Determination of Forming Limit Curves - Strain Path and Failure Analysis

Kohl D, Merklein M (2022)

Publication Type: Conference contribution, Conference Contribution

Publication year: 2022

Publisher: Published by Trans Tech Publications Ltd

City/Town: Switzerland

Book Volume: Vol. 926

Conference Proceedings Title: Key Engineering Materials

Event location: Braga, Portugal

DOI: 10.4028/p-up90fy


With the goal to define a cost-effective and efficient process to identify adequate materials
for sheet metal forming processes, it is crucial to evaluate the formability of materials. Forming limit
curves (FLC) are used to analyze the forming and failure limits of sheet metals and dependence of
the major (ε1) and minor strain (ε 2) from the uniaxial stress-strain area through the plane-strain point
to the biaxial strain area. According to ISO 12004-2, the FLC is performed by Nakajima or Marciniak
tests. Due to the experimental setup and the preconditions, pre-stretching occurs in the specimens and
bending and friction effect are the result. The determination of the onset of necking (FLC) results
mathematically from a “best-fit inverse parabola” on section lines. In addition, the failure point, i.e.
the maximum strain value one frame before failure, is also analyzed. In contrast, tensile, notched
tensile and hydraulic bulge tests, which are known in the literature as basic tests for the determination
of the FLC, exhibit a linear strain path evaluation. These tests are used in this investigation as an
alternative for the determination of the FLC. The behavior of the various strain paths of Nakajima
and the alternative methods are examined for necking and cracking. Furthermore, the fracture surfaces
are investigated by confocal laser scanning microscopy to identify influences of the different FLC
methods on the fracture mechanics. FLCs were conducted with the Nakajima and the alternative FLC
characterization method for a ductile steel (DX54D). To ensure transferability, the tensile tests are
also performed with a high-strength steel (DP800). The FLC of the ductile steel, generated through
the alternative method, exhibits a similar shape to the Nakajima generated FLC with the advantage
of a constant strain rate leading to linear strain paths and a lower number of tests. The same results
are achieved for the uniaxial strain tests with DP800.

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How to cite


Kohl, D., & Merklein, M. (2022). Determination of Forming Limit Curves - Strain Path and Failure Analysis. In Key Engineering Materials. Braga, Portugal: Switzerland: Published by Trans Tech Publications Ltd.


Kohl, Doris, and Marion Merklein. "Determination of Forming Limit Curves - Strain Path and Failure Analysis." Proceedings of the 25th International Conference on Material Forming 2022 (ESAFORM 2022), Braga, Portugal Switzerland: Published by Trans Tech Publications Ltd, 2022.

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