Ejector pin-related high-cycle fatigue fracture of the critical die corner during automatic multistage cold forging of an automotive wheel nut

An experimental and numerical study on the ejector pin's mechanics during automatic multistage cold forging (AMSCF) of an automobile wheel nut is conducted. The traditional, decoupled die structural analysis method (DDSM) of analyzing die structures as one of the post-processing functions is criticized, which uses the tractions exerting on the die parts predicted from the forging simulation under the rigid die assumption. To cope with the matter of the DDSM, a multibody treatment scheme (MBTS) is proposed to simulate the AMSCF process, emphasizing the ejector pin's mechanics, using an implicit elastoplastic finite element method. The experiments qualitatively validate the finite element predictions. It is shown that the asymmetric sheared material in AMSCF greatly influences the ejector pin's mechanics, which is characterized by its lateral and longitudinal displacements because of its structural flexibility. It is emphasized that the detailed understanding of the ejector pin's mechanics may not only give a helpful connection towards smart manufacturing because of its mechanical flexibility and sensitivity to the excitations and responses, but it also reveals the reason for the die's high-cycle fatigue (HCF) fracture of the critical die corner (CDC) near at the end of the ejector pin.