The metal fracture mechanism in cup ironing process was investigated, taking into account the effect of the galling. Three types of metal cracking were observed, which are distinguishable from each other in crack appearance. The axial fracture stress measured at the ironed cup wall decreases as the galling becomes heavy, though the tensile strength of the ironed wall metal is scarcely decreased by the surface damage caused by the galling. In such cases, a stress concentration is always caused by the uneven circumferential distribution of the friction at the die-metal interface. The cracking begins at the circumferential position where the axial stress exerted on the ironed cup wall partially reaches the axial tensile strength of the ironed wall metal. The mean axial fracture stress is thus decreased. This also yields the decrease of crack propagation velocity and consequently the difference in crack appearance. The interaction between the tribological phenomena and the metal fracture behavior in the ironing process was clarified.

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