On the formability limits of thin-walled tube inversion using different die fillet radii

This paper is focused on the external thin-walled tube inversion using different die fillet radii and is aimed at analysing its formability limits by local buckling, necking and fracture in principal strain space. The investigation draws from a methodology that was recently developed by the authors for general tube forming and focuses on the specificities that characterize its application to the deformation conditions of external tube inversion. Strain loading paths and formability limits at the onset of buckling, necking and fracture are determined by combination of experimental strain analysis with digital image correlation (DIC), thickness measurements, force-displacement evolutions and finite element modelling. Results show the paramount importance of the curling radius on the formability limits of external tube inversion. This information is relevant for designing energy absorbers based on external tube inversion and the novelty of the paper stems from the use of experimental techniques commonly employed in metal forming to the analysis of formability and failure in typical deformation modes of energy absorbers.