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Hydraulic bulging experiments are performed in order to evaluate the mechanical parameters of cold rolled steel (DC04) and aluminium alloy (EN AW 6016-T4) sheet materials. The biaxial yield stresses, and the biaxial anisotropy coefficients are derived from the biaxial stress-strain curves and the ratio between the strains in the transverse and in the rolling direction, respectively. The mechanical parameters resulted from the bulge test in combination with the results from the tensile tests are used to determine the yield loci of the two materials. The effect of the number of input parameters on the capability of the BBC 2008 yield criterion to predict the yield locus is also discussed in the paper. Keywords: hydraulic bulge test, mechanical properties, yield surface IntroductionThe most commonly used tests for the determination of the biaxial yield stress are the biaxial tensile test of cruciform specimens [1][2][3][4] and the hydraulic bulge test. A review of biaxial tensile tests using cruciform specimens is presented in the papers [5,6]. The disadvantages of this method are the complicated geometry of specimens as well as the complexity and high cost of the equipment. An alternative to this test is the hydraulic test. The most important advantage of the hydraulic bulge test is the absence of the contact (and therefore of the frictional interactions) between tools and specimen in the area of interest, which simplifies the analytical solutions for the calculation of stress and strain, but also ensures the repeatability of the test. The hydraulic bulge test is the subject of many scientific papers and has been investigated by other authors such as Hill [7], who developed analytical models for the calculation of polar thickness and curvature radius. He neglected the influence of the fillet radii of the die. The accuracy of the formulas proposed by Hill has been improved by Chakrabarty [8] by taking into account the hardening effects. Furthermore, Shang [9] extended the formulas proposed by Hill in order to take into account the fillet radius of the die insert. Atkinson [10] also tried to improve the accuracy of the analytical predictions referring to the polar thickness and dome radius. Kruglov [11] developed a formula for the calculation of the polar strains. Banabic [12] developed analytical models for the computation of the pressure-time relationship for the bulging of both strain hardening and superplastic materials trough elliptical dies and Vulcan [13] and Banabic [14] for superplastic forming of aluminium sheets for the cone-cup test. Lăzărescu [15][16][17] developed analytical models for the determination of stress-strain curves using dies with circular and elliptical apertures. Koç [18] performed experimental studies for the

The accuracy of the finite element simulation of sheet metal forming processes is mainly influenced by the shape of the yield surface used in the mechanical model and, in particular, by the number of input values used in the identification of the yield surface. This paper investigates the effect of the input values used for identifying the BBC 2005 yield criterion on the accuracy of the finite element predictions. The accuracy assessment of the simulation is based on the comparison of the numerical predictions obtained using the commercially available FE programme AUTOFORM and experimental measurements obtained from the hydraulic bulging of sheet metals. Thickness and strain distributions, as well as the geometry of the bulged specimen were taken as comparison parameters. The accuracy of the finite element predictions obtained using the Hill-48 and Barlat-89 yield criteria is also studied and discussed in comparison with the results provided by the BBC 2005 yield and the experimental data.

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