Finite element simulation of springback in sheet metal forming

in Journal of Materials Processing Technology (2002), 125(Sp. Iss. SI), 785-791

Although finite element analysis (FEA) is successful in simulating complex industrial sheet forming operations, the accurate and reliable application of this technique to springback has not been widely ... [more ▼]

Although finite element analysis (FEA) is successful in simulating complex industrial sheet forming operations, the accurate and reliable application of this technique to springback has not been widely demonstrated. Several physical parameters, as well as numerical, influence this phenomenon and its numerical prediction. In this paper, we investigate the impact of these parameters on the springback appearing in a 2D U-draw bending. (C) 2002 Elsevier Science B.V. All rights reserved. [less ▲]

Prédiction numérique de l’angle de surformage en profilage à froid

Report (2002)

Actes du Congrès ACOMEN 2002, Second international Conference on Advanced Computational Methods in Engineering, Liège, Belgium, May 28-31, 2002.

Book (2002)

Détermination automatique de la taille du pas de temps pour les schémas implicites en dynamique non-linéaire

in Mécanique & Industries (2002), 3(1), 63-77

Pour les problèmes caractérisés par de fortes non-linéarités, ainsi que des phénomènes d'impacts et de contacts, une stratégie d'intégration à pas de temps variables est particulièrement intéressante. Ces ... [more ▼]

Pour les problèmes caractérisés par de fortes non-linéarités, ainsi que des phénomènes d'impacts et de contacts, une stratégie d'intégration à pas de temps variables est particulièrement intéressante. Ces phénomènes sont par exemple rencontrés lors de l'étude dynamique d'une interaction aube-carter d'un moteur d'avion, le cas le plus critique étant la perte de l'aube. Une stratégie d'intégration implicite à pas de temps constant donne rarement satisfaction du fait qu'il est pratiquement impossible de déterminer une durée de pas qui ne conduise pas à la divergence ou à un coût de calcul prohibitif. Une gestion automatique du pas de temps, qui tient compte de l'histoire récente des accélérations dans le corps considéré, est proposée. En fait, l'algorithme est basé sur la mesure de l'erreur d'intégration des équations d'équilibre. Cela permet d'intégrer correctement les phénomènes transitoires avec un pas de temps très long (en régime) ou très petit (lors de la perte d'aube), en garantissant une bonne précision en un temps de calcul raisonnable. De plus, un algorithme qui décide automatiquement de recalculer ou non, la matrice hessienne est proposé. Cet algorithme permet d'éviter un nombre important de remises à jour de cette matrice, ce qui permet de réduire le coût de calcul tout en assurant la convergence. Enfin, un critère de divergence des itérations est proposé. Afin d'illustrer l'efficacité des algorithmes développés, des simulations numériques sont présentées. Il s'agit aussi bien de problèmes académiques que de problèmes industriels (contacts aubes carter). [less ▲]

Self-adapting time integration management in crash-worthiness and sheet metal forming computations

in International Journal of Vehicle Design (2002), 30(1-2), 67-114

Variable step strategies are especially well suited to deal with problems characterized by high non-linearity and contact/impact, and resolved with an implicit scheme. Both phenomena are typical of ... [more ▼]

Variable step strategies are especially well suited to deal with problems characterized by high non-linearity and contact/impact, and resolved with an implicit scheme. Both phenomena are typical of dynamic simulations of contact-impact problems, as well as sheet metal forming. Constant step size strategies do not give a satisfactory answer for this kind of problem, since it is very difficult, if not impossible, for the user to find an appropriate time step that does not lead to divergence nor generate extremely costly computations. An automatic time stepping algorithm is proposed, which takes into account the recent history of accelerations in the deformable bodies under consideration. More precisely, the adaptation algorithm is based on estimators of the integration error of the differential dynamic balance equations. This allows for adaptation of the step size to capture correctly the transient phenomena, with characteristic times which can range from relatively long (after contact, or during sheet metal forming) to very short (during contact-impact). thus ensuring precision while keeping the computation cost to a minimum. Furthermore, we will see that this strategy can be used in explicit schemes. Additionally, the proposed algorithm automatically takes decisions regarding the necessity of updating the tangent matrix or stopping the iterations, further reducing the computational costs especially, when the Augmented Lagrangian method is used. As an illustration of the capabilities of this algorithm, several numerical simulations (shock absorber devices for vehicle crash-worthiness or sheet metal forming) problems will be presented. Other simulations pertaining to the sheet metal forming for vehicle structures will also be investigated, thus demonstrating the versatility, the capabilities and the efficiency of the proposed strategy. [less ▲]

Implicit-explicit time integration algorithms for the numerical simulation of blade-casing interactions

in Rajendran, A. M.; Jones, B. N.; Brebbia, C. A. (Eds.) Structures under Shock and Impact VII (2002)

In order to solve fast dynamic problems, an explicit method is the most adapted. But for slower dynamics, an implicit method is more stable. Typical industrial problems are governed by high frequency, e.g ... [more ▼]

In order to solve fast dynamic problems, an explicit method is the most adapted. But for slower dynamics, an implicit method is more stable. Typical industrial problems are governed by high frequency, e.g. impact, during short time intervals and slower dynamics, e.g. spring-back, during other time intervals. The optimal solution is then to have both implicit algorithm and explicit methods readily available in the same code and to be able to switch automatically from one to the other. Criteria that decide when to shift from a method to another have been developed here. Implicit balanced restarting conditions that annihilate numerical oscillations resulting for an explicit calculation are also proposed. [less ▲]

Unified stress update algorithms for the numerical simulation of large deformation elasto-plastic and elasto-viscoplastic processes

in International Journal of Plasticity (2002), 18(1), 91-126

This paper is concerned with unified stress update algorithms for elastoplastic and elasto-viscoplastic constitutive equations for metals submitted to large deformations. We present here a newly developed ... [more ▼]

This paper is concerned with unified stress update algorithms for elastoplastic and elasto-viscoplastic constitutive equations for metals submitted to large deformations. We present here a newly developed time integration algorithm which is, in the case of J2 flow theory material behavior, an extension to the viscoplastic range of the classical radial return algorithm for plasticity. The resulting unified implicit algorithm is both efficient and very inexpensive. Moreover, if there is no viscosity effect (rate-independent material) the presented algorithm degenerates exactly into the classical radial return algorithm for plasticity. (C) 2002 Elsevier Science Ltd. All rights reserved. [less ▲]

Optimization Methods for Inverse Problems in Metal Forming Simulation

in STEELS-Mechanical Technology (2001)

Optimization methods for inverse modeling of forming processes

in Habraken, Anne (Ed.) Proceedings of the Fourth International ESAFORM Conference on Material Forming (2001)

Augmented Lagrangian procedure for implicit computation of contact-impact between deformable bodies.

in International Journal of Crashworthiness (2001), 6(2), 209-221

This paper shows how efficiency can be improved by using an adequate Augmented Lagrangian procedure instead of the classical and well-known Penalty method for solving contact-impact problems between ... [more ▼]

This paper shows how efficiency can be improved by using an adequate Augmented Lagrangian procedure instead of the classical and well-known Penalty method for solving contact-impact problems between deformable bodies, including frictional contact, large deformations, dynamical effects and inelasticity phenomena The Augmented Lagrangian method has already enjoyed great success in solving constrained minimisation problems or incompressibility conditions. Alternatives to existing automation techniques for augmentations are presented. Starting from a Penalty method, it will be seen how the Augmented Lagrangian decreases ill-conditioning of governing equations and gives a more precise solution with a lower CPU-cost. Several original simultaneous criteria are proposed for optimising the number and the location of the augmentations in an incremental implicit resolution. Application of the method is done for two axisymmetric impact problems. [less ▲]