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Failures in Drawing
Published in Ganesh M. Kakandikar, Vilas M. Nandedkar, Sheet Metal Forming Optimization, 2017
Ganesh M. Kakandikar, Vilas M. Nandedkar
Numerical simulation using FEM with either an implicit or explicit integration method has become a prime tool to predict the buckling behavior in the sheet metal operation which involves complicated geometry and boundary conditions including friction. Using an implicit method to predict wrinkling is essentially an Eigen value approach, and it is hard to initiate wrinkles without initial imperfections, for example, a specific mode shape and/or material imperfection, built into the original mesh. Unlike the implicit solver, the explicit method as a dynamic approach can automatically generate deformed shapes with wrinkles due to the accumulation of numerical error. However, the onset and growth of the buckling obtained from the explicit code is sensitive to the input parameters in the FEM model, such as element type, mesh density, and simulation speed. Generally, three types of elements are employed in the sheet metal–forming simulation: membrane element, continuum element, and shell element. Membrane elements have been widely used to model the forming processes, due to their simplicity and lower computation time, especially in the inverse and optimization analysis, where many iterations of forming are required. However, it does not include bending stiffness, and therefore may not be appropriate in modeling processes where the buckling phenomenon is important, unless some special treatment (such as postprocessing) is applied.
Evaluation of residual stress for multi-point repeated forming technology
Published in Ships and Offshore Structures, 2020
Wei Shen, Renjun Yan, Yue Lin, Peiyong Li
In order to verify the numerical model, the cold bending forming of regular plates (circular plates and rectangular plates) was used for numerical analysis and experimental measurement in multi-square punch forming. Due to symmetry, only a quarter of the structure was modelled. Symmetrical boundaries and other constraints are shown in Figure 8. At present, there are three types of elements used in sheet metal forming simulation: membrane element based on membrane theory, shell element based on plate-shell theory and solid element based on continuous medium (Flanagan and Belytschko 1981). The membrane element, ignoring the bending effect of the sheet metal forming process, has simple expression and high calculation efficiency. But the accuracy of springback prediction is low, which limits the practicability. The shell element has good solving efficiency and precision, and is mainly suitable for sheet metal forming. For medium and thick plates in ships and marine engineering, three-dimensional solid elements are required for plastic forming simulation of such plates. Considering the calculation time and accuracy, eight-node brick element SOLID 164 was adopted and at least four elements were assigned in the thickness direction (Lackner and Mang 2002; Liu et al. 2006). The element sizes in the length and width directions were basically consistent with the thickness direction. In addition, hexahedral elements were recommended in order to reduce stress concentration.