Bridge Maintenance, Safety, Management, Resilience and Sustainability – Biondini & Frangopol (Eds)© 2012 Taylor & Francis Group, London, ISBN 978-0-415-62124-3

Strain meaasuremeent of bridge meembers usinguSttrain Vi sualizattionSticcker
S. Umemoto
U& H. Ohaata & N. Miyamoto
M& T. Okammoto
Keisoku Researrch Consultaant Co Ltd., Hiroshimaa, Japan

T. Takaki
T&K
K. Fujii & I. Ishii

Hirooshima Univversity, Hirooshima, JappanABSSTRACT: E
Evaluation ofo stresses acting
aon briidge membeers is extrem
mely import
rtant for asseessing the safety
sof brridges. As a simple, economicalestrain meassurement device, the authors
aproppose a Straain Visualizzation
Stickker which ddisplays num
merical charracters and moiré fringge patterns correspondicing to the sttrain withouut using conventionacal electrical elements su
uch as amp lifiers, straiin gauges, oro signal cabbles. Based on the prinncipleof th
he Moiré friinge, the device display
ys a visible enlarged patternpshow
wing the miccro-displaceement causeed bystrainn, thereby pproviding sttrain inform
mation in chharacters whhich are visiible to the nnaked eye. PrecisePvaluues ofstrainn can also bbe obtainedd from stickker images by image processing,pand strain vvalues can be measureed remoteely using a camera witth a telesco
opic lens. A tensile test of a test specimensannd bending test of a bridgebmem
mber were ccarried out using the Strain
S
Visuualization Stticker. Accurate strainn informatio
on was obtaained
non--contact in bboth tests. InI the tensille test, erroor was less thant
10 με, showing acccuracy equual to that ofo theconv
ventional strrain gauge method, whilewerror iin the bend
ding test waas as small as 30 με. The
T visibiliity ofstrain
n informatiion provideed by the Strain
S
Visuaalization Stticker was also investtigated in th
he bendingg test,
demo
onstrating tthat approxiimate strain
n informatioon can be reead with thee unassistedd eye. Thesee results dem
monstrated that the Strain Visuualization Sticker can pprovide an effective toool for evaluuations of structuralssooundness.
1 IN
NTRODUCCTIONcal elements suchs
as am
mplifiers, sig
gnal cables, andstraain gauges.

Ratio
onal, cost-eeffective maaintenance and controol ofaging
g social inffrastructure such as briidges and ootherstrucctures has now becoome an immportant issue,heighhtening thee need for efficient, effective teechniquues for invesstigating thhe soundness of structuures.
The main type of periodic inspection used in invvestigattions of struuctural sounndness is visual inspecctionof external
eapppearance. If
I the stressses actingg on
strucctural membbers due too external forcesfcouldd be
deterrmined at thhe same timme during thhese routinee inspections, this w
would provvide useful information
in forassesssing structuural soundnness.
Sttress measuurements ofo structurall members are
geneerally madee using ellectrical sen
nsors suchh asstrainn gauges. W
When electtrical sensorrs are instaalled
at a number off points, it isi necessary
y to supply externaal power to each measuurement pooint via the signal cable, or aalternativelyy, to provide an inteernal
poweer supply in the sensors. The measured ddata
fromm the sensorrs must alsoo be transmiitted by a caableor wirelessw
metthod. Thesee requiremeents tend too increasse the scale and cost off measurement systemss.
Inn order to ssolve this problem,pth
he authors pproposee a “Strain V
Visualizatioon Sticker” which is caapable of displayiing charactters corresp
ponding to theamou
unt of strainn without usinguconveentional elecctri-Figure 1. Concept of a prooposed strain visualization
vssticker.

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Figu
ure 1 showss the conceept of the proposed
pSttrainVisuualization Sticker. The sticker is easily attacched
to bridge mem
mbers, and approximatte strain innformation can be oobtained froom characteers and patteerns
that can be reaad with the naked eye.. Precise sttrain
values can alsoo be obtaineed from thee sticker im
mage
by image processing, andd remote measuremenmnt ispossible using a camera withw a telep
photo lens. The
Straiin Visualizaation Stickeer also has the advantaagesof a simple strructure and low manu
ufacturing ccost,w
to multi-ppoint
and thus is paarticularly well-suitedstrainn measurem
ment.
VerificationVexperimentts were perfformed in orrder
to co
onfirm the pperformancce of the deeveloped Sttrain
Visuualization Stticker. Thiss paper pressents an outtlineof thhe experimeents and the verificationn results.
2 STRAIN VISSUALIZATTION STICK
KER
Figure 2. Sttructure of thee strain visualiization stickerr.

2.1 Structure of the Strainn Visualizatiion Sticker
Figu
ure 2 shows the structuure of the Sttrain Visuallization Sticker. Thhe sticker comprises
ctw
wo film layyers.
Linee gratings arre printed ono the back side of the top
film and the froont side of thhe bottom film.
fIn ordeer toobtaiin a clear M
Moiré fringee, the back sides of the bbottom film is painnted white. The interfaace betweenn thetwo films is filled with oiil to prevennt separationn of
the films.
fWheen applied to an objecct of measuurementt, the Straiin Visualizaation Stickeer displays themicrro-displacem
ment x of thhe object enlarged M tim
mes
by th
he Moiré friinge, makinng it possible to determmine
the approximate
ae strain ε byy simple vissual inspecttion.
Assuuming the ddistance bettween pointts where Sttrain
Visuualization SStickers are attached iss the refereence
lengtth L, strain ε is obtaineed as ε = x/LL.

Figure 3. TheT developedd strain visuallization stickerr.

3 TENSILE
TTESTT

2.2 Developed Strain Visuualization Stticker

3.1 Test piecee

Figu
ure 3 shows the appearaance of the developeddStraiin Visualizaation Sticker. The dimeensions of thhis
devicce are lengtth: 57 mm, width:w
10 mm,m and thicckness: 0.44 mm. The film materialmis poolyester. Thheline gratings aree printed onn the films usingua phottomask
k technique. The finestt line which can be prinnted
by th
his techniquue is 0.01 mm. The left
l side off thestickker displays a simple MoiréM
fringee for use in image processing,pwhile the rightr
side shhows numerricalcharacters, which make itt possible tot read the apximate meassured strainn with the unassistedueeye.
prox
The magnificattion M of thet Moiré fringe
fis x1121,
and its
i pitch p, △p, and reepetition nu
umber n aree 0.1mm, 0.01 mm, and 12, reespectively. Similarly, themagnnification M of the num
merical strip
pe is x501, andits pitchpp, △pp, and repeetition num
mber n are 0.1mm, 0.01 mm, aand 50, resppectively.

N 14
Thee test piece for the tennsile test (JIS Z2201 No.
testt piece) is shown in Figure 4. For compaarison
purrposes, a strrain gauge was attach
hed to the centercof the test pieece. The SStrain Visuaalization Stticker
wass set on the flat bar so as to be possitioned oveer theslit, and the flat bar was fixed to thee test piece withscreews so that the positioon of the sliit coincidedd withthat of the strain gauge.
3.2 Outline off apparatuss for tensile test
As shown in Figure 5, tthe test pieece was sett in ampressive/teensile test ddevice (ratedd capacity: 1000comkN), and a dig
gital video ccamera for measuremeent of
the Strain Visu
ualization SSticker wass set up squuarely
ker at a possition 86.5 mm
m from thhe test
facing the stickpiece. The digital video caamera was a commerciallyavaailable produ
uct. The strrain gauge wasw connectted toa sttrain measurement devivice.

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3.3 Test method

4 BENDING TESTAs the test method, loading and unloading were performed using loading steps of 0 kN, 20 kN, 50 kN,
and 80 kN. At each loading step, strain was calculated by image processing from the photographed image of the Moiré fringe, and strain was also measured with the strain gauge using the strain
measurement device.

4.1 Strain Visualization Sticker
In the bending test, in addition to the developed
Strain Visualization Sticker (Sticker I), a newly conceived prototype sticker (Sticker II) which enables
easier visualization of strain was also used. Figure 7
shows the appearances of the two devices. Sticker II
has a structure in which the Moiré character gratings
arranged in the form of a scale, making it possible toread the amount of change in strain directly from thedevice. The reference length L of both devices was
50 mm.

3.4 Result of tensile test
The test results are shown in Figure 6. The relationship between tensile loading and strain is linear during both loading and unloading, and hysteresis does
not occur. The difference between the measurements
by the strain gauge and the Strain Visualization
Sticker was <10 με. This result confirmed that noncontact strain measurement is possible with theStrain Visualization Sticker with substantially thesame accuracy as with a conventional strain gauge.
Strain gauge

4.2 Outline of apparatus for bending test
The specimen used here was a steel I-beam having awidth of 150 mm, height of 300 mm, and length of4300 mm. Table 1 shows the sectional specificationsof the specimen. Figure 8 shows an outline of thetest apparatus. The distance between the supports is4000 mm. Roller-type bridge bearings were used on
both sides. Two-point loading was applied at loadingpoints 500 mm from the specimen center on bothsides of the center. A 750 kN jack was used. For
measurement of the Strain Visualization Sticker, adigital video camera was set up directly under thecenter of the specimen approximately 30 cm fromthe specimen. The camera was positioned so as tosquarely face the Strain Visualization Sticker. The
two Strain Visualization Stickers (I, II) were arranged on the bottom flange at the center of thespecimen, together with two uniaxial strain gauges
used for comparison purposes. The sensor arrangement is shown in Figure 9.
Video images of the Strain Visualization Stickers
were processed by a real-time image processing device, as shown in Figure 10, and strain was calculated in real time.

unit：mm7540
270400400105

Screw

105

200
Flat bar

Strain visualization sticker

Slit

Figure 4. Test piece for tensile test.

Figure 5. Outline of apparatus for tensile test.

500

Strain ε (x10-6)302520151050-5-10-15-20-25-30

Strain Visualization Sticker （loading）
Strain Visualization Sticker （unloading）
Difference between the strain gauge (loading)Difference between the strain gauge (unloading)

4003002001000-100102030 40 50 60
Tensile Loading (kN)

7080

90

StickerⅡ

Difference between the strain gauge ε (x10-6)600

StickerⅠ

Reference length
L=50 mmFigure 7. Strain Visualization Sticker for bending test.

Figure 6. Result of tensile test.

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Standard cross-sectional
dimension
H300

B
150

t1
8t2
13Table 1. Sectional specifications.
Cross-sectional
Geometricalarea
moment of inertia
cm2cm4
AI
61.58
9480

Section modulus
cm3
Z
632

Jack(750 kN)

Loading girder

Frame for preventing side topplingSpecimen60

Digital video camera

Figure 8. Outline of apparatus for bending test.

Specimen

Strain Visualization Sticker Ⅱ
Uniaxial strain gauge
Strain Visualization Sticker Ⅰ

Bottom flange

Figure 9. Sensor arrangement.

Digital video camera

PC

Figure 10. Real-time image processing device.

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4.3 Test method

4.4.2 Nonlin
nearity

his test, reppeated loadding was peerformed twwice
In thusing
g the loadinng steps shhown in Taable 2. In eeachstep,, the strain vvalues obtaiined from th
he Strain V
Visualizaation Stickeers and straain gauge werew
recordded,and images of tthe Strain Visualization
Vn Stickers wwererecorrded.

xt, nonlinearity was ccalculated, assuming rated
Nexouttput (εRO) representsrmmaximum generated strain
sandd △εL is th
he maximuum differen
nce betweenn theactu
ual measureed values annd the strain
n obtained fromfa sttraight line joining 0 an
and the max
ximum geneerated
straain. The average valuee of nonlineearity was 1.5%(ROO = 845 με)). Table 3 shhows the caalculated ressults.

4.4 Results of bbending testt

4.4.3 Visibilility

4.4.1
1 Comparrison with strain gauugegure 12 shows images of the Straain Visualizzation
FigSticckers at steeps of 200 με. Table 4 and Tabble 5
shoow the relattionship beetween the generated strainsandd Moiré frinnge (charactter grating) for Sticker I andSticcker II, resp
pectively. W
With Stickeer I, an inccreaseof oneo numberr for roughlyy each 200 με step couuld beconnfirmed. WithW
Stickerr II, the scale increm
ments(chharacters) diisappear at roughly eaach 200 με step.
Thee scale chaanges fromm 1000μ too 200μ, correcspo
onding to increases in ggenerated sttrain from 0με0 to800
0 με, and th
hus providess a visualizaation of gennerated strains up too 800 με.

The relationshipp between loading
landd the strain obtaineed with the Strain Visuualization Stticker is shoown
in Fiigure 11. Thhe differencce between the values obtaineed with the strain gaugee and the Sttrain Visuallization Sticker is also show
wn. The vaalues of sttrain
ge are used average vaalues. The relationshiprgaug
betweeen loading aand strain shhows linearrity during bbothloadiing and unnloading. However,Hsoome hysterresis
occu
urred. The difference between thhe strain gaauge
and thet Strain V
Visualization Sticker wasw 30 με inn thefirst test and 32 με in the seecond test.

Step
12
3
4
5678

Step

0
10
20
30
4050
60
70

910111213
141516Load
LkN
k
80
90
100
110
120
130
140
150

800

400-4
40-8
80-120200-1600△ εL

Output-2
200
150.0(a) Firstt test
Nonlinearitty
1.5 %
1.5 %
1.5 %

1000

Strain Visu
ualization Sticker （loading）
Strain Visu
ualization Sticker （unnloading）
Differencee between the strain gauuge (loading)Differencee between the strain gauuge (unloading)

800

Strain ε (x10-6)

13 με
13 με
13 με

50.0
100.0
Applied Lo
oad (kN)

2000
1660
1220800040

6000-440

400-880
200-1120-11600

Load

0.0

Nonlinearity

50.0
100.0
Applied Lo
oad (kN)(b) Seconnd test
Figu
ure 11. Resultts of bending test.

2222-2200
150.0

Difference between the strain gauge ε (x10-6)

εRO

12200

Table 3. Nonlinearity.
842 με
847 με
845 με166040060000.0

Firrst test
Seecond test
Avverage

2000800

Strain ε (x10-6)

LoadkN

Strain Visu
ualization Sticker （loaading）
Strain Visu
ualization Sticker （unnloading）
Difference between the strain gauuge (loading)Difference between the strain gauuge (unloading)Difference between the strain gauge ε (x10-6)

1000

Table 2. Loading step.

Table 4. Change in character display with Sticker I.

StickerⅠ

Strain0 με0 με200 με
400 με
600 με
800 με

StickerⅡMoiré character gratings

１１１１１２２２２２３３３３３４４４４４５５５５５６６６６６Table 5. Change in character display with Sticker II.
Strain0 με
200 με
400 με
600 με
800 με

StickerⅠ200 με

StickerⅡ

1000μ1000μ1000μ1000μ1000μ

Moiré character gratings
800μ 600μ 400μ 200μ800μ 600μ 400μ 200μ800μ 600μ 400μ 200μ800μ 600μ 400μ 200μ800μ 600μ 400μ 200μ0μ0μ0μ0μ0μ5 CONCLUSION
StickerⅠ

A “Strain Visualization Sticker” was proposed as asimple, inexpensive strain measurement device forbridges and other infrastructure. This device is basedon the principle of the moiré fringe and displays
numerical characters and moiré fringe patterns corresponding to the amount of strain. No electrical elements (amplifier, strain gauges, signal cables) areused. Tests to verify device performance confirmedthat approximate strain information suitable for use
in routine visual inspections can be visualized with
the Strain Visualization Sticker. Precise strain values
with substantially the same accuracy as with theconventional strain gauge can also be obtained byimage processing of sticker images acquired by noncontact means. These results verified that the Strain
Visualization Sticker can provide an effective toolfor evaluations of structural soundness. In the future,
the authors plan to improve the strain measurement
accuracy and visibility of the Strain Visualization
Sticker.

400 με
StickerⅡ

StickerⅠ

600 με

StickerⅡ

StickerⅠ

6 REFERENCES
Takaki, T. Omasa, Y. & Ishii, I. 2008. Force Visualization
Mechanism using moiré Fringe for Robot Grippers, Proceedings of the 28th Annual Conference of the Robotics Society of Japan, 1K3-07, 2008. (in Japanese)
Takaki, T. Omasa, Y. Ishii, I. Kawahara, T. & Okajima, M.
2010. Force Visualization Mechanism Using a moiré Fringe
Applied to Endoscopic Surgical Instruments, Proceedings ofthe 2010 IEEE International Conference on Robotics andAutomation, pp. 3648-3653, 2010.

800 με
StickerⅡFigure 12. Change in character display withincreasing strain.

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