- Email: [email protected]
Pulsed remote field technique used for nondestructive inspection of ferromagnetic tube
NDT&E International 53 (2013) 47–52
Contents lists available at SciVerse ScienceDirect
NDT&E International journal homepage: www.elsevier.com/locate/ndteint
Pulsed remote field technique used for nondestructive inspection of ferromagnetic tube Binfeng Yang a,, Xuechao Li b a b
Institute of Telecommunication Engineering, Air Force Engineering University, Xi’an 710077, China School of Electronics & Information Engineering, Xi’an Jiaotong University, Xi’an 710049, China
a r t i c l e i n f o
abstract
Article history: Received 4 November 2008 Received in revised form 12 January 2009 Accepted 22 January 2009 Available online 20 February 2009
Remote field eddy current (RFEC) technique is an effective method for measurement of ferromagnetic tube. However, traditional RFEC is unable to differentiate the internal and external defect and the probe has a long length. Pulsed eddy current techniques excite the induction coil with a pulsed waveform and have the richness of frequency harmonics. The wideband excitation is thought to be a potential in providing more information about the flaw. In this paper, pulsed RFEC technique is used to inspect ferromagnetic tube. The finite element analysis and experiment method is used to give a thorough analysis of the influence effect with the variations of the system parameters. Results show that this technique effectively combines the advantages of RFEC and pulsed excitation, which not only acquires more inspection information, including measurements of inner diameter of tubes, internal and external defects, but also reduces the length of probe and power consumption. The agreement between simulation and experiment shows that the present method is correct. & 2012 Elsevier Ltd. All rights reserved.
Keywords: Pulsed remote field Ferromagnetic tubes Nondestructive testing
1. Introduction
2. The principle of PRFEC
Nondestructive testing is highly important in down-hole inspection of oil-well casings because of harsh operating environment and ecological and economic risks associated with oil-well breakdown [1]. Remote field eddy current (RFEC) technique is an effective method for measurement of ferromagnetic tube, conventional RFEC technique uses sinusoidal excitation and coils separated 2–3 tube diameters, which has the same detecting sensitivity for internal and external defects. However, sinusoidal RFEC has some inherent drawbacks, for example, it cannot differentiate whether the wall thickness is changed by an internal or an external defect, and relatively high power consumption and long length of probe. The pulsed eddy current (PEC) nondestructive testing method is a new technology developed in recent years, which has been demonstrated to be capable of quantifying corrosion in the multilayer aircraft structure [2–4]. In this paper, the advantages of PEC and RFEC are combined together to form the pulsed remote field eddy technique (PRFEC). The effect of inner diameter, wall thickness, internal and external defect and the design of the magnetic route are analyzed with the finite element method.
The probe structure of PRFEC is similar with the conventional sinusoidal RFEC, as shown in Fig. 1, which consists of an exciting coil and a pick-up coil, PRFEC techniques excite the probe’s driving coil with a repetitive broadband pulse, usually a square wave. The resulting transient current through the coil induces transient eddy currents in the tube wall, which are associated with highly attenuated magnetic pulses propagating through the tube wall [5]. Because the eddy currents flow through the tube wall, by which are influenced the electromagnetic properties of the tube material (permeability and conductivity).
Corresponding author. Tel.: þ 86 029 847 98479; fax: þ 86 731 457 3385.
E-mail address: [email protected] (B. Yang). 0963-8695/$ - see front matter & 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ndteint.2009.01.015
3. Finite element analysis In order to investigate the relationship between the system parameter (the thickness and inner diameter of tube, the length of probe, etc.) and the resulting response signal, the method of finite element analysis is used, simulation is performed using FEMM freeware package [6]. In the simulation model, tube inner diameter is 120 mm, tube wall thickness is 5 mm, the length of tube is 550 mm, the tube material has relative permeability mr ¼ 100 and conductivity s ¼5 MS m 1. The inner diameter of exciting coil is 24 mm, the thickness is 8 mm, the length is 80 mm, and the number of turns is 1000. The inner diameter of pick-up coil is 104 mm, the thickness is 3 mm, the length is 2.4 mm, and the number of turns
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B. Yang, X. Li / NDT&E International 53 (2013) 47–52
Fig. 1. The principle of pulsed remote field eddy current.
is 2000. The amplitude of exciting pulse is 48 V, the repetition rate of excitation is 20 Hz, and the pulse duration is 5 ms. Voltage induced in the pick-up coil consists of two components, one (coupling component) is induced directly by the exciter magnetic field and the other (eddy current component) is induced by the magnetic field of the eddy currents [7]. In the case of pulse excitation, the coupling component exists only during the rise or fall of the excitation current. In the other times, the eddy current component attenuates slowly and dominates the response signal. As a result, decoupling ‘‘in time’’ is achieved. In the simulation analysis, the coupling component and the eddy current are analyzed to quantify the parameter of tube.
Fig. 2. The influence of response signal with the variation of wall thickness.
4. The results of simulation The changes of tube wall thickness and inner diameter, the length and the magnetic route of the probe, the depth of flaw will influence the propagation of PRFEC, thus the features of response signal are changed, the influence effects of these factors are analyzed with the simulation method. 4.1. The influence effect of tube parameters The information of inner diameter and wall thickness of tube are important for the tube NDT, the influence effects of these two parameters to the transient response signals are analyzed firstly. The results of response signals with the wall thicknesses of 5, 7, and 9 mm are shown in Fig. 2, the distance between the exciting coil and pick-up coil is 1.75 tube inner diameter. It can be seen that the peak of response signal shows very little change, but the zero-crossing time has a linear relationship with tube wall thickness, which increases with increasing wall thickness. This indicates that the zero-crossing time can be used as the feature to quantify the thickness of tube wall. Fig. 3 suggests how peak of response signal can be used for measurement of the tube inner diameter, the results of response signals with the inner diameters of 120, 130, and 140 mm are shown in Fig. 3, the wall thickness is 5 mm, the distance between the exciting coil and pick-up coil is 1.75 tube inner diameter. It can be seen that the peak of response signal increases with increasing inner diameter, but the change of zero-crossing time is negligible. This indicates that the peak can be used as the feature to quantify the inner diameter of tube.
Fig. 3. The influence of response signal with the variation of inner diameter of tube.
4.2. The influence effect of probe length Fig. 4 shows the response signals when the distances between the exciting coil and pick-up coil separately are 1.50, 1.75, and 2.00 tube inner diameter. It can be seen that the coupling
Fig. 4. The influence of response signal with the variation of distance between coils.
B. Yang, X. Li / NDT&E International 53 (2013) 47–52
component attenuates faster than the eddy current component, which indicates that the high frequency component is sensitive to the change of coil distance, which nearly does not influence the eddy current component. Then, a shorter length probe can be designed. Figs. 5 and 6 show the current of exciting coil and response signals of pick-up coil when the inner diameters of exciting coil separately are 24, 44, and 64 mm, the tube inner diameter is 200 mm, the wall thickness is 5 mm. It can be seen that the amplitude of response signal increases with increasing inner diameter of exciting coil. However, it can also be seen from Fig. 5 that the rate of change of the rising edge is lower as the increasing of inner diameter. The rate of change of the rising edge of the current pulse is crucial as it determines the frequency components contained in the excitation. The higher the rate of change, the more high frequency components generated and hence, more diagnostic information can be expected [8]. As a result, in order to get a strong response signal while does not reduce the detecting sensitivity, a large exciting voltage is necessary.
49
4.3. The influence effect of magnetic route It can be seen from Fig. 2 that eddy current component weakens with increasing wall thickness of tube, which shows that eddy current attenuates seriously while propagating through the thick wall, as a result, the amplitude of response signal is reduced. In order to overcome this problem, a new exciting coil is designed using the ferrite yoke, as shown in Fig. 7, thanks to the feature that ferrite core has a high permeability, more exciting
Fig. 7. The new exciting coil using the ferrite yoke.
Fig. 5. The influence of exciting current with the variation of inner diameter of probe.
Fig. 6. The influence of response signal with the variation of inner diameter of probe.
Fig. 8. The influence of response signal with the variation of thickness of ferrite core.
Fig. 9. The influence of response signal with the variation of height of ferrite core.
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B. Yang, X. Li / NDT&E International 53 (2013) 47–52
magnetic field will be collected to propagate in the tube wall. As a result, the coupling component decreases and the eddy current component increases at the same time, thus, the amplitude of response signal is enhanced. In Fig. 7, T and H separately represent the thickness and the height of ferrite core. Fig. 8 shows the response signals with the T of 5, 15, and 30 mm, it can be found that the amplitude of response signal increases with increasing thickness of ferrite core. Fig. 9 shows the response signals with the H of 40, 45, and 50 mm, it can also be found that the amplitude of response signal increases with increasing height of ferrite core. When T¼30 mm and H¼45 mm, the amplitude of response signal is 30 times than the probe which does not has the ferrite core. Fig. 10 shows the response signals when the distances between the exciting coil and pick-up coil separately are 0.90, 1.10, and 1.30 tube inner diameter as the probe using the ferrite yoke. Simulation result shows that the distance between the exciting and pick-up coil can be shorten to the 1.10 inner diameter of tube, which indicates that the design of a shorter length of probe can be realized with using the ferrite yoke.
Fig. 12. The influence of response signal with the variation of depth of external defect.
Fig. 10. The influence of response signal with the variation of distance between coils. Fig. 13. The relationship of zero-crossing time and the depth for internal and external defect.
4.4. The influence effect of internal and external defects Three circumferential notch defects are simulated as external and internal on the tube, Figs. 11 and 12 show the results for the pick-up coil is placed beneath the defect, the distance between the exciting coil and pick-up coil is 1.75 tube inner diameter, the exciting coil using the ferrite yoke, the wall thickness is 5 mm. It shows that the depth of defect can be differentiated by using the feature of zero-crossing time, which decreases with increasing depth of defect. Fig. 13 shows the change in zero-crossing time for internal and external defects. It can be found that the zerocrossing time has a fairly linear relationship with both kinds of defect depth, which can be extracted as the feature to quantify the depth of defect.
5. The results of experiment Fig. 11. The influence of response signal with the variation of depth of inner defect.
The PRFEC instrumentation used in this work consists of a pulser, power amplifier, data acquisition module and a probe.
B. Yang, X. Li / NDT&E International 53 (2013) 47–52
51
Fig. 14. The schematic diagram of the PRFEC instrumentation.
enhance the excited magnetic field. The transient signal of the pick-up coil is sampled at 1 MHz sampling rate using data acquisition module and is recorded for the purpose of off-line post-processing. The inner diameter of exciting coil is 50 mm, the thickness is 2 mm, the length is 80 mm, and the number of turns is 800. The inner diameter of pick-up coil is 90 mm, the thickness is 3 mm, the length is 2.5 mm, and the number of turns of pick-up coil is 2000. Fig. 15 shows the response signals with the tube inner diameters of 98, 103, and 108 mm, the wall thickness is 5 mm. It can be seen that the peak of response signal increases with increasing inner diameter, however, the zero-crossing time almost does not change. Fig. 16 shows the response signals with the wall thicknesses of 5, 7, and 9 mm, the inner diameter of the tube is 98 mm. It can be seen that the zero-crossing time of response signal increases with increasing wall thickness, however, the peak shows very little change. Figs. 17 and 18 show the results with internal and external defects on the tube, the wall thickness is 5 mm, the inner
Fig. 15. The influence of response signal with the variation of inner diameter of tube.
Fig. 17. The influence of response signal with the variation of depth of inner defect.
Fig. 16. The influence of response signal with the variation of wall thickness.
Fig. 18. The influence of response signal with the variation of depth of external defect.
Fig. 14 shows a schematic of the experimental setup. In this experimental system, Direct Digital Synthesizer (DDS) chip AD7008 is used to generate the exciting pulse. The amplitude of exciting pulse is 10 V, the repetition rate of excitation is 20 Hz and the pulse duration is 5 ms. A power amplifier is employed to
Exciting pulse
Power magnify
Oscillograph
Data acquisition
Signal magnify
Exciting coil
Pick-up coil
52
B. Yang, X. Li / NDT&E International 53 (2013) 47–52
diameter of the tube is 98 mm, the defect sizes are (length width depth): 15 mm 5 mm 2 mm, 15 mm 5 mm 3 mm, and 15 mm 5 mm 4 mm, the detector coil is placed beneath the defect. It can be seen that the zero-crossing time of response signal is linearly related to the depth of defect, for the tube with the external defects, the change of the peak with the defect depth is negligible comparing to the case of the tube with the internal defects. The general shapes of experiment and simulation accord with each other, this shows that the zero-crossing time can be used as the feature to quantify the depth of defect, and the internal and external defect can be differentiated by the feature of peak.
6. Conclusion Traditional sinusoidal RFEC inspection techniques have some inherent shortcomings, in order to overcome these problems, the RFEC probe is excited with the pulse waveform, PRFEC technique is applied to inspect the ferromagnetic tube in this paper, and the influence effects of system parameters on the inspection results are analyzed by the finite element and experiment method. Simulation and experiment results show that the PRFEC technique effectively combines the advantages of RFEC and PEC, which has a high sensitivity to the inspection of tube inner diameter and internal and external defects. At the same time, a new exciting coil is designed using the ferrite yoke, which not only enhances the amplitude of response signal, but also shortens the length of probe.
Acknowledgment The authors would like to thank Professor G.Y. Tian in University of Newcastle (UK) for his contribution to the work. References [1] Vasic D, Bilas V, Ambrus D. Pulsed eddy current nondestructive testing of ferromagnetic tubes. IEEE Transactions on Instrumentation and Measurement 2004;53(4):1289–94. [2] Smith RA, Hugo GR. Transient eddy current NDE for ageing aircraft—capabilities and limitations. Insight 2001;43(1):14–25. [3] Yang BF, Luo FL, Han D. Research on edge identification of a defect using pulsed eddy current based on principal component analysis. NDT and E International 2007(40):294–9. [4] Yang BF, Luo FL, Han D. Pulsed eddy current technique used for nondestructive inspection of aging aircraft. Insight 2006;48(7):411–4. [5] Sophian A, Tian GY, Taylor D, et al. A feature extraction technique based on principal component analysis for pulsed eddy current NDT. NDT and E International 2003(36):37–41. [6] Finite element method magnetics (FEMM) version 3.1. Available: femm.berlios.de [on-line]. [7] Vasic D, Bilas V, Ambrus D. Pulsed eddy current nondestructive testing of ferromagnetic tubes. In: IMTC 2003 instrumentation and measurement technology conference, 2003. p. 1120–5. [8] Sophian A, Tian GY, Taylor D, et al. Design of a pulsed eddy current sensor for detection of defects in aircraft lap-joints. Sensors and Actuators A 2002(101):92–8.
Contents lists available at SciVerse ScienceDirect
NDT&E International journal homepage: www.elsevier.com/locate/ndteint
Pulsed remote field technique used for nondestructive inspection of ferromagnetic tube Binfeng Yang a,, Xuechao Li b a b
Institute of Telecommunication Engineering, Air Force Engineering University, Xi’an 710077, China School of Electronics & Information Engineering, Xi’an Jiaotong University, Xi’an 710049, China
a r t i c l e i n f o
abstract
Article history: Received 4 November 2008 Received in revised form 12 January 2009 Accepted 22 January 2009 Available online 20 February 2009
Remote field eddy current (RFEC) technique is an effective method for measurement of ferromagnetic tube. However, traditional RFEC is unable to differentiate the internal and external defect and the probe has a long length. Pulsed eddy current techniques excite the induction coil with a pulsed waveform and have the richness of frequency harmonics. The wideband excitation is thought to be a potential in providing more information about the flaw. In this paper, pulsed RFEC technique is used to inspect ferromagnetic tube. The finite element analysis and experiment method is used to give a thorough analysis of the influence effect with the variations of the system parameters. Results show that this technique effectively combines the advantages of RFEC and pulsed excitation, which not only acquires more inspection information, including measurements of inner diameter of tubes, internal and external defects, but also reduces the length of probe and power consumption. The agreement between simulation and experiment shows that the present method is correct. & 2012 Elsevier Ltd. All rights reserved.
Keywords: Pulsed remote field Ferromagnetic tubes Nondestructive testing
1. Introduction
2. The principle of PRFEC
Nondestructive testing is highly important in down-hole inspection of oil-well casings because of harsh operating environment and ecological and economic risks associated with oil-well breakdown [1]. Remote field eddy current (RFEC) technique is an effective method for measurement of ferromagnetic tube, conventional RFEC technique uses sinusoidal excitation and coils separated 2–3 tube diameters, which has the same detecting sensitivity for internal and external defects. However, sinusoidal RFEC has some inherent drawbacks, for example, it cannot differentiate whether the wall thickness is changed by an internal or an external defect, and relatively high power consumption and long length of probe. The pulsed eddy current (PEC) nondestructive testing method is a new technology developed in recent years, which has been demonstrated to be capable of quantifying corrosion in the multilayer aircraft structure [2–4]. In this paper, the advantages of PEC and RFEC are combined together to form the pulsed remote field eddy technique (PRFEC). The effect of inner diameter, wall thickness, internal and external defect and the design of the magnetic route are analyzed with the finite element method.
The probe structure of PRFEC is similar with the conventional sinusoidal RFEC, as shown in Fig. 1, which consists of an exciting coil and a pick-up coil, PRFEC techniques excite the probe’s driving coil with a repetitive broadband pulse, usually a square wave. The resulting transient current through the coil induces transient eddy currents in the tube wall, which are associated with highly attenuated magnetic pulses propagating through the tube wall [5]. Because the eddy currents flow through the tube wall, by which are influenced the electromagnetic properties of the tube material (permeability and conductivity).
Corresponding author. Tel.: þ 86 029 847 98479; fax: þ 86 731 457 3385.
E-mail address: [email protected] (B. Yang). 0963-8695/$ - see front matter & 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ndteint.2009.01.015
3. Finite element analysis In order to investigate the relationship between the system parameter (the thickness and inner diameter of tube, the length of probe, etc.) and the resulting response signal, the method of finite element analysis is used, simulation is performed using FEMM freeware package [6]. In the simulation model, tube inner diameter is 120 mm, tube wall thickness is 5 mm, the length of tube is 550 mm, the tube material has relative permeability mr ¼ 100 and conductivity s ¼5 MS m 1. The inner diameter of exciting coil is 24 mm, the thickness is 8 mm, the length is 80 mm, and the number of turns is 1000. The inner diameter of pick-up coil is 104 mm, the thickness is 3 mm, the length is 2.4 mm, and the number of turns
48
B. Yang, X. Li / NDT&E International 53 (2013) 47–52
Fig. 1. The principle of pulsed remote field eddy current.
is 2000. The amplitude of exciting pulse is 48 V, the repetition rate of excitation is 20 Hz, and the pulse duration is 5 ms. Voltage induced in the pick-up coil consists of two components, one (coupling component) is induced directly by the exciter magnetic field and the other (eddy current component) is induced by the magnetic field of the eddy currents [7]. In the case of pulse excitation, the coupling component exists only during the rise or fall of the excitation current. In the other times, the eddy current component attenuates slowly and dominates the response signal. As a result, decoupling ‘‘in time’’ is achieved. In the simulation analysis, the coupling component and the eddy current are analyzed to quantify the parameter of tube.
Fig. 2. The influence of response signal with the variation of wall thickness.
4. The results of simulation The changes of tube wall thickness and inner diameter, the length and the magnetic route of the probe, the depth of flaw will influence the propagation of PRFEC, thus the features of response signal are changed, the influence effects of these factors are analyzed with the simulation method. 4.1. The influence effect of tube parameters The information of inner diameter and wall thickness of tube are important for the tube NDT, the influence effects of these two parameters to the transient response signals are analyzed firstly. The results of response signals with the wall thicknesses of 5, 7, and 9 mm are shown in Fig. 2, the distance between the exciting coil and pick-up coil is 1.75 tube inner diameter. It can be seen that the peak of response signal shows very little change, but the zero-crossing time has a linear relationship with tube wall thickness, which increases with increasing wall thickness. This indicates that the zero-crossing time can be used as the feature to quantify the thickness of tube wall. Fig. 3 suggests how peak of response signal can be used for measurement of the tube inner diameter, the results of response signals with the inner diameters of 120, 130, and 140 mm are shown in Fig. 3, the wall thickness is 5 mm, the distance between the exciting coil and pick-up coil is 1.75 tube inner diameter. It can be seen that the peak of response signal increases with increasing inner diameter, but the change of zero-crossing time is negligible. This indicates that the peak can be used as the feature to quantify the inner diameter of tube.
Fig. 3. The influence of response signal with the variation of inner diameter of tube.
4.2. The influence effect of probe length Fig. 4 shows the response signals when the distances between the exciting coil and pick-up coil separately are 1.50, 1.75, and 2.00 tube inner diameter. It can be seen that the coupling
Fig. 4. The influence of response signal with the variation of distance between coils.
B. Yang, X. Li / NDT&E International 53 (2013) 47–52
component attenuates faster than the eddy current component, which indicates that the high frequency component is sensitive to the change of coil distance, which nearly does not influence the eddy current component. Then, a shorter length probe can be designed. Figs. 5 and 6 show the current of exciting coil and response signals of pick-up coil when the inner diameters of exciting coil separately are 24, 44, and 64 mm, the tube inner diameter is 200 mm, the wall thickness is 5 mm. It can be seen that the amplitude of response signal increases with increasing inner diameter of exciting coil. However, it can also be seen from Fig. 5 that the rate of change of the rising edge is lower as the increasing of inner diameter. The rate of change of the rising edge of the current pulse is crucial as it determines the frequency components contained in the excitation. The higher the rate of change, the more high frequency components generated and hence, more diagnostic information can be expected [8]. As a result, in order to get a strong response signal while does not reduce the detecting sensitivity, a large exciting voltage is necessary.
49
4.3. The influence effect of magnetic route It can be seen from Fig. 2 that eddy current component weakens with increasing wall thickness of tube, which shows that eddy current attenuates seriously while propagating through the thick wall, as a result, the amplitude of response signal is reduced. In order to overcome this problem, a new exciting coil is designed using the ferrite yoke, as shown in Fig. 7, thanks to the feature that ferrite core has a high permeability, more exciting
Fig. 7. The new exciting coil using the ferrite yoke.
Fig. 5. The influence of exciting current with the variation of inner diameter of probe.
Fig. 6. The influence of response signal with the variation of inner diameter of probe.
Fig. 8. The influence of response signal with the variation of thickness of ferrite core.
Fig. 9. The influence of response signal with the variation of height of ferrite core.
50
B. Yang, X. Li / NDT&E International 53 (2013) 47–52
magnetic field will be collected to propagate in the tube wall. As a result, the coupling component decreases and the eddy current component increases at the same time, thus, the amplitude of response signal is enhanced. In Fig. 7, T and H separately represent the thickness and the height of ferrite core. Fig. 8 shows the response signals with the T of 5, 15, and 30 mm, it can be found that the amplitude of response signal increases with increasing thickness of ferrite core. Fig. 9 shows the response signals with the H of 40, 45, and 50 mm, it can also be found that the amplitude of response signal increases with increasing height of ferrite core. When T¼30 mm and H¼45 mm, the amplitude of response signal is 30 times than the probe which does not has the ferrite core. Fig. 10 shows the response signals when the distances between the exciting coil and pick-up coil separately are 0.90, 1.10, and 1.30 tube inner diameter as the probe using the ferrite yoke. Simulation result shows that the distance between the exciting and pick-up coil can be shorten to the 1.10 inner diameter of tube, which indicates that the design of a shorter length of probe can be realized with using the ferrite yoke.
Fig. 12. The influence of response signal with the variation of depth of external defect.
Fig. 10. The influence of response signal with the variation of distance between coils. Fig. 13. The relationship of zero-crossing time and the depth for internal and external defect.
4.4. The influence effect of internal and external defects Three circumferential notch defects are simulated as external and internal on the tube, Figs. 11 and 12 show the results for the pick-up coil is placed beneath the defect, the distance between the exciting coil and pick-up coil is 1.75 tube inner diameter, the exciting coil using the ferrite yoke, the wall thickness is 5 mm. It shows that the depth of defect can be differentiated by using the feature of zero-crossing time, which decreases with increasing depth of defect. Fig. 13 shows the change in zero-crossing time for internal and external defects. It can be found that the zerocrossing time has a fairly linear relationship with both kinds of defect depth, which can be extracted as the feature to quantify the depth of defect.
5. The results of experiment Fig. 11. The influence of response signal with the variation of depth of inner defect.
The PRFEC instrumentation used in this work consists of a pulser, power amplifier, data acquisition module and a probe.
B. Yang, X. Li / NDT&E International 53 (2013) 47–52
51
Fig. 14. The schematic diagram of the PRFEC instrumentation.
enhance the excited magnetic field. The transient signal of the pick-up coil is sampled at 1 MHz sampling rate using data acquisition module and is recorded for the purpose of off-line post-processing. The inner diameter of exciting coil is 50 mm, the thickness is 2 mm, the length is 80 mm, and the number of turns is 800. The inner diameter of pick-up coil is 90 mm, the thickness is 3 mm, the length is 2.5 mm, and the number of turns of pick-up coil is 2000. Fig. 15 shows the response signals with the tube inner diameters of 98, 103, and 108 mm, the wall thickness is 5 mm. It can be seen that the peak of response signal increases with increasing inner diameter, however, the zero-crossing time almost does not change. Fig. 16 shows the response signals with the wall thicknesses of 5, 7, and 9 mm, the inner diameter of the tube is 98 mm. It can be seen that the zero-crossing time of response signal increases with increasing wall thickness, however, the peak shows very little change. Figs. 17 and 18 show the results with internal and external defects on the tube, the wall thickness is 5 mm, the inner
Fig. 15. The influence of response signal with the variation of inner diameter of tube.
Fig. 17. The influence of response signal with the variation of depth of inner defect.
Fig. 16. The influence of response signal with the variation of wall thickness.
Fig. 18. The influence of response signal with the variation of depth of external defect.
Fig. 14 shows a schematic of the experimental setup. In this experimental system, Direct Digital Synthesizer (DDS) chip AD7008 is used to generate the exciting pulse. The amplitude of exciting pulse is 10 V, the repetition rate of excitation is 20 Hz and the pulse duration is 5 ms. A power amplifier is employed to
Exciting pulse
Power magnify
Oscillograph
Data acquisition
Signal magnify
Exciting coil
Pick-up coil
52
B. Yang, X. Li / NDT&E International 53 (2013) 47–52
diameter of the tube is 98 mm, the defect sizes are (length width depth): 15 mm 5 mm 2 mm, 15 mm 5 mm 3 mm, and 15 mm 5 mm 4 mm, the detector coil is placed beneath the defect. It can be seen that the zero-crossing time of response signal is linearly related to the depth of defect, for the tube with the external defects, the change of the peak with the defect depth is negligible comparing to the case of the tube with the internal defects. The general shapes of experiment and simulation accord with each other, this shows that the zero-crossing time can be used as the feature to quantify the depth of defect, and the internal and external defect can be differentiated by the feature of peak.
6. Conclusion Traditional sinusoidal RFEC inspection techniques have some inherent shortcomings, in order to overcome these problems, the RFEC probe is excited with the pulse waveform, PRFEC technique is applied to inspect the ferromagnetic tube in this paper, and the influence effects of system parameters on the inspection results are analyzed by the finite element and experiment method. Simulation and experiment results show that the PRFEC technique effectively combines the advantages of RFEC and PEC, which has a high sensitivity to the inspection of tube inner diameter and internal and external defects. At the same time, a new exciting coil is designed using the ferrite yoke, which not only enhances the amplitude of response signal, but also shortens the length of probe.
Acknowledgment The authors would like to thank Professor G.Y. Tian in University of Newcastle (UK) for his contribution to the work. References [1] Vasic D, Bilas V, Ambrus D. Pulsed eddy current nondestructive testing of ferromagnetic tubes. IEEE Transactions on Instrumentation and Measurement 2004;53(4):1289–94. [2] Smith RA, Hugo GR. Transient eddy current NDE for ageing aircraft—capabilities and limitations. Insight 2001;43(1):14–25. [3] Yang BF, Luo FL, Han D. Research on edge identification of a defect using pulsed eddy current based on principal component analysis. NDT and E International 2007(40):294–9. [4] Yang BF, Luo FL, Han D. Pulsed eddy current technique used for nondestructive inspection of aging aircraft. Insight 2006;48(7):411–4. [5] Sophian A, Tian GY, Taylor D, et al. A feature extraction technique based on principal component analysis for pulsed eddy current NDT. NDT and E International 2003(36):37–41. [6] Finite element method magnetics (FEMM) version 3.1. Available: femm.berlios.de [on-line]. [7] Vasic D, Bilas V, Ambrus D. Pulsed eddy current nondestructive testing of ferromagnetic tubes. In: IMTC 2003 instrumentation and measurement technology conference, 2003. p. 1120–5. [8] Sophian A, Tian GY, Taylor D, et al. Design of a pulsed eddy current sensor for detection of defects in aircraft lap-joints. Sensors and Actuators A 2002(101):92–8.