Inverse FE Analysis of Combined Tension–Torsion Tests Performed with a 90 m Hopkinson Bar

In this work, the results coming from the recently developed Split Hopkinson Tension–Torsion Bar have been post-processed according to finite element model updating approach. The aim is to assess the elastoplastic constitutive behaviour of the material subjected to a multiaxial state of stress in the framework of large deformations. The experimental test consists in the application of a simultaneous tensile and torsional load to a hollow cylindrical-shaped sample; pure tension and pure torsion tests have been conducted as well. Both displacement (elongation and twist angle) and load (axial force and torque) values are measured. In the tests with the Split Hopkinson Tension–Torsion Bar, an average strain rate of 100/s was reached. In addition, analogous tests with similar load-torque ratios were performed with a quasi-static multiaxial machine. The experimental test was replicated in an Abaqus/Explicit FEM model, where the constitutive parameters are iteratively varied until an adequate match was obtained with the experimental observations in terms of force–displacement law. In particular, a power law was used for the strain hardening description, combined with the classical von Mises yield criterion. The material of the sample was AA7075T6, whose Johnson–Cook strain rate sensitivity parameters were borrowed from the literature. A reasonably good matching was achieved between the numerical and experimental load–displacement and torque-rotation, meaning that the classical von Mises plasticity describes quite well the plastic behaviour of the material; the model was also able to capture the effect of the non-proportional loading path applied in the combined tension–torsion test.