Profiles made of high or high-strength steels are often manufactured by conventional bending processes such as air bending and die bending, or roll forming. Through the cold forming process and the resulting strain hardening of the material, the product shows a higher strength by unchanged product properties (material, weight). Besides the strain hardening and the contribution of residual stresses by the inhomogeneous strain distribution, damage occurs during forming. Damage is defined by void nucleation, growth and coalescence and affects the product performance. The strain hardening, the residual stresses, and the damage influence the mechanical properties and they each have an influence on the product behavior. The occurring damage is highly dependent on the bending process, and therefore dependent on the load path, that means the chronology of stresses, plastic strains, temperature, and the plastic strain rate for a material point during the process. The damage evolution is positively influenced by e.g. an additional superposition of compressive stresses. The inhomogeneous strain and stress distribution across the sheet thickness is characteristic for the bending process. The damage nucleation will evolve from the outer fiber because of the tensile stress state. Up to now, the correlations between bending process, load path, damage evolution, and product performance have not been systematically investigated in the state of the art.

The next aim of this project is the prediction and the control of damage in sheet metal bending. During the first funding period, air bending is analyzed because of its essential character. The used material during research is the dual phase steel DP800. The evaluation of the damage on the products starts at the beginning by the use of a damage criterion (Cockcroft-Latham) and with the help of established damage models (Lemaitre and Gurson-Tvergaard-Needleman). All used criteria and models need an identification of the parameters. The qualification of suitable and efficient characterization methods for the qualitative and quantitative description of damage in DP800 takes place in cooperation with the projects B02 and B05. The product performance will be researched in cooperation with project B01 by conducting fatigue tests. The influences of the strain hardening and the residual stresses have to be separated for the research of the influence by the damage on the product performance. Therefore, different measurements like the measurement of hardness and residual stresses, and the stress-relief annealing are necessary. From the second half of the first funding period the damage models will be replaced by further developed models of the research area C (in particular project C02). Semi-finished sheet metal products with known damage of the project A04 are used for bending experiments for the research of the influence of the forming history.

The main question for the first funding period is: How can damage during air bending be predicted or controlled? For answering this question, at first the following sub questions have to be researched:

  • How can the load path be affected during air bending by the variation of process parameters (e.g. sheet metal width, friction, tool radius or controlled stress superposition)?
  • How can the modifications of the loading path affect the damage evolution (e.g. void volume fraction, allocation over the sheet metal)?
  • How does the forming history affect the damage evolution?
  • In which way can damage controlled process routes or process modifications be derived by controlled stress superposition during air bending?
  • In which relation are damage and product performance (e.g. among fatigue tests)? In addition, in which way can the effects of damage and the effects of strain hardening and residual stresses be separated?

The aim of the second funding period is the technological implementation of the gained knowledge in damage-controlled processes, in new processes, or in developed processes (e.g. roll forming with compressive stress superposition). The focus during the third funding period is on the expansion of the materials spectrum under consideration of different temperatures during the process, and other sheet metal bending processes.

 

Project leader
Prof. Dr.-Ing. Dr.-Ing. E. h. A. Erman Tekkaya
Institute of Forming Technology and Lightweight Components (IUL), TU Dortmund University

Project coordinator
Rickmer Meya M. Sc.
Institute of Forming Technology and Lightweight Components (IUL), TU Dortmund University