Wyniki wyszukiwania - Biblioteka Nauki

1

Modeling of articular cartilage replacement materials

100%

Stoffel M. , Weichert D. , Muller-Rath R.

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2009

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tom Vol. 61, nr 1

69-87

EN

The development of replacement material for human articular cartilage exhibiting similar mechanical properties as the native tissue is a problem of high actuality in biomeclicine. In the present work a new condensed collagen material is investigated. The study aims at developing a mechanical model especially adapted to this particular collagen material. For this purpose, a viscoelastic-diffusion (VED) model is proposed, accounting for two different diffusion evolutions assumed. Moreover, the need for a gradient material description is discussed in order to cover fabrication influences leading to a variable Young's modulus for the material. On this background, a phe-nornenological law is presented to predict deformation-dependent diffusion behavior and internal reaction forces. Furthermore, the present approach allows a practible identification of diffusion parameters. The theoretical model is implemented into a finite element code and parameters are identified by tension tests. The simulation results are validated experimentally.

2

Effective complex permittivity of two-phase random composite media: a test of the two exponent phenomenological percolation equation.

100%

Myroshnychenko V. , Brosseau C.

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tom Vol. 46, nr 19

40-47

EN

The nature of percolation in continuum media inhomogeneous media is a current topic of debate. In this work, Monte Carlo and finite element simulations of the effective complex permittivity, e- e -ye, of two phase random composite media are analyzed using the two exponent phenomenological percolation equation (TEPPE) due to McLachlan. The continuum-percolation system consists of two-dimensional equilibrium distributions of randomly distributed monodisperse circular and partially penetrable disks (or parallel, infinitely long, identical, partially penetrable circular cylinders) throughout a host matrix. The study is performed on a set of calculations, covering wide ranges of various parameters including the intrinsic constituent permittivity, the surface fraction, and the degree of impenetrability. In our analysis, we first determine the parameters that characterize the critical behavior at the percolation threshold. Our data suggest that the phenomenological TEPPE does not fit the simulation data well over the entire range of surface fraction, and whatever the degree of impenetrability considered. This is attributed, in part, to the fact that the effective medium approximation (restricted to dipolar interactions only) ignores the local field fluctuations explicitly. Moreover, the mixtures exhibit clustering in equilibrium which is not conceptually incorporated in the TEPPE, i.e. the inclusions form cluster with a percolating spongelike topology accompanied by strongly dependent shape of the radial distribution function on the degree of impenetrability. It is argued that further efforts are still needed to fully grasp the numerically (and experimentally) observed features of the effective properties of dielectric heterostructures.

3

Development of numerical tools for the multiscale modelling of recrystallization in metals, based on a digital material framework

88%

Bernacki M. , Chastel Y. , Digonnet H. , Resk H. , Coupez T. , Logé R.

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2007

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tom Vol. 7, No. 1

142-149

EN

This work is currently under development within the framework of an American-European project (Digimat Project) whose principal purpose is to model recrystallization in metals using a multiscale approach. The modelling effort is centered around a digital material framework. This framework is based on a digital representation of the material structure, where data coming from different scales can be stored or probed. The digital representation can be converted into finite element meshes, which are then used to model plastic deformation and subsequent recrystallization. The local behaviour of individual microstructure components is computed through models operating at different scales. In particular, grain constitutive models are derived from crystal plasticity concepts, with appropriate hardening/recovery laws which are linked to lower scale approaches at the dislocation level. Grain boundary motion is similarly described by connecting the continuum mechanical and thermal fields to simulations at the atomistic/dislocations levels. A detailed confrontation of the multiscale approach with experiment will be done at the ESRF synchrotron facility in Grenoble (France). In this paper, the needed development of numerical tools is presented together with the first finite element simulations. The development of the DIGIMAT software, dedicated to the concept of digital material, is first detailed. The construction of the virtual material consists in building a multi-level Voronoi tesselation. A polycrystalline microstructure made of grains and sub-grains can be obtained in a random or deterministic way. The software is at a stage of its development where it is possible to cut the microstructure along given planes, to approximate the grain shapes by a set of fitting ellipsoids, to roughly optimize the digital microstructure, and to generate a coarse mesh of the microstructure at each level of the microstructure (level 1 = external shape, level 2 =grains, level 3 = sub-grains). A second part of the work concerns the first finite elements simulations of a uniaxial compression test under large strain. The initial mesh is fine and anisotropic, taking into account the presence of interfaces between grains and sub-grains. A level-set approach is used to follow the grain boundaries during the deformation. In fact, with this method, the interface is modelled by the zero level-set of a time dependant level-set function which moves according to the mesh velocity field. The most complex test case carried out to date is a multi-domains Stokes problem, deforming at 85\% a cubic Volume Element made of 250 grains. Boundary conditions use a constant velocity in the compression direction, and free motion in the plane perpendicular to that direction. The constitutive law is a viscoplastic power law with a rate sensitivity of 0.2, and a variable hardness from one grain to another. The constitutive law will soon be replaced by a crystallographic formulation. Automatic isotropic and anisotropic remeshing, and parallel computation were successfully implemented, both features being crucial with respect to the Digimat project objectives (large strain to induce recrystallization, and large number of elements for a good representation of the microstructure). Finally, the bases of our first recrystallization simulations will be explained, including the description of nucleation and grain growth.

PL

Praca jest realizowana w ramach Amerykańsko-Europeskiego projektu (Digimat Project). W artykule opisano rozwój numerycznych narzędzi przeznaczonych do cyfrowej reprezentacji struktur metalicznych oraz do generowania powiązanych siatek anizotropowych dla modelowania metodą elementów skończonych dużych odkształceń polikrystalicznych mikrostruktur. Metoda ustalonych poziomów stosowana do opisu mikrostruktury stanowi wspólną bazę dla wszystkich analizowanych rozwiązań.

4

Finite element analysis of the hemodynamics in the human carotid artery bifurcation

88%

Postelnicu A. , Minea D.

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2000

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tom Vol. 5, no 4

759-772

EN

The present paper brings new lights on blood flow patterns in rigid artery bifurcation models, differing in angle of bifurcation. Using the ANSYS 5.4/FLOTRAN package, a numerical analysis in two dimensions was performed, under physiologically relevant flow conditions. For two bifurcation models, one with large angle the other with a small one, the distributions of velocities, shear-stress and pressure in the domain of carotid artery bifurcation have been obtained. It is argued and proved that the vascular geometry at the bifurcation level represents a risk factor in atherogenesis, the relevant factors for this process being the flow separation, reversal flow regions and wall shear stress.

5

Experimental and numerical investigation of plywood progressive failure in CT tests

75%

Ivanov I. V. , Sadowski T. , Filipiak M. , Kneć M.

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2008

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tom vol. 2, nr 1

79-94

EN

The plywood is considered as a layered cross-ply unidirectional fibre reinforced composite. The experimental Compact Tension (CT) tests carried out in different directions of plywood fibre orientation show that the characteristics of damages are fibre bundle rupture, matrix cracking along the fibres, and delamination at the ply interlayers of glue. The plywood CT specimens are modelled by continuum shell and cohesive finite elements with damage evolution in material models. The Finite Element (FE) model simulates the experimental behaviour of plywood samples very well and allows deep investigation of the different types of damage development and interaction. The FE model of plywood is useful for its lay-up optimization and for development of very efficient in large-scale simulations computational models of plywood.