•New SPIF geometry with strain pairs in the shear fracture forming limit line (SFFL).•Reopen the discussion on plastic flow and failure in SPIF. The aim of this paper is twofold: firstly, to identify a new failure mechanism by in-plane shear cracking in single point incremental forming (SPIF) and, secondly, to present a simple geometry to be produced by SPIF that is capable of providing fracture strain pairs to define the in-plane shear fracture forming line (SFFL) corresponding to crack opening by mode II of fracture mechanics. The methodology draws from the analysis of several pioneering studies in SPIF, where strain loading paths evolve in the tension–compression quadrant of the principal strain space, to setting up an experimental work plan aimed at identifying the forming conditions where crack opening by thinning under tension is replaced with crack opening by in-plane shear (distortion with limited thinning). The overall procedure makes use of circle-grid analysis and thickness measurements before and after fracture at several locations along the cracks in order to obtain the ‘gauge length’ strains. It is shown that a truncated four-lobe conical shape with varying drawing angle can be successfully employed to define the SFFL and to reopen the discussion around the physics behind plastic flow and fracture in SPIF.