# The Rapid Detection Technology of Lamb Wave for Microcracks in Thin-Walled Tubes
## 1 Introduction  
### Purpose and Significance  
Thin-walled tubes are pressure vessels formed by stamping and drawing. In the production process, due to errors in the processing parameters and faults in the production equipment, microcrack defects form in the inner and outer walls of thin-walled tubes. However, thin-walled tubes must withstand large sudden changes in pressure during use. When the depths of the microcracks on the inner and outer surfaces are greater than a certain threshold, severe safety accidents can occur, such as cracking and crushing. Therefore, it is necessary to carry out nondestructive testing of thin-walled tubes in the production process to eliminate the potential safety hazards. For the nondestructive testing of thin-walled tubes, the commonly used testing methods include eddy current [1; 2], magnetic powder [3], X-ray [4; 5], liquid penetrant [6; 7; 8], and ultrasonic [9] methods. With eddy current detection, it is difficult to distinguish irregular surface defects and detect themquantitatively, which can easily lead to misjudgment [10]. Magnetic particle testing is more suitable for surface defect detection, and it is difficult to quantitatively detect the depth of a flaw [10]. The X-ray method is mainly used to detect the volume of the defect inside the object. For an area defect, there will be an overlap of the front and back, and the angle between the ray beam and the orientation of the crack cannot exceed 10\({}^{\circ}\); otherwise, it cannot be detected [10]. Penetration detection is limited to the defects of the surface opening. The penetrating liquid pollutes the parts and the environment and is not suitable for on-line detection [10]. Ultrasonic testing is sensitive to the area of the defect inside the object and can locate the relative size and location of the defect. The detection depth is much greater than other detection methods, and thus, it has been widely used [10]. For thin-walled tubes, ultrasonic Lamb waves are usually used for testing [11]. The ultrasonic detection principle of thin-walled tubes is the same as that of pipe, and there are many studies on the principle, method, and application of Lamb wave detection of pipe for reference. Therefore, based on the analysis of Lamb wave detection of pipe, the ultrasonic detection method of thin-walled tubes was explored. Defect types in pipe include lamination [12], corrosion [13], and cracking [14; 15; 16], which can occur on both inner and outer surfaces. The modes of Lamb waves in the pipe are mainly divided into longitudinal [17; 18], torsional [19; 20; 21; 22; 23], and flexural [24; 25]. According to the relevant references, the following work must be completed to realize the Lamb wave detection of pipe defects. (i) Based on analyzing and studying the dispersion characteristics of Lamb waves, the transducer frequency and Lamb wave mode with the highest sensitivity to the detected defect type should be selected. (ii) To study the interaction between defect and Lamb waves in the pipe, simulation should be carried out under a three-dimensional (3D) model. (iii) A test platform should be built to compare the difference between simulation and test results by scanning artificial defects in the pipe with transducers. (iv) Signal processing is used to determine the location and size of defects in the pipe. The difficulties in the detection process mainly focus on the two aspects: mode selection and 3D model simulation. On the one hand, the dispersion characteristics of Lamb waves in pipe have three modes to be analyzed and studied. Compared with only symmetric and antisymmetric modes in a thin plate, the difficulty of modal analysis and selection is increased. On the other hand, a 3D model is needed to analyze the interaction between defects and Lamb waves in the pipe. Compared with the 2D model of a thin plate, there are great challenges in terms of computational complexity.