Influence of Camera’s Optical Axis Non-perpendicularity on Measurement Accuracy of Two-dimensional Digital Image Correlation

The two-dimensional digital image correlation technique is very commonly used in a wide variety of solid mechanics applications for measuring in-plane deformations of planner surfaces. The perpendicularity of the camera¡¦s optical axis to the surface being observed is one of the basic conditions for the validity of the measurement. Small magnitudes of camera misalignment angles, up to two or three degrees, can go easily unnoticed during the initial setting of the experimental setup especially when the stand-off-distance between the camera and the surface is not small. In the work presented in this paper we investigate the errors in strain measurements caused by the non-perpendicularity of the camera¡¦s optical axis, with respect to the surface being observed, both theoretically and experimentally. In-plane rigid-body-translations in the directions perpendicular and parallel to the camera tilt axis are used for estimating the resulting strain errors. Results show that the non-perpendicularity of the camera causes errors in both the normal and shear strains. Misalignment angles as small as 2o are found to cause strain error greater than 10^3 µ-strains. The magnitude of strain error is found to increase linearly with both the misalignment angle and the magnitude of in-plane translation while it is inversely related to the stand-off-distance. The analyses show that simple in-plane rigid-body-translations experiments can be used to estimate the resulting strain errors and more importantly to detect and hence correct any existing non-perpendicularity between the camera¡¦s optical axis and the target surface. Experiments show that misalignment angles smaller than 1° can be detected using in-plane rigid-body-translations.