## 1 Introduction  
The real-time monitoring of structures during construction or operation stages is crucial for ensuring structural safety and preventing unforeseen accidents or failures. Current monitoring systems largely utilize electronic devices such as strain and displacement sensors to measure mechanical deformations by applying the concept of electrical resistance changes. Although these technologies can be sophisticated and accurate, they also have limitations such as high initial installation and maintenance costs, low durability, need for technical expertise, and the requirement of placing sensors in hard-to-access locations [1]. To overcome these obstacles, various structural health monitoring (SHM) systems have been introduced as potential diagnostic techniques for engineering structures. For example, acoustic emissions can effectively detect the development of a crack and its position in advance. However, the frequency components of acoustic emission signals are not always reliable indicators of material damage, as they can be affected by the wave propagation distance [2], making it difficult to interpret the signals. Fiber-optic sensors offer many advantages, such as low power loss, immunity to electromagnetic interference, compact size, and durability in harsh environments for SHM diagnosis systems; however, several challenges remain, including difficulties in interpreting signals (high noise and sensitivity for low-amplitude signals) [3] and complex wiring systems [4]. Mechanochromic substances, which change color in response to mechanical deformations, offer opportunities in the construction industry [5]. These substances can be attached to the components that make up the structure, and their discoloration provide an intuitive, immediate visual cue of potential failures [6; 7], enabling real-time, at-a-glance diagnostics. They can be applied as a part of the attachment process during or after the construction phase, thereby significantly simplifying the installation process and reducing the need for specialized equipment and technical expertise. Furthermore, because they do not rely on data transmission, they allow for immediate response and intervention. Additionally, the monitoring systems require minimal maintenance as they do not require power or electronic components to function, further reducing long-term operational costs. Therefore, such mechanochromic substances are particularly advantageous in the diagnosis systems of large-scale structures requiring real-time data transmission and present as a promising sensor for SHM [8; 9; 10]. [PERSON] et al. [5] fabricated a photonic crystal (PC) mechanochromic sensor by assembling polystyrene nanoparticles on a silicon wafer via spin coating. They discovered discoloration of the sensor from blue to red when it was stretched by 50%. The diffracted wavelength increased linearly from 450 to 670 nm when strain from 0 to 50% was applied, indicating a high degree of sensitivity in this strain range. [PERSON] et al. [11] employed a nanoimprinting technique to fabricate polystyrene nanoparticles based on a PC mechanochromic sensor. The discoloration of the sensor could be distinguished at strains below 30%, and it maintained its mechanochromic ability after 2000 cycles of a stretching test. [PERSON] et al. [7] developed a Fabry-Perot (F-P) mechanochromic sensor composed of metal-insulator-metal cavity structures. This sensor can be stretched over 100%, i.e., it can be deformed beyond the initial length before deformation. Also, this sensor displays distinct angle-insensitive color changes in response to mechanical strain (percentage of displacement relative to the initial length). Another study by this group [6] proposed a colorimetric method for detecting deformation in Al-based metals using a mechanochromic sensor with a strain capacity (maximum strain) of 120%. This method achieved an error rate of less than 15% compared to the actual strain. [PERSON] et al.