ABSTRACT

The application of Eddy Current Testing (ECT) to ferromagnetic materials is challenging owing to the difficulties stemming to ferromagnetism. Due to the non-linear magnetic permeability, the use of a strong external DC magnetic field is often necessary in order to bring ferromagnetic materials close to magnetic saturation where permeability is minimised; strongly improving eddy current’s penetration in the material volume. In this work, the main application studied is ECT on a ferromagnetic tube with an Inside Diameter (ID) coil. The effect of different levels of the DC magnetic field on the coil impedance changes is investigated both experimentally and by simulation. It is shown that DC field has a systematic influence both to magnitude and phase of the Eddy Current signal. In addition, for the different defect depths, a critical level is observed, where the signals from discontinuities become visible and stand out from the noise of non-linearity. Moreover, for a range of magnetisation field level, the changes of phase angle are distinct for different defects’ depths allowing thus the application of phase analysis for defect characterisation.

摘要

由于铁磁性带来的困难，涡流检测 (ECT) 在铁磁材料中的应用具有挑战性。由于非线性磁导率，通常需要使用强大的外部直流磁场，以使铁磁材料接近磁饱和，从而使磁导率最小化；大大提高了涡流在材料体积中的穿透力。在这项工作中，研究的主要应用是在带有内径 (ID) 线圈的铁磁管上进行 ECT。通过实验和仿真研究了不同水平的直流磁场对线圈阻抗变化的影响。结果表明，直流场对涡流信号的幅度和相位都有系统的影响。此外，对于不同的缺陷深度，观察到一个临界水平，其中来自不连续性的信号变得可见并从非线性噪声中脱颖而出。此外，对于一定范围的磁化场水平，相位角的变化对于不同的缺陷深度是不同的，从而允许应用相位分析来表征缺陷。