Usually, criteria of stretch flange crack were greatly affected by the edge treatment. In the case of machining edge, it was said that strain gradients along the radial direction are important parameters for the criteria. However, the mechanism is not clear.
In this study, hole-expansion-tests with machined holes were carried out experimentally. The relationship between strain gradient and fracture strain was investigated. As same as the conventional knowledge, it was confirmed that the fracture limit was improved with the increase of strain gradient. Moreover, focusing on the strain state and necking angle at the necking point, the necking occurred along the radial direction, and the distributed strain states are between plain strain and uniaxial strain. Taking into account of expected necking direction, the fracture limit strain on stretch flange crack could be predicted by the modified M-K model.