The forming limit curve (FLC) is a standard procedure for evaluating the formability of sheet metals. The experimental determination of such a diagram can be costly. CrachLab predicts the FLC with a numerical approach based on the strainhardening behaviour of the sheet.
CrachLab calculates the forming limit for metal sheets. Previous forming processes and heat treatments are taken into account.
The calibration of forming limit curves with CrachLab requires the definition of strain hardening, the strain-rate sensitivity and the sheet orthotropy as well as one experimental limit strain. CrachLab may be used as a virtual test laboratory to reduce the number of experiments. Typically, seven differently waisted Nakajima specimens are used; with CrachLab you need only standard tensile tests.
CrachLab can determine the external limit strain (ELS), which is measured on the torn specimen by means of an imprinted measuring grid, as well as the internal limit strain (ILS), which is obtained by extrapolation of the strain over the distance to the fracture. Both methods are regularly used in industry.
Scope of CrachLab
CrachLab offers several possible applications. First, it may be used to determine an initial FLC of a sheet without great experimental expenses. This FLC may account for previous deformations with non-linear strain paths. The obtained FLCs are shown graphically and may be used as curve in FE simulations.
Further, the safety factor of a forming process may be calculated. CrachLab may also be used as a fitting tool for experimental FLCs with a reduced number of specimen geometries.
An auxiliary module allows to approximate experimental hardening curves from tensile tests with the analytical hardening laws used in CrachLab (Swift and Hockett-Sherby).
Benefits of CrachLab
CrachLab allows to determine a forming limit diagram for the evaluation of formability from standard tensile tests. Alternatively, a reduced testing programme may be combined with CrachLab to determine FLCs.
CrachLab accounts for the physical effects in the sheet; it can therefore be used to predict FLCs where experiments cannot be carried out easily, e.g. at high strain rates or for hot forming.
In the FLC benchmark at Numisheet 2008 in Interlaken, the FLCs that were predicted by CrachLab for two steel sheets of different thickness and for an aluminium sheet showed the best conicidence with the experiments.
 Volk W., Wahlen A., Hora P., Keßler L., Hotz W: Benchmark 1 – Virtual Forming Limit Curves. Part B: Benchmark Analysis; in: Hora, P. et al.: Numisheet 2008 Benchmark Study, pp 11-42, Institute of Virtual Manufacturing, ETH Zurich (2008), ISBN 978-3-909386-81-9