High-Current Double Pulse ECT Technique for Inspection of Ferromagnetic Materials

The detection of surface cracks of conductive materials that have a magnetic permeability higher than μ0, are usually made using the Magnetic Flux Leakage (MFL) technique. It requires the saturation of the specimen so that some magnetic flux lines escape the material when a defect is present. However, saturating the material can be very power consuming and if there is motion involved, eddy currents induced due to motion decrease or even null this method sensitivity as speed increases, which can be a disadvantage in cases such as railroad inspection. This work proposes a new technique to inspect the surface of ferromagnetic materials based on eddy currents. It is denominated high-current double pulse (Hi-CDP) ECT. The technique creates two consecutive pulses of currents (up to 1500 A) in a coil in the vicinity of the sample. Fig. 1 shows the simulation model used and the corresponding magnetic flux obtained in a point in the axis of a pancake coil (25 turns, id=15 mm, od=25 mm, heigh=10 mm), and in the vicinity of the sample material. The first pulse (starts at 0.1 ms) saturates the material, making it behave almost like a non-ferromagnetic material. The second pulse starts at 0.25 ms when the maximum current of the first pulse occurs (when the material is most saturated). When the second pulse occurs, eddy currents are induced. As the material is saturated, the ferromagnetic properties almost do not interfere with penetration depth and distribution of eddy currents, making it suitable for eddy current testing. Fig. 2 shows the derivative of the magnetic field obtained in a point located between the windings of the coil and the sample material, for a case without defect and in the presence of two similar defects with different depths (0.5 mm and 1.5 mm deep) in the vicinity of the point. It is possible to observe that the second current peak contains a perturbation that is different according to the defect. The signal derivative was chosen in order to distinguish the MFL from eddy currents perturbations

Characterization of Complex Defects Using Eddy Currents with Uniform Field Probes and Giant Magneto-Resistor (GMR) Detectors

Some defects in metallic plates present complex forms that difficult their geometric characterization. This is the case of defects with ramifications where the eddy currents cannot penetrate some concavity areas. The illumination of the material under test with a coherent excitation field presents some advantages, such as the possibility of applying the excitation into different directions, thus obtaining different patterns that may be correlated to increase the definition of the acquired signals. The generation of the coherent excitation is obtained by using a planar coil that produces a uniform excitation field inside a given area. It is not easy to correlate the magnetic field patterns, obtained by using single component giant magneto-resistor (GMR) sensors, with the real geometry of the defects. The interpretation of the measured data is much easier if the data are inversed to obtain the geometry of the eddy current lines inside the conductor. The inversion process was performed using the discrete Fourier transform of the field data and of the elementary dipole current kernel. The inversion was followed by Tikhonov regularization and automated determination of the regularization parameter. Results obtained for a defect with three linear segments in a star configuration are depicte

High-Current Double Pulse ECT Technique for Inspection of Ferromagnetic Materials

The detection of surface cracks of conductive materials that have a magnetic permeability higher than μ0, are usually made using the Magnetic Flux Leakage (MFL) technique. It requires the saturation of the specimen so that some magnetic flux lines escape the material when a defect is present. However, saturating the material can be very power consuming and if there is motion involved, eddy currents induced due to motion decrease or even null this method sensitivity as speed increases, which can be a disadvantage in cases such as railroad inspection. This work proposes a new technique to inspect the surface of ferromagnetic materials based on eddy currents. It is denominated high-current double pulse (Hi-CDP) ECT. The technique creates two consecutive pulses of currents (up to 1500 A) in a coil in the vicinity of the sample. Fig. 1 shows the simulation model used and the corresponding magnetic flux obtained in a point in the axis of a pancake coil (25 turns, id=15 mm, od=25 mm, heigh=10 mm), and in the vicinity of the sample material. The first pulse (starts at 0.1 ms) saturates the material, making it behave almost like a non-ferromagnetic material. The second pulse starts at 0.25 ms when the maximum current of the first pulse occurs (when the material is most saturated). When the second pulse occurs, eddy currents are induced. As the material is saturated, the ferromagnetic properties almost do not interfere with penetration depth and distribution of eddy currents, making it suitable for eddy current testing. Fig. 2 shows the derivative of the magnetic field obtained in a point located between the windings of the coil and the sample material, for a case without defect and in the presence of two similar defects with different depths (0.5 mm and 1.5 mm deep) in the vicinity of the point. It is possible to observe that the second current peak contains a perturbation that is different according to the defect. The signal derivative was chosen in order to distinguish the MFL from eddy currents perturbations.</p

Characterization of Complex Defects Using Eddy Currents with Uniform Field Probes and Giant Magneto-Resistor (GMR) Detectors

Some defects in metallic plates present complex forms that difficult their geometric characterization. This is the case of defects with ramifications where the eddy currents cannot penetrate some concavity areas. The illumination of the material under test with a coherent excitation field presents some advantages, such as the possibility of applying the excitation into different directions, thus obtaining different patterns that may be correlated to increase the definition of the acquired signals. The generation of the coherent excitation is obtained by using a planar coil that produces a uniform excitation field inside a given area. It is not easy to correlate the magnetic field patterns, obtained by using single component giant magneto-resistor (GMR) sensors, with the real geometry of the defects. The interpretation of the measured data is much easier if the data are inversed to obtain the geometry of the eddy current lines inside the conductor. The inversion process was performed using the discrete Fourier transform of the field data and of the elementary dipole current kernel. The inversion was followed by Tikhonov regularization and automated determination of the regularization parameter. Results obtained for a defect with three linear segments in a star configuration are depicted</p

Investigation of Different Features for Baseline-Free RAPID Damage-Imaging Algorithm Using Guided Waves Applied to Metallic and Composite Plates

In guided-wave-based damage-imaging algorithms, damage reconstruction typically involves comparing the signals with and without a defect. However, in many cases, defect-free data may not be available. Therefore, in this study, baseline and baseline-free approaches were used for damage imaging, exploiting not only the amplitude of the signal as the feature but also five additional features, namely, the amplitude of the sparse signal after deconvolution, the amplitude of the coefficients at the excitation frequency from the re-assigned short-time Fourier transform, the time of flight determined from cross-correlation, kurtosis in the time domain, and kurtosis in the frequency domain. For this study, three different plates with different types of defects were considered: a metallic plate with a notch-type artificial defect, a pultruded type of composite plate with an impact defect, and a laminate composite plate with plexiglass serving as an added mass damper artificial defect. The Reconstruction Algorithm for Probabilistic Inspection of Damage (the RAPID algorithm) was used to characterize the defects on the three plates, and the defect parameters were then quantified by creating an ellipse after thresholding

Baseline-Free Damage Imaging of Composite Lap Joint via Parallel Array of Piezoelectric Sensors

This paper presents a baseline-free damage imaging technique using a parallel array of piezoelectric sensors and a control board that facilitates custom combinations of sensor selection. This technique incorporates an imaging algorithm that uses parallel beams for generation and reception of ultrasonic guided waves in a pitch–catch configuration. A baseline-free reconstruction algorithm for probabilistic inspection of defects (RAPID) algorithm is adopted. The proposed RAPID method replaces the conventional approach of using signal difference coefficients with the maximum signal envelope as a damage index, ensuring independence from baseline data. Additionally, conversely to the conventional RAPID algorithm which uses all possible sensor combinations, an innovative selection of combinations is proposed to mitigate attenuation effects. The proposed method is designed for the inspection of lap joints. Experimental measurements were carried out on a composite lap joint, which featured two dissimilar-sized disbonds positioned at the lap joint’s borderline. A 2D correlation coefficient was used to quantitatively determine the similarity between the obtained images and a reference image with correct defect shapes and locations. The results demonstrate the effectiveness of the proposed damage imaging method in detecting both defects. Additionally, parametric studies were conducted to illustrate how various parameters influence the accuracy of the obtained imaging results