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This work focuses on the effect of strain rate on the mechanical response and adiabatic heating of two austenitic stainless steels. Tensile tests were carried out over a wide range of strain rates from quasi-static to dynamic conditions, using a hydraulic load frame and a device that allowed testing at intermediate strain rates. The full-field strains of the deforming specimens were obtained with digital image correlation, while the full field temperatures were measured with infrared thermography. The image acquisition for the strain and temperature images was synchronized to calculate the Taylor-Quinney coefficient (β). The Taylor-Quinney coefficient of both materials is below 0.9 for all the investigated strain rates. The metastable AISI 301 steel undergoes an exothermic phase transformation from austenite to α'-martensite during the deformation, which results in a higher value of β at any given strain, compared to the value obtained for the more stable AISI 316 steel at the same strain rate. For the metastable 301 steel, the value of β with respect to strain depends strongly on the strain rate. At strain rate of 85 s −1 , the β factor increases from 0.69 to 0.82 throughout uniform elongation. At strain rate of 10 −1 s −1 , however, β increases during uniform deformation from 0.71 to a maximum of 0.95 and then decreases to 0.91 at the start of necking.Publisher's Note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Despite tensile testing being commonly used for investigating the mechanical behavior of materials, the occurrence of heterogeneous strain and increasing temperature at high strain rates make the experiment much more complex. This work presents a method integrating synchronous full-field stereo Digital Image Correlation (DIC) and Infrared Thermography (IRT). This method enabled high resolution investigations of the development of local temperatures and strains of the specimen during tensile loading of four steels at strain rates ranging from 2.5·10−4 to 900 s−1. The tests were monitored by a stereo setup of optical cameras and an infrared camera. Data acquisition was synchronized, and a pinhole camera model was used to translate the images from all cameras to the same three-dimensional space. The displacement vector fields from DIC were subtracted from the IRT images to represent the temperature maps in a Lagrangian coordinate system. The overall thermomechanical response of the materials was shown as 3D waterfall plots, which represent localized strain and temperature as a function of position and engineering strain. The results show that temperature increased homogeneously during uniform deformation at higher strain rates (10−2-900 s−1) and increased markedly with the onset of necking on the region of localized strain. At these strain rates, the localized increase of strain and temperature during necking were observed at the same global engineering strain and position, evidencing the spatial and temporal synchronization. The described method was used to accurately investigate the evolution of localized strain and temperature in both low and high strain rate regime.

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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.