Contact modelling of large radius air bending with geometrically exact contact algorithm

Usage of high-strength steels in conventional air bending is restricted due to limited bendability of these metals. Large-radius punches provide a typical approach for decreasing deformations during the bending process. However, as deflection progresses the loading scheme changes gradually. Therefore, modelling of the contact interaction is essential for an accurate description of the loading scheme. In the current contribution, the authors implemented a plane frictional contact element based on the penalty method. The geometrically exact contact algorithm is used for the penetration determination. The implementation is done using the OOFEM – open source finite element solver. In order to verify the simulation results, experiments have been conducted on a bending press brake for 4 mm Weldox 1300 with a punch radius of 30 mm and a die opening of 80 mm. The maximum error for the springback calculation is 0.87° for the bending angle of 144°. The contact interaction is a crucial part of large radius bending simulation and the implementation leads to a reliable solution for the springback angle

Advanced Modeling of Large Radius Air Bending

Air bending remains one of the most popular forming techniques within the sheet metal manufacturing domain. This forming process is known for its flexibility since one pair of tooling suffices for a wide range of forming angles. The traditional approach of conventional or small radius air bending is described by a 3-point bending model. However, small radius bending is not technically feasible for high strength steels. High strength steels have superior load bearing characteristics, however, this quality usually results in very limited ductility. In order to be able to form high strength steels, a reduced degree of deformation should be imposed during the forming stage, for instance by using larger radius punches. However, large radius bending brings about several difficulties, such as the so-called multi-breakage effect. This effect makes the prediction of the bending process more difficult and dissimilar to conventional air bending. The objective of this research is to investigate the effect of process and tool parameters in a large radius bending process using both physical and simulation models. The first step in this doctoral project, is an extensive experimental campaign, which is set up to refine the limits of the process window of large radius bending and to establish a cause-effect map on a number of process settings and quality indicators of the formed component. The experimental investigation contains more than 1000 bending tests. Two regression models are constructed, based on the phenomenological observations and on a circular approximation of the bent plate. This approach predicts the bending characteristics accurately, but it requires the availability of a significant experimental data set. The simulation part of the work is performed using finite element models which turn out to be quite accurate. Both mesh parameters and relevant material parameters are set to achieve close correlation with experiments. An analytical model predicts the bending characteristics based on a circular approximation approach. Only a limited number of input parameters is required for this model. The prediction quality of the analytical model is comparable with the regression approach and it is better than finite element analysis predictions. However, it does not require an extensive database as for regression analysis, or significant computational resources and commercial calculation tools as for the numerical analysis. In the final step, the possibilities of process window enlargement for air bending by radiant heating are addressed. Forming of thick plates with a small radius punch is possible while simultaneously decreasing the springback and required bending force.status: publishe

Contact Modelling of Large Radius Air Bending with Geometrically Exact Contact Algorithm

Usage of high-strength steels in conventional air bending is restricted due to limited bendability of these metals. Large-radius punches provide a typical approach for decreasing deformations during the bending process. However, as deflection progresses the loading scheme changes gradually. Therefore, modelling of the contact interaction is essential for an accurate description of the loading scheme. In the current contribution, the authors implemented a plane frictional contact element based on the penalty method. The geometrically exact contact algorithm is used for the penetration determination. The implementation is done using the OOFEM – open source finite element solver. In order to verify the simulation results, experiments have been conducted on a bending press brake for 4 mm Weldox 1300 with a punch radius of 30 mm and a die opening of 80 mm. The maximum error for the springback calculation is 0.87° for the bending angle of 144°. The contact interaction is a crucial part of large radius bending simulation and the implementation leads to a reliable solution for the springback angle