E-Journal of Advanced Maintenance Vol.2 (2010/2011) 191-199
Japan Society of Maintenology

Strain Monitoring and Defect Detection in Welded Joints by Using
Fiber-Optic Distributed Sensors with High Spatial Resolution
Hideaki MURAYAMA1,*, Kohei OHARA1, Nozomi KANATA1, Kazuro KAGEYAMA1 and
Hirotaka IGAWA2
1
2School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan
Japan Aerospace Exploration Agency, 6-3-1 Osawa, Mitaka-shi, Tokyo 181-0015, Japan

ABSTRACT
We have developed a fiber-optic distributed sensor that can measure strain or temperature distributions at anarbitrary position along long-length fiber Bragg gratings (FBGs) with the high spatial resolution of less than 1
mm. In order to determine the position of a grating in FBGs, at which the light of the particular wavelength isreflected, optical frequency domain reflectometry (OFDR) and a signal processing technique are used. Although
the length of an FBG is usually about 10 mm, that of the long-length FBG is more than 100 mm. We applied
this sensing technique to strain measurements and defect detection in a welded joint. We monitored the strain
distributions along the weld lines of the tubular joints of steel pipes subjected to tensile loads. We could
successfully measure the development of the strain distributions along the weld lines, where the sharply
fluctuating strain distributions resulting from some defects were observed. In this paper, we describe the sensing
principle and then show the results of strain monitoring in the welded joints.
KEYWORDS

ARTICLE INFORMATION

Welded joint, Strain distribution, Fiber-optic distributed sensor, FBG,
OFDR, Spatial resolution, Structural health monitoring

Article history:
Received 1February 2011
Accepted 17 February 2011

1.

Introduction

As structural joints are transferring loads in a structure, large stress often concentrates at thesejoints. Thus defects and damage in them can cause a critical degradation or collapse of the structure.
Among joints, welded joints have some uncertainties, such as remaining stress, initial defects, andheat-affected zone. Such uncertainties in a welded joint can also induce a strain fluctuation, when they
are subjected to a load or in a temperature environment. Conversely the strain fluctuation along theweld line may imply the presence of an uncertainty in the joint. This means that monitoring strain
distributions along the weld line can be an inspection method to assess the integrity and improve thereliability of the welded joint.
Recently optical fiber sensors have been actively developed by many researchers and one canmeasure many kinds of not only physical but chemical measurands of interests by them [1-3]. They
have some useful characteristics for condition monitoring of in-service structures as follows:
flexibility, small diameter, light-weight, electromagnetic immunity and no spark. In addition, they canbe distributed sensors that return a value of the measurand as a function of linear position along an
optical fiber, so they are regarded as a promising sensing technology in the research field of structural
health monitoring (SHM).
In recent years various techniques for distributed strain or temperature measurements have been
proposed and applied to structural monitoring. Among them, various types of the distributed sensing
technique based on Brillouin scattering have been proposed. Brillouin optical time domainreflectometry (BOTDR) was developed in the mid 1990s [4] and it was applied to strain monitoring ofinternational America’s cup class (IACC) yachts [5]. In the early stages the spatial resolution of the
distributed sensing system based on Brillouin scattering was more than 1 m and it was not enough toassess the structural integrity on the basis of the strain distribution. The spatial resolution, however,*Corresponding author, E-mail: murayama@sys.t.u-tokyo.ac.jp

ISSN-1883-9894/10/11 © 2010/2011 – JSM and the authors. All rights reserved.

191

H. Murayama, et al./ Strain Monitoring and Defect Detection in Welded Joints
by Using Fiber-Optic Distributed Sensors with High Spatial Resolution

has been dramatically improved in this decade. Recently Song et al. achieved the spatial resolution of
1.6 mm using Brillouin optical correlation domain analysis (BOCDA) [6]. While fiber Bragg gratings(FBGs) with some interrogation systems are ordinarily employed for quasi-distributed strain or
temperature measurements, they can be also applied to fully-distributed sensing along FBGs. A strain
distribution along a chirped grating is measured by the phase-based sensing method [7] or the
low-coherence reflectometry [8]. We have developed a distributed strain sensing system using
long-length FBGs, based on optical frequency domain reflectometry (OFDR) [9]. We confirmed bysimulation that the spatial resolution of this system is less than 1 mm [10]. In this sensing system we
employ long-length FBGs whose length is about 100 mm, although the length of usual FBGs is about
10 mm. The sensing region, in other words the gauge length, could be expanded up to 1,500 mm byserially-cascaded long-length FBGs [11]. These efforts to improve the spatial resolution of distributed
sensing open recent development in the SHM field.
In this study we measured the strain distribution along the weld lines of welded tubular joints ofsteel pipes by the distributed strain sensing system. We had two joint specimens with long-length
FBGs. An FBG with 100 mm length was bonded on the surface along the weld line of one specimen
and long-length FBGs which consisted of five 100 mm FBGs serially-cascaded to have thegauge-length of 500 mm was bonded on another specimen in a similar way. By conducting tensile
tests on the pipes, we investigated the applicability of the sensing system to strain monitoring anddefect detection in the welded joints. As results of the tests, we could successfully measure thedevelopment of the strain distributions and found that strain sharply fluctuated at several positions
where defects were observed by the CT scanning.

2.

Principle of distributed sensing system

2.1. FBGFBG is an intrinsic device with periodic perturbation of refractive index in the core of an optical fiber[12]. When spectrally broadband light is injected into the fiber with FBG, narrowband spectral component
at a specified wavelength, called Bragg wavelength is reflected as shown in Fig. 1. FBG is usually used asa filter devise in the fiber-optic communication system. The Bragg wavelength, λB, is given by

λ B = 2neff Λ ,(1)

where Λ is the pitch of the grating and neff is the effective index of refraction of the core. When thepitch of FBG is mechanically changed by the applied strain, we can observe the wavelength shift in
the reflected spectrum proportional to the change of the pitch. Therefore we can use FBG as a strain
sensor and strain of a structural member can be measured by monitoring the reflected light of FBG
bonded on the surface. It is well known that the Bragg wavelength shift at a rate of 1.2 pm/με around1550 nm [12]. The Bragg wavelength can be also shifted by temperature change.

Strain and TemperatureΛIncident light

λB
Fig. 1. FBG sensor

192

Core
Cladding

λTransmitted Light

Incident Light

λReflected Light

Bragg Grating

Reflected light

Transmitted lightλB

λH. Murayama, et al./ Strain Monitoring and Defect Detection in Welded Joints
by Using Fiber-Optic Distributed Sensors with High Spatial Resolution

2.2. Distributed sensing system with OFDRAn arrangement of our sensing system is shown in Fig. 2. The distributed sensing system consistsof a tunable laser source (TLS), two photo detectors (PD1, PD2), three broad band reflectors (R1, R2,
R3), three 3 dB couplers (C1, C2, C3), three long-length FBGs (FBG1, FBG2, FBG3), an A/D
converter (DAQ), and a computer. This arrangement is similar to the system with OFDR by Childers
et al. [13], except using the long-length FBGs. When a coherent source scans R1 and R2 as a functionof wavelength or wavenumber, the resultant interference signal from PD1 is obtained as given in thefollowing expression:

PD1 = 2{1 + cos (2neff LR k )},(2)

where LR is the path difference of the two paths through the interferometer, and k is the wavenumberof the light and is related to the wavelength, λ, and given by

k=

2π

λ

.

(3)

The signal at PD1 is used as a trigger signal on DAQ to acquire the interference signal PD2 at aconstant wavenumber given byΔk =πneff LR,(4)

and the signal at PD2 is expressed as

PD2 ≈ ∑ Ri (k ) cos(2neff Li k ) ,i(5)where Ri(k) is the spectrum reflected by i-th grating in the FBGs, Li is a path difference between
reflector R3 and the i-th grating. The interference signal on PD2 is shown in Fig. 3. In this case thelength of each FBG was about 100 mm and the interval was 10 mm.

Fig. 2. Distributed sensing system.

193

H. Murayama, et al./ Strain Monitoring and Defect Detection in Welded Joints
by Using Fiber-Optic Distributed Sensors with High Spatial Resolution

Fig. 3. Measured interference signal on PD2.

Fig. 4. Spectrogram of interference signal.

Fig. 5. Spectrum at 4.700 m in spectrogram.

Fig. 6. Center wavelength distribution.

In our sensing system we use a signal processing technique based on short time Fourier transform(STFT) to map the strain profile along FBGs. In the case that STFT is applied to the interference
signal shown in Fig. 3, time and frequency in usual STFT are corresponding to wavenumber andposition, respectively. So by Fourier transform of the data within a window and sliding the window
along the wavenumber axis, we can get a spectrogram as shown in Fig. 4. The horizontal axis isconverted from wavenumber to wavelength and the vertical axis represents the relative position, z, ofFig. 2. The color depicts the intensity of the reflected light at (λ, z). Wavelength spectrum of thereflected light at an arbitrary position can be extracted from the spectrogram. The spectrum at 4.700 m
in Fig. 4 is shown in Fig. 5. The center wavelength can be determined by calculating the center of thefull width at half maximum (FWHM) of the spectrum at each position. Figure 6 shows the center
wavelength along the FBGs. Finally strain distribution along the FBGs is determined by the
distribution of center wavelength shift between the initial condition and the current one.

3. Distributed strain sensing of welded tubular joints
We had two joint specimens with long-length FBGs. First the result of distributed strain sensing
with an FBG with 100 mm length is shown below. Then the result of sensing with long-length FBGs
which consisted of five 100 mm FBGs serially-cascaded to have the gauge length of 500 mm and wasbonded on the surface along the weld line of another specimen is described.
3.1.

Distributed strain sensing with 100 mm FBG

3.1.1. Test specimen and sensor arrangement
Dimensions of the test specimen with 100 mm FBG and the locations of the sensors are shown in194

H. Murayama, et al./ Strain Monitoring and Defect Detection in Welded Joints
by Using Fiber-Optic Distributed Sensors with High Spatial Resolution

Fig. 7. The specimen of carbon steel had a butt welded joint at the center. The grip regions of bothends were solid and their diameter was 31.8 mm. The thickness of the pipe region was 1.6 mm. A
long-length FBG with 100 mm length was bonded along the weld line. The distance between thecenter of the specimen, namely the center of the weld line, and the FBG was 5 mm. The width of theweld line was less than 5 mm, so the FBG was not on the weld line in this case. Four strain gauges
were bonded on the opposite side of the FBG along the weld line. Four biaxial strain gauges were also
bonded between the weld line and the grip regions. Therefore, eight strain outputs in thecircumferential direction (#1 - #8) and four strain outputs in the longitudinal direction (#9 - #12)
would be obtained from the strain gauges. In addition, two fiber optic Doppler (FOD) sensors (Lazoc
Inc.) [14] were bonded in order to detect acoustic emission (AE) which might occur with somedamage. The circumferential locations of sensors are represented as a developed figure. The test
specimen was subjected to static tensile loads, and the strain distributions along the weld line were
measured by the distributed sensing system based on OFDR. Table 1 shows the load cases in the static
tensile test.

Fig. 7. Dimensions of the specimen and locations of the sensors.
Table 1 Load cases in the static tensile test.
case #
I
II
III
IV
V
VI

Initial load, ton
0.0
0.0
0.0
0.0
0.0
0.0

Max. load, ton
0.5
1.0
2.0
3.0
4.0
5.0

Terminal load, ton
0.0
0.0
0.0
0.0
0.0
0.0

195

Step interval, ton
0.25
0.25
0.50
1.0 (0.0-2.0), 0.5 (2.0-3.0)
1.0 (0.0-3.0), 0.5 (3.0-4.0)
1.0 (0.0-4.0), 0.5 (4.0-5.0)

H. Murayama, et al./ Strain Monitoring and Defect Detection in Welded Joints
by Using Fiber-Optic Distributed Sensors with High Spatial Resolution

3.1.2. Test result
Figures 8, 9 and 10 show strain distributions along the FBG in the load cases of I, IV and VI,
respectively. When the specimen was subjected to a tensile load, compressive strain along the FBG
occurred due to Poison’s effect. This test was conducted in a room temperature. The strain distribution
is compensated by using the center wavelength of the reference FBG which was in the stress freecondition. The indexes A to A’ correspond to the indexes of the sensing positions along thecircumference of the specimen shown in Fig. 7. In these figures, strains of #1, #2, #3, #4, and #7 arealso shown at the positions of A, B, C, D, and A’, respectively.
As shown in Fig. 8, strains of the strain gauges linearly decreased in proportion to the load andthey were almost distributed uniformly. On the other hand, strains along the FBG linearly decreased inproportion to the load except for those adjacent to A and C and that between D and A’. In some ofthese areas, strains were drastically changed with AE from 2.00 to 3.00 tons, as shown in Fig. 9. Thesechanges were also remained after unloaded. These results imply that we could find the existence ofinitial defects even under a relatively lower load.
In the case of IV, several AEs were detected and at the same time the strain distributions
measured by the FBG drastically changed at several areas, while strains of the strain gauges shifted
without a noticeable variation. In the case of VI, large plastic deformation rapidly developed and anumber of AEs were detected from 4.00 to 5.00 tons. Consequently large strains over -10,000 με
remained along the FBG after unloaded as shown in Fig. 10. This means that our sensing system has awide measurement range. In all the cases, we confirmed that strains of #5 - #12 that were far from theweld line were almost in proportion to the applied loads.
After the static tensile test, we conducted micro CT scanning to the area around the weld line. In
order to set the specimen into the scanning area, we machined the specimen to the small piece which
included the weld line. The test piece observed and the scanning region are shown in Fig. 11. We took
thirteen cross-sectional images around the weld line and the interval of each image position and theslice thickness are 0.25 mm and 0.0610 mm, respectively. The scanning position of 105.212 mm and106.712 mm corresponded to the center of the specimen and the closest position to the FBG,
respectively. Figure 12 shows the cross-sectional image close to the center. We can see a defectbetween D and A, so this defect might cause the anomalous strains as shown in Fig. 9. In some areas,
however, we could not find noticeable defects in the images even if the measured strain distributions
sharply fluctuated (e.g., around 4.075 m). Since it was difficult to catch a defect whose direction wasparallel to X-ray, there might be defects in such areas.

Fig. 8. Measured strain distributions in case I.

Fig. 9. Measured strain distributions in case IV.

196

H. Murayama, et al./ Strain Monitoring and Defect Detection in Welded Joints
by Using Fiber-Optic Distributed Sensors with High Spatial Resolution

Scanning region: 103.712 – 106.712 mm
Scanning pitch: 0.25 mm
Slice thickness: 0.0610 mm
Weld line

FBG

Fig. 10. Measured strain distributions in case VI.

Fig. 11. Test piece and the scanning region.

D

3.25 mm
C

A

B

105.462 mm

Fig. 12. Cross-sectional image at 105.462 mm

3.2.

Distributed strain sensing with 500 mm FBG

The 500 mm FBG which consisted of five 100 mm FBGs serially-cascaded was bonded on thesurface of the steel pipe similar to that of the previous test. In this case the FBG was bonded just on
the weld line which was sanded to be flat. Figures 13 and 14 show the steel pipe with a butt welded
joint and the FBG bonded along the weld line, respectively. The circumference length of the pipe wasabout 100 mm, so the FBG could be winded along the weld line about 5 times in a spiral manner.
There were 6 load cases and all the measurements were conducted in a room temperature. In the thirdcase the specimen was subjected to tensile loads that were increased up to 1.00 ton and then decreased
to 0.00 tons. In the fifth case the maximum tensile load was 1.60 tons. Figure 15 shows the strain
distributions in the third case. Measured strains are proportional to the tensile load except for theregion between B and C. In this area, we can see the difference between strains of 0.40 tons and 0.40
tons (unload) and residual strains are observed in the distribution of 0.00 tons (terminal). This meansthat stresses were reconstructed in the load of 1.0 ton. When the applied load was increased in the fifthcase, large plastic deformation developed from 1.0 ton to 1.60 tons. We can see larger compressive
strains especially between B and C. It seems that the larger residual stresses had been arising in thearea during the welding process.

197

H. Murayama, et al./ Strain Monitoring and Defect Detection in Welded Joints
by Using Fiber-Optic Distributed Sensors with High Spatial Resolution

Fig. 13. Steel pipe with a butt joint.

Fig. 14. 500 mm FBG bonded along the weld line.

Fig. 15. Measured strain distributions in third case.

Fig. 16. Measured strain distributions in fifth case.

Fig. 17. Cross-sectional image above the center of the specimen.

The cross-sectional images had been taken by micro CT scanning before the tensile test. The
image above the center of the specimen is shown in Fig. 17 and the indexes A to D correspond to theindexes of the sensing positions along the circumference of the specimen shown in Fig. 15 and Fig. 16.
In Fig. 17, a noticeable defect can be seen between B and C. Although this defect might cause thelarge residual stresses, further study is required to determine the relation between the defect observed
by non-destructive inspection (NDI) and fluctuations in the measured strain distribution.

4. Conclusion
We have developed the distributed sensing system based on OFDR that can measure the strain
distribution along the long-length FBGs with the high spatial resolution. In this study, we applied thissystem to strain monitoring of the welded joints in the static tensile tests. We could successfully198

H. Murayama, et al./ Strain Monitoring and Defect Detection in Welded Joints
by Using Fiber-Optic Distributed Sensors with High Spatial Resolution

measure the strain distributions in the welded joints and the results could show the process ofdegradation of the specimen. In the first test with 100 mm FBG, the measured strain was about -14000
με at minimum. Additionally, we observed that strain sharply fluctuated with AE in the areas where
the anomalous distortions had been found in the measured strain distribution under the relatively
lower loads. In the test with another specimen we bonded the longer sensor along the weld line. We
could observe larger compressive strains near the area where micro CT scanning indicated theexistence of a defect. It seems that the larger residual stresses were arising in the area during thewelding process. We can say that the distributed sensing system has the applicability to strain
monitoring and defect detection for the welded joints. However, further study is required to determine
the relation between the defect observed by non-destructive inspection (NDI) and fluctuations in themeasured strain distribution.

Acknowledgement
The authors would like to thank Dr. Sunao Sugimoto of Japan Aerospace Exploration Agency(JAXA) who helped us to carry on micro CT scanning. The authors are grateful to Japan Society ofMaintenology for financially supporting a part of this work.

References[1] K.T.V. Grattan and T. Sun, Fiber Optic Sensor Technology: An Overview, Sensors and Actuators 82 (2000),
40-61.
[2] K.T.V. Grattan and B.T. Meggitt, Optical Fiber Sensor Technology, Kluwer Academic Publishers, Boston ,2000.
[3] J. Dakin and B. Culshaw, Optical Fiber Sensors –Applications, Analysis, and Future Trends, Volume Four,
Artech House, Inc., Boston, 1997.
[4] T. Horiguchi, K. Shimizu, T. Kurashima, M. Tateda and Y. Koyamada, Development of a distributed
sensing technique using Brillouin scattering, Journal of Lightwave Technology 13 (1995), 1296-1302.
[5] H. Murayama, K. Kageyama, H. Naruse, A. Shimada and K. Uzawa, Application of fiber-optic distributed
sensors to health monitoring for full-scale composite structures, Journal of Intelligent Material Systems
and Structures 14 (2003), 3-13.
[6] K.Y. Song, Z. He and K. Hotate, Distributed strain measurement with millimeter-order spatial resolution
based on Brillouin optical correlation domain analysis, Optics Letters 31 (2006), 2526-2538.
[7] M. Volanthen, H. Geiger and J.P. Dakin, Distributed grating sensors using low-coherence reflectometry,
Journal of Lightwave Technology 15 (1997), 2076-2082.
[8] S. Huang, M.M. Ohn and R.M. Measure, Phase-based Bragg intragrating distributed strain sensor, Applied
Optics 35 (1996), 1135-1142.
[9] H. Igawa, H. Murayama, T. Kasai, I. Yamaguchi, K. Kageyama and K. Ohta, Measurements of strain
distributions with a long gauge FBG sensor using optical frequency domain reflectometry, Proc. of SPIE
5855 (2005), 547-550.
[10] H. Igawa, K. Ohta, T. Kasai, I. Yamaguchi, H. Murayama and K. Kageyama, Distributed measurementswith a long gauge FBG sensor using optical frequency domain reflectometry (1st report, system
investigation using optical simulation model), Journal of Solid Mechanics and Materials Engineering 2(2008), 1242-1252.
[11] H. Igawa, H. Murayama, T. Nakamura, I. Yamaguchi, K. Kageyama, K. Uzawa, D. Wada, I. Ohsawa, M.
Kanai and K. Omichi, Measurement of distributed strain and load identification using 1500 mm gauge
length FBG and optical frequency domain reflectometry, Proc. of SPIE 7503 (2009), 750351-1-75035104.
[12] A.D. Kersey, M.A. Davis, H.J. Patrick, M. LeBlanc, K.P. Koo, C.G. Askins, M.A. Putnam and E.J. Friebele,
Fiber grating sensors, Journal of Lightwave Technology 15 (1997), 1442-1463.
[13] B.A. Childers, M.E. Froggatt, S.G. Allison, T.C. Moore, D.A. Hare, C.F. Batten and D.C. Jegley, Use of3000 Bragg grating strain sensors distributed on four eight-meter optical fibers during static load tests of acomposite structure, Proc. of SPIE 4332 (2001),133-142.
[14] K. Kageyama, H. Murayama, K. Uzawa, I. Ohsawa, M. Kanai, Y. Akematsu, K. Nagata and T. Ogawa,
Doppler effect in flexible and expandable light waveguide and development of new fiber-optic
vibration/acoustic sensor, Journal of Lightwave Technology 24 (2006), 1768-1775.

199