Improved sheet metal forming simulation using 3D materials modeling and extended shell formulations – Part 2

Project description

The state of the art in the simulation of sheet metal forming processes is the use of shell elements based on the Reissner-Mindlin model and simplified materials models. This modeling approach is based on a number of simplifying assumptions. For structural modeling, it is assumed that the cross-sectional fibers remain flat and that normal stresses in the sheet thickness direction are negligible. For materials modeling, direction-dependent materials behavior is only represented in the sheet plane, and anisotropic effects outside the sheet plane are therefore neglected. This modeling approach reaches its limits in certain sheet metal forming processes — including bending-dominated processes and forming processes with small radii — as some of the assumptions made no longer apply.

The “3D Sheet Metal Modeling II” project followed on from the completed AiF project 19707N “3D Sheet Metal Modeling” (create ProPro?). In this project, the prediction quality of critical sheet metal forming processes was improved by developing extended shell formulations in combination with 3D materials models.

The “3D Sheet Metal Modeling II” research project pursued the further development of an alternative approach to the simulation of sheet metal forming processes and the qualification of this approach for industrial use. To this end, three aspects relevant to practical application were investigated in greater depth: To improve the numerical efficiency of the developed shell formulations, three different methodological approaches were pursued in order to reduce the computing time to a level that is usable in industry. In order to appeal to a broader range of users, the method for identifying model parameters for anisotropic 3D materials models was extended to another class of materials that is relevant in practice. A third focus was the evaluation of the prediction quality of the newly developed modeling approach through practical model tests and using a real component.

As part of the project, higher-order 3D shell elements that are not subject to the limitations of the Reissner-Mindlin model were used to simulate sheet metal forming processes. These were combined with 3D materials models that take into account a complete three-dimensional strain and stress state. The higher-order 3D shell elements developed in the previous project were improved in various aspects. In addition, the virtual testing method was improved and qualified for another class of materials. Numerical benchmarks and comparisons with test results and simulations of real components demonstrate the increased quality of the results of 3D sheet metals modeling and its successful qualification for industrial use.

In the medium term, the results achieved can also be used in other fields of application within the sheet metal processing industry (e.g., crash simulation, electrical plug contacts).

Fraunhofer IWM subproject:

Work focus “3D materials modeling” (Fraunhofer IWM)

WP1: Materials characterization

• Tensile tests and bulge tests
• Microscopy and EBSD measurements

WP2: Adaptation of 3D materials models

• Setup and calibration of RVE
• Performance of virtual tests and adaptation of 3D flow models
• Extension to new materials class (high-strength steels)

In the “3D material modeling” work focus, the materials to be investigated were characterized and represented in the form of a microstructure model. The methodology developed in the previous project for identifying parameters for anisotropic 3D flow path models served as the basis for this and was further developed in this research project with a focus on a new materials class (dual-phase steels).

Work focus “Evaluation of 3D modeling for practical applications” (Fraunhofer IWM & IBB)

WP5: Model tests

• Practical model tests for comparison with WP6
• For example, tensile tests, Nakazima tests, bending tests, and strip drawing tests with drawing beads

AP6: Prediction quality of 3D sheet metal modeling

• Simulation of the model tests from AP5
• Analysis of a real component (body outer skin component with character edge)
• Evaluation of the prediction quality with regard to materials flow, springback, and forming limits
• Derivation of recommendations for forming practice

In the work focus “Evaluation of 3D modeling for practical application,” the previously developed aspects of 3D sheet metal modeling were brought together and evaluated in terms of practical application by comparing them with fully three-dimensional simulations based on hexahedral elements, as well as with test data. The evaluation was carried out on the basis of several model tests and a practical real component.