Analysis and modelling of plastic bending processes

Abstract: It is well known that anisotropic properties of material allect forming of sheet metals and that non-homogeneous deformation across thickness of a bend gives rise to residual stresses. The interaction between material and process parameters determines, in a very complex manner, the formability of material. Theoretical analyses of some problems related to bending processes were carried out by the incremental theory of plasticity in an attempt to achieve a better understanding. Study was also made of the Bauschinger effect that is manifested as a premature yielding of material on reversal of straining after plastic prestrain. The Bauschinger effect was analysed by subjecting sheet metals to uniaxial compression-tension tests. On the base of the continuum theory of plasticity and experimental results, parameters for revealing the combined effects of strain hardening and softening in the forward and reverse straining were defined. A hardening rule to formulate the Bauschinger effect in complex loading conditions was also presented. The desired bend angle in V-die bending is usually obtained by controlling the punch insertion depth, so that springback can be compensated for by the correct amount of overbending. A process model was developed for V-die bending of sheet metals. This model was proved to be especially valuable to control a press brake equipped with a computer integrated controlling system Pure bending of elasticplastic materials was simulated. Analysis of the springback and residual stresses on unloading of bending moment was carried out. The predicted residual stresses were compared with those obtained by the X-ray diffraction and good agreement was found. For large bending curvature, the Bauschinger and anisotropic effects were taken into account for rigidplastic materials. Two models were derived and implemented in a computer simulation of stress-strain distribution, thickness change and bending moment magnitude versus bending curvature. The analysis showed that different materials responded to the Bauschinger effect in different ways, the stronger the effect on a material, the greater the thinning in the bend. Pure bending tests confirmed the theoretical predictions. It was found that the anisotropic effect on material thinning in the bend was relatively small but the effect on the springback was significant.