Pipelines, as one of the five major transportation methods in our country, not only enable low-loss and high-efficiency transportation but are also unaffected by weather and other factors, making them widely used in the energy and chemical sectors. By 2014, China's oil and gas pipeline transportation industry had rapidly developed, with a total length reaching 107,000 kilometers. However, due to harsh working environments and long-term online operations, pipelines are prone to defects such as cracks and corrosion, posing safety risks. By detecting defects and taking corresponding measures, economic benefits can be improved, and accidents can be avoided. Therefore, regular non-destructive testing and long-term online monitoring of pipelines are of great significance.
　　Ultrasonic testing technology is currently the most frequently used and widely applied testing technology in the field of non-destructive testing. According to the type of ultrasound, ultrasonic testing is divided into bulk wave testing and guided wave testing. Compared with other ultrasonic testing technologies, guided wave testing has the following advantages: (1) Guided waves are constrained by the medium boundaries during propagation, resulting in minimal energy attenuation along the propagation path, thus enabling long-distance detection; (2) Compared to traditional point-by-point ultrasonic testing, guided wave testing employs a line scanning method, which improves detection efficiency; (3) The wave structure of guided waves is relatively complex, and by controlling the mode, corresponding wave structures can be selected to identify different types of defects. The guided wave studied in this project is an isotropic tubular structure with free boundary conditions. According to the vibration modes of particles, the guided waves in the pipeline can be classified into longitudinal modes, torsional modes, and bending modes. Among them, torsional guided waves are sensitive to longitudinal defects as particles vibrate circumferentially along the pipeline during propagation, and low-modulus torsional guided waves exhibit non-dispersive characteristics, making them widely used in guided wave pipeline defect detection.
　　Piezoelectric ultrasonic technology, magnetostrictive technology, and electromagnetic ultrasonic technology can all excite torsional guided waves in pipelines. Currently, piezoelectric guided wave sensors and magnetostrictive torsional guided wave sensors are typical sensors for exciting and receiving torsional guided waves. Compared to electromagnetic ultrasonic sensors, these two types of sensors have higher conversion efficiency and are suitable for long-distance pipeline defect detection. In terms of detection sensitivity, the sensitivity of the magnetostrictive guided wave detection method is higher. Structurally, piezoelectric guided wave sensors require fixtures, making them complex, large, and costly; magnetostrictive guided wave sensors consist of coils, magnetostrictive strips, and the measured components, making them simple, compact, and low-cost. In terms of coupling methods, piezoelectric guided wave sensors achieve tight contact with components by applying external force, resulting in lower coupling efficiency; magnetostrictive guided wave sensors achieve tight contact with the measured components through adhesive coupling, resulting in higher coupling efficiency. Therefore, magnetostrictive guided wave detection technology is characterized by high efficiency and low cost.
　　Magnetostrictive guided wave detection technology, as an efficient and rapid detection method, has gradually become a research hotspot.
　　According to "GB/T28704-2012 Non-destructive Testing Magnetostrictive Ultrasonic Guided Wave Testing Method", the magnetostrictive effect includes positive magnetostrictive effect and inverse magnetostrictive effect. The positive magnetostrictive effect refers to the change in size and shape of ferromagnetic materials under the influence of an external magnetic field; the inverse magnetostrictive effect refers to the change in the internal magnetic field of ferromagnetic materials when subjected to axial external force (in the length direction).
　　The excitation of magnetostrictive guided waves in pipelines is based on the positive magnetostrictive effect, where ferromagnetic materials undergo elastic deformation on the surface under the influence of an external magnetic field, propagating in the medium in the form of guided waves. Correspondingly, the reception of magnetostrictive guided waves is based on the inverse magnetostrictive effect, where the vibration of particles causes elastic deformation in ferromagnetic materials, leading to changes in their internal magnetic field.
　　According to the deformation modes of materials, guided waves in pipelines can be classified into bending modes, torsional modes, and longitudinal modes. Due to the complex wave structure of bending mode guided waves, they are generally not used for pipeline defect detection. As shown in Figure 2-1, the coil generates an alternating magnetic field distributed along the axial direction of the pipeline under the excitation of high-power alternating pulses. If magnetization is performed along the length direction of the magnetostrictive strip, a direct current bias magnetic field distributed along the circumferential direction of the pipe will be obtained, meaning the alternating magnetic field is perpendicular to the direct current bias magnetic field. Under the combined action of the two magnetic fields, torsional deformation occurs on the surface of the magnetostrictive material, exciting torsional guided waves. Conversely, if magnetization is performed along the width direction of the magnetostrictive strip, a direct current bias magnetic field distributed along the axial direction of the pipe will be obtained, meaning the alternating magnetic field is parallel to the direct current bias magnetic field. Under the combined action of the two magnetic fields, axial deformation occurs on the surface of the magnetostrictive material, generating longitudinal guided waves.
　　During propagation, guided waves will reflect multiple times at the boundaries of the medium. Due to the guidance of the medium interface, energy is concentrated, allowing for longer propagation distances. Therefore, compared to other ultrasonic technologies, guided wave detection technology can perform long-distance detection of the specimen cross-section with single-point excitation, resulting in higher detection efficiency. The magnetostrictive guided wave pipeline defect detection system is an ultrasonic guided wave detection system based on the magnetostrictive effect, featuring a simple sensor structure that is easy to install, effectively reducing detection costs.
　　The basic principle of the magnetostrictive guided wave pipeline defect detection system is illustrated as follows: the ultrasonic guided wave emission device excites the emission sensor to generate magnetostrictive guided waves. When the guided waves encounter defects during propagation, some energy will form reflected echoes, while some energy will continue to propagate through the defect. The echo signals are received, processed, and analyzed by the receiving sensor and receiving device, allowing for the acquisition of internal defect information in the pipeline.