| Pulsed eddy current testing (PECT) is a nondestructive testing technique that develops fast in the recent years. The pulsed excitation comprises a broad band of frequencies and the response signal provides more information about defect than that of traditional eddy current testing. Therefore, PECT will play an important role in the maintenance of aircrafts, nuclear power plants, oil and gas transmission pipes and so on. The thesis studies feature extraction of PECT signals and defect characterization schemes to enhance the capability of PECT in defect classification and thus improve PECT technology.Most literature studies PECT defect classification mainly in time domain, ignoring the rich frequency information of PEC signals. A pulse waveform can be expressed as a summation of a series of harmonics with unequal amplitudes. Subsurface defects only affect the low frequency components, whereas surface defects affect the high and low frequency components because of the skin effect. Features which can fully identify defect’s location, radius, and height were selected and the effects of different defect parameters on the features were analyzed. The thesis fully exploits frequency-domain information of PECT signal and proposes defect characterization schemes for characterizing corrosion and cracks around rivet sites. The characterization schemes are simple and clear. More importantly, they can be used to identify defect’s location, size, and depth.Numerical simulation is a very useful, flexible and low-cost method for better understanding of PECT technology. Two finite element models, namely axisymmetric model and three-dimensional model, are utilized for PECT simulation. The models are validated by experiments.The research will further ECT theories and promote the application and development of ECT. The achievements can be applied to detect small defects in aircraft skin and other components, which is meaningful for aircraft safety. |
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