Longitudinal strain and wrinkling analysis of variable depth flexible roll forming

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Flexible roll forming (FRF) enables the forming of high-strength sheet metal materials into complex shapes relevant to the transportation industry until recently, however, has been limited to the forming of variable-width profiles where the web section is flat with a curved flange. Recent developments in 3D flexible roll forming now enable forming of variable depth components that have a curved web with flat flanges; these shapes are better suited for automotive applications. However, wrinkling in the flange due to excessive compressive longitudinal stress remains a major challenge. Deformation strain and stress analysis has been applied in previous studies to estimate the wrinkling tendency, but current analytical equations are limited to variable-width shapes with limited accuracy, particularly regarding the level and distribution of longitudinal strain. In the current study, a new analytical model is developed to analyse the distribution and level of longitudinal edge strain for a variable depth component shape. The analytical predictions are compared with experimental measurements for key conditions. This shows that the new analytical equations developed in this study provide higher accuracy compared to existing models and enable for the first time the prediction of the distribution of longitudinal strain in the flange of a flexible roll forming component. For some of the tested conditions, flange wrinkling was observed and there the model correlation was poor given the theory assumes a wrinkle-free flange. The strain deviation observed here is associated with the shape deviation due to wrinkling. The analytical model predicts the strains that are required to form a perfect part shape. Wrinkling reduces the compressive longitudinal strains that are formed into the flange, and this leads to a poor correlation between the experimental and the predicted strain values if the wrinkling severity is high. The model brings some insights into the wrinkling behaviour in variable depth flexible roll forming by predicting the variation of the peak strain and overall distribution with key geometric parameters. The new analytical model for longitudinal edge strain presented in this study will enable a more rapid process design and optimisation before time-consuming FEA analysis is performed. It, therefore, presents a vital step towards a wider spread in the application of flexible roll forming in the automotive and transport industries.