Springback Compensation: Fundamental Topics and Practical Application

Now that the simulation of deep drawing processes has become more reliable the virtual\ud compensation of the forming tools has become reality. In literature, the Displacement Adjustment (DA)\ud algorithm has proved to be most effective. In this article it is shown how the compensation factor, required for\ud (one-step) DA depends on material, process and geometrical parameters. For this an analytical bar stretchbending\ud model was used. A compensation factor is not required when DA is applied iteratively and the\ud products geometrical accuracy is improved further. This was demonstrated on an industrial part. The\ud compensation varies over the product, leading to a reduction in shape deviation of 90% and more, a result that\ud could not have been achieved with one-step compensatio

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

Charged excitons in doped extended Hubbard model systems

We show that the charge transfer excitons in a Hubbard model system including nearest neighbor Coulomb interactions effectively attain some charge in doped systems and become visible in photoelectron and inverse photoelectron spectroscopies. This shows that the description of a doped system by an extended Hubbard model differs substantially from that of a simple Hubbard model. Longer range Coulomb interactions cause satellites in the one electron removal and addition spectra and the appearance of spectral weight if the gap of doped systems at energies corresponding to the excitons of the undoped systems. The spectral weight of the satellites is proportional to the doping times the coordination number and therefore is strongly dependent on the dimension.Comment: 10 pages revtex, 5 figures ps figures adde

Numerical Modeling of Advanced materials

The finite element (FE) method is widely used to numerically simulate forming processes. The accuracy of an FE analysis strongly depends on the extent to which a material model can represent the real material behavior. The use of new materials requires complex material models which are able to describe complex material behavior like strain path sensitivity and phase transformations. Different yield loci and hardening laws are presented in this article, together with experimental results showing this complex behavior. Recommendations on how to further improve the constitutive models are given. In the area of damage and fracture behavior, a non-local damage model is presented, which provides a better prediction of sheet failure than the conventional Forming Limit Diagram

Switching Casimir forces with Phase Change Materials

We demonstrate here a controllable variation in the Casimir force. Changes in the force of up to 20% at separations of ~100 nm between Au and AgInSbTe (AIST) surfaces were achieved upon crystallization of an amorphous sample of AIST. This material is well known for its structural transformation, which produces a significant change in the optical properties and is exploited in optical data storage systems. The finding paves the way to the control of forces in nanosystems, such as micro- or nanoswitches by stimulating the phase change transition via localized heat sources.Comment: 7 pages, 3 figures The AFM images for the inset in Fig.2 were replaced with new ones as obtained with tips having high aspect rati

Identification of plasticity model parameters of the heat-affected zone in resistance spot welded martensitic boron steel

A material model is developed that predicts the plastic behavior of fully hardened 22MnB5 base material and the heat-affected zone (HAZ) material found around its corresponding resistance spot welds (RSWs). Main focus will be on an accurate representation of strain fields up to high strains, which is required for subsequent calibration of the fracture behavior of both base material and HAZ. The plastic be-havior of the base material is calibrated using standard tensile tests and notched tensile tests and an inverse FEM optimization algorithm. The plastic behavior of the HAZ material is characterized using a specially designed tensile specimen with a HAZ in the gage section. The exact location of the HAZ relative to the center of the RSW is determined using microhardness measurements, which are also used for mapping of the material properties into an FE-model of the specimen. With the parameters of the base material known, and by assuming a linear relation between the hardness and the plasticity model parameters of base material and HAZ, the unknown HAZ parameters are determined using inverse FEM optimization. A coupon specimen with HAZ is used to validate the model at hand