Biaxial flexible frequency-selective surface strain sensor with crack self-detection function

Structural health monitoring depends on strain sensors to ensure infrastructure safety; however, traditional active sensors are limited by issues such as complex wiring and vulnerability to interference. Although flexible frequency-selective surface (FSS) strain sensors enable wireless sensing, they are susceptible to failure due to physical damage, such as cracks. Therefore, this paper proposes a dual-axis flexible FSS sensor that integrates strain sensing with crack self-detection capabilities. The sensor features a centrally symmetric patch structure, operates in the Ka band with a resonant frequency of 32.832 GHz, and exhibits polarization-insensitive characteristics.Simulations and experiments demonstrate that the sensor detects biaxial strain with a uniaxial tensile sensitivity of 700 MHz/mm. Under biaxial tension, the sensitivities in the x-polarization and y-polarization modes are 1155 MHz/mm and 1260 MHz/mm, respectively. The resonant frequency exhibits a linear blueshift with strain. Moreover, the sensor can effectively perform self-detection of cracks. Horizontal arm cracks can be identified through a significant redshift in the reflection characteristic curve under x-polarization mode. In contrast, vertical arm cracks can be detected via redshift under y-polarization mode. The self-detection capability is insensitive to variations in crack position and width, and cracks in the central region do not impact performance. Multi-crack experiments and 500 fatigue cycles validated the sensor's reliable self-detection capability and mechanical stability under complex damage conditions.