Research on Analysis of Tube Hydroforming with High Accuracy by Meshfree Method

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The conclusions of this research is summarized as follows.(1) As a high accurate analysis method for tube hydroforming, the existing meshfree technique is applied to the finite element method. As results, a novel assumed strain type finite element method is proposed considering geometrical nonlinearity, in which deformation gradient is assumed to be constant and averaged within an element. For further improvement of the accuracy, the moving least-square method is adopted as the approximation of displacement field. This method uses the same nodal arrangement as in the finite element method and an element is also used as an integration domain. Therefore, any kinds of mesh generation techniques are available. The accuracy of the method is verified by illustrated numerical analyses of linear and nonlinear problem.(2) For practical evaluation of tube hydroforming, a finite element analysis of thin-walled tube which is simultaneously subjected to axial load and internal pressure is conducted … More . As for constitutive law, visco-plastic model is adopted. Axial load and internal pressure have to be independently controlled so that the ratio of longitudinal and circumferential stresses could be kept constant. To enable this, a novel arc-length method with two different loading modes is proposed. In the method, the ratio between longitudinal and circumferential stresses is controlled as an additional constraint condition. Plastic instability condition of thin-walled tube under combined loading is also presented. A numerical example demonstrates that the proposed method can keep the stress ratio constant through the analysis and evaluate the maximum loading points of thin-walled tube under combined loading.(3) For accurate estimation of material behavior for tube hydroforming, the hydrostatic stress-sensitivity effect, which has been observed in several previous experimental studies, is incorporated into a recently proposed phenomenological plasticity model with vertex-type of effect. This plasticity model is implemented into an finite element method. To test the model, shear band developments predicted by the present model are directly compared to predictions by a crystal plasticity model with hydrostatic stress-dependence of crystallographic slip behavior. It is shown that the present phenomenological model has a sufficient ability to reproduce the crystal plasticity predictions. Furthermore, an effect of the hydrostatic stress-dependence of yielding on forming limits of thin metal sheets is investigated by use of a Marciniak, VKuczynski-type approach. Less