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Determining the limit states (both theoretically and experimentally) of formed parts in the sheet metal forming represents quite important material characteristics, which are commonly applied e.g. in the field of numerical simulations. Within the precise determination of the limit state, there is not used only a relatively large number of theoretical approaches (e.g. theoretical Keeler-Goodwin diagram or MK theory), but also various experimental approaches. Thus, in addition to the basically preferred approach (the so-called Nakazima test), other experimental methods are commonly used – e.g. stretching shaped workpieces by the hemispherical punch and circular die, tensile test of specimens with different notches, common hydrostatic bulge test with circular or elliptical die or e.g. utilization of punches with different radii. Two different experimental approaches were used in this paper to determine limit state - to be specific position of the forming limit curve (FLC) in the forming limit diagram (FLD) for tested deep-drawing material DX57D. The contact-less optical system MERCURY RT from the company Sobriety was used to measure the distribution of true strains (major and minor one). In the experimental part, methods of stretching shaped workpieces by the hemispherical punch and circular die (can be applied to both parts of the FLC) and tensile test of samples provided with different notches (this one can be applied only to the left part of the FLC) were used. From the values of final limit states in light of strain and their mutual comparison, it is possible to deduce the influence of the chosen methodology (results from the first experimental approach are taken as a reference state) and thus also their possible applicability and reliably in the experimental determination of FLC´s.
Keywords: Forming limit curve, sheet metal forming, deep-drawing material, contact-less optical deformation measurement, photogrammetry© This is an open access article distributed under the Creative Commons Attribution License which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.