Beyond necking - Novel characterisation method to investigate the tensile response of sheet metals at elevated temperatures and constant true strain rates

Characterisation of the strain hardening behaviour for metal forming simulations at elevated temperatures still poses a challenge for state of the art characterisation strategies, where often microstructural changes shall be investigated simultaneously. To do so, GLEEBLE thermo-mechanical simulators are commonly used to conduct physical testing of materials at thermal and mechanical loading at a wide range of loading rates. Such testing systems introduce a known challenge of non-uniform temperature into the specimen which leads to inhomogeneous strain distributions. This heterogeneous strain distribution consequently leads to an uncontrolled forming area together with an undesirable increase of strain rate. To address that challenge, this contribution presents and applied a novel approach to conduct locally controlled tensile tests by utilising a 3D-DIC system for a closed loop strain rate control. The titanium alloy Ti6Al4V with a thickness of 1.5 mm was investigated at temperatures between 600 °C and 900 °C and strain rates between 0.01 s -1 and 0.1 s -1 . The novel testing method leads to a change in the analyzed flow stress at 600 °C of up to 80 MPa and at 900 °C of up to 65 MPa.