Hole-flanging of sandwich sheets

Within the scope of this project, collar forming is used to create stiffened joints in sandwich sheets, which are used for subsequent joining processes. The aim is to develop a process understanding for the hole-flanging of sandwich sheets and to infer design guidelines in order to enable the reliable and economical production of joints in sandwich sheets.

Motivation

In order to meet the constantly increasing requirements for lightweight construction, different approaches exist. In many cases, modern composite materials are used which have better weight-specific properties than monolithic materials due to their specialized structure. The combination of rigid face sheets (mostly steel or aluminium) with light core materials (often polymers) to sandwich sheets enables a significant weight reduction for components that are primarily subject to bending loads while maintaining the same stiffness. Sandwich sheets with polymer cores also have noise-damping properties and are therefore predestined for facade and car body parts.

Although the production of sandwich sheets is state of the art, numerous challenges still have to be solved, especially with regard to joining and forming. For example, sandwich sheets have a more complex forming behavior than conventional sheets, which is characterized by additional failure mechanisms. Furthermore, the process temperatures required for thermal joining processes are significantly higher than the decomposition temperature of plastics and for pre-stressed joints there is a risk of loss of preload forces due to the creep of the polymer core.

[1] Exemplary process window for D Punch = 10 mm, R Punch = 1,5 mm und R Die = 0,45 mm. Experimentally produced hole-flanges (data points) and numerically estimated process limits (colored areas) are shown

Approach

At the beginning a numerical model is built up, which supports the development of a modular experimental tool and further investigations. In extensive test series, various process and geometry parameters are varied and their influences and process limits are investigated. At the same time, the FE model is being continuously extended and optimized.

After the successful implementation of the process, various application-relevant concepts for joints are designed and their strength is examined. The findings are combined in an analytical model which enables to design the process for material combinations that have not been explicitly examined.

[2] Cross section of a failure-free flange

Acknowledgment

The research presented here is taking place within the IGF project 18773 N of the European Research Association for Sheet Metal Working (EFB). It is supported via the AiF within the funding programme “Industrielle Gemeinschaftsforschung und -entwicklung (IGF)” by the Federal Ministry of Economic Affairs and Energy (BMWi) due to a decision of the German Parliament. Furthermore, we would like to thank all industry partners supporting the research project “Hole-flanging of sandwich sheets”:

  • Audi AG
  • BMW Group
  • Daimler AG
  • FILZEK TRIBOtech
  • Graepel Löningen GmbH & Co. KG
  • RAZIOL Zibulla & Sohn GmbH
  • Rhenus Lub GmbH & Co KG
  • Salzgitter Mannesmann Forschung GmbH
  • Siebenwurst Werkzeugbau GmbH
  • thyssenkrupp Steel Europe AG
  • Volkswagen AG