The Pulsed Eddy Current Differential Probe to Detect ...

The Pulsed Eddy Current Differential Probe to Detect a Thickness Variation in an Insulated Stainless Steel

Journal of Nondestructive Evaluation ISSN 0195-9298 Volume 29 Number 4 J Nondestruct Eval (2010) 29:248-252 DOI 10.1007/ s10921-010-0083-3

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Author's personal copy J Nondestruct Eval (2010) 29: 248–252 DOI 10.1007/s10921-010-0083-3

The Pulsed Eddy Current Differential Probe to Detect a Thickness Variation in an Insulated Stainless Steel C.S. Angani · D.G. Park · C.G. Kim · P. Leela · P. Kollu · Y.M. Cheong

Published online: 28 October 2010 © Springer Science+Business Media, LLC 2010

Abstract Non-destructive testing (NDT) plays an important role in the safety and integrity of the large industrial structures such as pipelines in nuclear power plants (NPPs). The pulsed eddy current (PEC) is an electromagnetic NDT approach which is principally developed for the detection of surface and sub surface flaws. In this study a differential probe for the PEC system has been fabricated to detect the wall thinning in insulated steel pipelines. The differential probe contains an excitation coil with two hall-sensors. A stainless steel test sample was prepared with a thickness that varied from 1 mm to 5 mm and was laminated by plastic insulation with uniform thickness to represent the insulated pipelines in the NPPs. Excitation coil in the probe is driven by a rectangular current pulse, the resultant PEC response which is the difference of the two hall sensors is detected. The discriminating features of the detected pulse, peak value and the time to zero were used to describe the wall thinning in the tested sample. A signal processing technique such as power spectrum density (PSD) is devised to infer the PEC response. The results shows that the differential PEC probe has the potential to detect the wall thinning in an insulated pipeline of the nuclear power plants (NPPs). Keywords Differential probe · Insulation · Time to zero · PSD

C.S. Angani · C.G. Kim · P. Leela · P. Kollu Department of Material Science and Engineering, Chungnam National University, Daejeon 305-764, South Korea C.S. Angani · D.G. Park () · Y.M. Cheong Nuclear Materials Research Division, Korea Atomic Energy Research Institute, Daejeon 305-600, South Korea e-mail: [email protected]

1 Introduction Nondestructive testing (NDT) is the most significant tool in the inspection of industrial components or systems such as pipelines in nuclear power plants (NPP’s) [1]. The wall thinning in pipelines can affect the reliability and safety of the plant [2], usually the pipelines are covered with insulation for low thermal loss, and hence it is necessary to detect the wall thinning without removing the insulation [3]. The electromagnetic NDT methods are the most common for conductive materials to test in a range of technological applications such as thickness measurement, coatings and surface treatments [4–7]. The PEC has been paid much attention in research and development [8, 9] for the detection of wall and coating thickness, surface and sub surface defects [10]. The conventional eddy current technique (ECT) which operates with single frequency sinusoidal excitation has gained a wide acceptance in the field of NDT [11] but this technique suffers from a limitation i.e., penetration depth √ or skin depth. The skin depth equation is given by δ = 1/πμσf where μ is the permeability, σ is the conductivity and f is the frequency of excitation, the penetration depth δ depends on excitation frequency f [12]. In contrast to traditional ECT, the PEC employs a non sinusoidal excitation such as a pulse or square wave instead of single frequency sinusoidal excitation. Since the Fourier transform of a pulse contains multiple frequency components [13, 14], therefore a pulse excitation generates numerous frequencies simultaneously in to work piece, so that a rectangular pulse can provide depth profile of a material under test [15, 16]. The usage of short current pulse excitation reduces the power consumption, which is the most desired specification in the development of portable instruments. Because of the potential advantages of the PEC, prevalent investigations on this technique have been done such as detection of wall thinning and corrosion in aircraft multilayer structures [3, 17].

Author's personal copy J Nondestruct Eval (2010) 29: 248–252

The PEC probe consists of a driving or excitation coil and a detecting sensor, the probe is placed on the conductive specimen and the driving coil is excited by a current pulse. The driving coil induces the eddy currents in the specimen (Faraday’s law of induction); the induced eddy current field was detected by the sensor. The sensor detects not only the eddy current’s field but also the field produced by the excitation coil. Usually the field produced by the excitation coil is very larger than that of induced eddy currents field, hence this excitation field dominate the perturbed eddy current fields which are generated due to the geometric changes in the sample such as metal loss, cracks, etc.,. So it is difficult to detect the flaws, geometric changes in the specimen. To overcome this problem i.e., the excitation field’s interference, two techniques are widely used, one is the reference signal subtraction [18, 19] and the other one is the differential probe [20, 21]. The advantage of differential probe is, it avoids the storage of reference signal before starting the measurements. In the present study, a differential-type PEC probe with two hall-sensors has been developed and proposed for the evaluation of wall thinning in an insulated stainless steel.

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Fig. 1 Design of the differential probe and PEC system

2 PEC System and Differential Probe The PEC system consists of a square wave generator with varying duty cycle and frequency, a pulse amplifier, a differential probe having excitation coil with two hall sensors H1 and H2 (HW-300A), two amplifiers to extract the signals from two hall sensors, a sensitive difference amplifier subtracts the outputs of the two hall sensors, A/D converter, X-Y scanner, a computer with data acquisition and signal processing soft ware. The output of the probe is interfaced to the computer through an analog to digital conversion PCI (peripheral component interface) card for the data acquisition. A real time Lab VIEW based data acquisition program is used to scan the probe on the insulated sample. The differential probe consists of an excitation copper coil of 120 turns wound on a cylindrical ferrite core. It has the dimensions of 22 mm inner and 26 mm outer diameter. The excitation coil in the probe has been driven by the pulse amplifier. To detect the PEC response, two hall-sensors H1 and H2 are placed at the top and bottom axial center of the excitation probe as shown in Fig. 1. The magnetic field detected by the two sensors is subtracted by a difference amplifier and the resultant signal is used as the probe signal. The calibration sample is a stainless steel (SS304) with a thickness variation from 1 mm to 5 mm. A plastic plate having 8 mm thickness is attached on the flat side of the samples to represent the thermal insulation of the pipelines as shown in Fig. 1. During the measurement the PEC probe is placed on the plastic insulation.

Fig. 2 Response from the individual hall-sensors H1 and H2 in the probe and differential signal when the probe in air and on insulated sample

3 Experimental Setup and PEC Response The excitation coil is driven by a current pulse of 500 mA, 500 µs width. When probe is mounted on conducting sample, the exciting pulse causes induced currents within the sample to flow in the direction where its self flux opposes the externally imposed flux (Lenz’s law); the ohmic dissipation causes the induced currents to decay exponentially with time [22, 23]. Figure 2 shows the response of individual hallsensors as well as the differential signal (Vdiff = H2 − H1), when the probe is in air the responses from H1, H2 are almost same, therefore the difference is nearly zero, now by mounting the probe on the insulated sample the responses from H1 raises slower than H2 to reach its maximum value because the hall-sensor H1 is nearer to the sample surface, so that the effect of induced eddy currents are more on H1 than H2. If there is an increase in the sample thickness

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Fig. 3 (a) The detected differential PEC probe response at different thickness of the tested sample with 8 mm insulation. (b) The PSD of the detected pulse responses from the tested sample

(3 mm) then the response from H1 raises even slower to its steady state value due to the large cross sectional conduction area leads to higher induced eddy currents [24], therefore if the sample thickens increases the raise time of the H1 also increases. As we are measuring the difference of two sensors responses which results in increase of amplitude as well as the time to zero (when the two hall sensor responses approach their steady value then differential signal is zero) of the difference signal with increasing the sample thickness.

4 Results and Feature Extraction The shape of the detected differential pulse is of interest to interpret the results. The important characteristics of the pulse, ‘peak value’ and ‘time to zero’ values are utilized to investigate the variation in the thickness of the sample.

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Technically we can understand the differential pulse is proportional to the induced eddy currents in the sample, because the effect of excitation field is nullified by the differential arrangement of two hall-sensors, only the effect of induced eddy current fields are detected by the probe. As shown in Fig. 2 the differential pulse appeared initially during the transition region of excitation pulse (according to Faraday’s law eddy currents induced during the transitions), as the excitation pulse approaches its steady state value (there is no rate of change in voltage) the induced eddy currents decay to zero, hence the Vdiff approaches to zero. Figure 3(a) shows the results which are measured on the insulated test sample, it can be observed that the differential pulse amplitude and the time to zero are increased with the thickness of test sample. In addition, to obtain a reliable parameter for PEC signal a feature extraction technique has been devised which is the power spectral density (PSD) of signal [25]. The PSD of a signal is the square of the magnitude of theFourier transform of the signal. Which ∞ 1 −iωt |2 where ω is the anguis given by  = 2π −∞ |f (t)e lar frequency and F (ω) is the Fourier transform of f (t). Figure 3(b) shows the PSD of the pulse responses from the insulated sample. To get more insight into PSD,  ∞ the energy of the signal which is defined as E = −∞ [f (t)]2 has been calculated, because according to the Parseval’s theorem, sum of the square (energy) of the signal is equal to the sum of the square of its transform. Figure 4(a) shows the peak value and time to zero values of the PEC response at different thickness of the tested sample, as the peak value and time to zero of the detected pulse are increased with increasing the sample thickness, it means that the area under the pulse is also increases therefore the energy of the detected pulse is increased with sample thickness as shown in Fig. 4(b). Since the energy and power spectrum density are based on the same physical basis, there is a linear correlation between these parameters.

5 Scanning of Test Sample The PEC probe is fixed to the X-Y scanner to perform the scanning on the flat side of the tested sample including the plastic insulation. A Lab VIEW based data acquisition program was developed to continuously monitor the variation in the thickness of the sample and is observed on the computer screen. The time domain feature which is the peak value of detected pulse is used for the scanning test to monitor the variation in the thickness of tested sample. After the completion of each scan, the data from the program can be stored in a text form, one can reproduce the graph which is displayed in the thickness monitoring window by plotting the stored data as shown in Fig. 5.

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Fig. 5 The scanning results show the variation in the thickness of the tested sample

material degradation and damages, as a part of long-term nuclear R&D program supported by the ministry of education science and technology (MEST), Korea.

References

Fig. 4 (a) The detected pulse peak value and time to zero values corresponding to thickness of the tested sample. (b) The energy of the detected pulse as a function of thickness of the test sample

6 Conclusion A differential probe which is used in PEC system has been fabricated for the detection of wall thinning in an insulated stainless steel pipe. The wall thinning of calibration sample without removing the insulation has been investigated. The time domain features of detected pulse such as pulse amplitude and time to zero were used to detect the wall thinning. The signal processing techniques such as energy of the detected pulse and PSD were derived to analyze and understand the PEC results, these parameters are well described the thickness variation. The automatic scanning results were successfully displayed on the computer monitor. The results show the proposed differential PEC technique has the potential to detect the wall thinning in the insulated pipelines. Acknowledgement This work was developed by the research project on the development of advanced diagnostic technique for micro-

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