E-Field And H-Field Perturbation Nondestructive Testing Method Based On Injecting Alternating Current Excitation

There is widely applying requirement of defects inspection for non-ferrous metal, ferrite and Carbon Fibre Reinforced Plastics materials. The conductor carrying alternating current (AC) can produce not only electric field but also magnetic field in the space, and the current in the conductor can be saturated and is stronger than that aroused from inducting excitation, which will bring new characteristic in the excitation of flaw detection and measurement. In the previous studies, most of time are spent to explore magnetic field inspection methods or sensors in the state of non-saturation current. This paper will focused the research whether the disturbed eleven field and magnetic field existed in the conductor carrying saturated alternating current, exploring a new NDT method.For this purpose, firstly theoretical analyses using a three-dimensional (3-D) finite element method (FEM) are presented to explain the disturbed electric field produced by defects in AC-carrying conductors. A disturbed E-field caused by defects can be decomposed into a non-conservative (or rotational) E-field that arises from a time-variant magnetic field and a conservative (or irrotational) E-field that results from free charge. The simulation results demonstrated that for metallic materials with high conductivity, the conservative E-field due to free charge could be neglected when the frequency of the exciting voltage is higher than60KHz compared to the non-conservative E-field produced by the time-varying magnetic field. Experiments applying a prototype, coplanar capacitive probe were also conducted, and the experimental results agreed well with those of the theoretical analyses, confirming the signal mechanisms of the alternating current electric flux leakage (AC-EFL) testing method.It is very difficult to seek proper electric field sensors to pick up the weak flaw signal. So the alternating current electric flux leakage technology is discussed using a coplanar capacitor probe. Physical experiments with a prototype capacitive probe were also carried out to observe and analyze the detection performance of AC-EFL. Effects of crack size of the conducting specimen, through holes size, flat-bottom holes depth, insulation thickness between the probe and the conducting specimen, and exciting signal frequencies were studied.For the engineering application of the alternating current electric flux leakage testing method, the magnetic field perturbation method and the alternating current electric flux leakage method are used to inspect the same specimen with defects. And two type materials are prepared, the one the Ni-Zn ferrite specimens and the other is CFRP(carbon fiber-reinforced plastics). The simulations and experiment results indicate that the magnetic field perturbation method lose efficacy for these two type materials, but the alternating current electric flux leakage method can successfully realize defect inspection, which adds new inspection method and to overcome the difficulties of defects inspection for these two type specimen.The magnetic field perturbation method and the alternating current electric flux leakage method are used to inspect the same specimen with defects. And three metal materials are prepared, the one is aluminum plate, another is copper plate and the last is stainless steel tube. The experiment results indicate that, for these metal materials with high conductivity, there is better inspection results and higher sensitivity for the magnetic field perturbation method comparing with the alternating current electric flux leakage method, and finally the contrastive analysis is taken between the magnetic field perturbation method and Eddy current testing, which has a certain guiding significance for flaw detection.