On the Numerical Assessment of Failure in Stretch-Flanging by SPIF Using Equivalent Strain Versus Triaxiality Diagrams

In the last few years, there has been a growing interest in the sheet metal forming community for analyzing the enhancement of formability attained in non-proportional forming processes such as incremental sheet forming (ISF), and especially in its dieless variant single point incremental forming (SPIF). To this regard, this analysis using classical forming limit diagrams (FLD) do not provide enough information for understanding the conditions upon which this postponed failure is attained. As an alternative, some recent studies made use of equivalent strain versus stress triaxiality diagrams, which proved to be more suitable for evaluating the non-proportional strain paths that lead to failure in SPIF. On the other hand, flanged parts are commonly used in the aircraft industry to provide stiffness to the manufactured components as well as a target area for the assembly to other metallic parts. To this regard, the authors have recently carried out research works aiming investigating the formability and failure modes of AA2024-T3 sheet 1.2 mm thickness in stretch and shrink flanging by SPIF. These studies were carried out combining experimentation and numerical simulation using Finite Elements (FE), with the aim of evaluating and assessing the different modes of failure that occurred. In this context, the present contribution consists on a numerical evaluation of failure by using a FE modeling of the stretch flanging process in LS-DYNA. To this purpose, Barlat’s anisotropy is considered for the assessment of the forming limit at fracture within the material equivalent strain versus stress triaxiality diagram. The resulting numerical model, calibrated using principal strains experimental results, allows predicting the sheet material fracture and the mode of failure attained, which can be either failure by fracture at the flange edge or at the flange corner depending on the set of process parameters.