Finite-element analysis to predict residual stress and strain in press-braked cold-formed steel section

Cold formed steel sections are normally produced by cold work manufacturing processes. The amount of cold work to form the sections induces residual stresses and strains in the sections, especially in the forming zone and that due to these effects the load carrying capability of the structural members can be significantly reduced. In design, the longitudinal residual strain has been an important indicator to determine the geometric behaviour of the formed section. Over the years, many attempts have been made to investigate the residual stress/strain behaviour: analytical, laboratory measurements and numerical predictions. Numerical method has been recommended by researchers as the appropriate method to overcome the analytical limitation and laboratory work drawback (e.g. tedious calculation in complex sections and material wastage). Most studies have investigated the effect of residual stresses induced from roll- forming operation and its influence on steel section behavior. However, press- braking operation has received less attention. Hence, finite element simulation was employed in this study to examine the press-braked forming response of thin- walled sections giving due consideration to the influence of element selection, section springback and to material isotropic and anisotropic behavior. The findings from this work highlighted the complete press-braking history of the sections from the onset of elastic pressing through the nonlinear elastic and elasto- plastic phases of behaviour to final braking and unloading. A detail account of the growth and distribution of residual stresses/strains along the length of the corner region and through thickness was given in the thesis. The results from the finite element simulations were shown to compare well with independent simulations using the 2D-FE method of analysis and also experimental work. The thesis concluded that the residual stresses were not always linear longitudinally (along the corner region). For the carbon steel, the maximum residual stresses existed near the middle surface of the plate. The middle surface contained a combination of compressive and tensile residual stresses. The comparison between the 3D-Solid FE of this study and the 2D-FE of the independent study demonstrated that the 3D-Solid FE results showed a variation in the transverse and longitudinal residual stresses along the plate length. Also, the longitudinal residual strain at the section corner edge was higher than those at the rest of the corner region. These strains at the edge were higher than the yield strain (εy) of the formed section which occurred due to the lack of transverse restraint. In addition, 3D-Shell FE technique could predict longitudinal residual strain at inner/outer surfaces as efficient as the 3D-Solid FE technique but the later was more capable in predicting the through-thickness residual stresses. In the anisotropic materials like stainless steels, the maximum tensile residual occurred at the corner outer surface for sections with the corner radius-to-thickness ratio equal or less than 2 (r/t ≤2).

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