A Critical Evaluation of Forming Limit Curves Regarding Layout of Bending Processes

Abstract

Forming limit curves (FLCs) are important to predict failure against biaxial deformation in sheet metal forming. Particularly crucial is the detection and evaluation of the stable/instable transition which indicates necking and ultimately leads to rupture. In the past, it has been observed that specific processes, in particular free-form bending processes, might not be predicted well by conventionally evaluated FLCs, i.e. the Nakajima experiment, where tapered sheet metal specimens are stretched over a hemispherical punch until material failure. In the present study, FLCs are evaluated from such Nakajima tests and from notched tension tests (NTT) highlighting large differences in between both results. The differences in between the evaluation methodologies are discussed with respect to contact and bending strain in the forming zone. The FLCs of three tested sheet metal materials are compared to fracture strain resulting from an incremental bending process with free forming zone demonstrating an increased failure prediction accuracy by the NTT FLCs than by the conventional ones. In the light of these results, it is therefore encouraged to critically assess the application of FLCs obtained from NTTs to various free-form bending processes.