Research Of Ferromagnetic Components Using Pulsed Eddy Current Nondestructive Testing

Ferromagnetic materials have been widely used in national industry and the construction of the infrastructure. Therefore, it’s necessary to detect the damage of ferromagnetic materials.Pulsed eddy current(PEC) owns so many advantages, such as rich spectrum, low price of equipment and fast response, and any conductive material can be detected. It is known to all that the detection of PEC in ferromagnetic materials has always been difficult. In this paper,the aim is to solve the problem of low signal noise ratio and the disturbance during the detection. It’s necessary to find the deficiency of current study from the related papers, use the basal theory of electromagnetic fields, look for physical nature in the simulation and design the corresponding experiment to verify the simulation. Finally, the study contents are as follows:(1)The influence of electromagnetic parameters on signals of pulsed eddy current Testing(PECT) for the ferromagnetic materials has been analyzed. The method of magnetic shielding is proposed and the distribution of eddy current and magnetic field can be acquired under different situations. The relative amount of magnetic fields is used to analyze the effect of PEC on defect detection of ferromagnetic materials.(2)The distributions of X,Y and Z component of eddy current density are compared when the rectangular probe is placed on the surface of the ferromagnetic and non-ferromagnetic materials. The appropriate component of current density is chosen and differential testing mode of the rectangular probe is presented. According to the distribution rule of eddy current,different differential modes are obtained.(3)Through the distribution of magnetic field near the defect between the ferromagnetic and non-ferromagnetic materials, PECT of rectangular probes for the ferromagnetic materials have characteristics of magnetic flux leakage testing. The modes of PECT and pulsed magnetic flux leakage(PMFL) are investigated and the optimal testing angle between the probe and defect is found. The testing effect of different angles between the defect and probe on the aluminum plate is studied and magnetic leakage effect exists in PECT of ferromagnetic materials.(4) The iron and aluminum shield are manufactured placed on the surface of the rectangular coil. Besides, the effect of magnetic shield on PECT for the surface and sub-surface of the specimen is analyzed. Furthermore, the rectangular differential probes with Hall and GMR are designed to obtain different components of magnetic fields, which are applied to detect the iron and aluminum plate. The experimental results show that the X and Y components of magnetic field will be better for quantitative analysis of defect. The rule of PEC response signals is researched when different angles between defects and the probe existand the optimal testing angle is found. Finally, the effectiveness of the results for the simulation is verified.(5)Low-pass filtering and wavelet transform are used to denoise of PEC signals on ferromagnetic materials. The effect of denoising under the wavelet basis of coif5 and soft threshold is optimized. Besides, the signal after denoising is close to the original signal, the delay of phase won’t occur and degree of distortion is lower.