Abstract
An investigation was performed into the effects of operating an absolute eddy-current testing (ECT) probe at frequencies close to its electrical resonance. A previously undocumented defect signal enhancement phenomenon, resulting from associated shifts in electrical resonant frequency, was observed and characterized. Experimental validation was performed on three notch defects on a typical aerospace superalloy, Titanium 6Al–4V. A conventional absolute ECT probe was operated by sweeping through a frequency range about the electrical resonance of the system (1−5MHz). The phenomenon results in signal-to-noise ratio (SNR) peak enhancements by a factor of up to 3.7, at frequencies approaching resonance, compared to those measured at 1MHz. The defect signal enhancement peaks are shown to be a result of resonant frequency shifts of the system due to the presence of defects within the material. A simple, operational approach for raising the sensitivity of conventional industrial eddy-current testing is proposed, based on the principles of the observed near electrical resonance signal enhancement (NERSE) phenomenon. The simple procedural change of operating within the NERSE frequency band does not require complex probe design, data analysis or, necessarily, identical coils. Therefore, it is a valuable technique for improving sensitivity, which complements other ECT methods.
Highlights
Eddy-current testing (ECT) is a well-established non-destructive testing (NDT) technique, routinely implemented in industry for the inspection of safety-critical metallic components, because of its high sensitivity to small surface defects.resolution to the smallest defects, for which higher frequencies must be used.Higher frequency inspections are not without their problems
When an inductor is brought into proximity of an electrically conducting material, it will electromagnetically couple to the eddycurrents generated in the material surface
This paper has highlighted a band of frequencies, outside the conventional operation range, and close to electrical resonance of an eddy current probe, where the magnitude of impedance signal-to-noise ratio (SNR) reaches a peak
Summary
Eddy-current testing (ECT) is a well-established non-destructive testing (NDT) technique, routinely implemented in industry for the inspection of safety-critical metallic components, because of its high sensitivity to small surface defects. Inspection times, complex operation and sophisticated signal processing algorithms [2,12,13] required As a result such techniques are not commonly used in ECAs. As a result such techniques are not commonly used in ECAs Authors such as Owston [14], Liu [15,16] and Ko [17,18] have recognized the potential of measuring the shifting of electrical resonant frequency, due to changes in its environment, as a highly sensitive means of measuring proximity, surface roughness and surface conductivity variations. The significant power transfer and large rates of change around high quality factor resonance peaks in the electrical impedance of an eddy current coil and cable, offer an extended dynamic range of impedance measurements Such advantages make operating at frequencies around resonance highly sensitive to even the slightest changes in an electromagnetically coupled system, but currently there is no documented account of authors exploiting resonance effects for defect detection. The research presented in this paper documents the initial findings of the investigation and highlights the implications that the work will have on future defect inspection techniques
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