Received: 28 September 2017 Revised: 14 March 2018 Accepted: 11 June 2018

RE S EARCH ART I C L E

DOI: 10.1002/stc.2231

Axial magnetized patch for efficient transduction oflongitudinal guided wave and defect identification inconcrete‐covered pipe risers

Zhou Fang | Peter W. Tse

The Smart Engineering AssetManagement Laboratory (SEAM) and theCroucher Optical Nondestructive Testingand Quality Inspection Laboratory(CNDT), Department of SystemsEngineering and EngineeringManagement, City University of HongKong, Hong Kong

CorrespondencePeter W. Tse, The Smart EngineeringAsset Management Laboratory (SEAM)and the Croucher Optical NondestructiveTesting and Quality Inspection Laboratory(CNDT), Department of SystemsEngineering and EngineeringManagement, City University of HongKong, Hong Kong.Email: Peter.W.Tse@cityu.edu.hk

Funding informationResearch Grants Council of the HongKong Special Administrative Region,China, Grant/Award Number:CityU_11201315; CityU, Grant/AwardNumber: 7004905

Struct Control Health Monit. 2018;25:e2231.https://doi.org/10.1002/stc.2231

Summary

Pipe risers partially covered by a wall are difficult to test because the concrete

causes large attenuation and mode conversion. Both reasons make it difficult to

analyze the signal. This paper reports the design of an axial magnetized mag-

netostrictive patch transducer (AM‐MPT) for efficient transduction of longitu-

dinal guided wave, which can be used to detect defects in concrete‐covered

pipe risers. First, the paper started with a theoretical background about the

influence of length‐to‐width ratio of the magnetized rectangular patch on the

demagnetizing factors and the magnetic field intensity of the patch. Second,

the simulation and experimental results proved that the length‐to‐width ratio

of the magnetized iron cobalt patch has a great influence on its magnetic field

intensity and signal amplitude of the AM‐MPT. Comparison experiments

proved that the static magnetic field of AM‐MPT provided by an iron cobalt

patch led to larger signal amplitudes than those provided by magnets. Third,

the attenuation of L(0,2) in concrete‐covered pipe riser were studied using

the AM‐MPT experimentally, which were according with disperse simulation

results basically. And the experimental results proved that AM‐MPT was more

suitable to be applied in concrete‐covered pipe risers testing than original MPT.

Finally, the AM‐MPT was applied in a defect identification of concrete‐covered

pipe riser sample. The experimental results for the AM‐MPT and the piezoelec-

tric transducer showed that the AM‐MPT can potentially be applied in pipe

riser defect identification.

KEYWORDS

axial magnetized MPT, concrete, defect identification, longitudinal mode, pipe riser

1 | INTRODUCTION

Pipe risers are widely used to transport water, oil, and gas. If severe erosion of the risers occurs because of the moistenvironment, the risers may cause natural gas leakage, resulting in an explosion. Parts of building pipe risers arecovered by wall, and thus, they cannot be observed. Pulse‐echo guided wave testing mode permits inaccessible areasto be investigated to detect corrosion in pipe risers. Ultrasonic guided wave (UGW)1-5 is a low‐attenuation, rapid, andaccurate nondestructive testing method. When defects occur in pipe risers, there are flaw echoes in the time domain.However, concrete causes the amplitude of the wave packet to drop dramatically and mode conversion. Mode

© 2018 John Wiley & Sons, Ltd.wileyonlinelibrary.com/journal/stc 1 of 16

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conversion and noise complicate crack recognition in short temporal domains because the pipe risers are short, whichmakes the signal difficult to analyze.

A buried wave‐guided structure causes large attenuation of the guided wave, which has been proven from theory,6-9

simulations,10-15 and experiments.3,16-18 Concrete causes a larger attenuation than clay, sand, and viscoelastic‐coatedcylindrical waveguides, which shortens the practical testing length of the pipe. Castaings and Bacon19 simulated atorsional mode propagating along a cast‐iron pipe running through a concrete wall by solving the equations of dynamicequilibrium in the frequency domain. Mustaph et al20 attached rectangular piezoelectric ceramics at both the exposedends of the pipe to trigger 50 and 75 kHz longitudinal guided waves to identify damage in rebar‐reinforced concretestructures through a time‐reversal process. However, the pitch‐catch guided wave mode is not practical in riser testing.Leinov et al21 studied transmission loss in fully and partially embedded concrete pipe by using less than 100 kHz T(0,1)mode through semianalytical finite element and experiments, whereas crack identification in concrete‐covered pipeshas not been studied. The selection of testing frequency is difficult in concrete‐covered defect testing because bothlow attenuation and sensitivity need to be considered. For this reason, few scholars have studied crack detection inconcrete‐covered pipes.

The axisymmetric longitudinal and torsional guided wave modes22 are most widely used for cylindrical waveguidestructure inspection. Kwun et al23,24 applied a longitudinal guided wave electromagnetic acoustic transducer (EMAT)based on the magnetostriction mechanism to pipes. Huang et al25 used a multibelt coil to implement pure L(0,2) mode.L(0,2) shows good attenuation performance because the axial displacement of the L(0,2) mode within a certainfrequency range is larger than its radial displacement. Also, as the fastest mode, L(0,2) can significantly exclude theinfluence of other modes in signal analysis. Moreover, fig. 1b in Leinov et al21 showed that the attenuation caused byconcrete on T(0,1) is larger than that on L(0,2). In this work, attenuation is a big concern. L(0,2) was used to studythe concrete‐covered pipe risers test when considering the result in Leinov et al.21

Piezoelectric transducers (PZTs) and EMAT are the two main kinds of UGW transducers. PZT has been widely usedin industry because of its high signal‐to‐noise ratio. EMAT is flexible, durable, easily implemented, and cost‐effective,which acts as an alternative to PZT in some cases. Magnetostrictive patch transducer (MPT) was proposed in whichthe patch acted as the medium between the transducer and the waveguide, which produced deformation in the patchthrough Joule and Villari effects; it then transformed the deformation to a cylindrical waveguide to generate a torsionalguided wave. The patch in this kind of MPT is circumferential magnetized as shown in Figure 1a, which is called asoriginal MPT in the following section. The team of Kim26-28 enhanced the performance of the MPT and applied it toother aspects, such as defect circumferential location29 and mode suppression.30,31 Liu et al32,33 employed a planarsolenoid array coil and a multisplitting meander coil combined with a nickel strip for MPT array design to improvethe signal amplitude and realize defect location in pipe. As the key part of MPT, when magnetostrictive patch is axialmagnetized as shown in Figure 1b, longitudinal guided wave would be generated. And the magnetic fieldintensity and the amplitude of the guided wave signal of the axial magnetized can be improved through enhancingthe length‐to‐width ratio of the magnetostrictive patch, which is seldom reported.

In this paper, an axial magnetized MPT (AM‐MPT) for the efficient transduction of the longitudinal guided wave inpipe riser was proposed. The magnetized iron cobalt patch was used to produce the static magnetic field. The relation-ship between the length–width ratio of the iron cobalt patch and magnetic field intensity was discussed through thetheory of demagnetizing, and then it was studied through simulations and experiments. The static magnetic field ofthe AM‐MPT provided by the iron cobalt patch was compared with that provided by a magnet. The dynamic magneticfield was provided by four sections of a flexible printed circuit (FPC), in which the distance between two adjacentsections could be adjusted to trigger the desired mode at a specific center frequency. The optimum combination of static

(a) (b)

FIGURE 1 Magnetized direction of (a) original MPT and (b) AM‐MPT. AM‐MPT: axial magnetized magnetostrictive patch transducer;

MPT: magnetostrictive patch transducer

FANG AND TSE 3 of 16

and dynamic magnetic field was confirmed through these experiments. Then the measured attenuation of L(0,2) in piperiser sample embedded in 100‐ and 200‐mm‐length concrete was compared with disperse simulation results. The orig-inal MPT was compared with AM‐MPT to test the concrete‐covered pipe riser without defect. Finally, concrete‐coveredsteel pipe riser sample testing experiments with artificial cracks were conducted using the developed MPT at its centerfrequency. Also, PZT testing of the concreted‐covered riser with defects was conducted to verify the performance of theAM‐MPT.

The remainder of paper is organized as follows. Section 2 describes the theory of demagnetizing. The workingprinciple of proposed AM‐MPT and the design and development of AM‐MPT are discussed in Section 3. In Section 4,concrete‐covered steel pipe riser sample testing experiments are conducted using the developed MPT. Our conclusionsare summarized in Section 5.

2 | THEORY OF DEMAGNETIZING

Magnetized magnetostrictive patch plays an important role in MPT. The principle of demagnetizing is worth introduc-ing first, which can help to improve the efficient of the transducer.

When a bar sample is magnetized by a field applied from left to right as shown in Figure 2a, then a north pole isformed at the right end and a south pole at the left. The magnetic induction lines radiating out from the north poleand ending at the south pole constitute a field both outside and inside the magnetized sample that acts from north tosouth and which therefore tends to demagnetize the magnetized sample. The demagnetizing field Hd acts in theopposite direction to the magnetization M, which is introduced in the section 2.7 of Cullity and Graham.34 Thedemagnetizing field is determined by fluxmetric and magnetometric factors N f and Nm. The demagnetizing factorsof rectangular patch as shown in Figure 2b can be calculated from either Mmid,vol or Hd,mid,vol together with theconstant susceptibility χ using

Nf ;m ¼ Ha

Mmid;vol−

1χ; (1)

Nf ;m ¼ −Hd;mid;vol

χ Hd;mid;vol−Ha� �; (2)

where the subscripts “mid” and “vol” stand for the average over the midplane and over the entire volume, respectively.Ha is the uniform applied field. Hd,mid,vol may be calculated from the sum of the prism midplane and prism‐volumeaveraged fields generated by all rectangular elements as

Hd;mid ¼ 8∑npi¼1

σi

μ0hiz;mid; (3)

Hd;vol ¼ 8∑npi¼1

σi

μ0hiz;vol: (4)

Numerical calculation results35-38 show that when c:a is larger, N f and Nm are smaller as shown in Table 1, thendemagnetizing field would be smaller and the flux density B would be larger as shown in Figure 2a. Figure 2c shows

FIGURE 2 Diagram of demagnetizing:

(a) the magnetic induction lines in

magnetized sample, (b) magnetized

rectangular patch, and (c) variation of the

demagnetizing field along the length of a

magnetized sample

(a) (b) (c)

TABLE 1 Fluxmetric demagnetizing factor N f and Nm of a square bar with dimensions (2a × 2b × 2c) magnetized along the c dimension37

c:a χ = −1 0 1.5 9 ∞

0.5 48,891/54,629 44,731/49,592 42,731/47,089 40,500/44,288 39,178/42,639

0.7 38,069/46,990 35,438/41,574 34,106/39,132 32,556/36,530 31,607/35,049

1 26,132/38,967 25,873/33,333 25,426/31,030 24,725/28,713 24,222/27,445

1.5 14,278/30,498 16,393/24,918 16,874/22,842 17,094/20,888 17,091/19,882

2 8,434.7/25,091 11,089/19,831 11,959/17,937 12,651/16,200 12,928/15,329

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the variation of the demagnetizing field along the length of a magnetized sample. The theory of demagnetizing is thefoundation for the transducer design in Section 3. The dimension of the patched can affect the efficiency of the MPTthrough the demagnetizing field. The analysis and calculation results in Figure 2 and Table 1 will be used to provethe reliability of simulation and experimental results in Section 3.

3 | DESIGN AND DEVELOPMENT OF AM ‐MPT

3.1 | Configuration and working principle of AM‐MPT

Figure 3a,b shows the configuration and working principle of the AM‐MPT. The AM‐MPT consists of two components,a 0.15‐mm‐thick iron cobalt patch, which is a magnetostrictive material, and a four‐belt FPC as shown in Figure 3a. Theprinciple of AM‐MPT that generates a longitudinal guided wave mode in a pipe riser is shown in Figure 3b. The ironcobalt patch is uniformly magnetized and tightly bound around the pipe riser to supply the axial static magnetic fieldthrough epoxy resin. The FPC circuit contains four belts. The distance D between adjacent belts and center frequencyof transducer meets the equation:

D ¼ λ2¼ VP

2f 0; (5)

where λ is the wavelength, VP is the phase velocity, and f 0 is the center frequency. The D is 13.6 mm, half the wave-length of 200 kHz L(0,2), with a steel pipe riser of 34 mm in outer diameter and 4 mm thick with zine, steel carbon,

(b)

(a)

FIGURE 3 Configuration and working principle of axial magnetized magnetostrictive patch transducer (AM‐MPT): (a) configuration of

the AM‐MPT testing concrete‐covered pipe riser and (b) cross section of the AM‐MPT. FPC: flexible printed circuit

FANG AND TSE 5 of 16

epoxy, and acrylic layers as shown in Figure 4b. The reason why the D is designed to 13.6 mm is the relatively low dis-persive behavior in the frequency range of 150 to 300 kHz.

The four belts design was used in Liu et al,33 and the relationship between the number of belt and the signal ampli-tude was studied as follows. The signal amplitudes of AM‐MPTs testing a 780‐mm pipe riser by using different hard coilsare as shown in Figure 5. The amplitude obtained from the peak value of the first end echo and the time difference (Δt)between the two peak values of the first end echo and trigger waveform was the elapsed time of the pulse echo. Theechoes of AM‐MPTs testing pipe riser widened as the number of belts increasing, which would cause severe waveformoverlapping. Moreover, the signal amplitude comparison of AM‐MPTs testing a 780‐mm pipe riser by using differenthard coils shows that the signal improvement decreases from four belts to five belts as shown in Figure 5f, which meansthe effect of the number of belts on signal amplitude is limited when the number of the belts is larger than four. To takethe width of the echo and the signal amplitude into consideration, the four belts design was applied into AM‐MPT.

Table 2 shows the mechanical properties and thickness of each layer of pipe riser used in disperse simulations.When alternating current is loaded into the FPC through two poles, the axial dynamic magnetic field is generated.The pulse current reversing direction in each fourth of the width of the FPC. The design can tune the center frequencyand improve the amplitude. Because the static and dynamic magnetic fields are parallel, the longitudinal deformation ofthe patch tightly bound around the pipe riser will be caused by magnetostriction. Then the patch deformation generatesthe longitudinal guided wave mode in the pipe riser because of the mechanical coupling.

(a) (b)

FIGURE 4 Dispersion curves of the zero‐order guided wave modes for pipe riser: (a) group velocity and (b) phase velocity

(a) (b) (c)

(d) (e) (f)

FIGURE 5 The signal amplitudes of axial magnetized magnetostrictive patch transducers testing 780‐mm pipe riser by using different hard

coils, (a) one belt, (b) two belts, (c) three belts, (d) four belts, and (e) five belts, and (f) the signal amplitude comparison of axial magnetized

magnetostrictive patch transducers testing 780‐mm pipe riser by using different hard coils

TABLE 2 Mechanical properties and thickness of each layer of pipe riser used in disperse simulations

Material Density (kg/m3) CL (m/s) CS (m/s) E (GPa) Poisson's ratio Thickness

Carbon steel 7,932 5,960 3,260 216.9 0.2865 13 mm/0.06 mm

Zine 7,100 4,170 2,410 103 0.2492 0.06 mm

Epoxy 1,170 2,610 1,100 3.9 0.392 0.2 mm

Acrylic 1,180 2,730 1,430 6.3 0.3109 0.04 mm

Concrete39 1,945 3,750 1,990 20.1 0.3 ∞

Concrete (disperse) 2,200 4,100 2,300 29.6 0.27 ∞

Concrete21 2,320 3,982.6 2,413 32.7 0.21 ∞

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3.2 | Axial magnetized magnetostrictive patch

A uniformly axial magnetized iron cobalt patch is used to supply the static magnetic field because iron cobalt showedgood magnetostrictive effect.40 As a critical part in the AM‐MPT, the length‐to‐width ratio of the magnetized iron cobaltpatch has a great influence on its magnetic field intensity and signal amplitude, which has been discussed in Section 2.Simulation models in Section 3.2.1 and experimental results in Section 3.2.2 would further prove it.

3.2.1 | Axial magnetized magnetostrictive patch simulations

The commercial finite element software COMSOL Multiphysics was used to simulate the relationship between thelength–width ratio and magnetic field intensity of the magnetized iron cobalt patch. Five different length–width ratioiron cobalt patch models were placed between two magnets to be magnetized. Two 100 mm (z axis) × 300 mm (xaxis) × 10 mm (y axis) magnet models were built, which were large enough to provide magnetic field as shown inFigure 6, and the magnetic field intensity of z axis direction was 750 A/m. The gap between the two magnets wasthe same (600 mm) to guarantee that the five different length–width ratio iron cobalt patches were magnetized underthe same conditions. The dimensions of the five square‐shaped patches were as shown in Table 3. The five iron cobaltpatches were meshed with 3.599 × 106, 2.717 × 106, 1.829 × 106, 2.716 × 106, and 3.596 × 106 elements through free tet-rahedrals. The material parameters and B–H curve of the ferrocobalt come from the manual: Engineering Materials andPractical Manual.41 The simulation results are as Figure 7 shows.

FIGURE 6 Diagram of magnetization

TABLE 3 The dimensions of the patches

c:a x axis (cm) y axis (cm) z axis (cm)

1:2 100 0.15 50

2:3 75 0.15 50

1:1 50 0.15 50

3:2 50 0.15 75

2:1 50 0.15 100

FIGURE 7 Simulations for five different length‐to‐width (c:a) ratios of magnetized iron cobalt patches: (a) 1:2, (b) 1:1.5, (c) 1:1, (d) 1.5:1,

and (e) 2:1

FANG AND TSE 7 of 16

Figure 7 shows the central slices of iron cobalt patches. The color bar on the right side displays the degree of mag-netic field intensity. The colors of the center parts of the five iron cobalt patch models were compared with each otherbecause the distances from the center part to the magnet were the same. The length c in Figure 7 is corresponding to thec in Figure 2b, and the width a in Figure 7 is corresponding to the a in Figure 2b. First, as the length–width ratioincreases, the maximal magnetic field intensity of the patches increases. Second, as the length–width ratio increases,the color of the center part approaches the red region. The simulation results show that as the value of the length–widthratio increases, the magnetic field intensity increases. That is because c:a is larger and the demagnetizing factors issmaller, which results in larger magnetic field intensity. Different sizes and magnetic field intensity of the magnetscan only change the absolute value of the magnetic field intensity of patch, which would not influence the conclusionsstated above.

The demagnetizing field is larger at the edge of the magnetized rectangular patch than those at the center of thepatch as shown in Figure 2c. That is why the magnetic field intensity is smaller at the edge of the magnetized iron cobaltpatch than that at the center of the patch in Figure 7. The simulation results fit with the theory results well.

3.2.2 | Axial magnetized magnetostrictive patch experiments

Before comparing the signal amplitudes of AM‐MPTs by using different length‐to‐width ratios of iron cobalt patches, theinfluence of patch pasting on the energy transferring efficiency of the transducer was studied. The three experiments ofthe AM‐MPT testing pipe risers were carried on as shown in Figure 8. The patch pasting processes were the same inthese three experiments. The pipe risers were 780 mm in length. The signal amplitudes of these three experiments were0.927, 0.939, and 0.969 V, separately. The discrepancies of the signal amplitude were small, which means that the influ-ence of patch pasting on the energy transferring efficiency of the transducer was limited.

To prove the simulation results presented above, the UGW signals of three different AM‐MPTs were recorded tocompare with each other. The length–width ratios of iron cobalt patches in three different AM‐MPTs were 1:2, 1:1,and 2:1, respectively. A pipe riser sample of 780 mm in length with a 34‐mm outer diameter and 4 mm thick was chosen

FIGURE 8 Three experimental results of the axial magnetized magnetostrictive patch transducer (AM‐MPT) testing pipe risers

8 of 16 FANG AND TSE

as the testing sample because this size of pipe riser is widely used. The c:a = 1:2 patch was wrapped around the piperiser sample, as Figure 9a shows. The c:a denotes the length‐to‐width ratio. Two patches were symmetrically pastedaround the pipe riser sample to trigger the symmetrical L(0,2) mode, as shown in Figure 9b,c. The length c inFigure 9 was corresponding to the c in Figure 2b, and the width a in Figure 9 was corresponding to the a in Figure 2b. Dynamic magnetic field was produced by the same FPC in these three experiments. The excitation frequency was200 kHz, and the experimental results were as shown in Figure 9.

The amplitude obtained from the peak value of the first end echo and the time difference (Δt) between the two peakvalues of the first end echo and trigger waveform was the elapsed time of the pulse echo. The group velocity (5,262 m/s)of L(0,2) at 200 kHz was used to calculate the pipe riser sample length. The calculated lengths were 792, 794, and792 mm, for Figure 9a–c, respectively, by multiplying the elapsed time and group velocity of the 200 kHz L(0,2) mode.The relative errors were 1.54%, 1.79%, and 1.54%, respectively. The calculated pipe riser sample lengths were in goodagreement with the actual lengths (relative error within 5%), which proved that the 200 kHz L(0,2) was triggered.

The experimental results show that as the length–width ratio increases, the magnetic field intensity increases, whichlead to larger signal amplitude. That is because c:a is larger and the demagnetizing factors is smaller, which results inlarger magnetic field intensity. The experimental results fit with the simulation and theory results well. However, theimprovements of the signal amplitude and magnetic field intensity are not completely consistent for two reasons.One is that iron cobalt patch pasting and manual magnetization would produce errors; the other is that the relationshipbetween the length extension of the magnetostrictive material and its magnetic field is nonlinear in magnetostrictionpatch as shown in Figure 10a.

Larger length‐to‐width ratio of iron cobalt patches is not discussed for two reasons. First, the magnetic field intensityof magnetized iron cobalt patch is limited under a given magnet. Second, the coupling range of static and dynamicmagnetic fields is limited, because the size of FPC is limited.

3.2.3 | AM‐MPT with magnet experiments

Magnets can be used to provide large static magnetic field. Figure 11a,b shows the comparison between two AM‐

MPTs, one employing magnets and the other with no magnet. The FPC was 5 mm from one end of the pipe risersample; Figure 11c shows the signal path. Wave packets a and c in Figure 11a were the results of the UGW signalpropagating along Paths 1 and 4, respectively, in Figure 11c. Wave packet b in Figure 11a was the overlap of theUGW signal propagating along Paths 2 and 3 in Figure 11c. The inner radius R of the permanent magnet was18 mm, the thickness d was 5 mm, the height h was 10 mm, and the center angle θ was 70°. Four identical permanentmagnets were placed evenly on both sides of the patch to generate a static magnetic field along the axial direction of

(a) (b) (c)

FIGURE 9 Experiments for three different c:a ratio magnetized iron cobalt patches: (a) 1:2, (b) 1:1, and (c) 2:1

(a) (b) (c)

FIGURE 10 Theory of magnetostriction: (a) typical magnetostrictive curve, (b) magnetostriction at A, and (c) magnetostriction at B2

(a) (b)

(c)

FIGURE 11 Comparison of the AM‐MPT (a) without and (b) with magnet and (c) signal path analysis for (a) and (b). AM‐MPT: axial

magnetized magnetostrictive patch transducer

FANG AND TSE 9 of 16

the pipe riser sample. The signal amplitude in Figure 11 was obtained from the peak value of the end echo. Thecalculated lengths for the pipe riser samples in Figure 11a,b were 792 and 789 mm, respectively, by using the elapsedtime (Δt) of the first end echo and a group velocity of 200 kHz for the L(0,2) mode. The relative errors were 1.54% and1.15%, respectively.

Figure 11a,b shows that the amplitude of signal is lower when magnets are employed. L is the length of the patch,ΔL is the deformation of the patch, and H is the magnetic field intensity in Figure 10a.2 Even though the static magneticfield intensity at point B is larger than that at point A, ΔL and the strain at Point A are larger than those at B, asshown in Figure 10a–c. The experimental results show that the static magnetic field intensity should be controlled ina suitable range.

3.3 | Flexible printed circuit

The FPC can fit different diameters of pipes and can be easily installed without requiring a free end on the inspectedpipe.42 For this reason, FPC was chosen to produce the dynamic magnetic field. The FPC circuit is as shown inFigure 3a. The center frequency of the AM‐MPT was designed to be 200 kHz because of the relatively low dispersivebehavior in the frequency range of 150 to 300 kHz, as shown in Figure 4a, which would facilitate signal analysis.

Frequency sweeping experiments varied from 150 to 300 kHz with incremental steps of 10 kHz were carried out. Thered circles in Figure 4a show the group velocity responses at the frequency range of 150–300 kHz, which well corre-spond to the theoretical values. The frequency response of the AM‐MPT is shown in Figure 12a, which shows theamplitude obtained from the peak value of the first end echo at different frequencies. The signal amplitude of200 kHz is the highest, which is consistent with the theoretical center frequency. The experimental results showed thatL(0,2) can be efficiently transduced by the proposed MPT and that the center frequency is 200 kHz.

AM‐MPT can trigger pure longitudinal guided wave in pipe riser efficiently. The signal amplitude of AM‐MPT canbe improved through employing larger length‐to‐width magnetized magnetostrictive patch. Based on these results, anAM‐MPT employing 100 mm × 50 mm axial magnetized iron cobalt patch and an FPC was used to conduct the exper-iments described in Section 4.

(a) (b)

FIGURE 12 Ultrasonic guided wave

testing of pipe riser embedded in different

length concretes: (a) frequency response

curve of the axial magnetized

magnetostrictive patch transducer under

different concrete length and (b) the

relationship between the attenuation of

L(0,2) at 200 kHz in concrete‐covered pipe

riser and concrete curing time

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4 | EXPERIMENTS OF CONCRETE ‐COVERED PIPE RISER SAMPLE

Figure 13 shows the experimental setup for the pipe riser sample inspection with the AM‐MPT. The setup consists of anarbitrary waveform generator Agilent 33250A, a high‐power ultrasonic measurement system RPR‐4000, an oscilloscope,and AM‐MPT. A five‐cycle Hanning window‐modulating sine wave was triggered by Agilent 33250A after beingamplified by RPR‐4000 and then transformed to the AM‐MPT. Guided wave signals were received by RPR‐4000. ThedB value in RPR‐4000 was 50.

4.1 | Attenuation of concrete

Experiments using 200 kHz L(0,2) guided wave testing of pipe riser samples covered with 100‐ and 200‐mm‐lengthconcrete were carried out. Flower mud was put under the PVC pipe to prevent the concrete from covering the lowerpart of the pipe riser sample. Green Island Cement Hong Kong (Portland cement CEM І 52.5N, BS EN197‐1:2000)was used. Although the cement was solidifying, the concrete‐embedded pipe riser sample was pulled out of the flowermud. The flower mud‐covered part of the pipe riser sample was 260 mm long. The cement would be fully curing after28 days. AM‐MPT was put on one end of the pipe riser sample. The signal amplitude was obtained from the peak valueof the end echo. The attenuation characteristic was calculated from the ratio of the signal amplitudes measured from thepipe end reflections:

FIGURE 13 Experimental setup of

concrete‐covered pipe riser sample

inspection

FANG AND TSE 11 of 16

α ¼ −

20 log10AC

A0

2L; (8)

where AC is the signal amplitude of the concrete‐covered pipe riser and A0 is pipe riser without concrete and L isthe length of the pipe embedded in concrete. The attenuation of L(0,2) in air was much smaller than that in con-crete in reference tests; hence, the attenuation of L(0,2) in the pipe section exposed to air is considered to benegligible.

The frequency responses of the AM‐MPT testing pipes with different lengths of concrete are shown in Figure 12a.The experimental results showed that as the concrete length increased, the signal amplitude decreased at all frequenciesand the practical frequency range narrowed. That is because of the energy leakage into the embedding concrete. Whenthe buried parts are more, the attenuation would be larger. Figure 12a shows the relationship between the attenuationof L(0,2) at 200 kHz in concrete‐covered pipe riser and concrete curing time. The attenuation shows a relative stabiliza-tion after roughly 20 days of curing.

The discrepancy between those measured attenuation points comes from experimental errors. First, the measuredpoints come from two experiments: pipe riser test with 100 mm concrete covered and pipe riser test with 200 mm con-crete covered. The properties of the concrete in these two samples are so hard to ensure completely consistent. Theattenuation in pipe riser test with 100 mm concrete covered is slightly different from that in pipe riser test with200 mm concrete covered. Second, the signal amplitudes in these two experiments are so low that are easily to beaffected by amplitude oscillation, which is caused by continuously triggering.

The group velocity and attenuation of L(0,2) in concrete‐covered pipe riser calculated from the experimental resultsabove were compared with disperse simulation results as shown in Figure 14. The mechanical properties of each layer ofpipe riser in disperse come from Table 2. Because the mechanical properties of concrete cannot be guaranteed to specialvalues every time, three different mechanical properties of concrete were chosen to study the attenuation of L(0,2) inconcrete‐covered pipe riser. The measured group velocity and attenuation are the average of the values obtained fromat least two different tests performed under same conditions. Error bars represent the variation between measurementsat a certain frequency.

Figure 14a shows that three group velocity curves calculated by three different mechanical properties of concreteare very closed. The measured group velocities approached the disperse simulation results. The lower and upperaverage attenuation values were fitted to the attenuation curves in the range of frequencies covered as shown inFigure 14b. The best‐fit attenuation curve is the middle attenuation curve (ρ = 2,200 kg/m3, CL = 4,100 m/s,CS = 2,300 m/s) for pipe riser embedded in 200‐mm‐length concrete case. The discrepancy between some measuredattenuation points and attenuation curves plotted by disperse come from experimental error. Because the attenuationin concrete‐covered pipe riser is large, the amplitudes of weak signals are easy to be affected, which has beenexplained in Section 4.1. Experimental and disperse simulation results show that concrete cause large attenuationin pipe riser.

(a) (b)

FIGURE 14 Group velocity and attenuation measurements as a function of frequency and disperse simulation fits (red line) for concrete‐

covered pipe (a) group velocity and (b) attenuation

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4.2 | Concrete‐covered pipe riser sample testing by using original MPT and AM‐MPT

The original MPT was compared with AM‐MPT to test the concrete‐covered pipe riser samples without defect. Theexperimental results are shown in Figure 15a,b, and Figure 15c,d shows the signal path analysis for Figure 15a,b.The iron cobalt patch in Figure 15a was circumferential pasted and magnetized, and T(0,1) was triggered. Althoughthe patch was axial pasted and magnetized in Figure 15b, L(0,2) was triggered. The dynamic magnetic fields were pro-vided by the same FPC as shown in Figure 3a. Because the distance D between adjacent belts was 13.4 mm, half thewavelength of 120 kHz T(0,1), with a steel pipe riser sample of 34 mm in outer diameter and 4 mm thick as shown inFigure 4b.

Figure 15a shows that the end echo is undistinguishable. The arriving time of the end echo should be around0.478 ms through the length of pipe riser sample and the group velocity of 120 kHz T(0,1) mode as shown inFigure 4a. The L(0,2) end echo is cognizable in Figure 15b. The calculated pipe riser sample length is 891 mm, andthe relative error is 1.54%. That is because the displacement and power flow of 120 kHz T(0,1) are larger in outer diam-eter of the pipe than those of 200 kHz L(0,2) as shown in Figure 16. There are more energy leakages into the embeddingconcrete through outer diameter of the pipe riser sample. Fig. 1b in Leinov et al21 also showed that the attenuationcaused by concrete on T(0,1) is larger than that on L(0,2). The comparison experiments proved that AM‐MPT is moresuitable to be applied in concrete‐covered pipe risers testing than the original MPT.

4.3 | Defect identification in pipe riser sample by using AM‐MPT

Figure 17c shows the signal path analysis of UGW testing of a concrete‐covered pipe riser sample with defects. There isan artificial circumferential crack with Length × Depth × Width = 23 mm × 3 mm × 2 mm in the pipe riser sample. Apatch of 22 mm × 18 mm vinyl electrical tape was pasted on the crack to prevent concrete from flowing in. The concretelength was 200 mm, and the experimental setup was the same as that described in Section 4.1. Guided wave signals wererecorded 28 days after casting. Figure 17a shows the experimental results of the 200 kHz L(0,2) testing of the pipe risersample with defects. The signal amplitudes in Figure 17a were obtained from the peak value of the waveforms. The

(a) (b)

(c) (d)

FIGURE 15 Ultrasonic guided wave

testing of concrete‐covered pipe riser

sample without defect: (a) T(0,1)

generated by original magnetostrictive

patch transducer (MPT) at its center

frequency of 120 kHz, (b) L(0,2) generated

by axial magnetized magnetostrictive

patch transducer (AM‐MPT) at its center

frequency of 200 kHz, (c) signal path

analysis for (a), and (d) signal path

analysis for (b)

FIGURE 16 Mode shapes of T(0,1) at

120 kHz and L(0,2) at 200 kHz

(a)

(c)

(b)

FIGURE 17 Ultrasonic guided wave testing of concrete‐covered pipe riser sample with defect: (a) Piezoelectric transducer (PZT) at its

center frequency of 130 kHz, (b) axial magnetized magnetostrictive patch transducer (AM‐MPT) at its center frequency of 200 kHz, and

(c) signal path analysis for (a) and (b)

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calculated crack position and riser length were 428 and 810 mm, respectively, for Figure 17a. The relative errors were4.39% and 3.84%, respectively.

First, the echo from the end and crack is recognizable in the concrete‐covered pipe riser sample in Figure 17a.Second, the attenuation of the end echo (65 dB) and defect echo (70 dB) fits the relationship between attenuationand 200 kHz L(0,2) in concrete‐covered pipe. The experimental results show that there are two advantages forAM‐MPT in pipe riser testing. One is that AM‐MPT can trigger 200 kHz L(0,2), which is low attenuation in pipe riser.The other is that the signal energy of AM‐MPT is large, because the signal amplitude is improved through increasinglength‐to‐width ratio of iron cobalt patch. The experimental results thus show that AM‐MPT has potential to be appliedin building pipe riser sample defect detecting.

PZT testing of a concrete‐covered pipe riser sample with defects was conducted to compare with the experimentalresults shown in Figure 17b. The dB value of RPR4000 was set to 20 because the energy of PZT is larger than that ofAM‐MPT, whose signal amplitude would be out of range. The lower dB can only decrease the signal amplitude but notaffect the transducer's energy. Sixteen PZT pieces with dimensions of 20 mm× 5mm× 0.5 mmwere pasted onto the otherend of the second riser evenly. The kind of PZT can trigger longitudinal mode efficiently, which has been used by manyscholars.3,20,43 Moreover, considering the perimeter of the pipe riser (106 mm) and the width (5 mm) of the 130 kHz PZT,the number of the pasted PZT was enough. More PZTs would make the pasting process difficult. The center frequency ofthe PZT was 130 kHz. The PZT with 200 kHz center frequency had been manufactured before, but the effect was not goodas the 130 kHz PZT in the pipe riser. Figure 17b shows the experimental result of 130 kHz L(0,2) triggered by PZT testingof the pipe riser sample with defect. The end echo is indicated by arrow. The calculated pipe riser sample length was824 mm, and the relative errors were 5.64%. First, other echoes seriously interfered with end echo identification. Theseechoes were produced by transformed guided wave modes interacting with pipe end, crack, and so on, which was so hardto analyze. Second, there is no defect echo in Figure 17b. The arriving time of the defect echo should be around 0.138 msthrough the position of defect and the group velocity of 130 kHz L(0,2) mode as shown in Figure 4a. A frequency sweepingexperiment was conducted, and there was no defect echo at any frequency. The experiments showed that even though thePZT triggered a higher energy than AM‐MPT, noise and interference wave packets would affect the testing results. Thus,the AM‐MPT is more suitable for concrete‐covered riser defect identification than the PZT.

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5 | CONCLUSIONS

In this paper, we reported a kind of MPT with an axial magnetized patch for the efficient transduction of longitudinalguided waves in pipe risers. A magnetized iron cobalt patch was used to supply the axial static magnetic field, whichitself produced the magnetostrictive effect. Theory of demagnetizing showed that larger length–width ratio of themagnetized rectangular patch lead to smaller demagnetizing factors, and the magnetic field intensity would be larger.Both the simulation and experimental results proved that magnetizing along the longer side of square patch resultedin a larger magnetic field intensity and signal amplitude and that as the length of the magnetized side increased, themagnetic field intensity and signal amplitude also increased. Then it was proven that the static magnetic field providedby the magnetized iron cobalt patch could trigger a higher signal amplitude than that provided by a magnet. Adynamic magnetic field was provided by an FPC, and frequency sweep experiments demonstrated that the frequencycharacteristics of the developed MPT were related to the distance between the adjacent belts of FPC. The distancebetween two adjacent sections was adjusted to make the center frequency of AM‐MPT 200 kHz. Based on these studies,a 100 mm × 50 mm × 0.15 mm iron cobalt patch and a 40‐line FPC were employed in the AM‐MPT to carry outconcreted‐covered pipe riser sample crack testing.

The measured group velocity and attenuation of L(0,2) in pipe riser sample embedded in 100‐ and 200‐mm‐lengthconcrete were according with disperse simulation results basically. The attenuation caused by concrete was provedlarge. As the length of the concrete increased, the signal amplitude decreased at all frequencies, and the practicalfrequency range narrowed. Experiments with a concrete‐covered steel pipe riser sample without crack showed thatend echo was obvious when employing the AM‐MPT, whereas end echo was undistinguishable when employing theoriginal MPT. The comparison experiments proved that AM‐MPT was more suitable to be applied in concrete‐coveredpipe risers testing than the original MPT. Finally, experiments with a concrete‐covered steel pipe riser sample withcrack showed that crack echo and end echo were obvious when employing the AM‐MPT, whereas crack echo wassubmerged in noise when employing PZT. Attenuation of concrete on the UGW waveform of the pipe riser sampleend and cracks was estimable. Experimental results proved that AM‐MPT can potentially be used in building pipe riserdefect identification.

ACKNOWLEDGMENTS

The work described in this paper was fully supported by a grant from the Research Grants Council of the Hong KongSpecial Administrative Region, China (Project No. CityU_11201315) and an internal grant from CityU (Project No.7004905).

ORCID

Zhou Fang http://orcid.org/0000-0002-7413-3397

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How to cite this article: Fang Z, Tse PW. Axial magnetized patch for efficient transduction of longitudinalguided wave and defect identification in concrete‐covered pipe risers. Struct Control Health Monit. 2018;25:e2231.https://doi.org/10.1002/stc.2231