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Based on the meso-structured Voronoi cell model and the meso-macro homogenization procedure between the discrete particle assembly and the porous continuum for wet granular materials, meso-hydro-mechanically informed effective pressure and effective stress measures for saturated and unsaturated porous media are defined. The meso-hydro-mechanically informed generalized effective stress for saturated porous continua taking into account the volumetric deformation of solid grains due to pore liquid pressure is derived. The Biot coefficient associated to the meso-hydro- mechanically informed generalized effective stress for saturated porous media is formulated. The differences of the definitions for proposed generalized effective stress and Biot coefficient compared with those defined in the generalized Biot theory of saturated porous continua and in averaging theories are discussed. The wet meso-structured Voronoi cell model, consisting of three immiscible and interrelated (i.e. solid grains, interstitial liquid and gas) phases, at low bulk saturation (below about 30%) is proposed. A meso-structural pattern with the binary bond mode of pendular liquid bridges is assumed in particular to derive the meso-hydro- mechanically informed macroscopic anisotropic effective pressure and effective stress tensors for unsaturated porous media. As the isotropic case of the wet meso-structured Voronoi cell model is considered, the meso-hydro-mechanically informed effective pressure tensor degrades to the scalar variable in the same form as in the theory of macroscopic unsaturated porous continua. The proposed meso-hydro-mechanically informed Bishop's parameter is derived and obtained as a function of saturation, porosity, and meso-structural parameters, without need to introduce any macroscopic phenomenological assumptions for the description of hydro-mechanical constitutive behavior.

A mixed finite element (FE) procedure of the gradient Cosserat continuum for the second-order computational homogenisation of granular materials is presented. The proposed mixed FE is developed based on the Hu---Washizu variational principle. Translational displacements, microrotations, and displacement gradients with Lagrange multipliers are taken as the independent nodal variables. The tangent stiffness matrix of the mixed FE is formulated. The advantage of the gradient Cosserat continuum model in capturing the meso-structural size effect is numerically demonstrated. Patch tests are specially designed and performed to validate the mixed FE formulations. A numerical example is presented to demonstrate the performance of the mixed FE procedure in the simulation of strain softening and localisation phenomena, while without the need to specify the macroscopic phenomenological constitutive relationship and material failure model. The meso-structural mechanisms of the macroscopic failure of granular materials are detected, i.e. significant development of dissipative sliding and rolling frictions among particles in contacts, resulting in the loss of contacts.

The paper addresses modeling of avascular and vascular tumor growth within the framework of continuum mechanics and the adopted numerical solution strategies. The models involve tumor cells, both viable and necrotic, healthy cells, extracellular matrix (ECM), interstitial fluid, neovasculature and co-opted blood vessels, nutrients, waste products, and their interaction and evolution. Attention is focused on the more recent models which are much richer than earlier ones, i.e. they address more aspects of this complicated problem. An important element is how the governing equations are obtained and how the many interfaces between the above listed components are dealt with. These considerations suggest the definition of different classes of models comprised of diffusion, single phase flow and multiphase flow models with or without a solid phase. A multiphase flow model in a deforming porous medium (ECM) is chosen as reference model since it appears to invoke the least number of simplifying assumptions and has the largest potential for further development. The strategies adopted in the choice of the many model dependent constitutive relationships are discussed in detail. Two applications referring to two different model classes conclude the paper.

SUMMARY Today multiphysics problems applied to various fields of engineering have become increasingly important. Among these, in the areas of civil, environmental and nuclear engineering, the problems related to the behaviour of porous media under extreme conditions in terms of temperature and/or pressure are particularly relevant. The mathematical models used to solve these problems have an increasing complexity leading to increase of computing times. This problem can be solved by using more effective numerical algorithms, or by trying to reduce the complexity of these models. This can be achieved by using a sensitivity analysis to determine the influence of model parameters on the solution. In this paper, the sensitivity analysis of a mathematical/numerical model for the analysis of concrete as multiphase porous medium exposed to high temperatures is presented. This may lead to a reduction of the number of the model parameters, indicating what parameters should be determined in an accurate way and what can be neglected or found directly from the literature. Moreover, the identification parameters influence may allow to proceeding to a simplification of the mathematical model (i.e. model reduction). The technique adopted in this paper to performing the sensitivity analysis is based on the automatic differentiation (AD), which allowed to develop an efficient tool for the computation of the sensitivity coefficients. The results of the application of AD technique have been compared with the results of the more standard finite difference method, showing the superiority of the AD in terms of numerical accuracy and execution times. From the results of the sensitivity analysis, it follows that a drastic simplification of the model for thermo-chemo-hygro-mechanical behaviour of concrete at high temperature, is not possible. Therefore, it is necessary to use different model reduction techniques in order to obtain a simplified version of the model that can be used at industrial level. Copyright © 2012 John Wiley & Sons, Ltd.

SUMMARY The aim of this paper is to present a three-dimensional (3D) finite element modeling of heat and mass transfer phenomena in partially saturated open porous media with random fields of material properties. Randomness leads to transfer processes within the porous medium that naturally need a full 3D modeling for any quantitative assessment of these processes. Nevertheless, the counterpart of 3D modeling is a significant increase in computations cost. Therefore, a staggered solution strategy is adopted which permits to solve the equations sequentially. This appropriate partitioning reduces the size of the discretized problem to be solved at each time step. It is based on a specific iterative algorithm to account for the interaction between all the transfer processes. Accordingly, a suitable linearization of mass convective boundary conditions, consistent with the staggered algorithm, is also derived. After some validation tests, the 3D numerical model is used for studying the drying process of a cementitious material with regard to its intrinsic permeability randomness. Copyright © 2011 John Wiley & Sons, Ltd.

Recent experimental tests on the model coils have shown that the behavior of Nb3Sn based cables is not as good as expected on the basis of the characteristics evaluated for the uncabled strands. This degradation of the cable performance seems to be due to various factors, among which the strain state of the filaments due to bending and contact phenomena inside the cable. After these results it was decided that high performance strands will be used for ITER magnets, even if they have never been tested on a full size cable. Therefore the capability of evaluating the coil performance from the strands characteristics becomes a crucial point in the Research and Development (R&D) activity. Recently some extrapolations were attempted, but the mechanical model used is rather simplified and needs some fitting parameters which are not known a priori. In this work we present a thermo-mechanical model suitably developed to evaluate the strain state of a Nb3Sn strand inside a superconducting (SC) cable. It is based on the idea of multiscale modeling, starting from a enriched formulation of the beam kinematics to take into account the fibrous nature of a multifilamentary strand. The method consists in performing a successive substitution of discrete models involving many beams with a single equivalent beam model, which behavior is identified from the preceding cabling stage. This recursive substitution allows to perform the analysis within a reasonable computational time. Once the stress and strain fields are obtained at the higher level, a suitable unsmearing technique gives the strain till the first level, on the scale of the SC filament. The method is applied to the real case of the 3 times 3 and 3 times 3 times 5 CICC sub-size samples tested at FZK in Germany.

Recent experimental results on the International Thermonuclear Experimental Reactor (ITER) Central Solenoid Model Coil and Insert Coil have shown that the critical characteristics measured on a single strand are significantly different from those of a cabled conductor. This performance degradation seems to be due mostly to the bending effects on the strands subjected to the electromagnetic forces: a strand inside the cable behaves like a beam supported by the neighboring strands and loaded by the transversal Lorentz force. Recently at the University of Twente (The Netherlands) and at the Japan Atomic Energy Research Institute (JAERI) some experimental tests have been performed to study this effect. In this study we analyze the strand strain field resulting from the cool down and the successive bending loads. Due to the material non linearity it is not possible to calculate the two effects separately. To analyze the strain field in a strand experiencing an electromagnetic load at 4.2 K, it is hence necessary to follow the evolution of the deformation from the strand reaction temperature to its working conditions. One single strand is taken into consideration, and it is studied with three different approaches: assuming Kirchhoff hypothesis, applying Timoshenko beam theory and using a composite, fibrous beam model. The resulting strain field is used as input data for the THELMA code to compare the results with the experiment performed at JAERI. This work is detailed in a companion paper

The THELMA (Thermal-Hydraulic-ELectro-MAgnetic) code has been developed with the aim of simulating the main aspects of superconductors to be used in the coils of the International Thermonuclear Experimental Reactor (ITER). An application of the code is presented here, where THELMA is used to simulate a single strand by considering, as cable elements, groups of superconducting (SC) filaments and the corresponding portion of the resistive matrix. This approach is used to reproduce the voltage-current characteristic of a Nb3Sn bronze route strand when subject to a bending mechanical load. Attention is focused particularly on the effect of the applied mechanical load on the critical current, which is considered a relevant item in the explanation of the degradation of the coil performance observed in several ITER Model and Insert Coil experiments. The longitudinal strain of the SC filaments is calculated by means of a composite beam model of the strand, taking into account the nonlinear, temperature-dependent material characteristics of the components. The whole load history is simulated, computing first the thermal strain due to cool-down, and then the mechanical strain due to the bending at 4.2 K

The paper analyses the interaction between two internal length scales during dynamic strain localization in multiphase porous materials. The first internal length is introduced in the mathematical model by the gradient-dependent plasticity for the solid skeleton, while the second one is naturally contained in the multiphase model and is due to the seepage process of the water via Darcy's law, which induces a rate-dependent behaviour of the solid skeleton. Numerical results of a one-dimensional example of water saturated porous medium demonstrate the competing effect between these two length scales. The porous medium is here treated as a multiphase continuum, with the pores filled by water and air, the last one at constant atmospheric pressure. Copyright © 2005 John Wiley & Sons, Ltd.

The paper presents a fully-coupled numerical model for the analysis of fracture initiation and propagation in a two dimensional non-homogeneous elastic medium driven by mechanical loads and transient thermal fields. Cohesive crack behaviour is assumed for the solid. The solution of the coupled problem is obtained by using the finite element method without using special approximation techniques nor interface elements. Evolution of the process zone results in continuous changes of the domain topology. This is accounted for by updating the boundary geometry and successive remeshing of the domain. Optimality of the shape of the finite elements generated is controlled and the mesh density is adjusted adaptively on the basis of an error estimator. Two numerical applications are presented, which demonstrate the effectiveness of the proposed procedure. In the first, comparison is made with a laboratory experiment, whereas the second handles a problem with crack path completely unknown.

A back propagation artificial neural network (BP ANN) is proposed as a tool for numerical modelling of the constitutive behaviour of a physically non-linear body. Training process of the ANN using experimental data is discussed in details and illustrated with an example. In particular, some difficulties in the constitutive description proposed in consistent, incremental form are discovered and two solutions are proposed to overcome them. Two numerical examples are presented. The first one deals with modelling of elasto-plastic hysteresis, the second shows the application of ANN to approximation of biaxial non-linear behaviour.

In this paper, an algebraic multigrid method is suggested for the fully coupled thermo‐hydro‐mechanical analysis in deforming porous media. The mathematical model consists of balance equations of mass, linear momentum and energy and of the appropriate constitutive equations. The chosen macroscopic field variables are temperature, capillary pressure, gas pressure and displacement. The gas phase is considered to be an ideal gas composed of dry air and vapour, which are regarded as two miscible species. Phase change as well as heat transfer through conduction and convection and latent heat transfer (evaporation‐condensation) are taken into account. The problem hence presents an interaction problem between several fields with very different response characteristics. Further, the matrices are non‐symmetric and not diagonally dominated. Numerical examples are given to demonstrate the efficiency of this method.

Multiphysics phenomena and coupled-field problems usually lead to analyses which are computationally intensive. Strategies to keep the cost of these problems affordable are of special interest. For coupled fluid–structure problems, for instance, partitioned procedures and staggered algorithms are often preferred to direct analysis. In this paper, we describe a new strategy for solving coupled multiphysics problems which is built upon the LArge Time INcrement (LATIN) method. The proposed application concerns the consolidation of saturated porous soil, which is a strongly coupled fluid–solid problem. The goal of this paper is to discuss the efficiency of the proposed approach, especially when using an appropriate time-space approximation of the unknowns for the iterative resolution of the uncoupled global problem. The use of a set of radial loads as an adaptive approximation of the solution during iterations will be validated and a strategy for limiting the number of global resolutions will be tested on multiphysics problems. Copyright © 2003 John Wiley & Sons, Ltd.

Existing tumor growth models based on fluid analogy for the cells do not generally include the extracellular matrix (ECM), or if present, take it as rigid. The three-fluid model originally proposed by the authors and comprising tumor cells (TC), host cells (HC), interstitial fluid (IF) and an ECM, considered up to now only a rigid ECM in the applications. This limitation is here relaxed and the deformability of the ECM is investigated in detail. The ECM is modeled as a porous solid matrix with Green-elastic and elasto-visco-plastic material behavior within a large strain approach. Jauman and Truesdell objective stress measures are adopted together with the deformation rate tensor. Numerical results are first compared with those of a reference experiment of a multicellular tumor spheroid (MTS) growing in vitro, then three different tumor cases are studied: growth of an MTS in a decellularized ECM, growth of a spheroid in the presence of host cells and growth of a melanoma. The influence of the stiffness of the ECM is evidenced and comparison with the case of a rigid ECM is made. The processes in a deformable ECM are more rapid than in a rigid ECM and the obtained growth pattern differs. The reasons for this are due to the changes in porosity induced by the tumor growth. These changes are inhibited in a rigid ECM. This enhanced computational model emphasizes the importance of properly characterizing the biomechanical behavior of the malignant mass in all its components to correctly predict its temporal and spatial pattern evolution.

Shear band dominated process in fully and partially saturated sand samples is simulated by means of dynamic strain localisation analysis together with a multiphase material model. The partially saturated medium is viewed as a multiphase continuum consisting of a solid skeleton and pores filled by water and air (vapour) which, once it appears, is presumed to remain at the constant value of cavitation pressure (isothermal monospecies approach). The governing equations are based on the general framework of averaging theories. A modified generalised plasticity constitutive model for partially saturated soils, developed from the general Pastor–Zienkiewicz sand model, has been implemented in a finite element code and used in the computational process. This model takes into account the effects of suction in the stiffness of the porous medium (solid skeleton) in partially saturated state. A case of strain localisation, which has been tested in laboratory observing cavitation of the pore water, is studied. Negative water pressures, which are of importance in localisation phenomena of initially fully saturated undrained samples of dilatant geomaterials, are obtained similarly to those observed experimentally.

Two different approaches are presented for the parallel implementation of computer programs for the solution of coupled thermohydromechanical problems. One is an asynchronous method in connection with staggered and monolithic solution procedures. The second one is a domain decomposition method making use of substructuring techniques and a Newton-Raphson procedure. The advantages of the proposed methods are illustrated by examples. Both methods are promising, but we actually have no comparison between the two because one works on a linear program with only two interacting fields and the other on a full nonlinear set of (multifield) equations.

Dynamic strain localisation in saturated and partially saturated porous media is investigated with a one-dimensional model in this paper. The porous medium is treated as a multiphase continuum, with the pores filled by water and air, this last one at atmospheric pressure. A gradient-dependent plasticity model is introduced to describe the plastic behaviour of the solid skeleton. Material instability due to the softening behaviour of the solid skeleton and the well-posedness of the initial value problem are studied. The advantages of the enhanced model are that the governing equations remain hyperbolic even in the softening regime and convergent solutions with mesh refinements are obtained. Moreover, the influence of permeability in the seepage process for the development of the localised zones is discussed. We find that the permeability plays an important part in the compressive wave propagation, but not in the shear wave cases. For numerical implementation of the present method, a parametric variational principle is introduced by which the original problem is reduced to a standard linear complementary problem in mathematical programming. The results of a one dimensional example are given to illustrate the efficiency of the techniques presented here.

Non-linear deformable porous media with sorption (capillary condensation) hysteresis are considered. An arti cial neural network with two hidden layers is trained to interpolate the sorption hysteresis using a set of experimental data. The performance of the ANN, which is applied as a procedure in the FE code, is investigated, both from numerical, as well as from physical viewpoint. The ANN-FE code has been developed and tested for 1-D and 2-D problems concerning cyclic wetting–drying of concrete elements. In general, the application of the ANN procedure inside the classical FE program does not have any negative e ect on the numerical performance of the code. The results obtained indicate that the sorption isotherm hysteresis is of importance during analysis of hygrothermal and mechanical behaviour of capillary-porous materials. The most distinct di erences are observed for the saturation and displacement solutions. The ANN-FE approach seems to be an e cient way to take into account the in uence of hysteresis during analysis of hygro-thermal behaviour of capillary-porous materials. Copyright ? 2001 John Wiley & Sons, Ltd.

This paper investigates reservoir compaction and resulting surface subsidence during the exploitation of a gas reservoir and in the period after well shutdown. A substantial difference in behaviour results when assuming elastic and elastoplastic constitutive relationships for the reservoir rock and surroundings. Further, a correction is introduced to account for capillary effects, which strongly modify the behaviour especially after shutdown and may account for the ongoing subsidence observed.

In this paper, a multi-frontal parallel algorithm is developed to solve fully coupled heat, water and gas flow in deformable porous media. The mathematical model makes use of the modified e⁄ective stress concept together with the capillary pressure relationship and takes phase change and latent heat transfer into account. The chosen macroscopic field variables are displacement, capillary pressure, gas pressure and temperature. The parallel program is developed on a cluster of workstations. The PVM (Parallel Virtual Machine) system is used to handle communications among networked workstations. The multi-frontal method has advantages such as numbering of the finite element mesh in an arbitrary manner, simple programming organization, smaller core requirements and shorter computation times. An implementation of this parallel method on workstations is discussed. The speedup and eƒciency of this method is demonstrated and compared with a general domain decomposition method based on band matrix methods by numerical examples. ( 1998 John Wiley & Sons, Ltd.

Dynamic strain localisation theory together with a multiphase material model is used to simulate shear band dominated processes in fully saturated sand samples. The fluid-saturated medium is viewed as multi-phase continuum consisting of a solid skeleton and pores filled by fluids. The governing equations are based on the general framework of averaging theories. A generalised plasticity constitutive model for fully saturated soils is adopted in the computational process. Both samples of medium-loose and dense sands are studied. Negative water pressures, which are important in localisation phenomena of fully saturated dilatant geomaterials, are obtained for dense sands, while positive water pressures result for medium-loose sands.

SUMMARY A fully coupled numerical model has been developed which describes multiphase fluid flow through soil: namely gas, water and a non-aqueous phase liquid (NAPL) in a deforming porous media for subsurface systems. A multiphase flow model, based on the two-phase flow model of Brooks and Corey, is presented to express the dependence of saturation and relative permeability on the capillary pressure. Non-linear saturation and relative permeability functions are incorporated into a Galerkin finite element model which is subsequently used to simulate multiphase immiscible fluid flow under saturated and unsaturated conditions in porous media. The governing partial diAerential equations, in terms of soil displacements and fluid pressures, which are coupled and non-linear, are solved by the finite element method. Numerical implementation of the formulation is discussed, and example problems demonstrate the model and solution procedure. #1998 John Wiley & Sons, Ltd. Commun. Numer. Meth. Engng, 14, 135‐149 (1998).

In this paper we present an efficient tool for simulation of a fire scenario in a tunnel. The strategy adopted is based on a 3D-2D coupling technique between the fluid domain and the solid one. So, the thermally driven CFD part is solved in a three dimensional cavity i.e. the tunnel, and the concrete part is solved on 2D sections normal to the tunnel axis, at appropriate intervals. The heat flux and temperature values, which serve as coupling terms between the fluid and the structural problem, are interpolated between the sections. Between the solid and the fluid domain an interface layer is created for the calculation of the heat flux exchange based on a "wall law". In the analysis of the concrete structures, concrete is treated as a multiphase porous material. Some examples of application of this fully coupled tool will be shown.

SUMMARY It is recalled that negative water pressures are of importance in localisation phenomena of fully saturated undrained samples of dilatant geomaterials. A model to simulate cavitation phenomena connected with such pore water tractions is developed and implemented in a simplified form in a dynamics code for partially saturated porous media. A case of localisation is studied from the onset of the instability up to the full developed shear band. The weak mesh dependence of the maximum effective plastic strain, due to the employed physical model, is also shown.

This paper presents a general model for the analysis of coupled thermo-hydro-mechanical problems in porous media with possible pollutant transport. The governing equations are described and discrete solution techniques using the finite element method in space and finite differences in time are shown. Emphasis is put on a direct solution procedure, where the coupled system of equations is solved without use of matrix partitioning. Both the Newton-Raphson method and fixed point method are employed. Application examples involving pollutant transport, heat and mass transfer in partially saturated geomaterials, dynamic strain localization and durability of concrete show the range of applicability of this model in the field of evironmental engineering.

The solution of contact problems involves great numerical efforts to satisfy non-penetration conditions. The search for numerical efficiency hence has limited the modelling of the real physical interface behaviour. Up to now mainly simple laws, usually formulated using constant coefficients, have been available to study contact problems in uncoupled from. Here a thermomechanically coupled contact element is presented which accounts for the real microscopic shape of the surfaces, the microscopic mechanism of force transmission and heat exchange. The contact element geometrical behaviour has been put together with experimental and theoretical well founded micro-mechanical and micro-thermal laws adapted to Finite Element Method (FEM) necessities. Based on these laws the macroscopic related stiffnesses are calculated and continuously updated taking into account changes in significant parameters. The linearization of the set of equations has been obtained using a consistent technique which implies computational efficiency.

This paper analyses the numerical difficulties commonly encountered in solving fully coupled numerical models and proposes a numerical strategy apt to overcome them. The proposed procedure is based on space refinement and time adaptivity. The latter, which in mainly studied here, is based on the use of a finite element approach in the space domain and a Discontinuous Galerkin approximation within each time span. Error measures are defined for the jump of the solution at each time station. These constitute the parameters allowing for the time adaptivity. Some care is however, needed for a useful definition of the jump measures. Numerical tests are presented firstly to demonstrate the advantages and shortcomings of the method over the more traditional use of finite differences in time, then to assess the efficiency of the proposed procedure for adapting the time step. The proposed method reveals its efficiency and simplicity to adapt the time step in the solution of coupled field problems.

The modeling of the effects of bending on single Nb3Sn strand DC performance (IC, n index) is presented for a bronze-route strand subjected to the same loading conditions as in an experiment performed at JAEA Naka, Japan [Y. Nunoya, et al., IEEE TAS 14 (2004) 1468–1472]. The strand is discretized in strand elements (SE) representing groups of twisted filaments in the bronze matrix, and in portions of the outer Cu annulus, electro-magnetically coupled in the THELMA code. The 3-D strain map in the filament region is computed with a newly developed, detailed thermo-mechanical model accounting for non-linear, temperature dependent material characteristics. With respect to our previous analysis [P.L.Ribani, et al., IEEE TAS 16 (2006) 860–863] several new updated ingredients, besides the new thermo-mechanical model, are used here, including more accurate thermal and mechanical properties for the materials, a jacket-like model for the outer Cu layer, IC and n index (interpolative) scaling from Durham University. The simulation results show an improved agreement with the experiments, in the degradation of the single-strand performance due to bending.

The behaviour of all geomaterials, and in particular of soils, is governed by their interaction with the pore fluid. The mechanical model of this interaction when combined with suitable constitutive discription of the solid phase and with efficient, discrete, computation procedures, allows most transient and static problems involving deformations to be solved. This paper describes the basic procedures and the development of a general purpose computer program (SWANDYNE-X). The results of the computations are validated by comparison with experimental results obtained on physical models tested in the Cambridge Centrifuge.

A novel model of mechanical performance of concrete at early ages and beyond, and in particular, evolution of its strength properties (aging) and deformations (shrinkage and creep strains), described in terms of effective stress is briefly presented. This model reproduces such␣ phenomena known from experiments like drying creep or some additional strains, as compared to pure shrinkage, which appear during autogenous deformations of a maturing, sealed concrete sample. Creep is described by means of the modified microprestress-solidification theory with some modifications to take into account the effects of temperature and relative humidity on concrete aging. Shrinkage strains are modelled by using effective stresses giving a good agreement with experimental data also for low values of relative humidity. Results of four numerical examples based on the real experimental tests are solved to validate the model. They demonstrate its possibilities to analyze both autogenous deformations in maturing concrete, and creep and shrinkage phenomena, including drying creep, in concrete elements of different age, sealed or drying, exposed to external load or without any load.

In this paper a 3D beam-to-beam contact element is presented, to deal with contact problems in the coupled electric mechanical fields. The beams are supposed to get in contact in a pointwise manner, the detection of the contact points and the computation of all contributions are carried out using a fully symmetric treatment of the two beams. Concerning the mechanical field, Hertz theory of contact for elastic bodies is considered. The contact area is varying according to the beamto-beam angle, being circular only in the case of perpendicular beams. This variation of the shape is taken into account too. The problem is semi-coupled: the mechanical field influences the electric one because of the dependence of the voltage distribution on the contact area. Within the finite element discretization, the mechanical and the electric treatment of the beam element is formulated in the usual way, considering nodal displacements and voltages as main unknowns. The electromechanical contact constraints are enforced with the penalty method. Starting from the virtual work equation the consistent linearization of all contributions is computed to achieve the quadratic convergence within the Newton-Raphson iterative scheme. The complete set of equations arranged in a matrix form suitable for the finite element implementation is solved with a monolithic approach. Finally some numerical examples are discussed to show the effectiveness of the model. 104 D.P. Boso, P. Litewka, B.A. Schrefler, and P. Wriggers

A fully coupled model of hygro-thermo-chemo-mechanical phenomena in concrete is presented. A mechanistic approach has been used to obtain the governing equations, by means of the hybrid mixture theory. The final equations are written in terms of the chosen primary and internal variables. The model takes into account coupling between hygral, thermal, chemical phenomena (hydration or dehydration), and material deformations, as well as changes of concrete properties, caused by these processes, e.g. porosity, permeability, stress-strain relation, etc.

Finite element analysis of non-isothermal elasto-plastic multiphase geomaterials is presented. The multiphase material is modelled as a deforming porous continuum where heat, water and gas flow are taken into account. The independent variables are the solid displacements, the capillary and the gas pressure and the temperature. The modified effective stress state is limited by the Drucker-Prager yield surface for simplicity. Small strains and quasi-static loading conditions are assumed. Numerical results of strain localization in globally undrained samples of dense, medium dense and loose sands and isochoric geomaterial are presented. A biaxial compression test is simulated assuming plane strain condition during the computations. Vapour pressure below the saturation water pressure (cavitation) develops at localization in case of dense sands, as experimentally observed. A case of strain localization induced by a thermal load where evaporation takes place is also analysed.

Previous results indicate that fibers in ribbons are sometimes affected by systematic birefringence superimposed on the random one, their relative weights depending on fiber position in the ribbon. We report new theoretical and experimental results on stress distribution in ribbons, which is shown to depend on thermal and mechanical properties of the common coating. Numerical simulations are based on the theory of elasticity and the finite-element method (FEM). Polarization dispersion measurements vs. temperature match very well with numerical results, and indicate that central fibers in the ribbon exhibit significantly larger birefringence than lateral ones.

The progress in advanced technology fields requires more and more sophisticated formulations to consider contact problems properly. This paper is devoted to the development of a new constitutive model for electrostatic-mechanical contacts, based on a micro–macro approach to describe the contact behaviour. The electric-mechanical contact constitutive law is obtained considering the real microscopic shape of the contacting surfaces, the microscopic behaviour of force transmission and current flow. Some thermo-mechanical macroscopic models based on microscopic characterizations have already been developed to compute the normal and tangential contact stiffness and the thermal contact resistance. On the basis of such macroscopic models, a similar model, suitable for the electric-mechanical field, is developed. With reference to the thermal constriction resistance the electric contact resistance is studied, assuming a flux tube around each contacting asperity, and choosing a suitable geometry for its narrowing at the contact zone. The contact element geometry is based on well known theoretical and experimental micro-mechanical laws, suitably adapted for the FEM formulation. The macroscopic stiffness matrix is calculated on the basis of the microscopic laws and it is continuously updated as a function of the changes in the mechanical and electric significant parameters. A consistent linearization of the set of equations is developed to improve the computational speed, within the framework of implicit methods. Copyright © 2005 John Wiley & Sons, Ltd.

A theory of microplasticity for anisotropic elastoplastic body is used for phenomenological description and analysis of superconducting cables under transversal loading. It reflects the possibility to have hysteresis loops on the curve of loading and unloading on any level of loading including small ones. It reflects further the fact that these hysteresis loops don't depend on the strain rate. At last it reflects phenomenological anisotropy which is a characteristic property of the superconducting cable, because latter is produced of a large number of strands, oriented along the cable axis. The problem of identification of the model parameters is formulated and solved.

In the present concept of ITER fusion reactor the toroidal field and the central solenoid coils are made of Nb 3 Sn based strands with the cable-in-conduit-conductor (CICC) technology. It is well known that the critical parameters of the Nb 3 Sn strand material are strain sensitive; experimental investigations on short samples of basic strands and subsize CICC cables already demonstrated significant effects of residual strain on the critical parameters. In this paper a method is proposed to analyse in detail the thermal strain induced by the cool down from the strand reaction temperature to the coil working conditions. The superconducting strand can be regarded as a very good example of a hierarchical structure, since there is a clear distinction between the micro-scale of the Nb 3 Sn filaments, the meso-scale of the SC filament groups and the macro-scale of the strand, where it can be regarded as homogeneous. A constitutive relation for the homogenised micro- and meso-components is deduced from the knowledge of the respective internal structures, starting from an accurate description of the single representative cells. This two-scales homogenisation technique is associated with an efficient finite element procedure for computing effective material coefficients to be used with standard orthotropic 3D elements in structural codes. Finally the finite elements routines developed for the unsmearing process provide the real stress and strain values over each single material, which are essential to catch the local features needed for engineering design.

International audience

National audience; Une nouvelle stratégie, basée sur la méthode LATIN, a été développée pour résoudre les problèmes multiphysiques et appliquée avec succès à la consolidation d'un milieu poreux saturé. Une version multiéchelle en temps a ensuite été présentée afin de prendre en compte les échelles introduites par les différentes physiques. Ici, on propose une approximation de toutes les inconnues sous la forme d'une superposition de champs radiaux.

International audience

The interaction of the cutting tools with different geological settings determines a different wear and consequently different economical costs for the excavation. We apply here a strategy for soil modelling which is based on dis- cretization of the continuum with rigid disks and suitable contact models. We concentrate at contact level the real mechanical behaviour of the soil. In order to carry out this strategy a ”micro” and a ”macro” level are established. This paper focuses on the micromechanical model.

Cables where several fibers (typically, 4-16) are collected in a ribbon allow cost reduction and mass splicing. Their wide use in the access network and, in perspective, in long-haul links, has drawn attention on all factors affecting their performance. Polarization mode dispersion (PMD) in fibers embedded in ribbons has already been investigated, but with contradictory results. All previous results appear to indicate that birefringence in fibers belonging to ribbons can be significantly affected by stress distribution in the ribbon itself, which depends on thermal and mechanical properties of the coating. We report new theoretical and experimental results, which reconfirm this conjecture. Birefringence is shown to depend strongly on the fiber position in the ribbon. Theoretical results are based on a finite-element-method numerical simulation, based on the theory of elasticity, and apply over the temperature range (15-31/spl deg/C) where viscosity in all materials can be ignored. Central fibers are found to be under compression in the ribbon plane, while lateral fibers do not experience significant compression, as the ribbon coating is softer than silica. Birefringence can be estimated in a simple way from the stress distribution. To validate the predictions experimentally, PMD measurements were performed on a 4-fiber ribbon.

This paper presents first the applications of uniqueness and strain localization analysis of saturated porous media, where localization of deformation into well defined narrow zones in a saturated porous medium is studied in terms of discontinuous bifurcation theory. A generalized plasticity constitutive model and a Mohr–Coulomb model are used in both the theoretical and numerical analyses of shear band formations. The critical hardening moduli and shear band angle for localization are computed, and quantitative results are given for both constitutive models. Numerical results previously obtained and new ones are confirmed by this analytical and numerical investigation. Copyright © 2002 John Wiley & Sons, Ltd.

International audience; Multiphysics phenomena and coupled-field problems usually lead to computationally intensive structuralanalyses. Strategies to keep these problems computationally affordable are of special interest. Forcoupled fluid-structure problems for instance, partitioned procedures and staggered algorithms are oftenpreferred to direct analysis (also called the monolithic approach), from a computational efficiency pointof view. Recently, a mixed domain decomposition method has been designed for parallel computing environments, and a multi-level pproach embedding a homogenization procedure makes it suitable for highlyheterogeneous problems. From the generalization of the concept of geometric interfaces between substructures to an interface between different physics, the Large Time INcrement method (LATIN) allows building an approach suited for solving coupled multiphysics problems.The proposed application concerns the consolidation of porous saturated soil, i.e. a coupled fluid-solid problem in the domain. The feasability of the method and its performance comparison with a standard partitioning scheme (the so-called ISPP) has been presented in a previous paper. As an improvement, the further step is to take into account different time scales arising from multiphysics problem. Thus, the present paper proposes a time multiscale strategy.

International audience