Passive wireless bolt loosening detection method based on a combined arc-shaped meandering microstrip transmission line

Bolt loosening is a primary cause of structural failure in critical infrastructure; conventional inspection methods are often labor-intensive and difficult to implement in hard-to-reach areas. Reliable, long-term monitoring requires a sensing solution that is not only wireless and passive but also resilient to environmental fluctuations such as thermal changes. This article presents a passive bolt loosening detection method based on a combined arc-shaped meandering microstrip transmission line (MTL). The sensing unit, which has temperature self-compensation and wireless interrogation capabilities, comprises two parts: an annular substrate and a sector-shaped combined substrate. Arc-shaped meandering MTLs are arranged on the upper surface of the ring-shaped substrate and the lower surface of the sector-shaped combined substrate. The relative rotation between the two sections of the meandering MTLs alters the resonant frequency of the meandering structure, enabling the detection of bolt loosening. In addition, a dedicated temperature compensation patch is integrated onto the annular-shaped substrate. The resonant frequency shift of this patch with temperature variation is used to achieve self-compensation of the sensor. This study investigated the relationship between the resonant frequency of the combined arc-shaped meandering transmission microstrip line, ambient temperature, and bolt rotation angle. The proposed meandering MTL was fabricated, and its sensing performance was validated through simulations and experiments. The sensing unit was subjected to temperature chamber tests to examine the effect of thermal variations. The results demonstrated that the resonant frequency of the meandering line varied nearly linearly with the nut rotation angle, reaching a wireless interrogation sensitivity of 18 MHz/°. The proposed sensing method aligns well with the development needs of modern structural health monitoring systems.