Luminescent photoelastic coating image analysis and strain separation on a three-dimensional grid

The luminescent photoelastic coating (LPC) technique is an optical technique to measure the full-field strain on three-dimensional (3D) structural components. A luminescent dye within a photoelastic binder is excited with circular polarized light, and the corresponding coating emission intensity is detected via a digital camera for loaded and unloaded states of the specimen to which the coating is applied. Images are processed to find the relative change in emission with respect to camera analyzer position, and, subsequently, analyzed to determine maximum in-plane shear strain and the principal strain directions. For 3D structures with moderate movement or deflection in the field-of-view, especially when implementing an oblique excitation approach to separate the principal strains while accounting for non-strain related polarization changes due to surface inclination, the image analysis is preferably performed on a 3D grid. This study describes such an approach and discusses the analysis procedures to separate the principal strains and to obtain full-field strain distribution. The theoretical results are compared to experimental data from a 3D test specimen.