Metamaterial-Core Probes for Nondestructive Eddy Current Testing

Eddy current testing (ECT) is one of the most important techniques in nondestructive evaluation. A higher probe sensitivity is always pursued in ECT. To improve the probe sensitivity, the near electrical resonance signal enhancement (NERSE) technique was recently proposed. In the NERSE technique, a coaxial cable connecting with the probe coil is introduced to provide a capacitance for the electrical resonance. However, the long coaxial cable is awkward and inconvenient in engineering applications, restraining the usability of such probes. Metamaterials (MTMs) are novel artificial electromagnetic media and have been proved to be effective in manipulating the impedance of a solenoid coil. In other words, in principle, MTMs can be used in the same way as that a coaxial cable in NERSE in ECT. Based on this argument, low-frequency magnetic MTMs are introduced as the probe cores to provide a new approach to achieve the electrical resonance for the NERSE technique. A promising feature of the proposed approach is that the detecting signals and sensitivity are significantly increased due to the extraordinary electromagnetic properties of the MTM. The most salient features of an MTM core over a ferrite core are: 1) effective permeability of the MTM core will vary sharply from a high positive to a high negative values when crossing the resonance frequency, resulting in not only a high magnetic flux density in the tested material but also an extremely large variation of the normalized impedance which is very essential in identifying a crack and 2) electromagnetic properties of an MTM will increase with the increase of frequency, whereas those of a ferrite material usually degrade sharply with the increase of frequency. To demonstrate the advantages of the proposed new probe, comprehensive numerical and experimental studies are conducted, and the results have shown the high performance of the proposed probe. In addition, the proposed MTM-core probe also exhibits a diverse versatility and convenience in engineering applications as compared with the existing NERSE technique.