Finite element predictions of residual stresses in press-braked thin-walled steel sections

Residual stresses in cold-formed sections may play a significant role in determining their behaviour and strength. Laboratory measurements of residual stresses by destructive methods are not only time-consuming but also of limited accuracy. This paper presents a finite element-based method for predicting residual stresses in press-braked thin-walled sections, which overcomes these difficulties. In this method, the effects of coiling and uncoiling are accounted for analytically, with the resulting residual stresses specified as the initial stresses in a subsequent finite element simulation of cold bending. The method provides residual stress distributions over the cross-section as well as across the thickness. Numerical results from this method are shown to agree closely with laboratory measurements, demonstrating the validity and accuracy of the method. Numerical results presented in the paper show that the maximum residual stresses in a press-braked section generally occur in the corner region and away from the surfaces, and their values can be much higher than those at the surfaces. This means that the conventional method of measuring the surface residual stresses in the laboratory and assuming a linear variation across the plate thickness may greatly underestimate the real residual stresses. The results also explain why residual stresses may differ considerably in otherwise identical sections. The method offers a powerful tool for exploring the effect of different forming parameters on the magnitude and distribution of residual stresses so that these forming parameters can be optimised.