Dário Pasadas

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.

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.

GMR versus differential coils in velocity induced eddy current testing

This paper presents a development on a new nondestructive testing method using eddy currents induced by velocity. This new method uses a constant magnetic field that attached to a moving media induces eddy currents in the conductive material to be tested. By measuring the opposing magnetic field generated by the eddy currents it is possible to obtain information regarding the presence of defects. Two different magnetic field sensors, GMR and differential pick-up coils, were used and compared in the detection of perpendicular components of the magnetic field created by disrupted eddy currents due to linear defects machined on an aluminum plate.

Regularization methods to assess the eddy current density inside conductive non-ferromagnetic media

This presentation describes two regularization methods that were applied to preview the current density induced in an aluminum plate. The images that result from the measurement of one magnetic field component by scanning the plate were used to determine the current density. The scanning was performed using a constant field eddy current probe and a rectangular area including the defect was covered. With the constant field probe a sinusoidal excitation was imposed to the plate, being the amplitude and phase invariant under a limited space translation. The obtained data was inverted and two regularization methods were applied, Tikhonov and total variation. The two methods were compared to conclude about their inclusion into nondestructive test and evaluation instrumentation.

Handheld instrument to detect defects in conductive plates with a planar probe

This paper presents a novel low-cost handheld instrument to detect cracks and other defects in a plate of conductive material. The instrument includes a planar eddy-current probe having a giant magnetoresistor (GMR) based sensor, a mouse pointing device that acts as the positioner, a dsPIC that controls the measurement process and that possesses also signal processing capabilities to enhance the accuracy of the system and all the electronic circuitry (power supplies, signal amplification, current generation, analog to signal conversion) necessary to carry out the defect detection. The visualization of the defect can be depicted in real-time on a LCD display or transmitted to a PC. The metrological performance and the effective cost make this handheld device well suited for several applications such as cracks detection at subsurface depths.

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.

Determination of linear defect depths from eddy currents disturbances

One of the still open problems in the inspection research concerns the determination of the maximum depth to which a surface defect goes. Eddy current testing being one of the most sensitive well established inspection methods, able to detect and characterize different type of defects in conductive materials, is an adequate technique to solve this problem. This paper reports a study concerning the disturbances in the magnetic field and in the lines of current due to a machined linear defect having different depths in order to extract relevant information that allows the determination of the defect characteristics. The image of the eddy currents (EC) is paramount to understand the physical phenomena involved. The EC images for this study are generated using a commercial finite element model (FLUX). The excitation used produces a uniform magnetic field on the plate under test in the absence of defects and the disturbances due to the defects are compared with those obtained from experimental measurements. In...

Faraday induction effect applied to the detection of defects in a moving plate

This paper presents a new nondestructive testing method. The conductive material being tested moves in a time constant magnetic field. Due to the movement the induced currents in the conductor generate a secondary magnetic field. When the eddy current flow pattern is disturbed by the existence of defects, variations of the magnetic field occur and information about the defects can be assessed by measuring the magnetic field. As far as the authors know this is an original method, which 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).

Evaluation of Subsurface Defects Using Diffusion of Motion-Induced Eddy Currents

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.

ECT image analysis applying an inverse problem algorithm with Tikhonov/TV Regularization

Eddy current testing is used to inspect and analyze potential defects in metallic structures that can damage its future usefulness. However, this technique still needs to be improved. In this paper, a probe including one planar coil, one GMR sensor and a tiny permanent magnet is attached to a two dimension positioning system to evaluate a metallic plate with a linear defect. A magnetic field perturbation around the defect is measured and image processing techniques, in conjunction with an inverse problem algorithm, are used to obtain the eddy current density present in the metallic surface. From the eddy current map it is possible to determine the geometrical characteristics of the defect. To apply the inverse problem algorithm, regularization to suppress noise and to overcome instability effects are required. In this paper, Tikhonov and Total Variation Regularization are compared regarding their performance and obtained results.