Improvements in FE-analysis of real-life sheet metal forming

An overview will be presented of recent developments concerning the application\ud and development of computer codes for numerical simulation of sheet metal forming\ud processes. In this paper attention is paid to some strategies which are followed to improve the\ud accuracy and to reduce the computation time of a finite element simulation. Special attention\ud will be paid to the mathematical modeling of the material deformation and friction, and the\ud effect of these models on the results of simulations. An equivalent drawbead model is\ud developed which avoids a drastic increase of computation time without significant loss of\ud accuracy. The real geometry of the drawbead is replaced by a line on the tool surface. When\ud an element of the sheet metal passes this drawbead line an additional drawbead restraining\ud force, lift force and a plastic strain are added to that element. A commonly used yield\ud criterion for anisotropic plastic deformation is the Hill yield criterion. This description is not\ud always sufficient to accurately describe the material behavior. This is due to the\ud determination of material parameters by uni-axial tests only. A new yield criterion is\ud proposed, which directly uses the experimental results at multi-axial stress states. The yield\ud criterion is based on the pure shear point, the uni-axial point, the plane strain point and the\ud equi-biaxial point

Modelling of Dynamic Strain Aging with a Dislocation-Based Isotropic Hardening Model and Investigation of Orthogonal Loading

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

A nonlinear dynamic corotational finite element model for submerged pipes

A three dimensional finite element model is built to compute the motions of a pipe that is being laid on the seabed. This process is geometrically nonlinear, therefore co-rotational beam elements are used. The pipe is subject to static and dynamic forces. Static forces are due to gravity, current and buoyancy. The dynamic forces exerted by the water are incorporated using Morison's equation. The dynamic motions are computed using implicit time integration. For this the Hilber-Hughes-Taylor method is selected. The Newton-Raphson iteration scheme is used to solve the equations in every time step. During laying, the pipe is connected to the pipe laying vessel, which is subject to wave motion. Response amplitude operators are used to determine the motions of the ship and thus the motions of the top end of the pipe

Tool Texturing for Deep Drawing Applications

The application of surface texturing on sheet metal is a widely used approach to improve lubrication and control friction in deep drawing applications. However, it has been shown that current texturing processes are not robust to produce uniform textures on the sheet due to rapid and severe wear on texture-rolls. Furthermore, in multi-stage forming processes, deterioration of the sheet texture even at the first stage of forming makes texturing of the sheet metal surface ineffective. Tool surface texturing is a new method to control friction and tool wear in metal forming industry. In the current study, a multi-scale friction model is adopted to investigate the effect of tool texturing on the evolution of friction during sheet metal forming operations. The multi-scale friction model accounts for surface topography changes due to deformation of asperities and ploughing of tool asperities on the sheet metal surface, mixed lubrication regime and furthermore the tool micro-texture effects on lubricant distribution at tool-sheet metal interface. The model is validated with respect to strip-draw experiments using different tool textures. The model is later applied to the simulation of a U-bend forming process. The results show that using textured tools, it is possible to reduce friction and punch force in sheet metal forming processes. The model can be used to tailor and optimize textures on stamping tools for complex parts