2012 年 53 巻 620 号 p. 836-841
Numerical simulation by a finite element method investigated the mechanism of roughness improvement in pierced holes using a press working (PW) punch. Two types of simulation, i. e. piercing and crack conjunction, were performed. Piercing simulation suggested that the PW punch suppresses both the generation and propagation of a crack in a sheet material around the punch edge because of the suppression of plastic strain localization, but promotes the generation and propagation around the die edge because of the high stress triaxiality induced. This result implies the suppression of the mismatch of the two cracks by the dominant crack from the die edge. Crack conjunction analysis based on bifurcation and stress triaxiality showed that crack mismatch increases the unevenness of a fracture area of pierced surfaces.