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摘要
This paper presents an original method of the stress-strain diagram acquisition named modified hydraulic bulging on the sphere. The method has been derived from the hydraulic bulge test. The flow stress-strain curves were established for an aluminium sheet on an experimental apparatus designed for both methods. For the purpose of comparing the two tested methods, appropriate expressions, i.e. expressions for kinematics and particular equivalent stresses, were derived. In both cases, the assumption of the membrane stress state was used and the flow rule was determined, which was necessary for establishing the flow stress-true strain curves. The comparison between the used methods showed the advantages of the new modified method. The new method allows more accurate determination of membrane stress in the deforming material. The assumption on the bulge geometry is no longer needed since the modification using sphere as an obstacle determines the radius of curvature required for the acquisition of membrane stress. Performing modified hydraulic bulging on just one test specimen yields three points of the stress-strain curve. The experimentally obtained points have different strains, covering different areas of strain, i.e. small, medium and large deformations, within a single experiment. The use of a small transducer integrated in tooling enables the acquisition of force and thus the acquisition of reliable stress using the known (spherical) geometry. Moreover, in an attempt to establish whether and to what degree the variable parameters of sheet thickness and the sphere diameter can be disregarded, two different sheet thicknesses and two different sphere diameters have been tested. In addition to applying the analytical approach and conducting experiments, the results were modelled using the finite element method. The recorded true stress-strain curve was used for the creation of a finite element model of the modified hydraulic bulging on the sphere. The modified method proved to be adequate for checking the numerical model, because it enables direct experimental measurement of true stresses in the material, which cannot be obtained when the hydraulic bulge test is performed.
- 出版日期2011
