Prediction of forming limit for sheet metals between equi-biaxial tension and uniaxial tension using a new ductile fracture criterion

Accurately predicting the forming limit (FL) of sheet metals between equi-biaxial tension (EBT) and uniaxial tension (UT) is a research focus in sheet metal forming field. However, up to now, there is still a lack of an uncoupled ductile fracture criterion (DFC) which not only can predict the FL of sheet metals between EBT and UT accurately but also can be easily extended to other DFCs for bulk and sheet metals, preventing embedding only one uncoupled DFC into finite element analysis (FEA) software to satisfy the application requirements of fracture initiation prediction within different stress state ranges. The aim of this paper is to address the issue. The problems encountered in recently developed DFCs are firstly revealed and then a new uncoupled DFC is presented according to the analyses of void evolution of sheet metals. To fully understand the proposed DFC, parametric studies are conducted. In addition, the proposed DFC and four recently developed DFCs are employed to forecast the FL strains of three different sheet metals and the extensibility of the proposed DFC is studied. Research results show that the proposed DFC can not only forecast the FL of various sheet metals between EBT and UT reasonably but can also be easily extended to a unified uncoupled DFC for bulk and sheet metals, enabling the implantation of one DFC into FEA software to meet the application requirements within different stress state ranges. Moreover, the capability of the presented DFC to forecast FL in actual forming process is also verified.