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Abstract. Safety-relevant components in the automotive industry must fulfill stringent requirements for performance, particularly in endurance. The height adjustment system in a car seat is one example. The height adjustment system is made up of sheet metal parts which are joined with steel tubes through axial compression – also referred to as upset bulge forming. As a result of the forming process, initial cracks occur at the inner bending radius of the tubes which, along with the forming history, decrease the fatigue life by 450%. The aim of this study is to identify whether the initial cracks occur due to shear bending generated by the process or elastic springback. The influence of the elastic springback on the crack initiation is investigated through bending experiments of DP600, Al99.5, and 1.4301. To prevent elastic springback, the specimens are secured with screws post-bending and are removed afterwards. The bending radius is investigated with respect to the equivalent plastic strain to determine a minimum without failure. Cyclic tests of bulges with two different forming histories are tested to investigate the influence of the forming pre-strain on the fatigue behavior. In all bent materials, initial cracks occur at the inner bending radius where the elastic springback increases the initial cracks and Scanning Electron Microscopy (SEM) analysis confirms shearing as the cause. It was also found the grain shape and orientation have a significant influence on the crack propagation in the forming zone of the tube through the upset bulging process.
Jan Bechler (Fri,) studied this question.