A high magnification UV lens for high temperature optical strain measurements

Digital Image Correlation (DIC) measures full-field strains by tracking displacements of a specimen using images taken before and after deformation. At high temperatures, materials emit light in the form of blackbody radiation, which can interfere with DIC images. To screen out that light, DIC has been recently adapted by using ultraviolet (UV) range cameras, lenses, and filters. Before now, UV-DIC had been demonstrated at the centimeter scale using commercially available UV lenses and filters. Commercial high-magnification lenses using visible light have also been used for DIC. However, there is currently no commercially available high-magnification lens that will allow images to be taken (a) in the UV range, (b) at a submillimeter scale, and (c) from a relatively long working distance separating a specimen inside a test chamber and a camera outside the chamber. In this work, a custom UV high-magnification lens is demonstrated to perform high-magnification, high-temperature DIC measurements. To demonstrate the capabilities of this lens, a series of thermo-mechanical tests was run on a stainless-steel ring specimen. Two UV cameras performed simultaneous measurements: one at lower magnification using a commercial UV lens, and one with the custom high-magnification UV lens. At room temperature, the custom lens produces sufficiently bright images to perform DIC, while at high temperature (demonstrated to 900 °C) the images retained sufficient contrast while avoiding oversaturation. The lens can detect submillimeter rigid motion and tensile strains from long working distances and high magnification. These tests show that the custom lens is suitable for use in high-magnification UV-DIC measurements.