Effect of inclusions on initiation and Propagation of Crack in small-Hole Punching of Fe-420/ONi Alloy. Mechanism of Burr Formation in Punching of Meta1 Sheet

摘要

The mechanism of burr formation in punchinghas been investigated with laboratory-produced Fe42%Nialloys which contain different amounts of sulfur,0 .001 and 0.008mass%. Cold-rolled 0.4mm-thick sheetswere punched to form 0.4mm-diameter holes with toolsmounted in an instron-type universal testing machine.The cross sections of the specimens which were removedfrom the tools at various stages of punching wereinvestigated by optical microscopy. Cracks are alwaysinitiated near the corner of the die and propagatetoward the corner of the punch. In the alloy whichcontains 0.00lmass% S, cracks do not propagate steadilyand the material flow near the corner of the die afterthe initiation of the first crack results in a finallarge burr of the hole. In the alloy which contains0.008mass% S, however, cracks rapidly propagate via MnSinclusions and only a small burr is formed. Thegeometric position at which the crack is initiated ispredicted well from the distribution of hydrostaticstress analyzed by visioplasticity.