Ultraviolet Image-based Techniques for Strain Measurement at Extreme Temperatures

Abstract: Extreme temperatures play a role in a growing number of engineering applications, including hypersonic flight, gas turbine engines, spacecraft re-entry, and next-generation nuclear reactors. In order to design for any of these applications, structural materials must be characterized to ensure that they can withstand the combined thermo-mechanical environment. One popular characterization method is digital image correlation (DIC), in which a deformable test specimen is patterned with a high-contrast surface pattern, then recorded with a high-resolution digital camera before and after deformation. An algorithm then compares the images to compute full-field displacements and strains. At extreme temperatures, materials emit light in the form of blackbody radiation which can saturate the camera sensors. This light is known to be brighter at longer wavelengths, and can be mitigated by using optical bandpass filters. In this work, it is shown that by using ultraviolet (UV) cameras, lenses, and filters the temperature range of DIC can be effectively extended. The UV-DIC technique is then applied to a variety of 2D and 3D applications in order to measure heterogeneous strains at various temperature, time, and length scales.

Additionally, standard 3D measurements using DIC usually involves the use of at least two cameras, which can be both costly and difficult to synchronize. More recently, other researchers have demonstrated a novel method known as Diffraction-Assisted Image Correlation (DAIC), which makes 3D measurements using only a single camera. This is accomplished by placing a diffraction grating between the camera and the specimen, resulting in multiple views that can both be captured by one camera. The diffraction grating requires that testing be performed using a monochromatic light source. DAIC has already been demonstrated at room temperature using monochromatic red light, but in principle other colors should work as well. In this work, DAIC is extended to extreme temperature conditions by substituting a UV monochromatic light source and filters for the red. This new method, UV-DAIC, provides a cheap and effective alternative to stereo UV-DIC for measuring 3D deformation and strain at a high temperature. The method is demonstrated to measure the out-of-plane deformation of a beam which bends while at non-uniform high temperature.