Flanging is a forming process that is commonly used in many high technology industrial applications such as the manufacturing of thin-walled aircraft structures as wing ribs that are formed using rubber-pad forming during the last forming stages with the aim of increasing the stiffness of the component edges. In addition, the performance of hole-flanged parts manufactured using incremental sheet forming (ISF) processes has been recently addressed, focusing especially in evaluating the formability enhancement, as well as the onset of failure utilising a variety of forming strategies. In this context, this work presents an experimental analysis of formability and failure within the forming limit diagram (FLD) of stretch flanges of AA2024-T3 performed by single point incremental forming (SPIF) for a wide range of process parameters. The numerical modelling of the stretch flanging process in combination with a theoretical framework based on Barlat's anisotropy plasticity criterion, allowed to assess the formability of the material within the triaxiality space, permitting to understand the stress/strain states attained in SPIF and the forming conditions upon which the onset of failure occurs. The results show that this triaxiality space, defined as the accumulative effective strain versus the average stress triaxiality, might be the most appropriated space for analysing the highly non proportional process of stretch flanging by SPIF.