| Non-destructive testing (NDT) is the most widely used method for ferromagnetic materials. Among many NDT methods, the MFL testing has the advantages of simple structure, no couplant and high sensitivity for testing tanks and pipelines. So the MFL testing has been the most widely used method.In this paper, we firstly introduced the development and characteristic of the MFL testing. On the basis of current technology, we proposed a method of AC-DC composite magnetization for testing steel plates. Because of the deep DC magnetization and shallow DC magnetization, this method utilizes the DC MFL field to find out a defect and in the meanwhile utilizes the AC MFL field to distinguish that whether the defect is inside or outside. Secondly, according to the principle of composite magnetization, we designed a corresponding testing system, including the mechanical structure, signal acquisition and data processing. This provides a testing platform for further researches on simulations and practical experiments. Thirdly, we carried out practical tests on the steel plates using the testing system. According to the testing results, we could find out the distribution of MFL field. In addition, we analyzed the relations between the MFL field, the size of defects and the lift-off values of sensors. Fourthly, we researched the skin effect in the alternative current MFL testing. Through the testing results using the new testing platform, we found that the skin depth approach a constant value when the excitation frequency is enough high. Lastly, we proposed a new method of MFL testing to overcome the disadvantages of large energy consumption and big volume in current MFL testing system. This method used alternative current component of MFL field instead of the change rate of direct current MFL field under the condition of low direct current excitation. We designed a testing platform according to the principle, and carried out a series of practical testing. As a result, it is proved as an effected way to detect a defect. |
