `slow' evaporation can be monitored in a time-resolved fashion by using a zero normal-force controlled protocol in a parallel-plate geometry. Solvent evaporation from the sample leads to a decrease of the normal force, which is compensated at all times by a decrease of the gap height between the plates. As a result, the sample maintains a constant contact area with the plates despite the significant decrease of its volume. We validate the method under both oscillatory and continuous shear by accurately monitoring the viscosity of water-glycerol mixtures experiencing evaporation and a relative volume decrease as large as 70%. Moreover, we apply this protocol to dehydrating suspensions. Specifically, we monitor a dispersion of charged silica nanoparticles undergoing a glass transition induced by evaporation. While the decrease in gap height provides a direct estimate of the increasing particle volume fraction, oscillatory and continuous shear measurements allow us to monitor the suspension's evolving viscoelastic properties in real-time. Overall, our study shows that a zero normal-force protocol provides a simple approach to bulk and time-resolved rheological characterization for systems experiencing slow volume variations., Comment: 10 pages, 6+4 figures

Colloidal gels made of carbon black particles dispersed in light mineral oil are “rheo-acoustic” materials, i.e., their mechanical and structural properties can be tuned using high-power ultrasound, sound waves with submicrometer amplitude and frequencies larger than 20 kHz. The effects of high-power ultrasound on the carbon black gel are demonstrated using two experiments: rheology coupled to ultrasound to test for the gel mechanical response and a time-resolved ultra-small-angle x-ray scattering experiment (TRUSAXS) coupled to ultrasound to test for structural changes within the gel. We show that high-power ultrasound above a critical amplitude leads to a complex viscoelastic transient response of the gels within a few seconds: a softening of its storage modulus accompanied by a strong overshoot in its loss modulus. Under high-power ultrasound, the gel displays a viscoelastic spectrum with glasslike features and a significant decrease in its yield strain. Those effects are attributed to the formation of intermittent microcracks in the bulk of the gel as evidenced by TRUSAXS. Provided that the shear rate is not large enough to fully fluidize the sample, high-power ultrasound also facilitates the flow of the gel, reducing its yield stress as well as increasing the shear-thinning index, thanks again to the formation of microcracks.

Time-dependent rock behavior can significantly affect the design and the construction method of tunnel support lining. As an innovation, a concrete liner covered with a compressible layer is thus of great interest to reduce the underground space project costs. Indeed, the compressible material usually exhibits very high deformability under oedometric loading (30–60% volumetric deformation under a weak variation in axial stress). This helps to absorb the convergence of the excavated wall of the squeezing rock, and thus limit the transmission of stress to the inner concrete lining. This study firstly aims at deriving a closed-form solution for a deep-circular tunnel excavated within the viscoelastic Burger rock and supported by a double-layer concrete/compressible material. The inner concrete layer is assumed to be linear elastic, while the outer compressible layer is described by a tri-linear elastic model. The analytical solution is derived under the integral equation form. Secondly, the effects of uncertainty of time-dependent mechanical properties of host rock on the failure probability of tunnel for a period of 100 years are studied by introducing the closed-form solution into the well-known Kriging-based reliability analysis AK-MCS (Active learning reliability method combining Kriging and Monte Carlo Simulation) method. Notably, the benefit of the compressible layer is highlighted from the reliability analysis.

We propose a system of conservation laws with relaxation source terms (i.e. balance laws) for non-isothermal viscoelastic flows of Maxwell fluids. The system is an extension of the polyconvex elastodynamics of hyperelastic bodies using additional structure variables. It is obtained by writing the Helmholtz free energy as the sum of a volumetric energy density (function of the determinant of the deformation gradient det F and the temperature [Formula: see text] like the standard perfect-gas law or Noble–Abel stiffened-gas law) plus a polyconvex strain energy density function of F, [Formula: see text] and of symmetric positive-definite structure tensors that relax at a characteristic time scale. One feature of our model is that it unifies various ideal materials ranging from hyperelastic solids to perfect fluids, encompassing fluids with memory like Maxwell fluids. We establish a strictly convex mathematical entropy to show that the system is symmetric-hyperbolic. Another feature of the proposed model is therefore the short-time existence and uniqueness of smooth solutions, which define genuinely causal viscoelastic flows with waves propagating at finite speed. In heat-conductors, we complement the system by a Maxwell–Cattaneo equation for an energy-flux variable. The system is still symmetric-hyperbolic, and smooth evolutions with finite-speed waves remain well-defined.

[EN] This paper examines the feasibility of cloaking an obstacle using Plate-type Acoustic Metamaterials (PAMs). We present two distinct strategies to cloak this obstacle, using either the near-zero-density regime of a periodic arrangement of plates or the acoustic doping phenomenon to achieve simultaneous zero-phase propagation and impedance matching. The strong limitations induced by viscothermal and viscoelastic losses that cannot be avoided in such a system are studied. A hiding zone is reported analytically, numerically, and experimentally. In contrast to cloaking, where zero-phase propagation must be accompanied by total transmission and zero reflection, the hiding configuration requires that the scattering properties of the hiding device must not be affected by the presence of the obstacle embedded in it. Contrary to cloaking, the hiding phenomenon is achievable even with a realistic PAM possessing unavoidable losses., This article is based upon the work from COST Action DENORMS (No. CA15125), supported by COST (European Cooperation). The authors would like to thank the support of the ANR-RGC METARoom (No. ANR-18-CE08-0021) project. J. Christensen acknowledges the support from the European Research Council (ERC) through the Starting Grant No. 714577 PHONOMETA and from the MINECO through a Ramon y Cajal Grant (No. RYC-2015-17156).

IF=2.199; International audience; In this paper, a previously developed viscoelastic-viscoplastic behaviour model for short-fibre-reinforced thermoplastics (SFRT) is enriched with matrix ductile damage and fibre/matrix interfacial debonding constitutive laws. Aiming at industrial applications, model development takes special care to ensure a relatively easy characterisation of material parameters, as illustrated with case of a short-glass-fibre-reinforced polypropylene. Numerical simulations are done to assess the capability of the model to account for all specificities of SFRT behaviour with a particular attention paid on the coupled influence of strain rate and microstructure configuration on the initiation and development of matrix damage and fibre/matrix debonding.

Mechanical tests can allow for the mimicking of the textural attributes of foods as perceived by humans. Here we report on the use of the mechanical compression test to study the reactions underlying the cooking of the cowpea seeds (beans) until doneness. The creep test was applied to deal with the time-dependence of the strain, e(t), at a constant force and to derive the elasticity, Ke and viscosity, Kv indices for the different heating times. The results, e(t), were fitted using mostly a stretched exponential with a linear asymptotic (time) function. The flow rate (ω) and the strain (e) are the constants of the asymptotic function. Both viscoelasticity indices decreased strongly before the first 30 min (from 562 N to 14 N for Ke and from 5.4 × 106 N s to 0.11 × 106 N s for Kv), but from 30 to 75 min ± 3 min, Ke remained almost constant (~14 N) while Kv increases (from 0.11 × 106 to 0.24 × 106 N s). Both viscoelasticity indices decreased strongly again after 75 min ± 3 min (from 14 N to 0.24 × 106 N s for Ke and Kv respectively). The heating time-dependence of the viscoelasticity indices clearly reflects the cowpeas beans transitioning to the cooked state and this may be correlated directly to the kinetic of water absorption, starch gelatinization and protein denaturation. This would correlate not only to the cooking time but also to the nutrient quantity balance. The results from this study shows that the samples mechanical properties with the simmering time correlate with the cooking reaction transitioning.

We consider an infinite chain of particles with nonlinear elastic and dissipative nearest neighbors interactions. Assuming the existence of a traveling front between uniformly compressed (or stretched) states, we obtain jump conditions relating the wave speed and limiting particle velocities to the relative displacements at infinity. Using this result, we characterise compression fronts in chains of touching beads, for viscoelastic contact laws that include a nonlinear elastic force (generalised Hertz contact) and viscous dissipation. We compute compression fronts numerically for the generalised Kuwabara–Kono model in which the viscous contact force is proportional to the derivative of the elastic force, without precompression of the chain. Steady fronts are obtained both as the end result of the compression of one end of the chain and using a shooting method which provides numerically exact traveling waves. Depending on the magnitudes of contact damping and strain applied downstream, we obtain either overdamped (monotonic) or underdamped (oscillatory) compression fronts. To explain this transition and approximate the front profiles, we consider a continuum limit valid when the exponent of the contact nonlinearity is close to (and above) unity. Using multiscale expansions, we formally derive two different amplitude equations for long waves, a Burgers equation with logarithmic nonlinearity, and a logarithmic Korteweg–de Vries (KdV)–Burgers equation for small contact damping. Both models possess traveling front solutions that are in good agreement with the front profiles computed numerically in the granular chain. The analysis of the logarithmic KdV–Burgers equation allows one to approximate the critical damping corresponding to the transition from underdamped to overdamped fronts.

International audience; We focus on the role of dynamical heterogeneities on the weak mechanical nonlinearities of amorphous polymers near and above the glass transition temperature T g by combining experiments and numerical coarse-grained simulations. The acceleration of the macroscopic nonlinear modulus relaxation resulting from the applied stress is measured below yielding. As a result of dynamic disorder, the macroscopic acceleration differs from the local acceleration. We obtain a good agreement of experimental measurements with simulations computed by using an exponential function of the square stress for the local acceleration. Further, the length scale of dynamical heterogeneities is deduced.

International audience; The research work presented in this paper aims to optimize the dynamic response of a carbon-epoxy plate by including into the laminate one frequency-dependent interleaved viscoelastic layer. To keep an acceptable bending stiffness, some holes are created in the viscoelastic layer, thus facilitating the resin through layer penetration during the co-curing manufacturing process. Plates including (or not) one perforated (or non-perforated) viscoelastic layer are manufactured and investigated experimentally and numerically. First, static and dynamic tests are performed on sandwich coupons to characterize the stiffness and damping properties of the plates in a given frequency range. Resulting mechanical properties are then used to set-up a finite element model and simulate the plate dynamic response. In parallel, frequency response measurements are carried out on the manufactured plates, then successfully confronted to the numerical results. Finally, a design of experiments is built based on a limited number on numerical simulations to find the configuration of bridges that maximizes the damping while keeping a stiffness higher than half the stiffness of the equivalent undamped plate.

International audience; Extending the mechanical lifetime of NBR (i.e., poly(acrylonitrile-co-butadiene)), a large-volume synthetic rubber, requires a better understanding of its structure–property relationships. We demonstrate that industrial-grade and uncross-linked NBR can be mechanically rejuvenated and physically aged due to an inhomogeneous distribution of monomers along the polymer chains. As opposed to its nonpolar SBR counterpart (i.e., poly(styrene-co-butadiene)), NBR experiences thermodynamic driving forces for microphase separation and kinetic barriers for processing like those of block copolymers. Extruding NBR at high temperature and shear results in a weakly microphase-separated nanostructure of low relaxation time and resistance to flow, whereas physically aging NBR leads to lamellar nanodomains, a more solid-like material, and delayed stress relaxation. This effect of rejuvenation and aging on the nanostructure and rheological properties of NBR has important consequences on processing and storage conditions, such as the formation of defect-free interfaces in multilayered parts by polymer interdiffusion.

International audience; Quantitative ultrasound techniques have been previously used to evaluate biological hard tissues, characterized by a large acoustic impedance contrast. Here, we are interested in the imaging of experimental data from different test-targets with high acoustic impedance contrast, using the Full Waveform Inversion (FWI) method to solve the inverse problem. This method is based on high-resolution numerical modeling of the forward problem of interaction between waves and medium, considering the full time series. To reduce the complexity of the numerical implementation, the model considers a fluid medium. Therefore, the aim is to evaluate the precision of the reconstruction under this assumption for materials with a different level of attenuation of shear waves, to study the limits of this hypothesis. Images of the sound speed obtained using the experimental data are presented, and the precision of the reconstruction is evaluated. Future work should include viscoelastic materials.