| Supported by National Defense Project to apply eddy current technique to inspect airframe structures and critical engine components, theoretic model of eddy current field-circuit coupled problem, method to suppress probe's lift-off noise as a significant obstacle and the design of a novel probes for inspection of the specific specimen in Conventional/Pulsed eddy current testing are studied in this paper. The brief of these researches and the novel approaches are as follows:Based on electromagnetic field theory and electric circuit theory, mathematical model of three-dimensional eddy current field-circuit coupled problem are derived for two typical cases that coil is driven by voltage source and connected to bridge circuit. For three-dimensional eddy current-circuit coupled problem with complex boundary conditions, potential equation and boundary condition formulation of eddy current field problem are expressed by using A—φ, A method. And circuit equation of eddy current coil connected into outer circuit network is presented. Furthermore, mathematic model of field-circuit coupled directly is derived based on coil's flux linkage. Numerical calculation is adopted here. Finite element formulation is derived by using Galerkin method as coil is driven by voltage source and connected to bridge circuit. Solution steps of finite element analysis are presented. Furthermore, some keys such as open boundary problem, mesh, element density, the degrees of freedom at a node and boundary condition are illustrated detailedly.A probe-coil's lift-off and tilt effects for the curved specimen with small curvature radius in practical eddy current testing are analyzed. An approach using phase rotation and signal enhancement technique to eliminate lift-off and tilt noise is presented. For three typical detected surfaces-concave, plane and convexity, the relevant finite element models of three-dimensional eddy current field-circuit coupled problem have been built. The surface shape and curvature radius effects on eddy current distribution in the conductor and coil impedance have been investigated. The results show that the coil impedance magnitude is changed by the specimen's shape and curvature radius, but its characteristic is not influenced. Furthermore, the characteristics of the impedance locus produced by coil's lift-off variation and lift-off effect on defect detection for three specimens have been analyzed thoroughly. The results show that lift-off locus is always straight line. Defect signal is characteristic of rotating clockwise on lift-off locus as test frequency is increased. And the angle between defect signal and lift-off locus is 90 degree at a certain frequency. The physical essence of this characteristic is interpreted. Electromagnetic coupling between coil and specimen caused by coil's tilt angle variation has been analyzed. The impedance locus produced by coil's tilt has been compared with lift-off locus. The results show that the tilt locus is approximately straight line and close with lift-off locus as coil's tilt angle is less than a certain angle (about 69°). The above-mentioned property shows that tilt noise is suppressed mostly with lift-off noise. On this merit, an approach using phase rotation and signal enhancement technique are presented to eliminate lift-off and tilt noise. Moreover, the mode of operation and element of the bridge effects on signal and the proposed method have been researched as coil is connected to bridge circuit. Numerical results and theoretical analysis are validated by experiments. It is showed that the proposed method to suppress the lift-off and tilt noise is feasible.For inspecting the crack breaking away from the aperture of a disk, two eddy current probes with different configuration-an absolute probe with a coil (connecting to bridge circuit) and a differential probe with three coaxial coils, are proposed and designed, and compared with another probe with two cross-axis coils. According to the geometry of disk aperture and the direction of a fatigue crack around a disk aperture, the plug-in detection mode is chosen. Moreover, two eddy current probes with different configuration are proposed and compared with another probe. For detecting crack around a disk aperture by three eddy current probes, three-dimension finite element models of eddy current field-circuit coupled problem have been built respectively. The excitation coil's dimension effects on the test sensitivity have been analyzed. The results show that the probe has best sensitivity if excitation coil height is 3.0mm higher than disk thickness. Furthermore, according to the uniqueness of three probes, the specific parameters effects on the test sensitivity and crack signal have been investigated thoroughly. The results show that the larger an absolute probe's inner and outer radius, the higher the probe sensitivity. But the absolute change of bridge circuit output voltage is increased inversely with coil inner radius. For a differential probe, it is advantageous to improve the sensitivity and enhance crack signal that the turn ratio of excitation coil to pickup coil is less than 1. For a cross-axis probe, defect signal can be enhanced by decreasing the ratio of pickup coil's height to diameter. But the probe's sensitivity has directionality. That is, its sensitivity to a crack parallel to pickup coil's central axis is best. And its sensitivity to the cracks with another directions decreases rapidly to zero. The advantage and disadvantage over test performance and sensitivity of three probes are presented. Eddy current probes have been fabricated to inspect the standard castor gear specimen. The test results show that the absolute probe and differential probe can be applied to detect small crack breaking away from the disk aperture accurately. And the sensitivity of the differential probe is better than the absolute probe's sensitivity, but in the differential probe, two pickup coils must be configured and located symmetrically about the excitation coil. Compared with the cross-axis probe, the proposed eddy current probes have no directionality in the test sensitivity. A probe's lift-off effects for the multi-layered conductive structure inspected by pulsed eddy current technique are analyzed. An approach using "phase jump point" to distinguish a defect from lift-off noise is proposed. In order to suppress probe's lift-off noise that masks signal produced by a defect beneath the surface of the first-layer conductor, the three-dimensional finite element model of transient eddy current-circuit coupled problem has been built. The time-domain reponse and signal energy distribution characteristic in the time-frequency plane of three test signals-Δξl caused by probe's lift-off variation,Δξc produced by a defect, andΔξl+c caused by a defect together with lift-off, have been investigated. The results show that the deeper the crack location, the smaller the peak value ofΔξc, the longer the starting time ofΔξc and signal energy distribution in the time-frequency plane tends to compress downward in the frequency axis and extend rightward in the time axis. Based on penetration property of transient field into the conductor, the essence of signal features has been interpreted. The starting time of lift-off signalΔξl has no delay. And its peak value increases rapidly with probe's lift-off. As probe's lift-off is changed greatly, signal energy, especially the high frequency portion of the signal, tend to shift slightly in the frequency axis and remain unchanged in the time axis.Δξc is distinguished fromΔξl. ButΔξl+c are similar extremely toΔξl in time domain and the time-frequency plane. Consequently, a defect can not be identified visually from test signal with lift-off noise. Furhermore, according to the characteristic of signal shape, the phase of each frequency component in the signals caused by lift-off and defect has been analyzed. The results show that amplitude is decreased obviously and phase is unchanged almost by increasing probe's lift-off. But phase of frequency components that penetrate into defect's location is shifted significantly. On this merit, an approach using phase information to discriminate defect from lift-off noise is proposed. And an effective tool-dual-tree complex wavelet transform is adopted to extract signal phase in the time-scale plane. The results show that there is an important feature-"phase jump point (PJP)" in phase curves of three signals. The PJP is the time that wavelet coefficient's phase of signal is from "negative" to "positive". The PJP ofΔξl caused by lift-off variation remains unchanged. The PJP ofΔξc produced by a defect appears later than the PJP ofΔξl. And the deeper the defect location is, the later the PJP ofΔξc is. The PJP ofΔξl+c caused by a defect together with lift-off is between the PJP ofΔξl and the PJP ofΔξc.On this merit, an approach using "phase jump point" to identify a defect from signal with lift-off noise is proposed. These conclusions and theoretical analysis are validated by experiments. It is showed that the proposed method is feasible.
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