Development of Precision Springback Measurement and Bending System of Sheet Metal

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In this project, we developed two precise instruments for measuring young's modulus after large plastic deformation and distribution of internal stress of the workpiece, and attempt to calculate the springback from the measured young's modulus anc stress. The instrument for measuring young's modulus consists of a laser interferometer and optical extensometer. The stress sensor consists of magnetic field generator and semi-conduct magnetic detector based on hall effect.Experimental results show that the young's modulus measurement instrument is capable of measuring young's modulus of the test piece at beginning and after more than 20% plastic deformation with the same accuracy, and the accompanying stress in the tensile test was proportional to measured magnetoresistance of workpiece. It is found that the young's modulus decreased according to increase of plastic strain, and the young's modulus decreased to 75% of initial value after deformation with 20% plastic strain. Furthermore, the … More variation in the young's modulus was applied to predict the springback value by using FEM simulation. The results show that prediction of springback with varied young's modulus was more accurate than using only initial value.In order to clarify the mechanism of the changes in elastic modulus due to the plastic strain, microscopic measurement of material properties was performed. The changes of through-thickness distribution for mild-steel sheets were measured by analyzing the cross-sectional microphotographs of material structure. Furthermore, the elastic modulus and hardness around a grain was investigated by a nano-indentation test for measuring distribution of regional material properties. The results show that the elastic modulus decrease near the grain boundary, and the decrease of the regional material properties could significantly influence on the macro material properties, such as springback value. It's caused by piled-up dislocations, which may lead to increase of spring-back.FE simulation of V-bending process is carried out for the practical V-bending die. The discrepancies between the simulated and experimental results are evaluated in both laboratorial and practical V-bending processes. An adaptive filter to compensate for the discrepancies is proposed. The modified simulation results are stored in a database and used in the intelligent process control system based on the database developed by the authors. The AI control system of the V-bending process was evaluated by bending several kinds of materials. The results show that the FEM simulation database with the online adaptive filter is effective for the precision process control, and compatible to the practical process. Less