Improvement of Numerical Modelling Considering Plane Strain Material Characterization with an Elliptic Hydraulic Bulge Test

Abstract

A precise characterization of material behavior is necessary to identify yield criteria or hardening laws for an accurate numerical design of sheet metal forming processes. Current models like Yld2000-2d or Hill'48 do not consider the plane strain state, though this condition is primary cause of failure in deep drawing. It is anticipated that an improved yield locus contour which considers the stress under plane strain conditions leads to better results in numerical simulations of a deep drawing process. Within this contribution, a new experimental setup to characterize both principal stress components under plane strain as additional input data for material modelling is presented. Therefore, hydraulic bulge tests are carried out with a novel elliptical die, which implements a plane strain condition. Moreover, the improvement of the material model is investigated exemplarily for the three sheet metal alloys DC06, DP600 and AA5182. The resulting material parameters are used to identify the yield locus for plane strain by varying the yield locus exponent of Yld2000-2d. The results prove that considering plane strain yield locus results in a better sheet thickness distribution in comparison to conventional modelling of the deep drawing process.

Evaluation of testing methods for the characterization of material properties under plane strain

“…The parameter α, for the homogeneous length, was set to α = 0.98. The yield stress calculation in the hydraulic bulge tests is done according to [8]. The resulting flow curves of both tests and the corresponding strain paths are shown in Fig.…”

“…This is an important attribute because the load paths are slightly different in the elliptic hydraulic bulge test and in the notched tensile test. In the notched tensile test, there is no second principle strain component while in the bulge test there is a ratio of 1:5 for the second to the first principle strain [8]. For this reason these two strain paths are adapted in the biaxial tensile test for this investigation by a special specimen design and different loading conditions.…”

“…The hydraulic bulge tests with an elliptical die are performed with the tool presented in [8]. The tool is adapted in a hydraulic press Hydrap HDPZb630 (Hydrap, Germany).…”

“…As valid for all die geometries in the bulge test, the initial yield stress cannot be calculated with sufficient accuracy due to the evaluation methodology. Possible applications of the material properties under plane strain are the identification of the yield criterion of Veger [6] or an implementation in other yield criteria, for example Yld2000-2d via variation of the yield locus exponent m [7] with the additional experimental data [8]. Hippke et al [5] proved the better mapping accuracy of the optimized yield locus model through a comparison of the prognosed stress values in the first quadrant to those characterized in biaxial tensile tests.…”

A Review of Characterization and Modelling Approaches for Sheet Metal Forming of Lightweight Metallic Materials

“…Lafilé et al [ 269 ] proposed a new method to directly determine material behavior using DIC data on the outer surface of the specimen, which is suitable for hydraulic bulging experiments with small opening diameters of dies. A stress–strain curve close to the plane strain state can be obtained by hydraulic bulging with a reasonably designed elliptical die [ 270 , 271 , 272 ].…”