Colorimetric sensors that respond to mechanical stimuli have garnered increased attention for various applications such as healthcare, display, electronic skin, and camouflage. To date, these mechanochromic sensors have been largely fabricated and studied with periodic structures such as surface relief patterns and photonic crystals. Although periodic structures can provide high wavelength selectivity, the strong viewing‐angle‐dependence of the color hinders their practical application in indoor and outdoor environments. Herein, a Fabry–Perot (F–P) interferometer composed of an elastomeric triblock copolymer as a dielectric spacer sandwiched between two metal layers is presented as an angle‐insensitive mechanochromic sensor. The planar structure of this device, formed on a flexible substrate, can be stretched to strains greater than 100%, and it reveals diverse colors according to the applied strain. The revealed colors are highly independent of the viewing angle, as confirmed both experimentally and theoretically. The developed sensor can be attached to any material including metals to visually detect mechanical failures (cracking, cleavage, and deformation) of the material. Therefore, this F–P mechanochromic sensor shows significant potential for real‐time structural health monitoring.

tively captured during deformation using one or more cameras, and the obtained digital image data (a series of photos) are analyzed using the DIC software. The displacements and strains of individual regions are determined by correlating the position of pixel subsets in the images, typically based on contrast (i.e., gray intensity levels). Combined with scanning electron microscopy (SEM), DIC can also be used to monitor microstructural changes in metals during deformation. [7][8][9] For DIC to work effectively, the surface of the specimen must have a random pattern. It is generally obtained by spraying black and white paint onto the specimen surface. Although DIC is an effective technique for generating full-field strain maps, its use in outdoor environments is challenging because a permanent setup is required for multiple measurements at different stages of deformation. There is a need for measurement techniques that can be easily implemented, and are highly accurate.In this study, we present a colorimetric method for measuring mechanical deformation in metals. This method is based on the color change in an attached thin-film mechanochromic sensor. The goal of this study is to develop a cost-effective and feasible technique for structural health monitoring. If mechanochromic sensors are attached to the surfaces of a metallic engineering structure, as shown in Figure 1a, the damage to the structure can be identified by the color changes in the sensor. This approach significantly helps prevent disasters by forecasting the potential failure of a structure. Sensors invisible to the naked eye may be captured using a camera with a telescopic lens. Difficult-to-access areas can be monitored using drones. Despite numerous, recent studies on mechanochromic sensors and their applications, [10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28][29] the quantitative measurement of mechanical deformation in metals through the use of mechanochromic sensors has not yet been reported. Many mechanochromic sensors developed to date employ periodic structures such as photonic crystals and surface relief patterns. [14][15][16][26][27][28] Although these periodic structures exhibit high wavelength selectivity, the iridescent nature of the revealed color and the difficulty in maintaining periodicity over a large area limit the practical application of these structures. Photonic glasses with a short-range order may exhibit angle-independent colors; [30][31][32] however, their reflection spectra are extremely broad with low reflectivity, thereby leading to weak and dark colors. In this study, a Fabry-Perot (F-P) resonator structure comprising

Detecting Mechanical Deformation A colorimetric method for detecting mechanical deformation in metals is presented by Myeongkyu Lee and coworkers in article number 2100479, which uses the color change in an attached thin‐film mechanochromic sensor. This method enables full‐field strains to be mapped by capturing the sensor image using a smartphone and analyzing its color components. The presented colorimetric approach is promising for the structural health monitoring of metallic engineering structures.

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