# A Direction-Sensitive Microwave Sensor for Metal Crack Detection
## 1 Introduction  
Structural Health Monitoring (SHM) involves the detection and identification of damages in structure through sensors and data processing techniques, which has drawn great interest in recent years. Generally, metal materials will be corroded and damaged after long-term use, which results in metal cracks. Additionally, the existence of cracks will greatly affect the safety of the device. To realize SHM, nondestructive detection has become an important way to prevent serious accidents caused by cracks in the metal structure [1, 2]. In recent years, various nondestructive detection methods for metal crack have been developed. Eddy current testing (ECT) is one of the most important techniques in nondestructive detection. A higher sensitivity is always pursued to improve the ECT probe performance, and many approaches including probe designing, coil geometry optimization, signal postprocessing, and so on have been reported in the literature [3, 4, 5]. However, eddy current testing is difficult to quantify cracks, and can only detect cracks near the surface, which makes it difficult to detect some unreachable areas. Ultrasonic testing (UT) has the ability to detect the cracks deep beneath the surface, and is suitable for long-distance detection. However, ultrasonic testing requires transmitting and receiving devices, and the signal processing process is complicated [6, 7, 8]. Optical fiber sensors (OFS) with multiple unique advantages have shown the potential to replace conventional sensors in certain applications in these decades [9, 10, 11]. For example, fiber Bragg grating (FBG) sensor is a special type of OFS. The sensitive element of the Bragg grating is contained in the fiber core, which serves as a narrow-band reflection filter. FBG sensors are suitable for detecting elastic ultrasonic waves in solid materials. An FBG can selectively reflect a part of theincident light around its central wavelength referred to as the Bragg wavelength. However, the measurement accuracy of an FBG sensing system is critical for long-term monitoring. In the meantime, microwave sensors have the advantages of quantified detection, easy assembly, and stable detection performance. With the development of microwave technology, a great variety of microwave sensors for metal crack nondestructive detection have been proposed [12; 13; 14; 15], such as the waveguide sensor, coaxial sensor and microstrip sensor. Regarding the waveguide sensor, a high-sensitivity near-field waveguide sensor loaded with a dielectric resonator for metallic crack detection is demonstrated in [16]. Both the numerical and experimental results reveal that this dielectric resonator-based waveguide sensor can effectively detect cracks on a metallic surface. The maximum sensitivity for crack depth and width detection are 0.71 GHz/mm and 1.24 GHz/mm, respectively. As for coaxial sensor, a method based on the resonance frequency shift in coaxial resonator is presented in [17] to detect metal surface cracks. Additionally, this sensor will produce a frequency shift of 2.16 GHz when detecting a 0.2 mm-wide crack. Generally, waveguide and coaxial sensors have a higher sensitivity of crack width and depth. However, both kinds of sensors are unable to detect the orientation of crack and have a higher profile. Compared with waveguide or coaxial sensor, microstrip sensor has the advantages of low profile, light weight and easy integration. In [18], an enhanced sensor based on symmetrical split ring resonator (SSRR) functioning at microwave frequencies has been proposed for detecting and characterizing the cracked surface of the metal materials. The minimum detection limit is investigated which is around 10 \(\upmu\)m width or depth where the minimum shift in reflected frequency is recorded at 6.2 MHz and 3 MHz for crack width and depth, respectively. In 2020, a smart coating based on the frequency selective spoof surface plasmon polaritons was introduced for submillimeter crack monitoring [19].