Experimental investigation of the Mullins effect in swollen elastomers

International audienceNatural rubber distinguishes itself by its particular mechanical properties. It has become an almost irreplaceable important component part in industrial applications such as vibration isolator, sealing system, flexible piping or structural bearing. During the service, these components are subjected to fluctuating mechanical loading. Under cyclic loading conditions, rubber exhibits strong inelastic responses such as stress-softening due to Mullins effect. It is believed that such inelastic response plays major role in determining the durability in service of rubber component. In engineering applications where the components are concurrently exposed to aggressive solvent, further material degradation in the form of swelling occurs. Thus, it is essential to investigate the effect of swelling on the stress-softening due to Mullins effect in rubber like materials for durability analysis. In this study, the Mullins effect in swollen carbon black-filled natural rubber under cyclic loading conditions is investigated. The swollen rubbers are obtained by immersing initially dry rubber in solvent at room temperature for various immersion durations. The stress-strain responses for both dry and swollen rubber are found qualitatively similar. However, the stress-softening in swollen rubbers are notably lower compared to that in the dry one. This work is later extended for future modelling purpose by adapting the concept of Continuum Damage Mechanics (CDM) [Chagnon, G., Verron,E., Gornet, L., Markmann, G., Charrier, P., 2004. On the relevance of Continuum Damage Mechanics as applied to the Mullins effect in elastomers. J. Mech. Phys. Sol. 52, 627-1650] and pseudo-elastic model [Ogden, R.W., Roxburgh, D.G., 1999. A pseudo-elastic model for the Mullins effect in filled rubber.Proc. R. Soc. A. 455, 2861-2877]

Design of Polyurethane Fibers: Relation between the Spinning Technique and the Resulting Fiber Topology

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Fatigue of swollen elastomers

International audienceThe compatibility of the properties of elastomer with conventional diesel fuel has made it favourable in many engineering applications. However, due to global energy insecurity issues, there is an urgent need to find alternative renewable sources of energy as replacements to conventional diesel. In this respect, biodiesel appears to be a promising candidate. Hence, research into the compatibility and fatigue characteristics of elastomers exposed to biodiesel becomes essential. The present paper introduces the first attempt to investigate the effect of different solvents on the fatigue of swollen elastomers. The filled nitrile rubbers are immersed in the palm biodiesel and conventional diesel to obtain the same degree of swelling prior to the application of uniaxial fatigue loading. Field Emission Scanning Electron Microscopy (FESEM) analysis is carried out to observe the fracture surfaces. Stretch-N curves are plotted to illustrate the fatigue life duration. These curves showed that the fatigue lifetime of rubber is the longest for dry rubber and the least for rubber swollen in biodiesel. FESEM micrographs reveal that the loading conditions have no effect on the crack initiation and propagation patterns regardless of the swelling state

The role of infarct transmural extent in infarct extension: a computational study

Infarct extension, a process involving progressive extension of the infarct zone (IZ) into the normally perfused border zone (BZ), leads to continuous degradation of the myocardial function and adverse remodelling. Despite carrying a high risk of mortality, detailed understanding of the mechanisms leading to BZ hypoxia and infarct extension remains unexplored. In the present study, we developed a 3D truncated ellipsoidal left ventricular model incorporating realistic electromechanical properties and fibre orientation to examine the mechanical interaction among the remote, infarct and BZs in the presence of varying infarct transmural extent (TME). Localized highly abnormal systolic fibre stress was observed at the BZ, owing to the simultaneous presence of moderately increased stiffness and fibre strain at this region, caused by the mechanical tethering effect imposed by the overstretched IZ. Our simulations also demonstrated the greatest tethering effect and stress in BZ regions with fibre direction tangential to the BZ–remote zone boundary. This can be explained by the lower stiffness in the cross-fibre direction, which gave rise to a greater stretching of the IZ in this direction. The average fibre strain of the IZ, as well as the maximum stress in the sub-endocardial layer, increased steeply from 10% to 50% infarct TME, and slower thereafter. Based on our stress–strain loop analysis, we found impairment in the myocardial energy efficiency and elevated energy expenditure with increasing infarct TME, which we believe to place the BZ at further risk of hypoxia

Définition d'une nouvelle grandeur prédictive pour la durée de vie en fatigue des matériaux élastomères

Ces dernières années, de nombreux progrès ont été faits dans le domaine de la simulation numérique de pièces en élastomère. Ces avancées sont en grande partie motivées par la nécessité d'améliorer les délais et les coûts de conception dans des domaines industriels fortement compétitifs, et notamment pour les pièces anti-vibratoires automobiles. Dans ce contexte, si les logiciels classiques de type éléments finis sont capables de prédire efficacement l'histoire des déformations et des contraintes au sein des pièces, la prédiction de la durée de vie de celles-ci en fatigue reste un problème ouvert. Le processus de rupture par fatigue dans les élastomères se fait en deux phases : une premère phase d'initiation durant laquelle des micro-fissures apparaissent au sein du matériau, puis une phase de propagation durant laquelle ceux-ci croissent jusqu'à la rupture. Des travaux récents ont démontré que la première phase est prédominante pour la fin de vie des pièces anti-vibratoires. Ainsi, la construction d'une grandeur prédictive effcace doit être capable de déterminer les zones propices à l'apparition de micro-défauts au sein d'une pièce. Les trois grandeurs prédictives classiquement utilisées pour estimer la durée de vie en fatigue des élastomères sont la déformation principale maximale, la contrainte principale maximale et l'énergie de déformation. Si celles-ci s'avèrent effcaces pour des cas de chargement uniaxiaux, leur utilisation pour des cas de chargement multiaxiaux pose problème. Afin de prédire la rupture en fatigue des pièces élastomères en service, une grandeur prédictive efficace dans le domaine mul-tiaxial est nécessaire. Celle-ci doit être indépendante du mode de déformation, motivée par la physique des phénomènes mis en jeu, théoriquement bien fondée et finalement aisément implantable dans les outils numériques. La présente thèse propose de construire une telle grandeur. Des observations expérimentales réalisées préalablement et permettant de comprendre les phénomènes physiques mis en jeu nous ont conduit à considérer le tenseur des contraintes configurationnelles établi par Eshelby en 1951. Dans le domaine élastique, le nouveau prédicteur proposé est la plus petite valeur propre de ce tenseur, la direction propre associée étant la direction normale au plan d'ouverture de la fissure. L'extension de ces travaux au cas inélastique est aussi développée. Afin de vérifier le bien-fondé de cette théorie, des données expérimentales classiques de la bibliographie ont été utilisées ; les résultats obtenus démontrent l'efficacité de notre approche notamment pour unifier les résultats multiaxiaux en fatigue.The last decade has experienced a major advance in the development of finite element based tools for the simulation of a wide range of industrial rubber parts. This is mainly motivated by the need to improve time and cost efficiencies in highly competitive industries particularly in automotive Anti-Vibration Systems (AVS) industry. While the basic concept of finite element method capable of predicting stress and strain histories has been well established, the use of these histories to estimate fatigue life of rubber parts in service remains a critical issue. Typically, the fatigue failure process involves a period during which cracks nucleate in regions that were initially free of observed cracks, followed by a period during which nucleated cracks grow to the point of failure. For AVS, the former is the most important one. The three most widely used predictors for rubber crack nucleation are the maximum principal stretch, the maximum principal stress and the strain energy density. However, they fail to give satisfying prediction for multiaxial problems. In order to prevent fatigue failure of rubber parts in service, an efficient and well-defined multiaxial fatigue life predictor is required, i.e. independent of deformation state, physically motivated, theoretically well-formulated and easy to implement into finite element software. Thus, the purpose of this study is to develop a new fatigue life predictor which can meet these requirements. Experimental observations were conducted to understand physical phenomena which take place during fatigue crack nucleation and growth in rubber. Based on these observations, we consider that the configurational stress tensor introduced by Eshelby in 1951 is an appropriate continuum mechanics quantity to develop a relevant fatigue life predictor. In elasticity, the new predictor is given by the smallest eigenvalue of this tensor and the normal of the crack plan is the eigenvector associated with the smallest eigenvalue. An extension to the case of inelasticity is also proposed. To verify its efficiency, experimental data issued from the literature are considered. Results demonstrate that the proposed predictor is capable of unifying multiaxial fatigue data.NANTES-BU Sciences (441092104) / SudocNANTES-Ecole Centrale (441092306) / SudocSudocFranceF

Viscoelastic Characterization of Short Fibres Reinforced Thermoplastic in Tension and Shearing

6 pagesInternational audienceThe present work can be regarded as a first step toward an integrated modelling of mould filling during injection moulding process of polymer matrix composites and the resulting material behaviour under service loading conditions. More precisely, the emphasis of the present research is laid on the development of a mechanical model which takes into account the processing-induced microstructure and is capable to predict the mechanical response of the material. In the Part I, a set of experiments which captures the mechanical behaviour of an injection moulded short fibre reinforced under different strain histories is described. Three mechanical testing are conducted: Dynamic Mechanical Analysis (DMA), uniaxial tension and simple shear. Tests show that the material exhibits complex responses mainly due to non-linearity, anisotropy, time/rate-dependence, hysteresis and permanent strain. Moreover, the relaxed state of the material is characterized by the existence of a so-called anisotropic equilibrium hysteresis independently of the prescribed strain rate

Mechanical response of a short fiber-reinforced thermoplastic: Experimental investigation and continuum mechanical modeling

International audienceThe present work can be regarded as a first step toward an integrated modeling of mold filling during injection molding process of polymer composites and the resulting material behavior under service loading conditions. More precisely, the emphasis of the present paper is laid on how to account for local fiber orientation in the ground matrix on the prediction of the mechanical response of the composite at its final solid state. To this end, a set of experiments which captures the mechanical behavior of an injection molded short fiber-reinforced thermoplastic under different strain histories is described. It is shown that the material exhibits complex response mainly due to non-linearity, anisotropy, time/ratedependence, hysteresis and permanent strain. Furthermore, the relaxed state of the material is characterized by the existence of an equilibrium hysteresis independently of the applied strain rate. A three-dimensional phenomenological model to represent experimentally observed response is developed. The microstructure configuration of the material is simplified and assumed to be entirely represented by a distributed fiber orientation in the ground matrix. In order to account for distributed short fiber orientations in a continuum sense, a concept of(symmetric) generalized structural tensor (tensor of orientation) of second order is adopted. The proposed model is based on assumption that the strain energy function of the composite is given by a linear mixture of the strain energy of each constituent: an isotropic part representing Phase 1 which is essentially related to the ground matrix and an anisotropic part describing Phase 2 which is mainly related to the fibers and the interphase as a whole. Hence, taking into account the fiber content and orientation, the efficiency of the model is assessed and perspectives are drawn. 2010 Elsevier Masson SAS. All rights reserved

Recent advances on fatigue of rubber after the literature survey by Mars and Fatemi in 2002 and 2004

Subjected to multiaxial mechanical loading and hostile environment, rubber experiences degradation over a period of time. Therefore, it is of utmost importance to prevent failure of rubber components during the service. As highlighted in Mars and Fatemi [Mars, W.V, Fatemi, A., 2002. A literature survey on fatigue analysis approaches of rubbers. Int. J. Fatigue 24, 949–961; Mars, W.V, Fatemi, A., 2004. Factors that affect the fatigue life of rubber: A literature survey. Rubber Chem. Technol. 77, 391–412], a large number of works focused on the durability of rubber. Furthermore, it has been expanding rapidly until today. For this reason, the present work focuses on collecting and analyzing the vast amount of works on fatigue of rubber conducted in the last 15 years since the review of Mars and Fatemi in 2002 and 2004. To this end, three bibliographic databases are consulted: Google Scholar, Scopus and Web of Science. The collected works are analyzed with the objective to identify the current and future trends and needs in the study of rubber fatigue

Constitutive modeling of randomly oriented electrospun nanofibrous membranes

In this paper, a simple phenomenological model describing the macroscopic mechanical response of electrospun nanofibrous structures is proposed. Motivated by the experimental observation, the model development starts from the description of membrane response at fiber scale in order to capture individual fiber response and irreversible inter-fiber interactions using hyperelastic and large strain elasto-plastic frameworks, respectively. The macroscopic response is subsequently obtained by integrating the fiber responses in all possible fiber orientations. The efficiency of the proposed model is assessed using experimental data of PVDF electrospun nanofibrous membranes. It is found that the model is qualitatively in good agreement with uniaxial monotonic and cyclic tensile loading tests. Two other deformation modes, i.e., equibiaxial extension and pure shear (planar extension), are simulated to further evaluate the model responses. Finally, the deformation-induced fiber re-orientation is investigated for different modes of deformations. © 2018, Springer-Verlag GmbH Germany, part of Springer Nature