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The need for inspecting conductive materials using nondestructive methods faster and more efficiently has always been a challenge. This paper presents a method to evaluate the subsurface defects in nonferromagnetic materials using the diffusion of motion-induced eddy currents. A probe, including a permanent magnet and a magnetic field sensor, is moved with a constant speed in the vicinity of an aluminum plate with subsurface defects machined at different depths. The constant magnetic field due to the magnet, which moves relative to the plate, induces eddy currents that diffuse inside the material. At higher speeds, the eddy current diffusion creates a tilted transition zone. The location of the interaction between the frontiers of this transition zone with the defect shall be used to determine the depth of the subsurface defect. This study was carried out using a numerical model to verify the progression of the eddy currents along the depth of the conductive plate as it moves relatively to the probe and obtain the magnetic field perturbation due to the presence of the subsurface defects. The validation of the model is done comparing the results obtained experimentally with those obtained by simulation for identical situations at lower speeds. [ABSTRACT FROM AUTHOR]

This paper proposes a method to detect and evaluate surface and subsurface cracks in aluminum multilayer structures using a planar eddy current testing (ECT) probe and processing an inverse problem algorithm. The proposed excitation method using this ECT probe allows the induction of eddy currents with different orientations on the metal surface without rotating the probe during the scan. An inversion algorithm was applied to evaluate the geometry of the cracks. The main result of this inversion algorithm is the determination of the shape of the cracks using the reconstructed eddy current pattern inside the specimen. These reconstructed patterns give the indication of the length, orientation, and geometry profile of the crack. In this paper, complex geometry cracks at different depths are inspected. Experimental data were obtained around a star crack at different depths in a stack of four aluminum plates where each plate has a thickness equal to 1 mm. The presented work shows that the distance between the sensor and the layer under analysis must be adjusted in the inversion process in order to obtain the best reconstructed images when subsurface cracks are under study. This consideration affects the quality of the resulting images. [ABSTRACT FROM PUBLISHER]

Detection of a wall thickness change in pipelines is extremely important to avoid unwanted leakages and accident reduction in industrial structures. In the present study, transient eddy current oscillation (TECO) nondestructive testing (NDT) method has been used to detect the thickness of stainless specimen. Damped free running oscillations that are generated by cyclic exchange of energy between a capacitor and an induction coil in the TECO probe, are used to induce the eddy currents into the test specimen. The resultant magnetic field is detected by using a Hall sensor in the vicinity of the test specimen. The experimental results are analyzed in the frequency domain by calculating the power spectral density (PSD) of the detected decay of oscillations response from the probe. Two features, peak frequency and −3 dB bandwidth of the PSD, are used to interpret the thickness variation in test specimen. The TECO method has been successfully applied to measure the thickness of stainless steel plates. [ABSTRACT FROM AUTHOR]

Detection of corrosion and measurement of the thickness of stainless steel in industrial structures, such as pipelines, has been an important issue to avoid unwanted leakages. A recently proposed transient eddy-current oscillations (TECO) method has been used in this paper to measure thickness in non-magnetic stainless steel. The TECO method works based on the electromagnetic induction principle, as the conventional eddy-current testing method. An inductor (excitation coil) connected in parallel with a capacitor to generate damped oscillations is used to induce eddy currents into the test material beneath the coil. A Hall sensor, which is fixed at the bottom side and at the axial center of the excitation coil, detects the resultant magnetic field due to the induced eddy currents. Experiments were performed on the stainless steel specimens, having different thicknesses to test the potential of the TECO method. The results were analyzed using the fast Fourier transform. This paper proposes a lift-off point of intersection in the frequency domain that can be used to eliminate the false indications due to the lift-off variations during the thickness measurement of the test material. The results reveal that the proposed method can be applied to detect the thickness variations in metal structures covered with non-conductive paints or insulation coatings. [ABSTRACT FROM AUTHOR]