Tensile test of a HSLA steel at high strain rates with two different SHTB facilities

The dynamic stress-strain curves obtained by Hopkinson bar testing can be subjected to a variety of possible approximations including hardware-based issues and/or accuracy reductions due to the procedures for the analysis and the post-processing of the experimental data. In order to highlight the possible occurrence of such approximations and to eventually assess their magnitude, quasi-static and dynamic tensile tests have been carried out on a 38MnSiVS6 steel, adopting two different Split Hopkinson Tensile Bars (SHTB) adopting different mechanical solutions for the input bars clamping / releasing systems. High speed video acquisition with frames extraction and image analysis delivered the evolving specimen geometry and its dimensions all over the test duration. Slightly different procedures are also adopted for the analysis of the raw experimental data leading to the relationships between stress, strain and strain rate for each test. The stress–strain curves and the strain rate–strain curves are derived from both series of experiments according to the engineering approach and to the true approach, the latter being obtained according to different hypotheses for the post-necking evolution of the true curve. High speed camera helped in measuring via image analysis the current evolving area of the neck section allowing to derive the true strain, true strain rate and true stress to be compared to the similar variables determined from the strain gauges on the bars.