## 1 Introduction  
Carbon-fiber-reinforced polymer (CFRP) has high strength-to-weight ratio, high stiffness-to-weight ratio, light-weight, corrosion resistance, fatigue resistance, designability and other properties, and is widely used in aerospace, energy buildings, sports equipment, land transportation, marine fields, etc. In the context of the depletion of mineral energy, the global natural gas trade is active, and the market is increasing. Accordingly, composite wound gas cylinders commonly used for hydrogen storage or energy storage are also being used more widely and have a better prospect [1, 2]. The appearance of commonly used composite gas cylinders is shown in Figure 1. However, improper storage, prepreg production and preparation of CFRP during material processing may lead to fiber fracture, matrix debonding, fiber migration and other damages. During component manufacturing, fiber delamination, kinking, blistering, pollution, holes and other damages are caused due to improper laying, curing, machining or assembly. In service engineering, fiber delamination, fracture, erosion, matrix cracking and other damages may be caused by collision, chemical corrosion, local overload, fatigue and harsh environments. Due to the accumulation of impact, load, service time, environment and other factors, fatigue damage is the most dangerous and the main form of mechanical structure failure in the service process, accounting for 50-90% of the total mechanical structure failure [3]. At the initial stage, there will be slight fatigue damage in the structure, which is difficult to detect. The internal fatigue damage gradually expands, and when it expands to a certain extent, it will break, and when it is serious, it will cause huge economic losses and endanger personal safety [4]. Because of this, composite gas cylinders also have high safety risks, and now many accidents caused by the health problems of gas cylinders have aroused everyone's attention. Therefore, it is of great significance to monitor and evaluate the health status of CFRP in real time, and efficient detection and monitoring technology has become the focus of current research. Currently, many people are studying experiments on composite plates and cylinders. Generally, basic research can be carried out on plates before being extended to gas cylinders. At present, major research institutions have developed many structural health monitoring (SHM) methods. Common SHM methods include the acoustic emission method, electromechanical impedance method, vibration method, strain method, Lamb wave method, etc. Among them, the acoustic emission method depends on the acoustic emission signal when the damage occurs and expands, which is suitable for online use; The electromechanical impedance method is low in cost and simple in structure, but it is difficult to detect minor damage with low accuracy and small monitoring range [5,6]. Although the vibration method can monitor large-area damage, it requires more sensors, is insensitive to small damage, and is easily affected by the external environment; The strain method is only applicable to the monitoring of large-scale damage to structures. Compared with other SHM methods, because Lamb wave propagates in the measured structure with small signal attenuation, long propagation distance, sensitivity to minor damage, large area monitoring, high accuracy, low cost and other advantages, the structural health monitoring method based on Lamb wave is widely used in the industrial field and favored by researchers at home and abroad [4]. Piezoelectric (PZT) is the main sensing element used in Lamb wave experiments at present and is widely used in signal generation and the acquisition of Lamb waves. A piezoelectric ceramic chip is a dual-mode transducer made of zirconium, titanium and lead oxides after chemical reaction. The structure includes a piezoelectric ceramic sheet and metal vibration sheet, which have obvious piezoelectric characteristics and can be used for ultrasonic excitation and reception.