| Multi-point forming(MPF) is a new means for sheet metal forming. Its main idea is dividing the curved surface of the traditional stamping dies into many pairs of matrices of elements, and then the three-dimensional shape of sheet metal is formed by the curved surface of elements. Relative to the traditional stamping technique, the MPF has saved a lot of time for die design, die manufacturing and debugging. It has shortened the exploiting cycle and reduced the costs. Now, the MPF technology can be applied extensively in many fields, such as the skin of high-speed locomotive, ship shell plates, automobile panel, large sculpture pieces in the city. In the MPF the principal loading part is elements, so the part possibly appears dimples owing to the excessively centralizing load on the sheet metal. To suppress dimples, elastic cushion technology is adopted, which is the blanket between punches and blank to distribute the pressure on the blank by itself large deformation filling the interspaces among elements during the forming course. But the parameters and thickness of elastic cushion have great impact on not only dimples but the precision of the part. So to obtain the ideal forming results, elastic cushion should be selected reasonably. With computer applications and the maturity of nonlinear finite element, the numerical simulation technology of sheet metal forming is developed quickly. Now the technology is widely applied in the die design for forecasting forming defects. In MPF, the dynamic explicit finite element arithmetic is very suited to simulate the sheet metal forming because the contact between the elements and the sheet is discontinuous and the loading condition is very complicated. So a dynamic explicit finite element software—LS-DYNA is introduced to simulate the MPF for sheet metal forming, including the creation of finite element model, the choice of material model, the establishment of boundary conditions and the treatment of contact friction and so on. The dimples defect and its controlling methods are detailedly analyzed and studied in this paper. The main contents and results are as follows: 1) The research on the element model of sheet metal and the material model of elastic cushion in the MPF From the point of deformation, the area around a dimple is 3-D deformation state. Therefore solid element model is especially suited to the dimples analysis. However when blank is relatively thin, if solid element is employed, the number of elements will be extraordinarily large. It will cost a very long of CPU time. On the other hand, the dimple is main convexity-concavity dimple in the thinner sheet of MPF, so shell element for simulating the dimples is feasible. The paper adopts different element models to simulate the forming of sheet metal with different materials, different thicknesses, and different shape pieces and reasonable element model for dimple simulation is set up. The results indicate that the element model can be chosed according to the ratio of thickness and the length, and the choice has little anything to do with materials and shapes of sheet metal. When the ratio of thickness and length is less than 0.015, the shell element is recommended, and when the ratio is more than 0.0015, solid element recommended. Elastic cushion technology is an available means to suppress dimples, but the numerical simulation of MPF becomes more complex. Previous jobs mostly adopt liner elastic model to simulate elastic cushion, that simplified the computus and decreased precision. The paper builds Mooney-Rivlin nonlinear elastic model for the cushion using the experiment date of polyurethane and compares with linear elastic model. When the compress ratio of elastic cushion is less than 35%, the computing error of linear elastic model is small, on the other hand the greater the error is, the larger the compress ratio is. In this condition the nonlinear elastic model is recommended. 2) The numerical simulation of the effect on elastic cushion suppressing...
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