Showing papers by "A.H. van den Boogaard published in 2008"

An optimisation strategy for industrial metal forming processes

Abstract: Product improvement and cost reduction have always been important goals in the metal forming industry. The rise of finite element (FEM) simulations for processes has contributed to these goals in a major way. More recently, coupling FEM simulations to mathematical optimisation techniques has shown the potential to make a further giant contribution to product improvement and cost reduction. Much research on the optimisation of metal forming processes has been published during the last couple of years. Although the results are impressive, the optimisation techniques are generally only applicable to specific optimisation problems for specific products and specific metal forming processes. As a consequence, applying optimisation techniques to other metal forming problems requires a lot of optimisation expertise, which forms a barrier for more general industrial application of these techniques. In this paper, we overcome this barrier by proposing a generally applicable optimisation strategy that makes use of FEM simulations of metal forming processes. It consists of a structured methodology for modelling optimisation problems related to metal forming. Subsequently, screening is applied to reduce the size of the optimisation problem by selecting only the most important design variables. Finally, the reduced optimisation problem is solved by an efficient optimisation algorithm. The strategy is generally applicable in a sense that it is not constrained to a certain type of metal forming problem, product or process. Also, any FEM code may be included in the strategy. Furthermore, the structured approach for modelling and solving optimisation problems should enable non-optimisation specialists to apply optimisation techniques to improve their products and processes. The optimisation strategy has been successfully applied to a hydroforming process, which demonstrates the potential of the optimisation of metal forming processes in general and more specific the proposed optimisation strategy.

Time reduction in implicit single point incremental sheet forming simulation by refinement - derefinement

Abstract: This paper presents the implementation of a refinement - derefinement (RD) approach to reduce the computing time in single point incremental sheet forming (SPIF) simulation. The results of this approach are compared to a reference model that has a fine enough mesh to satisfy the process requirements. The fine mesh is created by refining an intermediate coarse mesh entirely. The RD approach performs one level of refinement–derefinement on the same intermediate coarse mesh based on a geometrical error indicator. The integration point data is mapped by a least squares method. The simulation of forming a 45 degree pyramid is considered as reference test. The refinement–derefinement approach reduces the computing time for the reference model by 50%. The achieved equivalent plastic strain by this approach shows a good agreement with the reference model.

Tensile tests with bending: a mechanism for incremental forming.

Abstract: In incremental sheet forming (ISF) large uniform strains can be obtained well above the common forming limit. Bending-under-tension has been proposed as a possible mechanism. This paper describes tension tests with repetitive bending to simulate this effect. Indeed very large levels of uniform elongation have been obtained, up to 300 %. The maximum strain increases with decreasing bending radius, and is reached at a certain optimum pulling speed. The material hardening seems little affected by the cyclic bending operations, and a hardening curve for large strains could be constructed.

Experimental validation of numerical sensitivities in a deep drawing simulation

Abstract: Deep drawing of a benchmark B-pillar is numerically modelled and experimentally performed with varying blankholder force and several blank shape parameters. The most influential parameters are selected for optimisation. Direct application of Autoform sigma software was used to determine sensitivities, as well as indirect application using response surfaces. Interesting nonlinear sensitivities were found that will be missed with simple linear screening techniques.

Modelling of dynamic strain aging with a dislocation-based isotropic hardening model and investigation of orthogonal loading

Abstract: Based on experimental results, a dislocation material model describing the dynamic strain aging effect at different temperatures is presented. One and two stage loading tests were performed in order to investigate the influence of the loading direction as well as the temperature influence due to the hardening mechanism. Bergstrom’s theory of work hardening was used as a basis for the model development regarding the thermal isotropic behavior as well as the Chaboche model to describe the kinematic hardening. Both models were implemented in an in-house FE-Code in order to simulate the real processes. The present paper discusses two hardening mechanisms, where the first part deals with the pure isotropic hardening including dynamic strain aging and the second part involves the influence of the loading direction regarding combined (isotropic and kinematic) hardening behavior.

Measurements and calculations on yield surfaces in tension-simple shear experiments

Abstract: For simulations of sheet metal forming processes a yield criterionwellsuitedtotheused material is required to obtain accurate results. In this article results of orthogonal strain path changes on mildsteelarepresented, bothexperimentalaswithsimulations, showing the shape of the yield locus. For the experiments a biaxial tester is used that loads a specimen on simple shear and plane strain tension in any arbitrary amount. Tests are performed with a two stage strain path; rst plane strain tension followed by simple shear. In the transition from tension to shear the stress state translates over the yield surface, hereby indicating the local shape of the yield surface. The experiments are simulated with the Vegter yield function, to investigate the correspondence of the material model and the actual response of the material. It shows that the in!uence of strain path changes is relatively strong, even with lower amounts of prestrain the actual response of the material could not be simulated with a regular hardeningmodel

A dislocation based constitutive model for warm forming of aluminum sheet

Abstract: The formability of aluminum sheet can be improved considerably by increasing the temperature. At elevated temperatures, the mechanical response of the material becomes strain rate dependent. To accurately simulate warm forming of aluminum sheet, a material model is required that incorporates the temperature and strain rate dependency. In this paper, the dislocation based Alflow hardening model is used. The model incorporates the influence of the temperature and strain rate effect on the flow stress by means of the storage and dynamic recovery of dislocations. It also includes the effects of solute level, particle fraction and grain size. Cylindrical cup deep drawing simulations are presented using shell elements. The anisotropic behavior of the sheet is described by using the Vegter yield locus. Experimental drawing test data is used to validate the modeling approach, where the model parameters follow from tensile tests.