Finite element-based method for residual stresses and plastic strains in cold-formed steel hollow sections

Residual stresses and plastic strains in steel tubes are induced by their manufacturing processes and play an important role in determining their mechanical and structural behaviour. The manufacturing process of cold-formed steel hollow sections can be divided into four main stages: (1) the coiling and uncoiling of a steel strip, (2) the transverse bending of the uncoiled strip into a circular shape, (3) the welding of the circular tube along the longitudinal strip edges, and (4) the shaping of the welded circular steel tube into a specific cross-sectional shape. In this paper, a finite element-based method is presented for predicting residual stresses and equivalent plastic strains in cold-formed steel hollow sections (including elliptical, square, and rectangular shapes of both normal grade steel and high strength steel). In this method, residual stresses and equivalent plastic strains due to the coiling, uncoiling and transverse bending operations are predicted analytically, and applied as the initial state for the subsequent finite element simulations of both welding and shaping operations. Predictions from this method show satisfying agreement with experimental measurements, which demonstrate the validity of the proposed method. This method can predict the distributions of residual stresses and equivalent plastic strains across the thickness and over the cross section, which are too complex to be measured completely in laboratory. The method provides a tool for investigating the effects of different material and forming parameters on the resulting cold work and structural behaviour.