| Summary: | 碩士 === 國立臺灣科技大學 === 機械工程系 === 94 === A methodology of formulating an elasto-plastic three-dimensional finite element model, which is based on Prandtl-Reuss flow rule and Hill’s yield criterion respectively, associated with an updated Lagrangian formulation, is developed to simulation sheet metal flanging process. The shape function derived from a four-node quadrilateral degenerated shell element associated into the stiffness matrix to constitute the finite element model. An extended rmin algorithm is proposed to formulate the boundary condition, such as nodal penetration and separation, strain increment and rotation increment and altered elasto-plastic state of material.
In order to verify the reliability and accuracy of the FEM code, the fractured thickness of a specimen in the simple tension test is adopted as the fracture criterion of forming limit in simulation. The numerical simulation results include relationship between punch load and punch stroke, distribution of the thickness, distribution of the stress, distribution of the strain, deformation history and the forming limit etc. in elliptical hole-flanging forming process.
According to the simulation and experiment results, the maximum stress and the minimum thickness are concentrated on the contact regions between workpiece and punch major axis. The maximum punch load decrease, as the profile radius of elliptical punch and initial hole of blank increase remarkably. The initial hole of blank has great influence on the forming limit, but it is independent of the profile radius of elliptical punches. The limiting forming ratio (LFR) is defined between the circumference of elliptical punch profile and the circumference of the minimal initial elliptical hole of the blanks. The LFR amounts to about 1.4627 for penetration in the elliptical hole-flanging process of this study. The different of the ratio of the punch major axis to the punch minor axis, as the aspect ratio of punch increase, the punch major axis regions is closed to the fractured thickness and the expanding height is low too.
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