Closure of the research project »Unfolded Length«


The research project »Optimized Calculation of the Unfolded Length in Bending Sheet Metal to Profiles and Tubes« was successfully competed at the end of February 2015.

State of the art methods for calculating the unfolded length in bending produce different results for the same task. Material properties as well as influences of different bending processes (die bending, folding, roll-forming) are generally neglected by state of the art calculation methods. Process design therefore relies on experience to avoid trial and error loops. During the past two years a research project was conducted aiming at the definition of an improved calculation method enabling a reliable calculation of the required initial sheet width in bending of sheet metal.

The project started off with an analysis of the basic assumptions of the state of the art calculation methods showing the need for an improved calculation method. This project aimed at improving existing calculation methods by conducting experimental and numerical investigation of different bending processes. As a prerequisite for experimental investigations on the shift of the unlengthened fiber in bending, an experimental approach to detect this shift needed to be designed. Using this experimental method enabled the determination of the unlengthened fiber in die bending, folding, and roll-forming processes. Furthermore, numerical studies of different bending processes were employed to define suitable design parameters for numerical simulations of bending experiments. Combining experimental and numerical results indicated the main factor of influence on the position of the unlengthened fiber. Finally, interpolating the test results provided the opportunity to create an improved calculation method for determining the unfolded length.

The ratio of bending radius to sheet thickness was identified to be the main factor of influence on the position of the unlengthened fiber. Furthermore, experimental results in roll forming suggest that material strength affects the position of the unlengthened fiber und thus the unfolded length if constant radius forming method is employed.

The results of the numerical studies suggest that the use of elements featuring quadratic shape functions is necessary to identify the position of the unlengthened fiber. A number of eight elements in thickness direction is required to identify the location of the unlengthened fiber in numerical simulation if the bend ratio is 2.5.

The suggestions for the unfolded length calculated by the improved calculation methods were confirmed in first test in industrial processes. Thus, the objective of this research project was accomplished.

For further information on the project and its results please contact our staff member Mr. Traub. The final report of the project will be available at Research Association for Steel Application (FOSTA) in the middle of the year.


The IGF-Research Project 17702 N supported by the Research Association for Steel Application (FOSTA), was funded in the scope of the “Progamm zur Förderung der Industiellen Gemeinschaftsforschung” (IGF) via the German Federation of Industrial Research Associations (AiF) by the German Ministry for Economy and Energy as decided by the German parliament.