Nondestructive Testing for Shallow Defect of Ferromagnetic Objects Based on Magnetic Probe Structure.

摘要

Summary form only given. The Electromagnetic inspection technique plays an important role in the nondestructive testing (NDT) for many decades. Today, this NDT area is rather wide and significant. The magnetic flux leakage (MFL) technology is one of the most widely used electromagnetic nondestructive testing (NDT) techniques. MFL tools use permanent magnets to magnetize the detected object near to saturation flux density [1], [2]. Generally, the magnetizer mechanism of MFL are bulky and heavy. Although the shape of the opening and the depth profile of an arbitrary three-dimensional (3-D) defect from MFL measurements can be estimated [3], the inversion method is complicated and susceptible by the magnetization factors. For different shapes and different sizes of detected object, the magnetizer mechanism of MFL should be designed individually, and this requires a lot of time and experimentation. In this paper, a simple and portable magnetic detection device is designed with permanent magnets, magnetic probe structure and the Hall sensors. Compared with the magnetizer mechanism of MFL, the magnetic detection device is very light, low cost and easy to design and manufacture. The magnetic detection device can make qualitative, and quantitative evaluation for shallow defect of ferromagnetic objects. The designed magnetic detection device is made of permanent magnets, magnetic probe structure and the Hall sensors in Fig. la. The designed magnetic detection device is not to magnetize the detected object near to saturation fl ux density, instead use the permanent magnets to generate the magnetic field that perpendicular to the surface of the detected object. The magnetic probe structure is made of high permeability materials, is used to gather the magnetic field. Through the magnetic probe structure, the direction of magnetic field is better, and the magnetic field intensity is stronger than generated by permanent magnets. During the testing process, there is no defect on the detected object, and there is no change of the direction of magnetic field under the probe structure. If there is defect on the detected object and nearby the magnetic probe structure. Because the magnetic resistance of th e defect is far greater than the magnetic resistance of the intact surface, the balance of the magnetic circuit of detection system is destroyed, it causes the direction of magnetic field under the probe structure be changed. The Hall sensors detect the changes of the magnetic field under the probe structure to qualitative and quantitative evaluation the shallow defect of ferromagnetic objects. In this paper, the above phenomenon is analyzed by FEM in Fig. lb, and through the simulation analysis, we optimized the magnetic circuit to high sensitivity and precision of the detection system. The magnetic detection device based on magnetic probe structure for the plate, wire -rope and pipe are designed (in Fig. 2a and 2b). Through simulations and experiments, we find the magnetic detection device can make qualitative and quantitative evaluation for shallow defect of ferromagnetic objects. The waveform of defect on the plate is shown in Fig. 2c, and the result of detection of wire -rope is shown Fig. 2d. For wire -rope detection, the weight of the detection device is one thirtieth of the weight of the magnetic leakage detector, and the qualitative detection rate of defect is the same as the magnetic leakage detection. At the same time, it has certain quantitative detection precision.