E-Journal of Advanced Maintenance Vol.9-1 (2017) 44-51
Japan Society of Maintenology

Ultrasonic Guided Wave Approach for Inspecting Concave Surface of theLaser Butt-welded Pipe
Akinori FURUSAWA1,*, Akihiko NISHIMURA1,† Toshihiko TAKEBE2,
Masaki NAKAMURA3, Yusuke TAKENAKA4, Shingo SAIJO5, Hiroyuki NAKAMOTO6
1

Applied Laser Technology Institute, Japan Atomic Energy Agency, 65-20, Kizaki, Tsuruga-shi, Fukui,914-8585, Japan
2
SUGINO MACHINE LTD., 2410 Hongo, Uozu City, Toyama, 937-8511, Japan
3
Laserx Co., Ltd., 7, Kofukada, Shin-Bayashi-tyo, Chiryu-shi, Aichi, 472-0017, Japan4
A-tech Co., Ltd., 2-11-7,Funaishikawaeki-nishi, Tokai-mura, Naka-gun, Ibaraki, 319-1116, Japan
5
Sanritz Automation Co., Ltd., 4-21-6, Minaminaruse, Machida-shi, Tokyo, 194-0045, Japan
6
Graduate School of System Informatics, Kobe Univ.,1-1 Rokkodai-tyo, Nada-ku, Kobe-shi, Hyogo,657-8501, Japan
ABSTRACTThe aim of this work presented here is to investigate the applicability of ultrasonic guided wave for evaluationof laser beam butt-welding quality. First, ten in total test pipes having welding seam are prepared. Two piece ofpipe are jointed and continuous laser beam is irradiated on the edges, varying laser irradiation power, welding
side and surface profile of the adjacent edges of the pipe. Second, ultrasonic guided wave testing experiment isperformed on the pipes. Torsional mode guided wave is excited by Electromagnetic Acoustic Transducers.
Finally, the experimental results are analyzed and issues are discussed. The reflection wave bullet from the poorinterface of the welding seam is clearly observed, whereas no reflection from fine welded line. From the aspectof laser irradiation power, welding side and surface profile of the adjacent edges, the relation between theinterface condition and detection wave bullet are analyzed. It is found that the ultrasonic guided wave
technologies have the potential for evaluating laser beam butt-welding seam.
KEYWORDS

ARTICLE INFORMATION

Structural health monitoring, Laser processing, Laser cladding,Hybrid optical fiber scope, Electromagnetic Acoustic Transducer,
EMAT, Nondestructive testing

Article history:
Received 7 November 2017
Accepted 27 April 2017

1. Introduction
Advanced maintenance technologies for pipework become more important as the number ofageing industrial plants increase in Japan. To overcome these emerging problems, JAEA started
developing robotic maintenance devices[1,2] based on laser fine processing. To date, as a result of
improvements, it has been enabled to butt-weld pipes and repair cracks on the surface from the innerside of pipework. Fig. 1 shows a prototype robotic device, its operation screens and schematics ofwork procedure. Elongated ‘torch’ is inserted from end of the pipe, irradiating with laser beam on thesurface. The laser irradiation point and the edges are observed through hybrid optical fiber scope
remotely. However, the welding and repair quality are mainly evaluated with visual images, remoteand quantitative evaluation method being expected.
Laser beam butt-welding process, important conditions that have to be controlled is thepenetration depth and integrity. The desired transition of the adjacent edges of the pipe is smooth andflat, as shown in Fig. 2(a)[3]. Concave, convex interface, lack of fusion and porosities shown in
Fig.2(b), Fig.2(c) are the sign of the poor quality welding. Typically, these surfaces are caused frompoor seam tracking, mismatch of the adjacent edges, excessive or incomplete laser irradiation power.
Nondestructive testing for evaluating interface of the welding line has been investigated widely.
In recent study, Phased array ultrasonic[4], Time of Flight Diffraction (TOFD) method[5] and laser
based ultrasonic[6] are found to be promising technology, being tackled by many researchers.
*

Corresponding author, E-mail: furusawa.akinori@jaea.go.jp

ISSN-1883-9894/10 © 2010 – JSM and the authors. All rights reserved.

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A. Furusawa, et al./ Ultrasonic Guided Wave Approach forInspecting Concave Surface of the Laser Butt-Welded Pipe

However, phased array and TOFD techniques are mainly targeting detection and sizing of cracks, lackof fusion or porosities and the systems are stationary not geared toward online and remote enough.
Laser based ultrasonic is attractive because of its flexibility by optical fiber and compatibility to ourdevice. Many researchers report its online and remote application[3,7], remaining the need to beminiaturize and improve signal noise ratio.
Guided wave[8-10] is one of the most promising technologies for rapid nondestructive testing for
pipework because of its characteristics of propagating long distance along the longitudinal direction.
Generally, guided wave responds sensitively to welding line because of its excess weld seen inexperiments in past study[11]. Welding lines are possible to be the noise source of the guided wave
testing, various approaches reported to avoid or utilize such influence by many researchers. Li Zhang
et. al.[12] are trying to focus guided wave beyond welding line and Zheng Fan et. al.[13] focus on thecharacteristics that welding line concentrate and guide the energy of the guided wave. Laser
butt-welded interface does not have such excess weld. It is easy to assume that the fine conditioninterface reflects no guided wave and clear guided wave signal reflects from poor.
The goal is to be able to detect the all defects could present under laser butt-welding process. At
the current time, we focus on the concave surface between the adjacent edges caused from incomplete
laser power. This paper investigates the applicability of the guided wave to such concave interface
evaluation of the laser butt-welding. After brief review of the guided wave testing method and thesystem, the experimental setup for guided wave testing is demonstrated. The details of the test pipes
are shown in same section. Finally, experimental results are analyzed and issues concerning with theapplicability is discussed.

(a)

(b)

(c)
Fig.1 (a) Overview of the pipe welding and repair robotic device (b) Operation screens of the device via its
hybrid optical fiber scope. (c) Schematic illustration showing how the device works

(a)

(b)
Fig.2 Schematic illustration of the butt-welding conditions.

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(c)

A. Furusawa, et al./ Ultrasonic Guided Wave Approach forInspecting Concave Surface of the Laser Butt-Welded Pipe

2.

Guided Wave Method for Defect Measurement

Ultrasonic guided wave has been studied as nondestructive testing or evaluation technique forlocating the discontinuity of the pipe. Almost of all guided wave testing methods based on thereflection of its waves caused by changes in properties of medium which it travels. Generally, a pairof guided wave transmitter and receiver is set on the surface of the pipe, measuring the time of flight(TOF) to be the discontinuity is localized. To setup field-practical ultrasonic guided wave testing
system, it is obvious that the need remains to develop adequate transducer for its excitation andreception. We have tackled the problem by using EMAT, demonstrated the low-cost, environmentallytolerant system which is able to generate and receive both T-mode and L-mode[15,16]. Fig. 3 shows
the schematic illustration of the guided wave testing system and the method.
Vibration mode of the guided wave is one of the most important aspects, it because each mode
has each propagation velocity varying with its frequency. Analytical solution of the guided wave
mode are achieved by Gazis[17] and the calculation method is demonstrated by Rose [8,9]. Sample
calculations of the dispersion curve of guided wave on hollow cylinder are shown in Fig. 4. As seenclearly in Fig.4, Torsional; T(0,1) mode guided wave is not dispersive, meaning it is easily applied tonondestructive testing. Then, at the current study, T(0,1) mode guided wave is used.
.

Fig.3 Schematic illustration of the ultrasonic guided wave testing system, its defect measurement method
and typical signal profile.

Fig.4 Group velocity dispersion curve of the hollow cylinder; the medium, outer radius and thickness are
SS400 steel, 0.0605[m] and 2.3[mm], respectively. Black solid line, red solid line and blue dashed-line areL(0,1) mode, L(0,2) mode and T(0,1) mode, respectively.

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A. Furusawa, et al./ Ultrasonic Guided Wave Approach forInspecting Concave Surface of the Laser Butt-Welded Pipe

3. Experimental Setup
In order to investigate the applicability of ultrasonic guided wave to detect poor condition
between laser beam butt-welded edges, the experimental system is setup as seen in Fig.5. The system
is composed of the pipe under the test, two EMAT ring arrays: one is transmitter the other is receiver,pre-amplifier (Japan Probe Inc.: PR-60), pulser-receiver (Japan Probe Inc.: JPR-600C) and a personal
computer as controller of the pulser-receiver. The pipe medium is SS400 steel and samarium-cobalt
magnets are used for EMAT. Each EMAT ring is composed of eight EMATs. Each EMAT are seriesconnected of the ring for transmitter, whereas parallel connected of the ring for receiver to improvesignal noise ratio. EMATs are set as its static magnetic field flows along the circumferential direction
so that excite T-mode guided wave. Experimental parameters for guided wave excitation andreception are summarized in Table.1. Detail specifications of the EMAT under this experiment isreferred in ref.[15].
It is clear that laser beam irradiation power is crucial parameter for the seam quality. In addition,
pipe-wall-thickness, welding speed and profile of the adjacent surface are also important factors. Then,
10 test pipes are manufactured as listed in Table.2. Test pipes are divided into two groups. The firstgroup is the group to investigate the relation between laser irradiation power and the detection signal
strength which consist of the pipes welded from outer side. Three laser irradiation powers and thewelding speeds are determined by preliminary experiment on the plate of same medium. The second
group consists of the pipes welded from the inner side using our robotic device. Two types of surface
profiles; smooth and rough are in this group to be investigated its effect to the guided wave signal,
where ‘smooth’ and ‘rough’ means polishing up with sanding machine and just cutting with saber saw,
respectively. Each welding seam, ‘smooth’ surface and ‘rough’ surface profiles are shown in Fig.6.

(a)

(b)
Fig.5 (a)Ultrasonic guided wave testing system and enlarge view of the EMATs arranged for torsionalmode excitation. (b) Schematic illustration of the system and the dimensions.

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A. Furusawa, et al./ Ultrasonic Guided Wave Approach forInspecting Concave Surface of the Laser Butt-Welded Pipe
Table.1 Experimental parameters of the guided wave pulser/receiverpulserpulse type
Frequency
Voltagewave cycles

number
no.1
no.2
no.3
no.4
no.5
no.6
no.7
no.8
no.9
no.10

square burst
100[kHz]
400[V]
5

receiverlow pass filter
high pass filter
gain

200[kHz]20[kHz]25[dB]

Table.2 List of pipes under the guided wave testingwelding side
thickness laser power
welding speed surface profile
outer
1.6[mm]
1500[W]
16.5[mm/s]
smoothouter
2.3[mm]
1500[W]
8.0[mm/s]
smoothouter
1.6[mm]1000[W]
12.5[mm/s]
smoothouter
2.3[mm]1000[W]
5.5[mm/s]
smoothouter
1.6[mm]
500[W]
12.5[mm/s]
smoothouter
2.3[mm]
500[W]
5.5[mm/s]
smoothinner
1.6[mm]300[W]
1.5[mm/s]
smoothinner
2.3[mm]300[W]
1.0[mm/s]
smoothinner
1.6[mm]300[W]
0.5[mm/s]
rough
inner
2.3[mm]300[W]
0.5[mm/s]
roughFig.6 Welding seam of the test pipes and edge profiles

4. Results and Discussion
Experimental results are shown in Fig.7 and Fig.8, where amplitude of the waveforms isnormalized with reference to the peak value of the initial wave bullet. Fig.7 shows the comparison
between waveform of the no.2 pipe and that of the no.10. The waveform of the no.2 pipe is plottedwith red solid line and that of the no.10 is with green. Large wave bullets clearly seen in both
waveforms are initial wave bullet and pipe-end wave bullet referred in Fig.2, respectively. Time offlights of each wave bullet are 0.169[ms] and 0.669[ms] respectively, propagation velocity of theguided wave being calculated to be 3200[m/s].
Clear wave bullet only to those two seen in Fig.7 is detection wave bullet of the welding seam ofpoor condition interface. It is identified as with its time of flight; 0.480[ms] and guided wave
propagation velocity calculated above. No.10 pipe is welded on 300[W] of lowest irradiation powerand has ‘rough’ surface profile, its welding seam reflecting intense wave bullet. No detection wave
bullet is seen in the waveform of the no.2 pipe, it is because no.10 pipe is welded on 1500[W] of
adequate laser irradiation power and has ‘smooth’ surface profile, being considered to have finecondition interface.
Fig.8 shows the collection of the experimental waveforms. (a) is a set of the waveforms of
1.6[mm] thickness pipes and (b) is a set of the 2.3[mm] thickness. Numbers in the figures arecorresponding to the pipe number listed in Table.2. As Fig.8 shows, it is found that the welding seam48

A. Furusawa, et al./ Ultrasonic Guided Wave Approach forInspecting Concave Surface of the Laser Butt-Welded Pipedetection wave bullets come into being detectable from no.5 and no.6 pipes that is welded on 500[W]of laser irradiation power. That suggests pipes that is welded less than 500[W] laser irradiation power
have poor interface of concave or lack of the penetration depth.

Fig.7 Comparison between experiment waveform of no.2 pipe and that of no.10. Red and green solid lines
are no.2 pipe and no.10 pipe, respectively.

(a)(b)
Fig.8 Experimental waveforms. (a) is a set of 2.3[mm] thickness pipes (b) is a set of 1.6[mm]. Numbers ineach figure correspond to the pipes listed in Table.2.

Among the pipes that are able to reflect detection wave bullet, from pipe no.7 to pipe no.10 are
welded from inner side. As Fig.8 shows, no. 9 pipe reflects more intense detection wave bullet than
no.7 pipe and so too with no.8 pipe and no.10 pipe. The reason is that no. 9 pipe and no. 10 have‘rough’ surface profile to have poorer interface condition. These waveform of no.7 pipe to no.10 pipes
indicate that welding seam of 300[W] reflects more intense detection wave bullets than that of
500[W] and that surface profile of adjacent edges is able to affect the seam condition.
With respect to pipe wall thickness, intensity of the detection wave bullet grows as the pipe wall
becomes thicker. It is caused by the proportion of the welded to lack of fusion. Penetration depth ofthe laser welding is determined by the laser irradiation power so that 2.3[mm] thickness pipes reflect
more intense wave bullet.
Appearing in no.5, 6, 7 pipes prominently, initial wave bullets have the ‘tail’. In current
experiments, transmitter EMAT ring array consists of eight EMATs shown in Fig.5(b). There isinterspace between each EMAT, which is able to generate unintended higher mode guided wave.

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A. Furusawa, et al./ Ultrasonic Guided Wave Approach forInspecting Concave Surface of the Laser Butt-Welded Pipe

Superposition at the pipe-end is considered to be another reason of the tails. During exciting a wave,
excited wave just before reflects from nearby pipe-end, affecting the initial wave profile. Adding one
more thing, no. 2 pipe is seen to have slight detection wave bullet while no. 1 pipe does not have. The
possible reason is that the convex interface reflects the slight detection wave bullet. No.2 pipe iswelded slower than no.1 pipe, being overheated and formed such interface. Findings from this work
are summarized as follows;(1) Fine condition of the laser butt-welding reflects no detection wave bullet.
(2) Poor condition of the laser butt-welding is possible to be detected with ultrasonic guided
wave. Concave or lack of fusion of the interface caused by shortage of laser irradiation
power reflects detection wave bullet corresponding its degree.
(3) Surface profile of the adjacent edges is important factor of the seam condition and affects
the intensity of the detection signal. Notches of the surface prevent the edges to be fused,reflecting much detection wave bullet.

5. Conclusion and Futurework
We have developed the robotic maintenance device, which makes it possible to butt-weld andrepair with laser from inner side of the pipe remotely. To extract potential of the device, we have tried
to develop non-destructive testing method for the butt-welded seam. Focusing on the concaveinterface condition caused by shortage of the laser irradiation power and surface profile of theadjacent edges, it was investigated whether such conditions can be evaluated with ultrasonic guided
wave testing. As a result, fine condition of the laser butt-welded seam does not reflect detection wave
bullet while poor condition interface does. It is shown that ultrasonic guided wave has applicability toremote and rapid testing for laser butt-welded seam. We expect that our work helps the nuclear andother plant maintenance.
There remain some challenges in both the robotics device and non-destructive method. The
robotics device only transmits 300[W] laser energy. To apply this robotics device to thicker pipes, it isneeded to be refurbished to transmit higher laser energy. With respect to guided wave testing method,
it should be investigated that the relation between the weld penetration depth and intensity of thedetection wave bullet. In addition, limit of guided wave detection needs to be investigated. These
challenges are under study.

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