Profile defects caused by inhomogeneous longitudinal strain distribution in roll forming

Roll forming is a sheet metal forming operation that incrementally forms flat sheets into a desired profile geometry. The process is characterized by a high material utilization and a high output quantity. Concomitant with these advantages, profile defects such as bow and twist of the profile can occur. In the literature, an inhomogeneous longitudinal strain distribution across the profile cross-section is considered to be the cause of these defects. However, a quantitative cause and effect analysis is missing up to now. This paper presents an analytical model that shows a quantitative relationship between profile defects and the underlying longitudinal strain distributions. The model can be used to calculate the longitudinal strain distribution of a roll-formed profile across its cross-section based on given values for bow and twist or vice versa. It is compared with results from simulations and experiments and clearly reveals the cause for twist and bow in roll forming.