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Selection principles The main principles for selecting non-destructive testing techniques are as follows: First, effectively combine non-destructive testing with destructive testing. Experience in practical applications shows that non-destructive testing techniques enable inspections to be carried out without damaging the structure of the material, yet it should be noted that such techniques cannot completely replace destructive testing ； Second, it is necessary to ensure that non-destructive testing is carried out at the appropriate time, and before proceeding with the testing, relevant information such as the purpose of the testing, the structure of the object, and the manufacturing process must be clearly defined ； Third, ensure that the selection of non-destructive testing methods is appropriate. Generally speaking, different pressure vessels have their own unique application characteristics; therefore, the testing methods must also meet their specific requirements as much as possible, so as to ensure that the test results are of the best quality. Ultrasonic testing: As one of the commonly used non-destructive testing methods, ultrasonic testing relies on the propagation of mechanical waves. The frequency of ultrasonic waves typically ranges from 1.4 to 3.6 MHz, and this method offers advantages such as strong penetration capability and fast inspection speeds. For example, when performing longitudinal testing on pressure vessels, ultrasonic waves can start from the surface of the vessel and travel inward at an angled direction, propagating in a serrated pattern within the wall of the vessel ； When performing lateral testing on pressure vessels, ultrasonic waves can be transmitted along the axis at an inclined angle, moving in a zigzag pattern. At the same time, according to relevant survey data, ultrasonic testing is generally used in the process of detecting defects in forgings; it enables accurate identification of defects such as inclusions, cracks, and lack of fusion, allowing for timely implementation of appropriate measures to resolve these issues effectively. Magnetic particle testing – this testing technique is also often referred to as magnetic particle inspection. It involves using the magnetic properties of certain magnetic materials to carry out the inspection process. It boasts a high level of sensitivity; the principle behind its application is that, during the inspection, the surface of the workpiece subjected to magnetic influence undergoes local deformation, which in turn creates a leakage magnetic field that attracts magnetic particles present on the surface of the workpiece. This allows for accurate identification of the exact location of defects as well as the extent of damage to those defects. At the same time, magnetic particle non-destructive testing technology also has the following advantages: First, it is not affected by the size or type of the workpiece during the non-destructive testing process, which facilitates identifying container defects in the most detailed manner ； Second, the operational process is simple, resulting in high detection efficiency. Electromagnetic eddy current testing: Eddy currents refer to circular currents that are generated as a result of electromagnetic induction in the object being inspected when an alternating magnetic field is applied near it. These currents facilitate accurate inspection of pressure vessels. It is these circular currents that are known as eddy currents. During this process, technicians must also conduct a detailed examination of the shape of the currents generated by the object under inspection, in order to accurately assess any disturbances in the internal electromagnetic field and thus identify any defects present in the pressure vessel. For example, when testing pressure vessels, if it is found that the eddies within the vessel exhibit a layered structure and are distributed evenly, this indicates that such eddies will induce electromagnetic phenomena in the object being tested. Should there be any defects in this system, the eddies will change accordingly, altering the vertical magnetic field significantly, which in turn facilitates the obtaining of accurate test results.