Piezoelectric and mechanical properties of a high performance thermoplastic composite

To protect aircraft and satellite structures from mechanical solicitations such as vibrations, a piezoelectric-based passive damping concept is studied. Most of the time, the piezoelectric elements are surface bonded or embedded in the host structure that needs to be damped. There are integrated with an external shunted circuit. When the piezoelectric material is de-formed, it generates an electrical potential that is dissipated by Joule effect in a resistive ele-ment. More precisely, the purpose of this work is to integrate this passive macroscopic damping con-cept to the composite scale. For this, a hybrid piezoelectric composite based on high perfor-mance thermoplastic polymer as structural matrix is developed. This structural matrix belongs to the PAEK (Poly Aryl Ether Ketone) family. The high glass transition temperature of these thermostable polymers is a critical parameter in the choice of the piezoelectric ceramic. Its Cu-rie temperature has to be higher than the Tg of the composite. In addition, one of the main challenges is to ensure homogeneous particle dispersion with a sufficiently low content to maintain the matrix ductility. We propose to present the piezoelectric and mechanical proper-ties of a thermoplastic polymer / micronic piezoelectric particle composites as a function of their chemical composition

New hybrid polymer nanocomposites for passive vibration damping by incorporation of carbon nanotubes and lead zirconate titanate particles

A new hybrid nanocomposite for vibration damping has been elaborated. Ferroelectric lead zirconate titanate particles and carbon nanotubes are dispersed simultaneously in an engineering semi-crystalline thermoplastic matrix by an extrusion processing. Ferroelectric particles have been made piezoelectric once incorporated into the polymer matrix through a poling step. The dynamic response of nanocomposites has been characterized by dynamic mechanical analysis and vibration test. The shear mechanical modulus exhibits an increase of the conservative and dissipative components after the poling step of nanocomposites. By vibration test, the first bending mode of the frequency response function has been followed and a significant damping inherent to poling is also recorded. These evolutions are heightened by the use of two constrained elastic layers. For the first time, a synergy between poled piezoelectric particles responsible for the transduction phenomena and conductive particles allowing a local dissipation of electric charges has been revealed by two complementary techniques for the improvement of the polymer damping

Caractérisation de la résistance d'une pâte fromagère lors de la croissance de bulles

National audienceL'objet de cette étude est la compréhension des mécanismes mis en jeu lors de la fabrication de fromages à pâte pressé, plus particulièrement la caractérisation de la résistance de la pâte fromagère lors de la croissance d'une bulle, jusqu'à la rupture. Des tests permettant de mesurer les propriétés rhéologiques de la pâte ont été mis en place, dans différentes conditions depuis les petites (non destructives) jusqu'aux grandes déformations (à la rupture). Des échantillons de fromage affiné à pâte pressée (G/S de 47%) ont été prélevés à l'aide d'un emporte pièce de diamètre D au coeur du fromage de façon à éviter tout effet gradient des zones sous la croûte. Des tranches de hauteur H ont ensuite été découpées dans les cylindres obtenus, dans un rapport D=2H. Un rhéomètre MCR 301 (Anton Paar, Autriche) équipé d'une géométrie plan/plan a été utilisé en mode texturomètre. La traverse a été pilotée à vitesse décroissante pour maintenir une vitesse d'élongation biaxiale έb constante, dans la gamme [0,004s-1 ; 0,5s-1]. Ces conditions reproduisent celles de croissance de bulles dans la pâte fromagère lors du procédé de fabrication, de vitesse d'expansion très lente. Des essais menés à différentes vitesses d'élongation biaxiale aux petites déformations ont permis l'obtention de courbes d'écoulement pour plusieurs déformations biaxiales, donnant l'évolution de la viscosité en fonction de la vitesse d'élongation. Les profils obtenus, sous la forme de lois en puissance, sont caractéristiques d'un comportement rhéofluidifiant avec un indice n de l'ordre de 0,2. Aux grandes déformations, des valeurs de contrainte σf et de déformation εf à la fracture ont été relevées en fonction de la vitesse d'élongation biaxiale utilisée. En effet, les courbes σ=f(ε) ont permis le repérage de deux étapes clef : le seuil de plasticité au sortir des petites déformations et la fracture lors de la rupture des échantillons. Il apparaît que la déformation à la fracture n'est pas influencée par la vitesse d'élongation, contrairement à la contrainte à rupture, qui augmente avec la vitesse έb. Enfin, des tests d'anisotropie aux grandes déformations ont pu être menés à une vitesse d'élongation biaxiale de 0,02s-1, permettant de déterminer si la pâte fromagère, structurée en grains de caillé orientés, ne possèderait pas des points de plus faible résistance que d'autres, lors d'une mise sous contrainte telle que la croissance d'une bulle, par exemple

Multi-scale study of industrial semi-hard cheeses: better understanding the eye growth phenomenon

International audienceSome semi-hard cheeses are downgraded because their eyes are not mastered, as regards to their number, size, shape or repartition. The eye growth phenomenon relies on empirical knowledge and needs to be better understood and formalized. To do so, an original multi-scale investigation strategy was designed, based on the combination of Magnetic Resonance Imaging (MRI) to monitor eye growth during ripening [2], rheology to provide information on the cheese matrix resistance [3], biochemistry to quantify metabolites [5] and microscopy to analyze the microstructure [1, 4]. However, semi-hard cheeses are very heterogeneous products due to their process, which makes the combination of methods even more essential, in order to characterize spatial differences inside a product (eyes, curd grains, protein network, bacteria, etc), so that eye growth mechanisms could be studied using both time and spatial dependency monitoring. A strong opening gradient was identified between under the rind and the core zone of cheese, with smaller and fewer eyes under-rind, growing slower core ones [4, 6]. Several leads were investigated to explain these differences. The microstructure of the cheese matrix in these two zones did not present any significant difference, so the only parameters left that could cause the opening gradient were the salt content and the rheological properties. The combination of MRI and gas pressure measurement demonstrated that the salt content influenced the eye growth gradient through the CO2 production, by slowing down the growth and metabolism of bacteria. The higher firmness under the rind assessed using Lubricated Squeezing Flow tests could strengthen the opening gradient. Insights on crack development were also found by observing different grain patterns at the periphery of eyes and showing that the cheese matrix possessed strain-hardening properties, maximized at the eye’s apex. All experiments were led at an industrial scale to avoid scale-up issues

Rheological characterisation of semi-hard cheese using lubricated squeezing flow test

In semi-hard cheeses, the pressure increase occurring during eye growth can create cracks leading to product downgrading. Rheological properties of semi-hard cheeses were investigated using lubricated squeezing flow (LSF), a test specifically chosen to mimic conditions as close as possible to the real eye growth in cheese. Flow curves showed a power law profile, characteristic of a shear-thinning behaviour. Moreover, stress/strain curves obtained at a constant biaxial extension rate showed a strain-hardening behaviour. Structural changes in the matrix during eye growth giving higher resistance towards deformation were indicated, and a schematisation of microstructure changes due to eye growth was postulated. Some threshold values were also identified during ripening, such as the biaxial yielding stress and strain; these were not impacted by cheese age or by the anisotropic organisation of the matrix. However, the yielding strain and Young's modulus increased with the biaxial extension rate

Design of a multi-scale texture study of yoghurts using rheology, and tribology mimicking the eating process and microstructure characterisation

Physical properties of 13 yoghurts were investigated combining rheology, tribology, particle size measurements and confocal laser scanning microscopy. New protocols were designed to improve mimicking of the eating process. In addition to classically acquired rheology properties, viscosity was measured under a constant shear rate and increasing temperature from 10 degrees C to 25 degrees C, corresponding to in-mouth conditions before swallowing. For friction measurement, force load and velocity were determined to be discriminating and relevant to oral conditions. Friction coefficient was the only instrumental indicator significantly correlated to fat content; viscosity was mostly linked to protein content even at high concentrations. Moreover, tribology and rheology indicators were uncorrelated with one another, bringing complementary information. The combination of oral-process-mimicking protocols and microstructure evaluation, performed on products purposely covering wide ranges of fat (0-10%, w/w) and protein (3-10%, w/w) content, allowed good characterisation of such diversity through a multi-scale approach

Investigation of curd grains in Swiss-type cheese using light and confocal laser scanning microscopy

The mechanisms of eye growth in Swiss-type cheeses have mostly been explained empirically. Greater understanding may be needed to anticipate structural changes that may lead to cracking instead of bubble formation. The microstructure of a Swiss-type cheese was therefore investigated using a multi-scale approach. Two techniques were used: light microscopy to study curd grain patterns, and confocal laser scanning microscopy (CLSM) to study the organisation of the fat and protein network in the cheese matrix around eyes. Anisotropy was revealed according to the direction of examination, with a different organisation of curd grains along or perpendicular to the pressing axis of the cheese. The curd grains appeared to be highly stretched around the openings. CLSM revealed a difference in composition inside curd grains and at the surface of eyes. Moreover, holes were observed at the interfaces of the grains, possibly corresponding to small eyes or to the origin of cracks. (C) 2013 Elsevier Ltd. All rights reserved

Investigation of curd grains in Swiss-type cheese using light and confocal laser scanning microscopy

International audienceThe mechanisms of eye growth in Swiss-type cheeses have mostly been explained empirically. Greater understanding may be needed to anticipate structural changes that may lead to cracking instead of bubble formation. The microstructure of a Swiss-type cheese was therefore investigated using a multi-scale approach. Two techniques were used: light microscopy to study curd grain patterns, and confocal laser scanning microscopy (CLSM) to study the organisation of the fat and protein network in the cheese matrix around eyes. Anisotropy was revealed according to the direction of examination, with a different organisation of curd grains along or perpendicular to the pressing axis of the cheese. The curd grains appeared to be highly stretched around the openings. CLSM revealed a difference in composition inside curd grains and at the surface of eyes. Moreover, holes were observed at the interfaces of the grains, possibly corresponding to small eyes or to the origin of cracks. © 2013 Elsevier Ltd

Characterization of the resistance of a Swiss-type pressed cheese under stress as regards to the organization of the curd grains

International audienceThe objective of the present study is to follow the evolution of a Swiss-type cheese’s paste when put under stress up to rupture from both a microstructural and a rheological point of view, in order to achieve a better understanding of the cracks’ formation in Swiss-type cheese during ripening and storage. A binocular observation (x5) and some confocal scaning laser microscopy (protein and fat labeling, X10 and X63) were carried out. Cheese paste showed an anisotropic organization of the pressed curd grains, depending on their position regarding the press axis. They have an ovoid shape with a principal axis orientated perpendicularly to the press direction. Borders of the curd grains showed a locally higher protein concentration. That is why their mechanical properties are probably different from the protein network inside the curd grains. Stress/strain curves of cheese’s samples from small to high level of strain were carried out. Samples of ripened commercial Swiss-type cheese (fat/dry 47%, water content 42%) were studied using a MCR 301 rheometer (Anton Paar, Austria) used in a texturometer mode. The crosshead was driven in a decreasing speed mode (dH/Hdt = constant) in order to ensure a constant biaxial extensional rate in the cheese sample. These experimental conditions added to the lubricated conditions of the tests represented a Lubricated Squeezing Flow. There were chosen in order to mimic the real conditions of eyes’ growth in cheese. Tests led at different constant biaxial extensional speeds from small to high level of strain allowed the determination of threshold values on the stress/strain curves σ=f(ε). These two noticeable points were the yield stress and the rupture point, where values of stress (σy and σr) and strain (εy and εr) could be found. They were used as rheological indicators in order to test the cheese paste anisotropy effect on its rheological properties at large deformations. It appeared that the rupture strain depends on the direction of sampling, in good accordance with the anisotropic organisation of the curd grains in the cheese

Rational design of a versatile lab-scale stirred milk gel using a reverse engineering logic based on microstructure and textural properties

International audienceThe textural properties of stirred yogurts (i.e. stirred milk gels) are directly linked to their microstructure. In this study, a logic of reverse engineering was used to design a lab-scale process producing real-like stirred milk gels (1.5 kg batch). A reference of stirred fermented milk gel was produced at pilot scale (100 kg batch). The conditions of lab-scale homogenization (sonication), acidification (glucono-delta-lactone concentration) and stirring (filter, pumping rate, ultra-smoothing) were adjusted to obtain size distributions of fat droplets and of microgels and rheological properties similar to those of the pilot reference. The final lab-scale process was validated by comparing the properties of stirred milk gels produced at lab and pilot scales with two fat contents (6 and 10 wt %). Although the stirred milk gels obtained at the two scales differed slightly in microgel sizes and in rheological properties, they were similarly ranked according to their fat content