yQuantitative Evaluation of the Relationship between Strain and Color Change in Opal Photonic Crystal Films and Application into Complex Specimen Geometries

The color change of opal photonic crystal films (OPCFs) due to deformation was quantitatively evaluated using digital image correlation (DIC) analysis. OPCFs were pasted on specimens of three different gauge geometries, and random patterns were formed on the opposite side of each specimen for DIC analysis. To assess the applicability of using OPCFs-based strain characterization for analyzing steel structural components and associated metallurgical analyses, smooth, width -gradient, and holed specimens were prepared in this study. As deformation increased in the smooth specimen, the color of the OPCFs changed significantly. The color change in the OPCFs could through Hue value analysis. Heterogeneous strain distributions could also be quantitatively analyzed using OPCFs-based analysis at the submillimeter or millimeter scale. When the strain gradient is too high, for example, near a stress concentration site such as a hole, local peeling of the OPCFs away from the specimen surface can occur. Consequently, for quantitative characterization, we must take proper care when measuring this upper limit of the "strain gradient" as well as strain, which would depend on the adhesion and surface condition of the specimen.