Thermomechanical Behavior of Steels in Tension Studied with Synchronized Full-Field Deformation and Temperature Measurements

Abstract

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.

Simultaneous Full-Field Strain and Temperature Measurements in Tensile Hopkinson Bar Experiments at Extreme Temperatures

“…The IRT images were deformed using the displacement vector fields obtained from DIC so that both temperature and strain data could be analysed in the reference frame of the undeformed sample. A more detailed description of the temporal synchronization, spatial calibration, and representation on the data in the same frame of reference can be found in reference [2]. At elevated temperatures, the specimens did not always break during the first loading pulse.…”

“…DIC enables measurements of localized strains and IRT allows the measurements of local temperatures during mechanical loading, and therefore a much more comprehensive description of the thermomechanical response of the material to dynamic loading can be described with high spatial resolution. Although there already are works that use DIC and IRT to investigate material behaviour at high strain rates [2,3], full-field strain and temperature measurements at elevated temperatures have only been possible at lower acquisition rates [4][5][6]. The use of DIC and IRT for measuring the full-field strains and temperatures at high strain rates and elevated temperatures has not yet been reported on the literature.…”

Combined thermographic phosphor and digital image correlation (TP + DIC) for simultaneous temperature and strain measurements

“…Again, see Section S4.2 in the Supporting Information for details. Note that the emissivity calibration was performed here on an undeformed specimen, which is typical of emissivity calibrations in the literature (see, e.g., Soares et al and Cholewa et al [8,9] ). This emissivity characterization does not take into account potential errors from paint changing emissivity and/or cracking during testing.…”

“…Spatial registration of the two data sets is also particularly important as the IR temperature data is in an Eulerian framework while the DIC strain data is in a Lagrangian framework. Transforming both data sets to the same threedimensional coordinate system and point cloud can be aided by imaging a dot-grid calibration target with the DIC and IR cameras simultaneously, as in Soares et al and Cholewa et al [8,9] However, this process requires a calibration target with contrast in both the visible and long-wave IR wavelengths, which we did not have for this study. Therefore, we registered the two data sets in post-processing using the process outlined in Section S4.3 in the Supporting Information.…”

“…For combined thermal and strain measurements, infrared thermography has been combined with DIC (IR + DIC) and is commonly employed. [2][3][4][5][6][7][8][9][10][11] This IR + DIC methodology, while successfully demonstrated, has a few obstacles, including: (1) the necessity to carefully characterize the specimen emissivity, (2) the requirement of multiple high resolution and often costly cameras, (3) the geometric constraints on positioning the cameras to obtain the desired field of view in all images, (4) a limitation on the maximum image acquisition speeds, particularly for IR cameras, (5) the potentially mismatched synchronicity between IR and visible images, and (6) the post-processing requirements to register measurements from both diagnostics onto a common point cloud in 3D space. In part due to these obstacles, researchers have also sought unique ways to utilize single IR cameras or even single CCD chips for combined thermomechanical measurements.…”