A. Lopes Ribeiro

Detection and characterization of defects using GMR probes and artificial neural networks

This work presents an eddy-current testing system based on a giant magnetoresistive (GMR) sensing device. Non-destructive tests in aluminum plates are applied in order to extract information about possible defects: cracks, holes and other mechanical damages. Eddy-current testing (ECT) presents major benefits such as low cost, high checking speed, robustness and high sensitivity to large classes of defects. Coil based architecture probes or coil-magnetoresistive probes are usually used in ECT. In our application the GMR sensor is used to detect a magnetic field component parallel to a plate surface, when an excitation field perpendicular to the plate is imposed. A neural network processing architecture, including a multilayer perceptron and a competitive neural network, is used to classify defects using the output amplitude of the eddy-current probe (ECP) and its operation frequency. The crack detection, classification and estimation of the geometrical characteristics, for different classes of defects, are described in the paper.

Liftoff Correction Based on the Spatial Spectral Behavior of Eddy-Current Images

In this paper, a new processing scheme for compensating the liftoff effect is proposed to increase the signal-to-noise ratio of measurements carried out when a duralumin plate with artificially manufactured defects was scanned with eddy-current probes. The mathematical algorithm is based on the spatial frequency behavior of the output signals with the liftoff distance (the distance between the probe used for inspection and the sample) and its equivalent form (deconvolution) in the spatial domain. Results are presented for measurements performed at three different distances for the same defect of the duralumin plate.

Using the skin effect to estimate cracks depths in mettalic structures

This paper is concerned with the characterization of depth profiles defects in metallic structures. Experimental work using giant magnetoresistors (GMR) scanning automatically a metallic plate with eddy currents induced by an excitation coil has been carried out and is described. The results are analyzed in comparison with the simulation results obtained with a commercial finite element model for some test cases.

Thickness measurement using transient eddy current techniques

This paper describes a pulsed eddy current (PEC) based non-destructive testing system used for experimental evaluation of material thickness and properties. Both classical and modern sensing methods of magnetic field have been carried out. An inductive as well as a giant magnetoresistive sensor based (GMR) probe have been used for sensing and the results obtained are presented and compared. The use of GMRs for this type of application is innovative and the post processing method too, since it evaluates the thicknesses of the conductive samples or of its non-conductive coatings, from the values obtained with the integration of measured signals.

Velocity induced eddy currents technique to inspect cracks in moving conducting media

This paper presents a new nondestructive testing technique based on velocity induced eddy currents. When a time constant magnetic field moves along a conductive material velocity induced eddy currents are generated in the media. The technique developed within this work uses these eddy currents in the same way as any eddy current testing method where the flow pattern disturbed by the existence of defects creates variations of the secondary magnetic field that can be assessed by a magnetic field sensor. This magnetic field provides the information about the defects. Within this paper the magnetic field parameters are measured using “giant” magnetoresistors (GMR). As far as the authors know this is an original method, that has not yet been reported in the literature, and presents undoubted advantages when the material to be tested is in motion relative to the test sensor (like when inspecting a rail track with a bogie) because the sensitivity of the method increases with speed. To validate the results simulations of the actual testing conditions are also presented using a commercial finite element model.

GMR based eddy current sensing probe for weld zone testing

The detection of flaws in the weld zone of conductive plates is an important issue and, in this field, different solutions using eddy current probes are mentioned in the literature. However, when eddy current pancake probes are used, it is difficult to detect the flaws or the welding non-uniformities due to the lift-off effects. Thus, in the present work we present a novel uniform eddy current probe architecture based on a tangential excitation coil with a rectangular geometry and highly sensitive giant magnetoresistances (GMR). The induced magnetic field is measured using a GMR sensor fixed on the axis midpoint inside the coil. The sensor sensitive axis is perpendicular to the excitation coil axis in order to measure only the induced magnetic fields caused by the presence of flaws. We developed an automatic testing system to characterize the sensor and to perform tests on aluminum plate specimens with flaws included in the weld zone.

Image analysis for crack detection

Evaluation of cracks using eddy currents (ECT) is one of the most used methods for non destructive testing (NDT). However, this technique still needs to be improved. One of the problems is related to the fact that eddy currents tests are so sensitive that a reproducible background noise signal is always present mitigating the informative data. This paper presents new developments in the area of signal processing that promise significant improvements to an accurate data interpretation. The steps carried out in the processing algorithm are illustrated using the experimental data acquired in the detection of a crack in an aluminum plate.

Remote field eddy current inspection of metallic tubes using GMR sensors

This article describes the inspection of metallic tubes using the remote field eddy current method. The excellent performance of giant magneto-resistance sensors in what concerns sensitivity, linearity, large bandwidth and direct assess to the magnetic field makes it a good choice for defect inspection in the applications where the measurement of low amplitude and low frequency magnetic fields is necessary. Experimental results with an artificial machined defect were included in the paper.

Sub-surface defect detection with motion induced eddy currents in aluminium

Nondestructive testing in situations where there is a moving media has always been a challenging task. Motion induced eddy current testing is a good solution for testing metallic surfaces, as it does not require contact with the sample. This work presents a method to detect sub-surface defects in non-ferromagnetic material with motion induced eddy currents. A numerical model was used to verify the progression of eddy currents along the depth of a conductive plate and to obtain the magnetic field perturbation in the presence of sub-surface defects. A probe, including a permanent magnet to induce eddy currents and a magnetic sensor was moved in the vicinity of an aluminium plate with sub-surface defects to obtain experimental data and the results compared to those obtained with the numerical model. At higher speeds, the time of diffusion of deeper eddy currents takes, makes it possible to also estimate the depth of a sub-surface defect by measuring how far away the perturbation is from the moving magnet.

Eddy current image normalization in ndt of aluminum plates

This paper presents a normalization method that allows the comparison between data taken from different eddy current experiments. The experimental data were acquired by scanning an aluminum plate containing a machined long crack. Results show that with two data sets acquired for orthogonal directions of sensor orientation it is possible to preview the data for any other measurement direction.