Dual Parameter FSS-Based Sensing for Structural Health Monitoring Applications

frequency selective surfaces (FSSs) are periodic arrays of conductive elements or apertures that reflect and/or transmit incident electromagnetic energy. Their response depends on parameters, such as element shape, unit cell dimensions, dielectric properties, and the local environment, making them suitable for structural health monitoring (SHM) applications. This article presents a dual-parameter FSS-based sensor design capable of measuring small-scale uni-directional longitudinal strain (0%–0.5%) and temperature ( $23~^{\circ }$ C– $223~^{\circ }$ C). The sensor integrates two-unit cells: 1) a patch-based cell on a thin substrate for strain sensing, offering enhanced strain transfer and superior sensitivity (~16–18 MHz/0.1%) and 2) a loop-based cell with a temperature-sensitive dielectric for temperature measurements, achieving a sensitivity of ~0.54 MHz/°C. The dual-measurand capability is achieved by designing the sensor with two distinct resonant frequencies, each corresponding to a specific parameter. Simulation and measurement results demonstrate that the proposed sensor achieves greater strain sensitivity as compared to existing FSS-based strain sensors while maintaining temperature sensitivity on par with existing FSS temperature sensors. The study also characterizes thermal expansion-induced errors through simulation and proposes a compensation approach that successfully improves sensitivity. Overall, this work demonstrates the potential of FSS-based sensors as compact, multimeasurand solutions for SHM applications, offering high sensitivity and reliability with minimal cross-sensitivity effects.