Process window of tube-end inversion: experimentation and numerical analysis

Abstract

A systematic methodology for determining the formability limits of thin-walled tubes based on strain analysis has been recently presented by the authors with the aim of evaluating formability and failure within the principal strain space, analogously to the conventional procedures used in sheet metal. This methodology for establishing the formability limits in tube-end expansions combined numerical simulations and analytical approaches. Preliminary investigations have been conducted with the aim of applying the same principles to other tube forming processes. In this context, this paper focuses on the investigation of the formability limits of tube-end external inversion and combines the experimental determination of strain paths by means of digital image correlation (DIC) with finite element (FE) modelling. Results show that the process window is limited by local buckling and by fracture as a function of the inversion radius of the forming die.