Strength properties of nanoporous materials: Molecular Dynamics computations and theoretical analysis

Since the recent arising of advanced nano-technologies, as well as of innovative engineering design approaches, nanoporous materials have been extensively studied in the last two decades, leading to a considerable worldwide research interest in both industrial and academic domains. Generally characterised by high specific surface area, uniform pore size and rich surface chemistry, nanoporous materials have allowed for the development of challenging ultra-high performance devices with tailorable properties, finding widespread application in several technical fields, including civil and environmental engineering, petroleum and chemical industries, biomechanics, molecular sieving and sensoring. In order to fulfil to these promising applications, one of the most fundamental research aspect consists in characterising and predicting the strength properties of these materials, as dependent on the size of voids. Since the current lack of an exhaustive benchmarking evidence, as well as of a comprehensive theoretical modelling, the central purpose of the present paper consists in: -) investigating strength properties of an in-silico nanoporous sample via Molecular Dynamics computations. In detail, a parametric analysis with respect to the void radius and for different porosity levels has been carried out, by considering different loading paths with a wide range of triaxiality scenarios. As a result, the influence of void-size effects on the computed strength properties has been clearly quantified, also highlighting the dependence of the predicted material strength domain on the three stress invariants; -) establishing an engineering-oriented theoretical model able to predict macroscopic strength properties of nanoporous materials, by properly accounting for void-size effects. To this end, a homogenization procedure based on a kinematic limit-analysis is performed addressing a hollow-sphere model comprising a rigid-ideal-plastic solid matrix and undergoing axisymmetric strain-rate boundary conditions. Void-size effects are accounted for by introducing an imperfect-coherent interface at the cavity boundary. Both the interface and the solid matrix are assumed to obey to a simplified form of the general yield function proposed by Bigoni and Piccolroaz [Int J Solids Struct; 41: 2855-2878], thereby allowing for an extreme flexibility in describing triaxiality and Lode-angle effects. A parametric closed-form relationship for the macroscopic strength criterion is obtained as the unique physically-consistent solution of an inequality-constrained minimization problem, the latter being faced via the Lagrangian method combined with Karush-Kuhn-Tucker conditions. Any possible choice of local-yield-function parameters is carefully addressed, by clearly highlighting the effects of a specific local plastic behaviour on the material macroscopic response. Finally, several comparative illustrations are provided, showing the influence of model parameters on the proposed yield function, as well as the model capability to describe the macroscopic strengthening, typical of nanoporous materials, induced by a void-size reduction for a fixed porosity level

Micromechanical modeling-based self-consistent scheme of polymer-layered silicate nanocomposites

Although few investigations recently proposed to describe the overall elastic response of polymer-clay nanocomposites using micromechanical-based models, the applicability of such models for nanocomposites is far from being fully established. In this communication, we present a micromechanical approach for the prediction of the overall moduli of polymer-clay nanocomposites using a self-consistent scheme based on the double-inclusion model. The efficiency of the proposed model to predict the experimental elastic response of various polymer-clay nanocomposites is pointed out

Multi-scale modeling of mechanical behavior of polymer nanocomposites reinforced with montmorillonite clay : micromechanical approach and molecular dynamics simulation

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

Strength properties of nanoporous materials: molecular dynamics computations and theoretical analysis

Since the recent arising of advanced nano-technologies, as well as of innovative engineering design approaches, nanoporous materials have been extensively studied in the last two decades, leading to a considerable worldwide research interest in both industrial and academic domains. Generally characterised by high specific surface area, uniform pore size and rich surface chemistry, nanoporous materials have allowed for the development of challenging ultra-high performance devices with tailorable properties, finding widespread application in several technical fields, including civil and environmental engineering, petroleum and chemical industries, biomechanics, molecular sieving and sensoring. In order to fulfil to these promising applications, one of the most fundamental research aspect consists in characterising and predicting the strength properties of these materials, as dependent on the size of voids. Since the current lack of an exhaustive benchmarking evidence, as well as of a comprehensive theoretical modelling, the central purpose of the present paper consists in: -) investigating strength properties of an in-silico nanoporous sample via Molecular Dynamics computations. In detail, a parametric analysis with respect to the void radius and for different porosity levels has been carried out, by considering different loading paths with a wide range of triaxiality scenarios. As a result, the influence of void-size effects on the computed strength properties has been clearly quantified, also highlighting the dependence of the predicted material strength domain on the three stress invariants; -) establishing an engineering-oriented theoretical model able to predict macroscopic strength properties of nanoporous materials, by properly accounting for void-size effects. To this end, a homogenization procedure based on a kinematic limit-analysis is performed addressing a hollow-sphere model comprising a rigid-ideal-plastic solid matrix and undergoing axisymmetric strain-rate boundary conditions. Void-size effects are accounted for by introducing an imperfect-coherent interface at the cavity boundary. Both the interface and the solid matrix are assumed to obey to a simplified form of the general yield function proposed by Bigoni and Piccolroaz [Int J Solids Struct; 41: 2855-2878], thereby allowing for an extreme flexibility in describing triaxiality and Lode-angle effects. A parametric closed-form relationship for the macroscopic strength criterion is obtained as the unique physically-consistent solution of an inequality-constrained minimization problem, the latter being faced via the Lagrangian method combined with Karush-Kuhn-Tucker conditions. Any possible choice of local-yield-function parameters is carefully addressed, by clearly highlighting the effects of a specific local plastic behaviour on the material macroscopic response. Finally, several comparative illustrations are provided, showing the influence of model parameters on the proposed yield function, as well as the model capability to describe the macroscopic strengthening, typical of nanoporous materials, induced by a void-size reduction for a fixed porosity level

Limit analysis and homogenization of nanoporous materials with a general isotropic plastic matrix

In this paper, a closed-form expression of a macroscopic strength criterion for ductile nanoporous materials is established, by considering the local plastic behavior as dependent on all the three isotropic stress invariants and by referring to the case of axisymmetric strain-rate boundary conditions. The proposed criterion also predicts void-size effects on macroscopic strength prop- erties. A homogenization procedure based on a kinematic limit-analysis is performed by addressing a hollow-sphere model comprising a rigid-ideal-plastic solid matrix. Void-size effects are accounted for by introducing an imperfect-coherent interface at the cavity boundary. Both the interface and the solid matrix are assumed to obey to a general isotropic yield function, whose parametric form allows for a significant flexibility in describing effects induced by both stress triaxiality and stress Lode angle. Taking advantage of analytical expressions recently provided by Brach et al. [Int J Plasticity 2017; 89: 1–28] for the corresponding support function and for the exact velocity field under isotropic loadings, a parametric closed-form relationship for the mac- roscopic strength criterion is obtained as the solution of an inequality-constrained minimization problem, the latter being faced via the Lagrangian method combined with Karush-Kuhn-Tucker conditions. Finally, several comparative illustrations are provided, showing the influence of local-yield-function parameters on the established criterion, as well as the model capability to describe the macroscopic strengthening, typical of nanoporous materials, induced by a void-size reduction for a fixed porosity level

Limit analysis and homogenization of nanoporous materials with a general isotropic plastic matrix

In this paper, a closed-form expression of a macroscopic strength criterion for ductile nanoporous materials is established, by considering the local plastic behavior as dependent on all the three isotropic stress invariants and by referring to the case of axisymmetric strain-rate boundary conditions. The proposed criterion also predicts void-size effects on macroscopic strength prop- erties. A homogenization procedure based on a kinematic limit-analysis is performed by addressing a hollow-sphere model comprising a rigid-ideal-plastic solid matrix. Void-size effects are accounted for by introducing an imperfect-coherent interface at the cavity boundary. Both the interface and the solid matrix are assumed to obey to a general isotropic yield function, whose parametric form allows for a significant flexibility in describing effects induced by both stress triaxiality and stress Lode angle. Taking advantage of analytical expressions recently provided by Brach et al. [Int J Plasticity 2017; 89: 1–28] for the corresponding support function and for the exact velocity field under isotropic loadings, a parametric closed-form relationship for the mac- roscopic strength criterion is obtained as the solution of an inequality-constrained minimization problem, the latter being faced via the Lagrangian method combined with Karush-Kuhn-Tucker conditions. Finally, several comparative illustrations are provided, showing the influence of local-yield-function parameters on the established criterion, as well as the model capability to describe the macroscopic strengthening, typical of nanoporous materials, induced by a void-size reduction for a fixed porosity level

Limit analysis and homogenization of porous materials with Mohr–Coulomb matrix. Part II: Numerical bounds and assessment of the theoretical model

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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