On the evaluation of dimensional accuracy in rotary stretch bending

Abstract

The main objective of this work is to quantify the impact of the most important parameters on dimensional accuracy in stretch forming, combining analytical models and experiments conducted in industry-like stretch forming operations. Using the deformation theory of plasticity, an analytical moment-curvature model for prediction of deformations of a curved flange under the action of longitudinal stretching and transversal (local) bending was established. By combining suitable equilibrium and compatibility equations, the model was used for prediction of cross-sectional distortions. The overall results formed the basis for the development of a closed-form relationship for prediction of cross-sectional distortions. This model explicitly expresses the most important influential parameters and their impact on cross-sectional distortions. The results show that anisotropy, as represented by a Hill (1948) relationship, is the most important material characteristic with regard to suck-in of individual cross-sectional members. The strain hardening characteristic and the initial yield stress level of the material have very limited impact on local distortions. Moreover, geometrical parameters such as flange width, flange thickness, cross sectional height and web depth, play an important role with regard to dimensional accuracy. The analytical model also shows that the flange width is the single most important design parameter with regard to the nominal value of the flange’s suck-in, whereas the flange thickness is the main source to variability, i.e. noise parameter. The analytical results show remarkably good correlation with experimental results obtained in a full-scale rotary stretch forming setup.