Real-Time Structural Surface Cracks Monitoring Using SRSR-Based Microwave Sensor

A slotted rectangular spiral resonator (SRSR) based array sensor is presented for real time metallic structure surface cracks monitoring on carbon fiber reinforced polymer (CFRP) or metallic structures. The SRSR-based unit sensor utilizes a fringing field sensing capacitor (FFSC) and transforms into an array by arranging cascaded sensor elements, enabling the detection cracks. The sensor array comprises 10 uniform rectangular spiral resonators connected to a 50 mm-long transmission line with a $50 \Omega$ characteristic impedance. The sensor operates either as a band-stop or band-pass filter, depending on the presence or absence of cracks in structure. The sensor functions within the frequency range of 5 to 6.5 GHz and has the capability to detect cracks with dimensions as small as 2.5 mm in terms of width, length, or height having any orientation. The sensor arrangement detailed in this study presents a compact and affordable remedy that could function as a technique for real-time detection of cracks in structures like bridges, oil or gas pipelines, and more.