10_Defect edge identification with rectangular pulsed eddy current sensor based on transient
Defect edge identi?cation with rectangular pulsed eddy current sensor based on transient response signals
Yunze He n ,Feilu Luo,Mengchun Pan,Xiangchao Hu,Bo Liu,Junzhe Gao
College of Mechatronics and Automation,National University of Defense Technology,Changsha 410073,China
a r t i c l e i n f o
Article history:
Received 5November 2009Received in revised form 4March 2010
Accepted 24March 2010
Available online 30March 2010Keywords:
Pulsed eddy current (PEC)Rectangular coil Edge identi?cation Feature extraction
Transient response signal
a b s t r a c t
The Pulsed Eddy Current (PEC)testing is an increasingly emerging nondestructive testing &evaluation (NDT&E)technique.The main purpose of this study is to improve the performance of defect edge identi?cation of C-scan imaging technique utilizing the rectangular PEC sensor.When sensor scans along the defect,peak waves of response signals always present a crest and a trough in direction of magnetic induction ?ux,while present different shapes in direction of exciting current.The maximum and minimum values of peak waves in direction of magnetic induction ?ux are corresponding to the moment of sensor entering and leaving the length edge of defect,which provides us a way to evaluate the length edge of defect.To evaluate the width edge of defect,we obtain and analyze the C-scan imaging results in direction of magnetic induction ?ux.For improving the identi?cation of width edge of defect,we proposed news features from response signals and differential response signals.Experiment results have shown that the width edge of defects on surface can be identi?ed effectively by selecting and normalizing the appropriate features in time domain.Therefore,both length edge and width edge of defect can be evaluated effectively in direction of magnetic induction ?ux.The rectangular PEC sensor is helpful for C-scan imaging inspection technique and has a good prospect in ?eld of nondestructive testing &evaluation.
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1.Introduction
The pulsed eddy current (PEC)nondestructive testing is an effective technology that has been demonstrated to be capable of quantifying defect in the aging aircraft structure [1–3].PEC testing possesses many advantages against the conventional single frequency eddy current testing,including more extended detection depth,richer information about defects and higher robustness of anti-interference [4].Meanwhile,pulses can be easily generated and controlled and the other advantage of PEC method over multi-frequency eddy current method is less expensive instrumentation.Therefore,the PEC testing has been particularly developed and devised for sub-surface crack mea-surements,crack reconstruction,and depth estimation [5].In 1994,Rose et al.[6]described the defect detection in time domain for both air-core and ferrite-core PEC probes with one ?at coil.In 1996,Moulder et al.[1]developed a new scanned pulsed eddy current instrument for nondestructive inspection of aging aircraft and Tai et al.[7]determined the thickness and conductivity of conductive coatings on metal plates.In 2001,Giguere et al.[8]
illustrated three features adopted in PEC testing to quantify defects and exemplify their application.In 2002,Yang and Tai [9]determined the thickness or conductivity of metallic coatings on a metal substrate for the case when either the coating or substrate is magnetic.In 2003,Sophian et al.[10]introduced the application of principal component analysis in extracting information from PEC response.In 2004,Tian et al.[11]investigated the dynamic behavior of pulsed eddy current techniques and the feasibility of their use in an on-line inspection system.In 2005,Tian and Sophian [12]also presented a new feature called as rising point time to identify the different defect types and lift-off and Tian et al.[13]presented a new approach for defect classi?cation and quanti?cation by using pulsed eddy current sensors and integra-tion of principal component analysis and wavelet transform for feature based signal interpretation .In 2007,Li et al.[4]introduced development of differential probes in pulsed eddy current testing.In 2008,Chen et al.[14]extracted and selected appropriate features for defect classi?cation of pulse eddy current.In 2009,Fan et al.[15]made a research about analytical modeling for transient probe response in pulsed eddy current testing .In 2009,the defects in riveted structures are detected effectively utilizing the differential hall/coil probe in our labora-tory [16].In all these studies,the cylindrical driving coil is excited by repeated pulses and the response signal is measured with a
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n
Corresponding author.Tel.:+86134********.E-mail address:hejicker@https://www.sodocs.net/doc/1812158880.html, (Y.He).
NDT&E International 43(2010)409–415
sensor,which may be the driving coil,single pick-up coil, differential coil or a hall-effect sensor[17].
The Alternating Current Field Measurement(ACFM)technique is an electromagnetic technique capable of detecting and sizing defects in metallic components.It was developed within the NDE Centre,department of mechanical engineering at university college London[18]and is an extension of the Alternating Current Potential Drop(ACPD)crack detection and measurement techni-que.It is based on the thin-skin theory developed by Lewis, Michael,Lugg and Collins(LMLC theory)[19]and is widely applied for the detection of near-surface defects.The ACFM probe usually incorporate an induction coil for the generation of surface current and the pick-up coils for the measurement Bx and Bz in the vicinity of a defect[20].Associated with the current?owing in the surface,there is a magnetic?eld above the surface. The magnetic?eld will be disturbed in the presence of a defect and the pick-up coils will induce the disturbed magnetic?eld [21,22].
In our laboratory,rectangular exciting coil which was widely used in ACFM method was proposed and researched in PEC testing.The experimental results have indicated that it is also capable of detecting and evaluating defects.In2006,Yang et al. [23,24]proposed peak value and over-zero time of response signal,respectively,to measure the length and depth of defect in aircraft multi-layered structure.In2007,they also did a research on edge identi?cation of a defect using pulsed eddy current based on principal component analysis[25].In our previous studies, defects on surface can be identi?ed and evaluated effectively utilizing the2D butter?y-shape graph in both directions of sensor scanning in2009[26].The main purpose of this paper is to detect the edge of defects on surface and to enhance the accuracy of edge identi?cation based on rectangular exciting coil.
The rest of the paper is arranged as follows.Firstly,experi-mental set-up including hardware,software,and specimen is established in Section2.Next,different directions of rectangular sensor scanning are introduced and peak waves in different directions are analyzed in Section3.Then,the results of C-scan imaging are displayed and features from response signals are extracted to detect the edge of defect.Experiment results of edge identi?cation are shown in Section4.Finally,conclusions and further work are outlined in Section5.
2.Experimental set-up and specimen
The PEC experimental set-up used in this research consists of pulse generator,power ampli?er,signal conditioning module, data acquisition,X–Y displacement system,and PC-based soft-ware.The generator module is used to generate the exciting pulse, whose frequency range is10Hz–1MHz and minimum spacing adjustment is10Hz.The power ampli?er is employed to enhance the power of exciting signal,which is used to excite the rectangular coil.Then,the response signals from pick-up coil are ampli?ed and sampled by data acquisition module with 100kHz sampling rate.Operation of the set-up is controlled by a Windows XP-based program,which is programmed by Microsoft Visual C++ 6.0,combined with Matlab7.0[16,26].In the experiment of defect edge identi?cation,the amplitude of the exciting pulse is10V,the repetition rate of the excitation is 100Hz and the pulse duration is5ms,whose schematic diagram is shown in authors’another paper Ref[16].
An aluminum specimen whose thickness is2mm is designed to verify performance of the proposed method.On the surface, two slots(named defect one and defect two)are manufactured to simulate corrosion type of defects in real situation.The length?width?depth of defect one and defect two,respectively are15?5?1.5and15?2.5?1.5mm3.
3.Rectangular pulsed eddy current sensor
The probe we designed in our research consists of one rectangular exciting coil and one pick-up coil.The length,width and height of rectangular exciting coil,respectively,are50,45and 45mm.The number of turns is400.The pick-up coil is located orthogonally in the centre at the bottom of the exciting coil, which are reeled with inductive coil to induce the change of magnetic?elds along the scanning path.The turn of pick-up coil is 1000[26].
In our previous studies,peak waves of magnetic?eld are analyzed in different directions of sensor scanning.One is the direction of magnetic induction?ux;the other is the direction of exciting current.As shown in Fig.1,considering the orientation of the coil to the sample,a Cartesian coordinate system is introduced.The direction of magnetic induction?ux is parallel to X-axis and the direction of exciting current is parallel to Y-axis. In the course of probe scanning,the response signal of each pick-up coil is sampled in real time and the periodic peak voltage of each signal is extracted at the same time.Consequently,peak waves form after peak voltages are connected[26].
To observe the difference of peak waves in different directions, a defect is detected utilizing the PEC sensor in both directions.As shown in Figs.2and3,there are some kinds of scanning forms when sensor scans along the defect in each direction.The arrows point the scanning direction of rectangular sensor.Peak waves of response signals on different forms in each direction are shown in Figs.4and5,respectively.The horizontal coordinates represent the scanning time;the vertical coordinates represent the amplitude of peak scanning waveforms.It can be seen from the plots in Figs.4and5that the peak waves are distorted as the defect is scanned.
When sensor scans along the defect in the direction of magnetic induction?ux,peak waves caused by defect in specimen are shown in Fig.4.As shown in Fig2(5),when eddy currents in specimen induced by exciting pulse are disturbed by defect whose resistance is bigger than that of aluminum specimen,they will ?ow to the two ends and bottom of the defect.However,the?ow directions of eddy currents in two ends are contrary.One is clockwise,while the other is anti-clockwise.Consequently,the magnetic force induced by changed eddy currents in two ends is also opposite.Hence,the currents in pick-up coil will change adversely when sensor is on the different ends of defect.In
other Fig.1.The diagram of PEC probe and scanning direction.
Y.He et al./NDT&E International43(2010)409–415 410
words,as the probe scans along the defect,a crest and a trough always appear on the peak waves [27].In addition,when the pick-up coil is away from the defect,such as the form 1and 5,peak waves still present a crest and a trough.
When sensor scans in the direction of exciting current,peak waves caused by defects are shown in Fig.5.If eddy currents in specimen are disturbed by defect,they will ?ow to the two sides and bottom of the defect,which is shown in Fig.3(5).However,the ?ow directions of eddy currents in two sides are contrary.Consequently,the magnetic force induced by changed eddy currents in two sides is also opposite.So the induced currents in pick-up coil will change adversely when sensor is on the different sides of defect.In other words,when defect is on the left of pick-up coil (form 1and 2),a broad crest appears on peak waves.In contrast,when defect is on the right of pick-up coil (form 4and 5),a broad trough appears on peak waves.When defect is on the centre of sensor (form 3),because the eddy current ?ow to the bottom of defect,the magnetic force induced by eddy current is vertical with the normal of the pick-up coil.So,the variation of induced current in pick-up coil is small and the voltage of peak wave is approximate constant [27].Furthermore,it can be seen that even the pick-up coil is away from the defect,peak waves in the form 1and 5still present a crest or a trough.
Comparing the results in Figs.4and 5,we can ?nd that even though the pick-up coil is not on the defect,the response signals still changed.Fortunately,in Fig.4,the maximum and minimum values of peak waves are corresponding to the moment of sensor entering and leaving the length edge of defect [18–21].In other words,the length edge of defect can be evaluated in magnetic induction ?ux.If we can identify the width edge in magnetic induction ?ux,the defect edge can be detected.Therefore,in next section,we analyze the C-scan imaging results in direction of magnetic induction ?ux.
4.Results and discussion 4.1.C-scan imaging results
The specimen designed in Section 2is used in the experiment of C-scan imaging detection.As shown in Fig.6,the arrows
point
Fig.2.The scanning forms of sensor in direction of magnetic induction ?ux:(1)and (2)pick-up coil is on the right of defect;(3)pick-up coil is on the centre of defect;(4)and (5)pick-up coil is on the left of
defect.
Fig.3.The scanning forms of sensor in direction of exciting current:(1)and (2)pickup coil is on the right of defect;(3)pickup coil is on the centre of defect;(4)and (5)pickup coil is on the left of
defect.
Fig.4.Peak waves on different forms in direction of magnetic induction ?ux.
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the scanning direction of rectangular sensor.We make a research of 15scanning forms when sensor scans along the defect in direction of magnetic induction ?ux.The step distance between two adjacent forms is 1mm,which is controlled by X–Y displacement system.The C-Scan imaging results of defects are shown in Figs.7and 8.It can be seen that the length of defect can be evaluated effectively.Unfortunately,the width of defects is evaluated large than the real width.As shown in Fig.7,the average voltage of defect-free is 420mV.If selecting the 40%(168mv)of average voltage as the threshold voltage (588mV),the width edge of defect one will appear between form 2and 14.And then,the estimated width edge (from form 2to 14)is 12mm,which is obviously bigger than actual width (5mm)of defect one.
As the same in Fig.8,the estimated width edge (from form 3to 13,10mm)of defect two is also clearly bigger than actual width (2.5mm).In other words,the width edge of defect in C-scan imaging results is overestimated.So,the new method is needed to improve the estimation of the width edge of defects.4.2.Features from defect response signal
As shown in Fig.9,the regular features of response signals in time domain are as follows:time to rise,time to peak,the rising time,and peak amplitude [16,27].The peak amplitude and the time to peak are the main features used in extracting information from PEC response signals.It is found that the peak amplitude is valuable to identify the edge of defect in experiments.However,the only one feature is not enough to identify the edge of defects.4.3.Features from differential response signal
In Section 4.1,the peak amplitude is proposed to identify the edge of defects in C-scan imaging results.For improving and enhancing the performance of edge identi?cation,we extract three new features from differential response signals,which
are
Fig.5.Peak waves on different forms in direction of exciting
current.
Fig.6.The scanning forms of sensor against
defect.
Fig.7.The C-scan imaging results of defect
one.
Fig.8.The C-scan imaging results of defect two.
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computed by subtracting a defect-free signal from the defect signals.Defect-free signal is obtained when the probe is located on a known defect-free sample [16].As shown in Fig.10,the ?rst feature is called as differential peak.The second feature is differential time to peak,which is the time to peak amplitude of the differential response signal.The last feature is the differential time to zero,which is also the time when the defect response signal (blue line in Fig.10)intersects with defect-free response signal (black line in Fig.10).In experiment,it is found that the three features of differential response signals can be used to identify the edge of defect.Therefore,the three new features are extracted and combined with peak amplitude to identify the edge of defect in direction of magnetic induction ?ux in next subsection.4.4.Edge identi?cation
The main purpose of this subsection is to identify the width edge of defects on surface by exacting the appropriate features in time domain.The four features proposed in Sections 4.1and 4.2(peak,differential peak,differential time to peak,and differential time to zero)are used and processed.The four features on
maximum point of peak waves from 15scanning forms (1–15in Fig.6)of defect one are shown in Figs.11and 12.The horizontal coordinates represent the state 1–15;the vertical coordinates represent the volt amplitude and time,respectively.Obviously,the waves of peak and differential peak present a crest.On the contrary,the waves of differential time to peak and differential time to zero present a trough.If normalizing the four feature waves,they will intersect at some time each other,which may be useful to identify the width edge of defect.
The results of edge identi?cation of defect one after the normalization in direction of magnetic induction ?ux are shown in Fig.13.The horizontal coordinates represent the scanning form 1–15;the vertical coordinates represent the normalization amplitude.There is no doubt that the differential peak wave (blue line)and the wave of differential time to zero (black line)intersect at two junctions.One is between form 5and 6;the other is between form 10and 11.The distance of two junctions is 5mm,which is the same to the width of defect one.Hence,the width edge is evaluated correctly,which is better than estimated width (12mm)in Section
4.1.
Fig.9.The features of defect response signal in time
domain.Fig.10.The features of differential response
signal.
Fig.11.The waves of peak and differential peak of defect
one.
Fig.12.The waves of differential time to peak and differential time to zero of defect one.
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For validating the performance of method,the peak waves of defect two are processed as the defect one.The results of edge identi?cation are shown in Fig.14and the estimated width is 3mm.Although it is slightly bigger than the actual width of defect two (2.5mm),it is further better than the estimated width (10mm)in Section 4.1.
Comparing the results in Figs.13and 14with the results in Figs.7and 8in Section 4.1,we can ?nd that the identi?cation of width edge is improved remarkably by proposed method.So,the method provides us a better way to identify the width edge of https://www.sodocs.net/doc/1812158880.html,bining with the length edge,which has been evaluated by the crest and trough on peak waves in Section 3,the edge of defect can be identi?ed effectively in direction of magnetic induction ?ux.
5.Conclusions
In this paper,the further study of PEC rectangular probe proposed in author’s previous work has been made to identify the edge of defects on surface.When sensor is on the different positions against the defect,peak waves of response signals
present the same shape in direction of magnetic induction ?ux,while present different shapes in direction of exciting current.The maximum and minimum values of peak waves in direction of magnetic induction ?ux are related to the moment of sensor entering and leaving the length edge of defect,which gives us a way to estimate the length edge of defect.Therefore,we put our emphasis on the C-scan imaging results in direction of magnetic induction ?ux.Unfortunately,the estimated width edge of C-scan imaging results in direction of magnetic induction ?ux is unperfected.For improving the identi?cation accuracy of width edge,we proposed news features from response signal and differential response signal.Experiment results show that the width edge of defects can be identi?ed effectively by selecting and normalizing the appropriate features in time domain.Thus,the length edge and width edge can be evaluated in direction of magnetic induction ?ux.To sum up,the PEC rectangular sensor is valuable for inspection and identi?cation of defects in ?eld of nondestructive testing and evaluation.Future research of the authors will include the real-time defect identi?cation,in situ defects evaluation,and defects reconstruction.
Acknowledgements
The authors would like to extend their appreciation to BinFeng Yang,Ping Xu,and TingTing Feng for contributions when they were in National University of Defense Technology.And the authors wish to thank the Subject Editor,Gerd Dobmann and the reviewers,for these very valuable comments and suggestions.The authors also thank National University of Defense Technol-ogy,China,for funding the study.
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