Time to failure prediction in rubber components subjected to thermal ageing: A combined approach based upon the intrinsic defect concept and the fracture mechanics

In this contribution, we attempt to derive a tool allowing the prediction of the stretch ratioat failure in rubber components subjected to thermal ageing. To achieve this goal, the mainidea is to combine the fracture mechanics approach and the intrinsic defect concept. Using an accelerated ageing procedure for an Ethylene–Propylene–Diene Monomer (EPDM), it is first shown that the average molar mass of the elastically active chains (i.e. between crosslinks) can be used as the main indicator of the macromolecular network degradation. Byintroducing the time–temperature equivalence principle, a shift factor obeying to an Arrhenius law is derived, and master curves are built as well for the average molar mass as for the ultimate mechanical properties. Fracture mechanics tests are also achieved and the square root dependence of the fracture energy with the average molar mass is pointed out. Moreover, it is shown that the mechanical response could be approximated by the phantom network theory, which allows to relate the strain energy density function to the average molar mass. Assuming that the fracture of a smooth specimen is the consequence of a virtual intrinsic defect whose the size can be easily estimated, the stretch ratio at break can be therefore computed for any thermal ageing condition. The estimated values are found in a very nice agreement with EPDM experimental data, making this approach a useful tool when designing rubber components for moderate to high temperature environments

Phenomenological nonlinear modelling of glassy polymers

Compared with the numerous works into the constitutive equations for the mechanical behaviour of metals, very little attention has been devoted to those of polymers. However, a model is required to describe both the complex shape of the stress–strain curves and strain rate sensitivity of glassy polymers. In this Note, a unified viscoelastic-viscoplastic model is presented in which the nonlinear pre-yield behaviour, the strain softening and the strain hardening are described by internal variables, in analogy with the models developed for metals. In order to check the predictive capability of the model, the numerical results are compared with the experimental data (monotone, creep and relaxation tests) of a typical amorphous glassy polymer

Improving estimates of water demand at scheme level using knowledge on farmers' practices

International audienceAccurate estimation of water demand at the irrigation scheme scale is a key requirement for water management, which is made difficult by the large diversity of crops and production systems. The main objective of this study was to estimate irrigation water demand at the farm scale, taking different types of knowledge into account: (i) database study, where water demand is supposed to be the supply (L0); (ii) actual cropping pattern (L1),; (iii) actual irrigation techniques and cropping pattern (L2); and (iv) actual irrigation practices, actual irrigation techniques, and cropping pattern (L3). Farm typology makes easy this estimation as it takes into account various farm characteristics. Firstly, a farm typology was established based on 115 farms surveyed in the irrigation scheme of Borj Toumi Toungar (Tunisia), selected for the study. Secondly, climatic crop water irrigation requirements were estimated using the agro-meteorological water balance model Pilote. Typology results were used to estimate water demand at the scale of family farms taking into account different levels of knowledge Six classes of farming areas were identified based on irrigation subsystems, cropping pattern (fruit tree area, cropping vegetable area and field crop area), intensification crop level and cropped surface. Results showed that supply could not entirely cover the climatic water demand. However, it could nearly meet actual water demand at the farm scale. According to the water demand estimation at different levels, results showed that there were few changes in water demand at farm scale when moving from L1 to L2. At the opposite, actual water demand (L3) represented about half the climatic water demand (L2). Calculations based on farm classes highlighted the importance of actual farm practices. Within the same class, the difference between water demand estimation at different levels of knowledge was remarkable. Network rehabilitation is thus necessary to enable farmers to increase water delivered to crops and limit water stress; the collective network was conceived to deliver a flow of 0.34 l/s/ha in this sector. Given changes in farming and varietal choices, this flow proved insufficient to face the climatic water demand

Modélisation du vieillissement thermique et mécanique d'une protection externe en EPDM de jonctions rétractables à froid

L'objectif de cette thèse est l'étude des conséquences de la thermo-oxydation sur la structure chimique et le comportement mécanique d'un Ethylène Propylène Diène Monomère (EPDM). Afin de déterminer les modifications à différentes échelles structurales, quatre formulations modèles sont étudiées : la gomme pure, les matrices vulcanisées stabilisée et non stabilisée et l'élastomère industriel. L'ensemble des échantillons est vieilli entre 70 et 170C dans l'air ou sous vide puis caractérisé par divers outils analytiques. La thermogravimétrie donne accès aux variations de masse résultant de l'incorporation d'oxygène et l'émission de composés volatils. L'analyse infrarouge permet de suivre les évolutions des espèces chimiques. Les essais de gonflement, de chromatographie et de spectrométrie mécanique permettent de calculer les nombres de coupures de chaînes et d'actes de réticulation à chaque instant. Sur la base de ces résultats, un modèle cinétique général de thermo-oxydation de la matrice EPDM est proposé et en partie validé. Les conséquences du vieillissement thermique sur le comportement mécanique de l'élastomère industriel sont mises en évidence par des essais de traction uniaxiale et de multi-relaxation à température ambiante et vitesse de déformation initiale de 10-3 s-1. L'impact du vieillissement thermique sur les propriétés ultimes et les réponses à l'équilibre et hors équilibre est examiné. Un critère prédictif de rupture basé sur la mécanique de la rupture est proposé. Enfin, le couplage vieillissement thermique - contrainte mécanique est étudié par des essais relaxation de contraintes continues entre 130 et 170C dans l'air. Les modifications de la microstructure pendant le vieillissement thermique sont intégrées dans les équations constitutives du modèle mécanique macroscopique afin de proposer un outil de prédiction du comportement à long terme de l'élastomère industriel. La simulation numérique montre une bonne adéquation avec les résultats expérimentaux.The aim of this work is to study the consequences of the thermal oxidation on the chemical structure and mechanical behavior of an Ethylene-Propylene-Diene Monomer (EPDM). In order to determine the structural changes at different scales, four model formulations have been considered: free additive gum, stabilized and unstabilized vulcanized matrix and industrial rubber. All samples were aged between 70 and 170C in air or vacuum and characterized by several analytical tools. Thermogravimetry gives access to weight variations due to oxygen grafting and volatile compounds release. Infrared analysis is used to follow chemical species evolutions. Swelling tests, chromatographic and mechanical spectrometry tests allow calculating the number of chain scission and cross-linking events at any time. Based on these results, a general kinetic model is proposed and partially validated for EPDM matrix thermal oxidation. The consequences of thermal ageing on the mechanical behavior of the industrial rubber are pointed out by monotonic tensile and stress relaxation tests at room temperature and a 10-3 s-1- initial strain rate. The impact of thermal ageing on ultimate properties and equilibrium and non-equilibrium response are examined. Finally, the coupling between thermal ageing and mechanical stress is studied by continuous stress relaxation tests between 130 et 170C in air. The microstructural modifications during thermal ageing are introduced into the constitutive equations of the macroscopic mechanical model in order to propose a predictif tool of the long time behavior of the industrial rubber. The numerical simulation is in good agreement with experimental results.PARIS-Arts et Métiers (751132303) / SudocSudocFranceF

Contribution à l'étude de la déformation et de l'endommagement des polymères vitreux homogènes et à renforts caoutchoutiques

Ce travail constitue une contribution visant à décrire le comportement non-linéaire et l'endommagement des polymères vitreux homogènes et renforcés par l'adjonction de particules d'élastomère sphériques. Pour caractériser le comportement fortement non-linéaire des polymères vitreux, un modèle viscoplastique à variables internes a été développé et implémenté dans un code de calcul par éléments finis. Le modèle viscoplastique développé a été ensuite couplé à différentes formulations micromécaniques pour caractériser le comportement cohésif ou à l'endommagement de mélanges de polymères. Les résultats des simulations numériques ont été ensuite validés expérimentalement par comparaison avec les données expérimentales obtenues sur différents polymères vitreux typiques. Par ailleurs, l'activation des mécanismes de déformation a également fait l'objet d'une attention particulière.LILLE1-BU (590092102) / SudocSudocFranceF

On the cavitation problem and the influence of the surface energy in a viscoplastic medium

In this Note, the solution for spherically symmetric cavitation in a viscoplastic material is analysed. To ensure of the reality of the physical behaviour of the material, the problem is studied by considering a hollow sphere whose matrix obeys to a modified Bodner and Partom model. This local phenomenon is understood in the sense of the rapid growth of a pre-existing void and a particular attention is made to understand the influence of the surface energy on the critical dilative stress

Modélisation multi-échelle du comportement mécanique de nanocomposites polymères à renforts d'argile de type montmorillonite (approche micromécanique et simulation de dynamique moléculaire)

Les nanocomposites à matrice polymère et à renforts d argile ont pris une grande importance au cours de ces deux dernières décennies. Ceci trouve son explication d une part, dans la grande disponibilité et le faible coût de production de la phase renforçante, et d autre part dans les remarquables améliorations de propriétés physiques et mécaniques. Ces améliorations sont observées même à de très faibles quantités de renforts comparées à celles de leurs homologues microcomposites. Le développement de ces nouveaux matériaux suscite un fort engouement tant au niveau de la recherche académique qu industrielle. Cependant, les mécanismes responsables de ces améliorations de propriétés demeurent mal compris et restent l une des principales préoccupations des chercheurs. Il s agit dans ce travail de thèse, d apporter une contribution à la compréhension et à la mise au point d outils prédictifs du comportement mécanique de nanocomposites polymères à renforts d argile de type montmorillonite. Pour y parvenir, deux approches de modélisation sont utilisées : la micromécanique des matériaux hétérogènes et la simulation de dynamique moléculaire. Du point de vue analytique, un modèle micromécanique basé sur l approche auto-cohérente est développé. Le modèle proposé est validé par nos données expérimentales et celles issues de la littérature. Un protocole de simulation de dynamique moléculaire est proposé pour la modélisation à l échelle atomique de ces nanomatériaux. Cette approche nous a permis, entre autres, de faire la lumière sur les interactions moléculaires entre les différents constituants, et de déterminer les propriétés élastiques effectives du nanocomposite.Polymer nanocomposites reinforced with clay minerals have attracted a great consideration during the last two decades. That can be explained, firstly, by the availability and the reduced production cost of the reinforcing phase, and secondly, by the remarkable improvements in physical and mechanical properties. These improvements are observed even at very low amounts of reinforcements compared to their microcomposite counterparts. The development of these new materials creates a keen interest both in academic and industrial research. However, the mechanisms responsible of these property improvements are still poorly understood and remain a major concern of researchers. This work contributes to the understanding and to the development of predictive tools of the mechanical behavior of polymer nanocomposites reinforced with montmorillonite clay using two modeling approaches: the micromechanics of heterogeneous materials and the molecular dynamics simulation. An analytical micromechanical model based on the self-consistent approach is developed. The proposed model is validated by our experimental data and those from the literature. A new molecular dynamics simulation protocol is proposed for the modeling of these nanomaterials at the nanometric scale. This approach has allowed us, inter alia, to get insight into the molecular interactions between the different components and to determine the effective elastic properties of the nanocomposite.LILLE1-Bib. Electronique (590099901) / SudocSudocFranceF

Temperature and filler effects on the relaxed response of filled rubbers: Experimental observations on a carbon-filled SBR and constitutive modeling

International audienceThe self-heating temperature of filled rubbers under cyclic loading at environmental conditions is well-known. This increase in temperature seriously affects the constitutive stress–strain behavior by producing a thermal softening of the rubber compound. Although this feature is well-recognized and considered as important to its function, few constitutive thermo-mechanical models attempt to quantify the stress–temperature relationship. In this work, a physically-based model is developed to describe the large strain relaxed response of filled rubbers over a wide range of temperatures. The non-linear mechanical behavior is described via a Langevin formalism in which the temperature and filler effects are, respectively, included by a network thermal kinetics and an amplification of the first strain invariant. Experimental observations on the relaxed state of styrene-butadiene rubber hourglass-shaped specimens with a given carbon-black content are reported at different temperatures. A hybrid experimental–numerical method is proposed to determine simultaneously the local thermo-mechanical response and the model parameters. In addition, the predictive capability of the proposed constitutive thermo-mechanical model is verified by comparisons with results issued from micromechanical simulations containing different arrangements of the microstructure. The results show that the model offers a satisfactory way to predict the relaxed response of filled rubbers at different temperatures

Structural and thermodynamics properties of organo-modified montmorillonite clay

International audiencePolymer clay nanocomposites (PCNs) have been seen as the most novel materials in engineering applications since they exhibit significant improvement in mechanical and physical properties. Indeed, with few amount of organoclay, PCNs exhibit enhanced mechanical, optical, thermal and liquid or gas barrier properties compared to pure polymers and to their counterpart microcomposites. Thus, organoclays are extensively used as precursors in the preparation of PCNs. They are the best candidate in reinforcing PCNs because of the lightweight and the high availability of clay minerals in the nature. However, structure and physical phenomena arising at molecular level in organoclays, and subsequently in PCNs, are not completely or difficultly accessible with existing experimental techniques. In this work, molecular dynamics (MD) simulation was conducted using the combination of two force fields (CLAYFF and CHARMM) to evaluate the thermodynamics and structural properties of organoclay such as heat capacities, isothermal bulk modulus, density, basal spacing and chains arrangement in the interlayer spacing. Our results regarding the basal spacing and density are in fairly good agreement with available experimental data. This allows us to validate the use of the two force fields to represent interactions in organoclays. The effect of the cation exchange capacity (CEC) on the basal spacing and the thermodynamics properties is assessed. We found, through our MD simulation, that the calculated isothermal bulk modulus is in good agreement with the density value of organoclays with two different CEC

Methodological Pathways to Improvements of Evaluation Approaches: The Case of Irrigated Agriculture Performance Evaluation

Background:  Irrigated agriculture is often evaluated but few reviews of evaluation methodology adapted to this object are available in the literature. Besides, recommendations to improve evaluation in this field are lacking.   Purpose: The purpose of the paper is to contribute filling this gap.   Setting: Not applicable.   Intervention: Not applicable. Research Design: Not applicable.   Data Collection and Analysis: Desk review.   Findings: This review shows the evolution of evaluation methodology in the field of irrigated agriculture pointing out a trend towards more comprehensive methodologies. The review also suggests some methodological tools to improve evaluation process. Keywords: irrigated agriculture; evaluation methodology; plurality of evaluation; intended users; viewpoint  &nbsp