Conventional Eddy Current Techniques Research Articles

A Magnetic Shielding-Type PEC Sensor With a Canister Structure and Magnetic Core

The pulsed eddy current (PEC) technique exhibits considerable benefits over conventional eddy current techniques. The structure and dimensional parameters of PEC sensors considerably influence the PEC systems. Conventional PEC sensors are susceptible to background noise of a magnetic excitation field. To strengthen the magnetic gathering ability and improve detection sensitivity, a magnetic shielding shell structure was introduced to PEC sensors. In this study, a novel canister structure and magnetic core-based PEC sensor with magnetic shielding was designed. Finite-element simulation and experimental study were performed subsequently to optimize the design. A 3-D finite-element simulation model was established according to the eddy current detection mechanism and electromagnetic shielding principle. The influence of the PEC sensor structure and size on its detection performance was observed. Next, a 2-D axis-symmetric model was established, and the size of this PEC sensor was optimized by using the orthogonal approach. The results revealed that the detection performance of this PEC sensor could be improved by incorporating an appropriate magnetic shielding mechanism and through the optimization of the sensor size. The research results show that compared with the PEC sensor without magnetic shield, the PEC sensor with magnetic shield developed in this article can obtain higher signal amplitude and can be better applied to surface and subsurface defect detection.

Eddy current and inspection of coatings for storage tanks

The aim of this work is to make a survey of typical flaws presented by modern anticorrosive coatings commonly employed for inside protection of petrochemicals storage tanks and discuss what can be expected from nondestructive inspection with conventional eddy current technique. In this sense, advantages and limitations of the method are discussed taking into account the characteristics of the coatings. Controlled defects were machined in carbon steel substrates in order to simulate localized corrosion. The detectability of the eddy current technique was evaluated for as-applied insulating coatings (with different thicknesses) in presence and absence of iron oxide inside the simulated corrosion defects. Comparing the results obtained with the original C-Scan maps and after the image processing, the machined defects were successfully detected. In this comparison, it was observed a significant improvement for the false defects elimination using image processing.

Finite element modeling of second layer crack detection in aircraft bolt holes with ferrous fasteners present

Small cracks originating near bolt holes in the inner layers of aircraft lap joints cannot be detected by conventional eddy current techniques with ferrous fasteners present. The current work employs finite element modeling to investigate the effectiveness of pulsed eddy current (PEC) in detecting deep-lying cracks of different sizes and orientations. Differential signals analyzed using a modified Principal Components Analysis (PCA) method reveal the potential success of a PEC-PCA combination to produce scores that can be used to determine the presence of cracks. The method can also identify signals from an off-centered probe. Results are supported by experiment.

Research on edge identification of a defect using pulsed eddy current based on principal component analysis

It is difficult to identify the edge of a defect in nondestructive testing (NDT), which affects the result of a C-scan image acquired by arrays of pulsed eddy current sensors (APECS). To overcome the problem of low accuracy in identification of the edge of a defect using the conventional eddy current techniques, which only extract features in time domain, features from pulsed eddy current that has a broadband frequency are extracted both in time and frequency domains. A method based on principal component analysis (PCA) is proposed to enhance the accuracy in identification of the edge of a defect, the agreement between theory and experiment shows that the proposed method is applicable and preferable to APECS.

Eddy-current lockin-thermography: Method and its potential

The basic idea is that an induction coil induces eddy currents in conducting materials where the involved resistive losses are used for heating. The remote modulated excitation generates thermal waves that interact with boundaries thereby revealing defects. As a thermography camera monitors the temperature modulation on top of the surface, the response to the coded excitation allows for fast imaging of defects in larger areas without the need of slow point-by-point mapping. This is an essential advantage as compared to conventional eddy current techniques. As compared to inductive heating with visual inspection of the thermographic sequence, the advantage is local phase information resulting in phase angle images that are independent of most artefacts. Due to the Fourier analysis performed at the lockin frequency on the temperature image sequence, the signal to noise ratio in the obtained images is significantly better than in single temperature shots. The inspection method provides images of defects within several seconds, but the applicability is confined to conductive materials. However, it can be applied not only to metals (crack detection etc) but also to carbon fibre reinforced laminates where even impact damage can be detected on the front surface.

Bolt-Hole Corner Crack Inspection Using the Photoinductive Imaging Method

We applied a laser-excited eddy current (EC) imaging technique, or so-called photoinductive (PI) imaging, to characterize corner cracks at the edge of a bolt hole. Crack images with excellent signal-to-noise-ratios were obtained. The PI signals revealed the geometrical shape of the electrical-discharge-machined (EDM) notches that were either triangular or rectangular. The results show that this technique is promising to characterize the length, as well as possibly the depth and shape, of corner cracks. In this paper we present measurement results of 0.25-mm, 0.50-mm, and 0.75-mm rectangular and triangular EDM notches. We also show measurement results of a very small notch (<0.25 mm) which would be difficult to detect with conventional eddy current techniques. The dependencies of PI signals on laser chopping frequencies and eddy current frequencies are also examined. To demonstrate the photoinductive imaging capabilities to image actual cracks, we display images of fatigue cracks grown in a Ti-6Al-4V hole specimen. Finally, we present comparisons of the photoinductive imaging results with usual eddy current images obtained from a 0.75-mm triangular EDM notch using a rotating bolt-hole scanner. This article intends to verify experimentally that the photoinductive imaging technique has a potential to become a useful nondestructive testing method.

Thickness Measurement of MCrAlY High-Temperature Coatings by Frequency Scanning Eddy Current Technique

Nondestructive characterization of nonserviced, high-temperature coatings can be considered one of the important factors to achieve a higher level of structural integrity of advanced gas turbines. The present paper describes an innovative eddy current technique especially developed for measuring the thickness of metallic (MCrAlY) coatings applied by vacuum plasma spray on Ni-base superalloys. Conventional eddy current techniques, well established for quality control of coating thickness, are not applicable in this case because of the low differences of electrical conductivities of coating and base materials, which is a consequence of their quite similar physical and chemical properties. The new technique employs fast frequency scanning of the electromagnetic field in the range 100 kHz–10 MHz, corresponding to probing depths from 1 mm to 0.1 mm. Dedicated hardware has been developed featuring high sensitivity, stability, and harmonic rejection. Analysis of the measured data (i.e., probe impedance versus frequency), in order to estimate the relevant diagnostic parameters (coating thickness, coating, and base metal electrical conductivities), is carried out on the basis of a theoretical model of interaction between a plane electromagnetic wave and test piece. The results of tests performed on a nonserviced first stage blade are reported and compared with reference destructive data. Reliability, accuracy and practical applicability of the method meets the requirements for in-shop quality control.

Techniques for depth-selective, low-frequency eddy current analysis for SQUID-based nondestructive testing

Conventional eddy current techniques are widely used for detection of surface-breaking cracks in metal structures. These techniques have limited success in the detection of deep, nonsurface-breaking flaws that require low frequency eddy currents, for which inductive pick-up probes have drastically reduced sensitivity. High resolution, Superconducting QUantum Interference Device (SQUID) magnetometers, which are very sensitive to do or low frequency magnetic fields, have been developed for detection of subsurface flaws. We have now extended SQUID NDE by utilizing a sheet inducer to produce an extended eddy current parallel to the surface in a conducting plate. The magnitude of the induced current density inside the plate reduces with the depth; however, the current component at a certain phase angle may increase with the depth. At a particular phase angle, the current density on the surface becomes zero, while the current inside the plate is large, so that the magnetic signal at that phase angle due to the surface structures can be minimized. With this method, we have detected simulated cracks in the sides of plugged holes in a thick plate, a hidden corrosion area in a specimen which consisted of two painted aluminum plates joined with sealant, as well as crack defects adjacent to fasteners in the second layer of lap joined aluminum plates. We present a theoretical model for simulation of the phase-related magnetic signal due to a flaw, which shows the relation between the phase angle and the depth of the flaw. The theoretical phase analysis is compared with the experimental results.

Self-scanning-line electro-elastic NDE technique

A contactless self-scanning-line electro-elastic nondestructive testing technique is described capable of determining in real time the presence, location, number, and size of defects present along a linear path on the surface of a conductive material under test. In one embodiment, the NDE probe consists of a magnetostrictive delay line placed in close proximity with the surface to be inspected. A current carrying wire placed adjacent and parallel to the delay line is used to induce eddy currents in the material. The presence of a crack creates transverse eddy currents interacting locally with the magnetostrictive delay line launching elastic waves in the delay line. The elastic waves are generated only at the crack edge locations along the delay line. Spatial information regarding crack edge locations is preserved in the time information. An ultrasonic receiver coupled to the delay line is used to detect the elastic waves traveling in the delay line. The self-scanning-line electro-elastic NDE probe is not susceptible to the presence of elastic waves propagating in the material under test. The new concept is applicable to flat and curved surfaces including pipe inspection. Conventional eddy current techniques are not self scanning along a linear path giving only an integrated response, indicating only the presence and not the location and number of defects. Application of the concept to automation will be discussed.