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

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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

Random distribution of polydisperse ellipsoidal inclusions and homogenization estimates for porous elastic materials

International audienceThis work proposes an extension of the well-known random sequential adsorption (RSA) method in the context of non-overlapping random mono-and polydisperse ellipsoidal inclusions. The algorithm is general and can deal with inclusions of different size, shape and orientation with or without periodic geometrical constraints. Specifically, polydisperse inclusions, which can be in terms of different size, shape, orientation or even material properties, allow for larger volume fractions without the need of additional changes in the main algorithm. Unit-cell computations are performed by using either the fast Fourier transformed-based numerical scheme (FFT) or the finite element method (FEM) to estimate the effective elastic properties of voided particulate microstructures. We observe that an isotropic overall response is very difficult to obtain for random distributions of spheroidal inclusions with high aspect ratio. In particular, a substantial increase (or decrease) of the aspect ratio of the voids leads to a markedly anisotropic response of the porous material, which is intrinsic of the RSA construction. The numerical estimates are probed by analytical Hashin-Shtrikman-Willis (HSW) estimates and bounds

Profil radiologique des traumatises de la voie publique : A propos de 420 cas.

Objectifs : évaluer les aspects épidémiologiques des traumatisés de la voie publique, de répertorie radiologiquement des lésions traumatiques observées et de préciser leur fréquence.Patients et méthode : Il s’est agi d’une étude transversale de 420 patients victimes d’accident de la voie publique reçus au service de Radiologie et d’Imagerie Médicale et au pavillon Scanner du CHU Sylvanus Olympio de Lomé de Juillet 2012 à Décembre 2012 pour une exploration radiologique.Résultats : notre échantillon comporte 308 hommes et 112 femmes avec un sex ratio de 2,75. La majorité de nos patients était des élèves et étudiants avec 26.90% des cas. La plaie a été le signe physique le plus fréquent représentée avec 236 cas (56,19%). A la radiographie standard les lésions ont été observées chez 287 patients (86,55%).Les fractures des membres inférieurs ont représenté 84,88%. Cent trente-six (136) patients ont réalisé le scanner crânio-encéphalique et 97 patients présentaient des lésions traumatiques. Il se dégageait une prédominance des lésions péri cérébrales (51,54%) dont 50% d’hématome sous-dural et 40% d’hématome extra-dural.Conclusion : les traumatismes par AVP restent élevés. Le profil des traumatisés de la voie publique est celui d'un adulte jeune de sexe masculin, pouvant être soit un passager soit un piéton. Ainsi, les traumatismes par accident de la voie publique demeurent un problème de santé publique dans notre pays.Mots clés: AVP, lésions, aspects radiologiques, radiographie standard, TDM, Afrique noire.ABSTRACTObjectives: To evaluate the epidemiological aspects of trauma of the highway, lists of radiologically observed injuries and specify their frequency.Patients and methods: It was a matter of a cross sectional study of 420 patients victims of highway accidents received Radiology Department and Medical Imaging and pavilion Scanner CHU Sylvanus Olympio Lome July 2012 to December 2012 radiological exploration. Results: Our sample includes 308 men and 112 women with a sex ratio of 2.75. The majority of our patients was of pupils and students with 26.90% of cases. The wound was the most common physical sign represented with 236 cases (56.19%). A standard radiography lesions were observed in 287 patients (86.55%). The lower extremity fractures accounted for 84.88%. One hundred and thirty six (136) patients completed the Cranioencephalic scanner and 97 patients had traumatic injuries. It gave off a predominance of brain lesions died (51.54%), 50% of subdural hematoma and 40% of extra-dural hematoma.Conclusion: AVP by trauma remain high. The public road traumatized profile is that of a young adult male, which can be either a passenger or a pedestrian. Thus, trauma by highway accident remains a public health problem in our country.Keywords: AVP, injuries, radiological aspects, plain radiography, CT, Black Africa