Strain rate jump tests on an austenitic stainless steel with a modified tensile Hopkinson split bar

¹ Tampere University of Technology, Laboratory of Materials Science, Korkeakoulunkatu 6, P.O.Box 589, FI-33101, Finland
² Materials and Simulation Methods, Fraunhofer Institute for High-Speed Dynamics, Ernst-Mach-Institut, Eckerstrasse 4, 79104 Freiburg, Germany

^* Corresponding author: naiara.vazquez@tut.fi

Published online: 7 September 2018

Abstract

This paper presents an improved experimental setup for high strain rate testing based on the modified Tensile Hopkinson Split Bar device developed previously at TUT. The test setup can be used to study the effects of a sudden large change in the strain rate on the stress flow of the material. The setup allows deforming the sample at a low rate and at isothermal conditions before the high rate loading. During the strain rate jump, the deformation rate is rapidly increased by approximately six orders of magnitude. In this work, the low and high rate deformation of the specimen was recorded with a combination of low and high-speed digital cameras and analyzed using the Digital Image Correlation technique. The measurement provides information about the effects of the strain rate jump on the macroscopic response of the material and allows accurate observation of the deformation of the sample just before, during, and immediately after the strain rate jump, when the conditions change from isothermal to adiabatic. In this paper, we present the results for a metastable austenitic stainless steel and discuss the effects of the strain rate jump on the strain-hardening rate, compare the experimental results with numerical results from a thermomechanical model, and evaluate the effects of the preceding deformation at a low strain rate on the strain localization. We conclude that the strain rate jump results in a clear decrease in the strain-hardening rate, the deformation following the jump is uniform along the gauge section, and that the strain localization is not significantly affected by the strain rate or the amount of pre-strain in the studied conditions.