2012 Volume 98 Issue 7 Pages 378-387
In order to clarify the governing factors of stretch flange-ability, a study to elucidate the mechanism of crack growths on pierced edges during hole- expanding tests was carried out and the following conclusions were obtained.
1) In the beginning and the middle stage of crack growth, cracks on the pierced edge propagate through its thin surface where a large strain has been introduced by piercing. On the following final stage one of them grows rapidly into deeper area where the material is not or only slightly strained by piercing. Hole-expansion ratio is characterized by the final rapid crack growth.
2) Materials with good stretch flange-ability have strong crack growth resistance which can be measured by the test method based on fracture mechanics. This is due to the fact that stretch flange-ability is characterized by the final rapid growth of edge cracks and its occurrence is basically dependent on the crack growth resistance based on fracture mechanics.
3) Thus measured crack growth resistance makes it possible to evaluate a material property which is strongly linked to stretch flange-ability of hot rolled high-strength steel sheets with smaller data scatterings than hole-expansion ratios.