Microscopic investigation of evolution of micro-crack in tight radius bending with advanced high strength steels

K. Jeong, Y. Jeong, J. Lee, G. Kim, J. Yoon
Hanyang University,

Keywords: 3-point-bending test, plane strain failure, VDA-238-100, bending failure, local formability


Fracture of sheet metal in bending mode exhibits unique characteristics such as constant triaxiality, no necking, and shear-band induced crack, which differ from tensile fracture phenomena. Particularly, for AHSS, the fracture limit in bending mode is difficult to predict using tensile fracture test results due to the influence of phase effects. Therefore, the bending fracture limit, often referred to as local formability, is evaluated with tight-radius 3-point bending (3PB) tests following the VDA-238-100 standard. However, the current failure criterion based on the force drop in the VDA-238-100 standard has been questioned by recent studies due to the continuous and slow nature of crack propagation in bending. To address this issue, this study proposes a failure point based on defect evolution and investigate its relation to the VDA-238-100 failure point. An integrated line-scanning system for the VDA-238-100 test was developed to quantify defect evolution alongside surface and cross-sectional image analysis. AHSS with strengths of 980~1500 MPa was tested in both RD and TD directions. The results reveal that for all materials and directions, the outer fiber of the specimen fails completely at the VDA-238-100 failure point, with the evolution of microcrack depth and width following an exponential growth pattern. The force drop point, however, is not the onset of crack, but rather a transition in the crack propagation direction. The true onset of crack is identified as a crack depth exceeding 1% of the sheet thickness.