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Abstract: In recent years, China has achieved remarkable results in research on the basic theories and applications of magnetic particle testing, development of related equipment and instruments, and standardization efforts. The testing technology has been widely applied in the production and use of industrial products, while methods of magnetic particle testing continue to be innovated. This article first introduces the basics of magnetic particle testing, including its fundamental concepts and principles ； Next, it focuses on the current magnetic particle inspection methods, introducing the circumferential magnetization method and the longitudinal magnetization method respectively ； Finally, it was analyzed that the ultimate direction for the development of magnetic particle inspection technology in the future should be fully automatic fluorescent magnetic particle inspection technology. 1. Overview of magnetic particle inspection Magnetic particle inspection is a non-destructive testing method used to detect defects on the surface or near the surface of magnetic materials. The parts that are typically inspected include pressure-resistant containers, welded components, parts that have been repaired, and semi-finished products. With the increasing demands for detection accuracy, magnetic particle testing equipment has also continued to evolve, resulting in improvements in both detection sensitivity and reliability. **Scholars in this field are also continuously improving their research on the theory of magnetic particle testing. Coupled with the emphasis placed on the personnel involved in such testing, these factors have collectively contributed to the development of magnetic particle testing technology in China. Magnetic particle inspection is a widely used method in non-destructive testing. It has a broad range of applications, and it can identify the main causes of defects such as cracks, inclusions, white spots, pores, and incomplete welding, enabling inspectors to take timely action for subsequent repairs. The principle of magnetic particle testing is to first magnetize the material to be inspected using a magnetic particle tester; thereafter, the magnetic particles adhering to the surface of the material arrange themselves in a certain pattern, which can be observed directly with the naked eye – these patterns are what we call magnetic traces. If magnetic traces are distorted, it indicates the presence of defects in that area. By observing them, it is possible to determine the location, shape, and size of the defects, and the cause of those defects can be identified based on the specific patterns of the magnetic traces. This method provides intuitive detection results and high sensitivity, and it is suitable for detecting magnetic workpieces of various shapes. Its drawbacks are that it is limited to detecting magnetic materials only, and it can only identify defects on the surface of the workpieces. 2. Common magnetic particle inspection methods 2.1 Circumferential magnetization inspection Circumferential magnetization inspection refers to a situation in which the direction of the magnetic field lines generated by magnetization is perpendicular to the direction of the magnetizing current, and also perpendicular to the longitudinal direction of the workpiece; the direction of the magnetic field follows the right-hand rule. This magnetization can detect longitudinally oriented defects that are essentially parallel to the workpiece axis. During circumferential magnetization testing, first clamp the test piece between the two clamps of the flaw detector as specified, gradually increase the magnetization current, and observe the indicated value of the circumferential magnetization current on the magnetic particle flaw detector; this value should reach the specified level. Using current transformers or shunts of appropriate specifications, standard ammeters and voltmeters with corresponding ranges are connected to the output terminal of the magnetizing current of the magnetic particle flaw detector. Adjust the magnetizing current, and at 30%, 60%, and 100% of the circumferential magnetizing current indicated on the indicator of the magnetic particle testing machine, record the readings of the standard ammeter to obtain measurement values corresponding to the ammeter readings of the magnetic particle testing machine. Repeat this measurement three times. Based on the obtained data and the transmission process of the test signal analyzed above within the system, the faulty component can be determined. Normally, at all stations for inspecting a workpiece, the same amount of current is used for sampling. After the signals are transmitted to the computer, the programmer adjusts them based on the specific parameters of the workpiece. This allows the entire workpiece to be compared under the same standards. If, after the correction, most workstations display abnormal results, it may be due to errors in the correction process; in such cases, the original signal needs to be corrected again. On the other hand, if there is a significant discrepancy between the readings of the standard meter and those of the sampling meter, the sampling meter may be malfunctioning; in such cases, it is necessary to consider replacing the sampling meter and conducting another test. 2.2 Longitudinal magnetization testing: Longitudinal magnetization testing refers to a magnetization method in which the direction of the induced magnetic field generated in the workpiece is parallel to the axis of the workpiece. This magnetization method is primarily used to detect transverse defects on the surface or near the surface of a workpiece, that is, defects that are perpendicular to or at an angle to the axis of the workpiece. The method for detecting longitudinal magnetization current is the same as that for detecting circumferential magnetization current, but depending on factors such as the shape and placement of the magnetization coil, it is generally required that the magnetic field strength be slightly higher than that for circumferential magnetization, measured in ampere-turns. The method for calculating the relative error is the same as that for calculating the relative error of the circumferential magnetization current. 3. Development trends in magnetic particle inspection. After decades of development, China has established a range of magnetic particle inspection equipment that includes portable yoke-based inspection devices, mobile magnetic particle inspection units, fixed magnetic particle inspectors, as well as specialized and semi-automated magnetic particle inspection systems. The performance parameters of some of these products have reached international advanced levels. There are various types of fixed magnetic particle flaw detectors in our country, such as the CJW AC series, CEW AC/DC series, CXW combined magnetization series, CZQ DC flaw detection-ultra-low frequency demagnetization series, and CDG multi-functional series, all of which can meet the magnetic particle inspection needs of various small and medium-sized parts. Specialized and semi-automated magnetic particle inspection machines have developed rapidly, with dozens of different types available on the market. These include bolt magnetic particle inspection machines, fluorescent magnetic particle inspection machines for pipe ends, gear magnetic particle inspection machines, and journal magnetic particle inspection machines, all of which meet the needs for semi-automated inspection during the mass production of specific workpieces. The highly sensitive fluorescent magnetic particles, oil-based carriers, and defect display films developed in our country are at the international advanced level. The performance of the self-developed teslameters (CT, HT102, and TYU-2000H), JXC pocket magnetometers, ST white light illuminometers, UV-A radiation illuminometers, magnetic powder property testing devices, standard test pieces (types A, C, D), and standard test blocks (types B, E) and other auxiliary equipment is on par with that of similar foreign products. The ultimate direction for the development of magnetic particle testing technology should be fully automated fluorescent magnetic particle testing technology. This technology was developed based on advancements in related technologies such as modern CCD cameras and optical scanning. It utilizes fluorescent magnetic particle imaging to achieve fully automated detection, replacing visual inspection by humans; this helps to minimize errors caused by human oversight. The core of this technology development lies in image acquisition and processing. Currently, it still faces challenges such as uneven grayscale in imaging, imaging of non-planar workpieces, false crack signals, degradation of ultraviolet illumination intensity over time, as well as issues related to the consistency and uniformity of the concentration of fluorescent magnetic powder in the suspension fluid, motion blur, selection of the effective imaging area, algorithm stability, and real-time performance. Current research focuses on defect identification and cause determination, and there is still a long way to go in future development. 4. Conclusion Over the past few decades, particularly in the last 30 years, magnetic particle and penetrant testing technologies in our country have seen rapid development, with fully automatic fluorescent magnetic particle inspection techniques also under development. Looking back at the past and looking forward to the future, it is necessary to carry out work in the following areas: first, to integrate various technologies in order to enhance their capabilities. By using ultrasonic vibration-PT technology, the penetration ability of the penetrant is enhanced, thereby improving the sensitivity and speed of PT detection ； Remote PT testing is carried out using remote-controlled robot-PT technology to accomplish the inspection of internal surfaces in special environments such as nuclear reaction systems ； MT/PT-endoscopy technology is used to perform inspections of deep holes and the inner walls of cavities. Second, make full use of digital technology to further improve the level of automation. Third, conduct research on micro-defect detection technologies to improve detection sensitivity. High-end products such as aero engines with high thrust-to-weight ratios have extremely strict requirements regarding the quality of component surfaces; for example, it is necessary to detect defects as small as 15 μm on the surface of ceramic components. The life-cycle design of aircraft also demands that even the smallest incipient defects be identified. Therefore, the magnetic powder and penetrant currently in use are no longer suitable for detecting such tiny defects; it is necessary to develop nanoscale materials as soon as possible and conduct research on corresponding detection techniques. The improvement of surface inspection technology in the future depends on breakthroughs in new technologies. Only innovation can enable our country to transform from a major nation in non-destructive testing into a powerful one in this field.