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See detailSimulation of crashworthiness problems with implicit time integration methods for non-linear dynamics
Noels, Ludovic ULg; Stainier, Laurent; Ponthot, Jean-Philippe ULg

Scientific conference (2005, February)

When studying impact problems, time integration of the equations of evolution occurs in the non-linear range. Usually, explicit algorithms are used in such a context. Nevertheless, due to its lack of ... [more ▼]

When studying impact problems, time integration of the equations of evolution occurs in the non-linear range. Usually, explicit algorithms are used in such a context. Nevertheless, due to its lack of stability in the non-linear range, and its limitation in the time step size, an implicit scheme could advantageously be used. The most widely used implicit algorithm is the Newmark algorithm. Nevertheless, with this algorithm used in the non-linear range, the conservation of the energy is no longer satisfied. To avoid divergence due to the numerical instabilities, numerical damping was introduced, leading to the generalized- methods. But these schemes can exhibit instabilities in the non-linear range too. Therefore a new family of integration algorithms for problems in structural dynamics has appeared that satisfies the mechanical laws of conservation (i.e. conservation of linear momentum, angular momentum and total energy) and that remains stable in the non-linear range. The first algorithm verifying these properties was described by Simo and Tarnow. They called this algorithm Energy Momentum Conserving Algorithms or EMCA. It consists in a mid-point scheme with an adequate evaluation of the internal forces. This adequate evaluation was given for a Saint Venant-Kirchhoff hyperelastic material. A generalization to other hyperelastic models was given by Laursen, who iteratively solved a new equation for each Gauss point to determine the adequate second Piola-Kirchhoff stress tensor. Another solution that avoids this iterative procedure leads to a general formulation of the second Piola-Kirchhoff stress tensor, was given by Gonzalez. This formulation is valid for general hyperelastic materials. The EMCA was recently extended to dynamic finite deformation plasticity by Meng and Laursen. Armero and Romero also introduced numerical dissipation in these conserving algorithms. This dissipation only affects the total energy but preserves the angular momentum. Moreover, it is proved to be stable in the non-linear range, contrarily to the generalized-algorithms. It is called Energy Dissipative Momentum Conserving algorithm or EDMC. Besides, Armero and Petöcz proposed a treatment of contact interactions in a consistent way in the non-linear range. All the conserving methods described above were established for hyperelastic materials. We have recently established a new expression of the internal forces for the hypoelastic materials using the final rotation scheme. When associated with the mid-point scheme, this expression ensures the conservation laws of the mechanics for a hypoelastic constitutive model. Moreover, using the radial return mapping, we prove that this adaptation remains consistent with the Drucker postulate when plastic deformation occurs. In this work we introduce numerical dissipation in a consistent way for such hypoelastic constitutive models. Moreover, we propose a method to enhance the contact simulation proposed by Armero and Petöcz to surfaces with discontinuous normal as is the case when the two bodies in contact are deformable and are thus discretized by finite elements. With such improvements, we are able to simulate complex simulations of impact such as a blade-loss in a turbo engine. [less ▲]

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See detailLarge strain elasto/viscoplastic constitutive model. Theory and numerical scheme
Ponthot, Jean-Philippe ULg; GARCIA-GARINO, C.; MIRASSO, A.

in MECOM 2005, Mecanica Computational (2005)

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See detailOptimisation methods for initial/tool shape optimisation in metal forming processes
Ponthot, Jean-Philippe ULg; Kleinermann, Jean-Pascal

in International Journal of Vehicle Design (2005), 39(1-2), 14-24

Nowadays, the computer simulations of metal forming processes using the finite element method (FEM), have reached some level of maturity. Inverse problems purpose is to determine one or more of these ... [more ▼]

Nowadays, the computer simulations of metal forming processes using the finite element method (FEM), have reached some level of maturity. Inverse problems purpose is to determine one or more of these forming processes simulations input data, leading to a desired result. A first example that has now become classical is called parameter identification. It consists in evaluating the material parameters for material behaviour laws that would lead to the most accurate model, minimising the difference between experimental and numerical results. Another example, which is much less extensively described in the literature is the initial geometry and tool shape design, which consists in determining the initial shape of the specimen and/or the shape of the forming tools, in order to provide the desired final geometry after forming process. In this paper, we will show how some optimisation methods are able to solve this shape optimisation problems. We will also compare the efficiency of the proposed numerical methods. [less ▲]

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See detailSimulation of Complex Crashworthiness Problems by Using a Combined Implicit/Explicit Time Integration.
Noels, Ludovic ULg; Stainier, Laurent ULg; Ponthot, Jean-Philippe ULg

(2005)

In this work we will combine the ®-generalized explicit time integration scheme with the Energy-Dissipating Momentum-Conserving (EDMC) implicit time-integration scheme.

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See detailA variational framework for nonlinear viscoelastic models in finite deformation regime
Fancello, E. A.; Ponthot, Jean-Philippe ULg; Stainier, Laurent ULg

in Journal of Computational & Applied Mathematics (2005), 215(2), 400-408

This work presents a general framework for constitutive viscoelastic models in the finite deformation regime. The approach is qualified as variational since the constitutive updates consist of a ... [more ▼]

This work presents a general framework for constitutive viscoelastic models in the finite deformation regime. The approach is qualified as variational since the constitutive updates consist of a minimization problem within each load increment. The set of internal variables is strain-based and uses a multiplicative decomposition of strain in elastic and viscous components. Spectral decomposition is explored in order to accommodate, into analytically tractable expressions, a wide set of specific models. Moreover, it is shown that, through appropriate choices of the constitutive potentials, the proposed formulation is able to reproduce results obtained elsewhere in the literature. Finally, numerical examples are included to illustrate the characteristics of the present formulation. (C) 2007 Elsevier B.V. All rights reserved. [less ▲]

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See detailA variational formulation of constitutive models and updates in nonlinear finite viscoelasticity
FANCELLO, E.; Stainier, Laurent ULg; Ponthot, Jean-Philippe ULg

in 7ème Colloque National en calcul des Structures (2005)

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See detailAnalyse numérique des tests expérimentaux à grande vitesse de déformation
Jeunechamps, Pierre-Paul ULg; Ponthot, Jean-Philippe ULg; BOUVIER, S. et al

in 7ème Colloque National en calcul des Structures (2005)

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See detailA variational framework for nonlinear viscoelastic and viscoplastic models in finite deformation regime
Stainier, Laurent ULg; FANCELLO, E.; Ponthot, Jean-Philippe ULg

in MECOM 2005, , Mecanica Computational (2005)

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See detailHeat generation due to friction and plastic dissipation – a coupled thermomechanical approach for impact and metal forming simulation
Adam, L.; Ponthot, Jean-Philippe ULg

in Proceedings of Int. Conf. on Computational Methods for Coupled Problems in Science and Engineering – COUPLED PROBLEMS 2005 (2005)

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See detailFinite element simulation of cold roll-forming processes
BUI, Q. V.; Ponthot, Jean-Philippe ULg

in ACOMEN 2005 (2005)

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See detailNumerical simulation of springback using enhanced assumed strain elements
Bui, Q. V.; Papeleux, Luc ULg; Ponthot, Jean-Philippe ULg

in Journal of Materials Processing Technology (2004), 153(1), 314-318

The quality of springback prediction for a sheet metal forming process depends on a precise estimate of the elasto-plastic stress distribution throughout the metal sheet. The use of low-order conventional ... [more ▼]

The quality of springback prediction for a sheet metal forming process depends on a precise estimate of the elasto-plastic stress distribution throughout the metal sheet. The use of low-order conventional finite elements may be, without any proper treatment, responsible for low quality prediction because of volumetric and shear lockings. In this study, the enhanced assumed strain technique will be exploited for locking removal. The quality of the numerical simulation is evaluated through a comparison with other popular techniques like selective and uniform reduced integration. In contrast to the latter, and thanks to a full numerical integration scheme, the enhanced assumed strain element is very efficient in accurately capturing the development of plastic flow. This enables a reliable prediction of springback even with a rather coarse mesh. (C) 2004 Elsevier B.V. All rights reserved. [less ▲]

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See detailEfficient algorithmic approach for mixed-lubrication in cold rolling
Stephany, A.; Ponthot, Jean-Philippe ULg; Collette, C. et al

in Journal of Materials Processing Technology (2004), 153(1), 307-313

A model of cold rolling taking into account lubrication in mixed mode was developed. The main objective is to obtain a numerical tool that permits us to optimise a mill configuration from the lubrication ... [more ▼]

A model of cold rolling taking into account lubrication in mixed mode was developed. The main objective is to obtain a numerical tool that permits us to optimise a mill configuration from the lubrication point of view. It means that lubricant viscosity but also roll diameter and roughness, rolling diagrams, etc could be modified to improve stability and efficiency of the mill. A modified slab method is solved iteratively and is coupled with a sophisticated tribological model. A general calculation of the modified Reynolds equation is presented in order to evaluate the fluid pressure for any piezoviscosity law. The coupling is made through the calculation of fluid thickness and friction along the roll bite. The evolution of the cylinder/strip contact area due to the asperity crushing along the roll bite is also considered. A parametric study has been undertaken in order to determine the influence of various process parameters on friction and lubricant flow. Applications to industrial cases obtained from Cockerill Sambre plant will show the applicability of the method. (C) 2004 Elsevier B.V. All rights reserved. [less ▲]

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See detailFinite Element Simulation of Lubricated Contact in Rolling using the Arbitrary Lagrangian-Eulerian Formulation
Boman, Romain ULg; Ponthot, Jean-Philippe ULg

in Computer Methods in Applied Mechanics and Engineering (2004), 193(39-41), 4323-4353

In this paper, the lubrication problem in numerical simulation of forming processes is considered. Two efficient convection algorithms are presented in order to update the value stored at the Gauss points ... [more ▼]

In this paper, the lubrication problem in numerical simulation of forming processes is considered. Two efficient convection algorithms are presented in order to update the value stored at the Gauss points during the Eulerian step of an arbitrary Lagrangian Eulerian computation in solid mechanics. They are based on the finite volume method and on the streamline upwind Petrov Galerkin method. After enumerating the difficulties encountered when trying to solve the lubrication problem with the finite element method, a generalization of the formulation introduced by Liu [Simulation of Ring Rolling Process by Arbitrary Lagrangian Eulerian Finite Element Method, vol. 20, p. 225; ASME Winter Meeting, Dallas, Tx, 1990, Int. J. Numer. Methods Engrg. 36 (1993) 855; Numerical Methods in Industrial Forming Processes, Valbonne, France, 1992, p. 731] for the thin film hydrodynamic lubrication regime is presented. This method is then applied to a strip rolling simulation, using the arbitrary Lagrangian Eulerian formalism. (C) 2004 Elsevier B.V. All rights reserved. [less ▲]

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See detailCombined implicit/explicit time-integration algorithms for the numerical simulation of sheet metal forming
Noels, Ludovic ULg; Stainier, Laurent ULg; Ponthot, Jean-Philippe ULg

in Journal of Computational & Applied Mathematics (2004), 168(1-2, Sp. Iss. SI), 331-339

In order to simulate stamping processes, an explicit method, which is conditionally stable, is generally thought to be the most adapted. Such an algorithm presents the advantage of being noniterative ... [more ▼]

In order to simulate stamping processes, an explicit method, which is conditionally stable, is generally thought to be the most adapted. Such an algorithm presents the advantage of being noniterative while, the contact configuration evolves rapidly, and the conditional stability is not a disadvantage since time steps must be small enough anyway for an accurate computation. But during the springback simulation, an implicit method, which is iterative, presents the advantage of unconditional stability. The optimal solution is then to have both implicit and explicit methods readily available in the same code and to be able to switch automatically from one to the other. Criteria that decide to switch from a method to another, depending on the current dynamic, have been developed. Implicit restarting conditions are also proposed that annihilate numerical oscillations resulting from an explicit calculation. (C) 2003 Elsevier B.V. All rights reserved. [less ▲]

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See detailOn the use of large time steps with an energy momentum conserving algorithm for non-linear hypoelastic constitutive models
Noels, Ludovic ULg; Stainier, Laurent ULg; Ponthot, Jean-Philippe ULg

in International Journal of Solids and Structures (2004), 41(3-4), 663693

This paper presents an extension of the energy momentum conserving algorithm, developed by the authors for hypoelastic constitutive models. For such a material, contrarily to hyperelastic models, no ... [more ▼]

This paper presents an extension of the energy momentum conserving algorithm, developed by the authors for hypoelastic constitutive models. For such a material, contrarily to hyperelastic models, no potential can be defined, and thus the conservation of the energy is ensured only if the elastic work of deformation can be restored by the scheme. In a previous paper, we proposed a new expression of internal forces at the finite element level which is shown to verify this property. We also demonstrated that the work of plastic deformation is positive and consistent with the material model. In this paper, the second order terms that were neglected in the previous work are now taken into account. Several numerical applications are presented to demonstrate the necessity of taking these terms into account once large time step sizes are used. The limitations of the Newmark algorithm in the non-linear range are also illustrated. (C) 2003 Elsevier Ltd. All rights reserved. [less ▲]

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See detailCombined implicit/explicit algorithms for crashworthiness analysis
Noels, Ludovic ULg; Stainier, Laurent ULg; Ponthot, Jean-Philippe ULg

in International Journal of Impact Engineering (2004), 30(8-9, SEP-OCT), 1161-1177

In order to simulate an industrial process, an explicit method, which is conditionally stable, is the most adapted while the non-linearities evolve rapidly (impact phase, stamping process, etc.). But when ... [more ▼]

In order to simulate an industrial process, an explicit method, which is conditionally stable, is the most adapted while the non-linearities evolve rapidly (impact phase, stamping process, etc.). But when the dynamics becomes quasi-linear (post-impact analysis, springback simulation, etc.), an implicit method, which is iterative, presents the advantage of unconditional stability. The optimal solution is then to have both implicit and explicit methods readily available in the same code and to be able to switch automatically from one to the other. Criteria that decide to switch from one method to another, depending on the current dynamics, have been developed. Implicit restarting conditions are also proposed that annihilate numerical oscillations resulting from an explicit calculation. (C) 2004 Elsevier Ltd. All rights reserved. [less ▲]

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See detailAn energy-momentum conserving algorithm for non-linear hypoelastic constitutive models
Noels, Ludovic ULg; Stainier, Laurent ULg; Ponthot, Jean-Philippe ULg

in International Journal for Numerical Methods in Engineering (2004), 59(1), 83-114

This paper presents an extension of the energy momentum conserving algorithm, usually developed for hyperelastic constitutive models, to the hypoelastic constitutive models. For such a material no ... [more ▼]

This paper presents an extension of the energy momentum conserving algorithm, usually developed for hyperelastic constitutive models, to the hypoelastic constitutive models. For such a material no potential can be defined, and thus the conservation of the energy is ensured only if the elastic work of the deformation can be restored by the scheme. We propose a new expression of internal forces at the element level which is shown to verify this property. We also demonstrate that the work of plastic deformation is positive and consistent with the material model. Finally several numerical applications are presented. Copyright (C) 2003 John Wiley Sons, Ltd. [less ▲]

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