Analysis of deformation during structural tube hydroforming.

Abstract

Tube hydroforming is a process used to form tubular sections of various geometries from initially circular tubes. This thesis focuses on the tube hydroforming for structural components. The main topics for this thesis are corner filling, wrinkling detection, and feeding optimization. The analysis contributes to a better understanding of the deformation in tube hydroforming, to the establishment of guidelines for the selection of the process variables, and to an efficient utilization of the process capabilities. The first part of the dissertation focuses on the role of friction at the tube/die interface in determining the distribution of the thickness during deformation. An analytical model is developed to determine the variation in the stresses and strains along the tube wall as the internal pressure increases to expand a circular tube into a square cross-section. A parametric study is conducted on the effects of the coefficient of friction and the material properties on the distribution of cross section thickness in the hydroformed tubes. The model explains the mechanism through which friction affects the deformation during tube hydroforming. The second part is concerned with the determination of the optimal end feeding scheme during tube hydroforming. The selection of loading curve, which relates the tube internal pressure with the end feeding, is crucial to the success of the hydroforming. One way to optimize the loading curve is adaptive loading using finite element simulation. An essential part of any adaptive loading simulation is a wrinkling indicator. The problem of wrinkling detection is addressed for tube hydroforming using a new geometrical wrinkling indicator. Wrinkling is detected based on the direction of the normal to the surface velocity and on the second order variation of the normal velocity, along axial lines on the tube surface. The application of wrinkling detection during tube bending is illustrated. The variation in the developed wrinkling indicator is correlated to the start and growth of wrinkling during tube bending. Adaptive loading simulation is applied on a complex geometry 3-dimensional hydroforming. The effect of the control parameters of the adaptive loading scheme on the final quality of the formed part is evaluated.