Effect of glass refraction on stereo-DIC measurements: a study using virtual experiments

Stereo digital image correlation (stereo-DIC) has been routinely used as a practical technique for high-temperature/low-temperature deformation measurements. However, the refraction of the optical path caused by the glass observation window in many high-temperature/low-temperature environments may introduce additional errors in stereo-DIC measurements. The stereo-DIC measurement errors caused by glass refraction are investigated using virtual stereo-DIC tests simulated by Blender, a powerful simulation tool that enables realistic modeling of optical distortions. Specifically, the virtual images are generated by rendering an object behind glass using Cycles renderer in Blender. First, the shape measurement error in the stereo-DIC system containing glass is compared with different stereo angles and glass thicknesses. Then, the displacement and strain measurement errors due to glass refraction are assessed by translation tests. In addition, a validation experiment was carried out to confirm the validity of the simulation. The simulation results show that the glass thickness and the stereo angle directly affect the shape errors of the 3D reconstructed surface. By reducing the stereo angle while avoiding the edges of the field of view, the glass-induced shape errors can be effectively reduced. The glass-induced displacement and strain errors can be minimized by adjusting the baseline to be perpendicular to the primary direction of in-plane deformation in the specimen. The results validate the effectiveness of Blender as an effective tool for assessing glass-induced stereo-DIC measurement errors and provide practical guidelines to reduce refraction errors.