Abstract
Hole expansion using a flat-bottomed cylindrical punch was carried out to clarify the fracture behavior of high-strength steel sheets by changing the initial hole diameter, d0. Experimental results showed that the hole expansion ratio, λ, became larger as the initial hole diameter, d0, increased. Moreover, the fracture site around the hole shifted considerably with the initial hole diameter, d0, in the case of high-strength steel sheets with high r-value. On the other hand, the analytical results exhibited poor coincidence with experimental ones when the initial hole diameter was small, whereas good correlation was observed with a large initial hole diameter. As a result, it was clarified that plastic deformation behavior during stretch flange forming performed using a flat-bottomed cylindrical punch is influenced not only by the planar anisotropy of the mechanical properties of steel sheets but also by the initial hole diameter.
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