Development of the strain visualization device and application example
Masakazu Omachi1, Shuji Umemoto1, Takeshi Takaki2, Keisuke Matsuo1, Masaki Yonemoto1, Takashi Okada1
1
Keisoku Research Consultant Co.,665-1 fukuda 1-chome higashi-ku Hiroshima-city,Japan
2
Hhiroshima University,1-3-2 kagamiyama Higashi-hiroshima-city,Japanemail: oomachi@krcnet.co.jp, umemoto@krcnet.co.jp, takaki@hiroshima-u.ac.jp, matsuo@krcnet.co.jp,
yonemoto@krcnet.co.jp, t-okada@krcnet.co.jp

ABSTRACT: The strain visualization device is a strain measurement sensor developed to contribute to maintaining the soundnessof structures.The principle of moiré fringes is used for the measurement principle of the strain visualization devicé, and thecharacteristic that the moiré fringes can magnify and display minute displacements makes it possible to measure minutedisplacements at the strain level. In addition, by simply utilizing the principle of moiré fringes, the most problematic power source
for sensors for Structural Health Monitoring is not required. The strain visualization device is truly the ultimate power-saving
sensor.In addition, it has unique features not found in conventional strain measurement sensors, such as (1) strain can be readdirectly by naked eye, (2) strain can be measured with a digital camera, (3) self-temperature compensation type, and (4) highenvironmental resistance performance for over 10 years.
The strain visualization device has a reading capacity of ±500 µε, a visualization resolution of 50 µε, and a measurement accuracyof ±10 µε ±10 µε (2σ), which satisfies the performance as a sensor for Structural Health Monitoring.
In this paper, we show the performance and measurement method of the developed strain visualization device in detail, and also
introduce the example applied to the actual structure.
KEY WORDS: Strain; Moiré; Health Monitoring; Visualization.
1

INTRODUCTION

In Japan, operation and maintenance standards for structures
were strengthened following the Sasago Tunnel ceiling panel
collapse accident in December 2012. Measures included
enactment of legal inspection standards for bridges and tunnels
and stricter periodic inspections with a frequency of once every
5 years.
Japan has approximately 720 000 bridges and 10 000 tunnels.
Although operation and maintenance of this massive social
capital is extremely important, at present, there is a possibility
that maintenance may become impossible due to reductions inmaintenance and repair budgets and the decreasing number ofexpert engineers engaged in maintenance work for bridges andother structures.
Because many parts of periodic inspections based on closerange visual inspection depend on manpower, and a shortage ofexpert engineers is foreseen in the near future, monitoring
technologies that enable even unskilled personnel to determine
the condition of a structure in a simple manner are strongly
desired. Conventionally, structural monitoring has been
performed by sensors connected to a data logger, but thismethod has various problems, as many structures areconstructed in locations with poor accessibility, sources ofelectric power are not readily available, and frequent
maintenance is not possible. As other problems, the cost of
introduction is high, and running costs are also incurred in thismethod. For these reasons, monitoring has generally been
limited to important, large-scale structures. However, among
the above-mentioned 720 000 brides in Japan, approximately
510 000, or 71 %, are located not on national highways but oncity, town, or village roads, and maintenance of these medium-and small-scale bridges is also extremely important forprotecting the lives of people.
Under such social circumstances, the authors focused on
strain, which is one monitoring item, and developed a strain
visualization device (Figure 1) by enabling simple, inexpensive
measurement by anyone, with the aim of establishing a remote,noncontact strain measurement technique [1][2][3]. The strain
visualization device is a new type of sensor for use in strain
measurement which based on the principle of moiré fringes,
and has the following features:i) Strain can be visualized quantitatively.
ii) Because absolutely no electrical elements are used, apower source is not necessary and electricalmalfunctions are not an issue.
iii) For the same reason, the device is not affected byelectrical noise.
iv) By using an ordinary digital camera, strain can bemeasured remotely, without contact, and with higher
accuracy than by visual inspection.
v) Maintenance is easy.
In work to date, the authors improved the measurement
accuracy of the device, developed a self-temperaturecompensated type, and also developed an image analysis
application to enable use of the strain visualization device infield measurements [1][2][3]. As described in this report, anumber of improvements were made to solve several problems
that arise in long-term monitoring, and practical application of
the strain visualization device was successfully achieved. This
paper presents details of the performance and measurement
method of the developed strain visualization device andintroduces an example of application to an actual structure.

Figure 1. Schematic illustration of the Strain
Visualization Device(a)Line grating1

Moiré fringe

Figure 3. Structure of Strain Visualization Device
Line grating2

+∆

(b)

(A)

Line grating1(= ∆Table 1. Specification of the strain visualization device(= ∆ ))DEVELOPED STRAIN VISUALIZATION DEVICE
Measurement principle

As shown in Figure 2(a), when a line grating 1 with a pitch p
is overlaid with a line grating 2 having a pitch p+Δp which isΔp (<<p) larger than the pitch of grating 1, a striped pattern witha pitch W larger than that of gratings 1 and 2 appears. Thispattern is called a moiré fringe. The strain visualization device
uses the principle of moiré fringes. The relationship between
the pitches of the line gratings and the pitch of the moiré fringes
is expressed by Equation (1). The display of original pitch p canbe visually enlarged by (p+Δp)/Δp times by the moiré fringes.
This magnification factor is termed M.
=

⊿
⊿

・

(1)

Equation (2) expresses the amount of movement Δxm of the
moiré fringes in direction A in case line grating 1 is moved Δx
in direction A, as shown in Figure 2(b). In other words, thedisplay of displacement Δx can be visually enlarged by a factorof M. This principle enables measurement of extremely small(microscopic) amounts of displacement and visualization ofstrain in the measurement object.
⊿

=

・⊿

105mm

Reading capacity±500με (F.S.=1000με)

Visualization resolution

50με

Nonlinearity±1% of F.S.

Repeatability (2σ)±10μεOuter dimensions

W:17×H:6.8×L:120mm

Structure and specifications

Figure 2. Example of a moiré fringe

2

Gauge length(2)

Figure 3 shows the structure of the strain visualization device.
The strain visualization device comprises mainly a glass front
plate and rear plate, which form the line grating or characters,
and a steel temperature compensating plate, and is assembled
into a unit by the fixing rings. The length, width, and thicknessof the strain visualization device are 120 mm, 17 mm, and 6.8mm, respectively, and the gauge length is 105 mm. In addition,
the strain visualization device has three types of display
patterns. The top row is a character pattern with a scale of 100με pitch, which makes it possible to read strain values directlywith the unaided eye, and the center row is a moiré fringe
display for strain measurement by image processing. The
bottom row is a fixed reference scale which is used to improveaccuracy in image analysis.
Table 1 shows the specifications of the strain visualization
device developed with the structure described above.
Nonlinearity and repeatability were confirmed in previous
laboratory tests using a precision stage [3].
Verification of performance
In long-term monitoring in outdoor environments, stablemeasurement for long time is required in the sensors, but the
influence of temperature is frequently a problem. In addition,
sensor life is sometimes reduced by deterioration caused byultraviolet radiation, or by corrosion. Therefore, in order toverify the environmental performance of the strain
visualization device, tests were conducted to verify thetemperature
characteristics,
weathering
resistance(weatherability), and corrosion resistance of the device. The
results are presented in the following sections.
(1) Temperature characteristics
When a strain visualization device is fixed to an object ofmeasurement by gluing, and a temperature change occurs in
addition to the strain caused by external force, apparent strain
is generated by the difference in the linear expansion
coefficients of the material being measured and the sensor usedin the strain measurement. The term “self-temperaturecompensated strain measurement sensor” refers to a type of
sensor which minimizes the apparent strain generated bytemperature changes. Among strain gauges, which are a
representative type of strain measurement sensor, those inwhich apparent strain is within ±1.8 με/°C in the specified
temperature range when affixed to a compatible material arecalled self-temperature compensated strain gauges. In thesedevices, self-temperature compensation is realized by adjusting
the temperature coefficient of resistance of the resistive
element of the strain gauge so as to be compatible with thematerial being measured.
In previous work, the authors developed strain visualization
devices with a self-temperature compensate function formeasurement of concrete members and steel members [2].
Figure 4 shows the relationship between the temperature of thetest specimen and apparent strain. The apparent strain of thestrain visualization device was less than ±1.0 με/°C, confirming
self-temperature compensation. It is particularly noteworthy
that the self-temperature compensation range of the developed
strain visualization device is significantly wider than that of theconventional strain gauge, as apparent strain was very smalleven under extreme temperature conditions, being only about
20 με under a high temperature of 70 °C and –20 με at –20 °C
below freezing. These results demonstrated that the strain
visualization device, in which a temperature compensatingplate was incorporated in the device structure, is a selftemperature compensated strain measurement sensor withtemperature characteristics superior to those of conventionalstrain gauges.
(2) Weathering resistanceAn accelerated weathering test was conducted to confirm
weathering resistance, that is, whether the various materials
that comprise the strain visualization device are degraded byultraviolet radiation (UV). The strain visualization device wasplaced in the testing device and irradiated continuously with axenon lamp (180 W/m2), and was also exposed to steam for
18 min during each 2 h period. The temperature in the testing
device was 63 °C (38 °C during exposure to steam), and therelative humidity was 50 %.

100

Apparentstrain (με)
見掛けのひずみ(με)

50

0

-50
No.1-1
No.2-1
No.3
No.4-2
No.5-2

-100

-150
-30

-20

-10

0

10

20

30

40

No.1-2
No.2-2
No.4-1
No.5-1
Strain Gauge
50

60

70

80

温 度(℃)(°C)
Temperature

Figure 4. Relationship of specimen temperature and apparent strain

Before corrosion resistance test

After corrosion resistance test (800 h)

Figure 5. Condition of strain visualization device before (left) and after (right) the corrosion resistance performance test

The sample was removed at specified time intervals, the strain
scales were photographed, and the operation of the device waschecked. The test was conducted up to a test time of about 2 000h.
In a test conducted under the conditions described above, 200
hours of test time is equivalent to about 1 year under a naturalenvironment.
As a result of this test, no abnormalities such as discolorationof the surface of the strain visualization device or the formed
pattern were observed, and no problems were encountered inreading the strain values by visual inspection or in analysis of
the photographed images during the test. Accuracy was also
verified by using a precision stage after the completion of theweathering test, confirming that the device has the required
performance. These tests confirmed that the weathering
resistance of the strain visualization device under a naturalenvironment is 10 years or more.
(3) Corrosion resistance
Because steel is used in some parts of the strain visualization
device, the possibility that corrosion of these steel parts might
affect the normal functioning of the device was a concern.
Therefore, an accelerated corrosion test was carried out toconfirm corrosion resistance.
The strain visualization device was placed in the testing
device and subjected to a repeated cycle of saltwater spraying(5 % NaCl, 35 °C) for 2 h, drying (60 °C, 25 %RH) for 4 h, andhumidity (50 °C, 98 %RH) for 2 h. The test was conducted up
to approximately 800 h.
Here, 80 hours under the above-mentioned test conditions areequivalent to about 1 year under a natural environment(exposure in a coastal region).
Figure 5 shows the condition of the strain visualization device
before and after the corrosion resistance test. During the test,
no obvious rust was observed on the steel parts of the strain
visualization device, and visual reading of the strain values andanalysis of photographed images were possible without any
problem. Accuracy verification using the precision stage wasalso performed after completion of the corrosion resistance test,
and it was confirmed that the device has the required
performance. These results confirmed that the strain

visualization device has corrosion resistance of 10 years ormore under the natural environment (i.e., exposure in a coastal
region).

3

MEASUREMENT METHODS
Measurement by visual inspection

Figure 6 shows an example of the change in the strain values
in the visualization part (numerical characteristic display). The
strain value is obtained by reading the number with the darkest
appearance among numerical values with intervals of 100 με.
When two numbers appear in same darkness, the strain value
can be obtained with 50 με resolution by reading between those
two numbers.
Measurement by digital camera
In measurement by digital camera, the camera is set up facing
the strain visualization device as squarely as possible, and thedevice is photographed so that the black circles located in thefour corners are sufficiently within the shooting screen. During
photography, the photographer must be sure that the flash of
the camera, the LED lighting and the background behind thephotographer are not reflected on the strain visualization device.
The strain value can be calculated by analyzing thephotographed image with the strain calculation application
shown in Figure 7. The analysis by the strain calculation app iscarried out by an import of a still image, extracting the moiré
fringes, and fitting the measured values and the theoreticalvalues of the luminance distribution of the moiré fringes. In
actual operation, strain values can be calculated simply by
selecting the black circles in the four corners of the strain
visualization device in order and clicking the “Analyze” button.
4

EXAMPLE OF APPLICATION TO EVALUATION OF
BRIDGE LOAD BEARING CAPACITY
To confirm the applicability of the strain visualization device

Extraction of
moiré fringes“Analyze”
buttonFigure 6. Change in strain value of visualization part

Select black circles in four corners in orderFigure 7. Stain calculation application

Table 2. Specifications of the bridge
Bridge length
102.230 m
Girder length
Span length
Effective width30.750 + 40.750 + 30.550 m
30.050 + 39.950 + 29.850 m
Vehicle road: 6.750 m, walkway: 1.500 m

Total widthSuperstructure
construction system

9.250 mPost-tensioned PC simple
T-girder bridge (5 girders)Year of construction

1975

Figure 8. Appearance of the bridge

A1

P1

Symbols: Strain visualization device

P2: Strain gauge: Deflectometer

Figure 9. Arrangement of measuring devices

in the field, the device was applied to a bridge in Kagoshima
Prefecture. Strain measurements were conducted during aloading test, and an evaluation of the load bearing capacity ofthe bridge was carried out.

Mounting jigs

Specifications of the bridge
Figure 8 and Table 2 show the appearance and specificationsof the target bridge. The bridge is a 3 span post-tensioned PC
simple T-girder bridge (5 girders) located in Kagoshima
Prefecture.
The bridge had been in service for 46 years since construction,
and a large number of cracks caused by the alkali-aggregate
reaction had occurred in the main girder in the bridge axial
direction.
Loading test procedure
In the evaluation of load bearing capacity, a reconstruction
design is prepared based on the design standard at the time of
construction and the current condition of deterioration of thebridge. Here, the validity of the reconstruction design was

Anchor bolts

Figure 10. Condition of installation of a strain
visualization device

verified by a loading test, and the load bearing capacity of thebridge against loading was confirmed.

CASE1

CASE4

Center of Span

A1

outbound line

outbound line

inbound line

inbound line

P1

CASE2

P2

A1

P1

P2

CASE5

Center of Spanoutbound line

A1

P1

Center of Spanoutbound line

inbound line

CASE3

Center of Span

inbound line

P2

A1

P1

P2

CASE6

Center of Span

A1

Center of Spanoutbound line

outbound line

inbound line

inbound line

P1

P2

A1

P1

P2

Figure 11. Loading cases
Table 3. Results of strain measurements in loading test
Strain Visualization Device（με）
A1-P1

CASE

Strain Gauge（με）

P1-P2

A1-P1

P1-P2

G3

G4

G1

G4

G3

G4

G1

G4

CASE1

24

54

-

-

32

55

-

-

CASE2

27

43

-

-

39

26

-

-

CASE3

52

64

-

-

70

81

-

-

CASE4

-

-

3

21

-

-

4

29

CASE5

-

-

8

20

-

-

27

19

CASE6

-

-

20

42

-

-

35

47

(1) Arrangement of measuring devices
Figure 9 shows the arrangement diagram of the measuring
devices. The targets of verification of load bearing capacity
were the A1-P1 span and the P1-P2 span. Strain gauges anddeflectometers were installed on the underside of each girder inthe center of the spans. Strain visualization devices were also
installed near the strain gauges on girders G3 and G4 of the A1P1 span and girders G1 and G4 on the P1-P2 span.
Figure 10 shows the condition of installation of a strain
visualization device. The device was installed using dedicatedmounting hardware, and was fixed to the concrete by using
anchor bolts.
(2) Measurement method
The measurement method used with the strain visualization
device was measurement with a digital camera, as described in
the above section 3.2. First, initial images were taken in theunloaded condition, and still images were then taken for eachloading case. The acquired images were analyzed by the strain
calculation app shown in Figure 7, and the strain was calculated.

The measurements by the strain gauges and deflectometers
were performed using dedicated instruments.
(3) Loading method
Figure 11 shows the vehicle (truck) loading cases in theloading test. The bridge was loaded with 200 kN dump truck(s)which had been weighed in advance, and the test was conducted
for the case of loading on the inbound lane, the case of loadingon the outbound lane, and the case of loading on both theinbound and outbound lanes on the respective spans. The
bridge was loaded by dump trucks so that the center of the rear
wheels of the truck was located at the center of the span in eachspan.
Results of loading test and evaluation of load bearing
capacityThis section presents the strain measurement results for theG3 and G4 girders of the A1-P1 span and the G1 and G4 girdersof the P1-P2 span, where both the strain visualization devices

and strain gauges were installed. Table 3 shows the results ofthe strain measurements during the loading test. The results
shown for the strain visualization devices are the average valueof the results of an analysis of multiple images for each loading
case.
With the exception of girder G4 of the A1-P1 span for Case 2,
the strain visualization devices showed slightly smaller values
than the strain gauges. However, generally similar results wereobtained in all cases, and the largest difference between thestrain visualization device and the strain gauge was only 19 με.
This result satisfies the overall accuracy of ±20 με(nonlinearity: ±1 % of F.S., repeatability (2 σ): ±10 με), whichis the specification of the strain visualization device, andverifies the applicability of the strain visualization device tofield measurements.

Results generally in agreement with those of conventionalstrain gauges were obtained when the developed device wasapplied in an evaluation of the load bearing capacity of an
actual bridge, and it was possible to confirm the validity of thereconstruction plan based on the current condition ofdeterioration of the bridge and the load bearing capacity of thebridge against loading. The strain visualization devices
installed on the bridge will be left in place, and monitoring oftime-dependent changes in strain is planned. In the future, theauthors intend to accumulate actual results of long-termmonitoring, and hope to contribute to health monitoring ofstructures.
REFERENCES[1]

5

CONCLUSION

The authors developed a strain visualization device thatenables simple health monitoring of structures even byrelatively unskilled personnel, and successfully applied thedeveloped device under practical conditions. Laboratory tests
verified that the strain visualization device has weathering
resistance and corrosion resistance of 10 years or more, which
is a necessary condition for long-term monitoring in the field.

[2]

[3]

Omachi, M. et al. (2015). Verification of Strain Visualization Sheet with
Aim of Field Application, Proceeding of the 7th International Conferenceon Structural Health Monitoring of Intelligent Infrastructure, Torino,Italy,
1-3 July 2015
Umemoto, S. et al. (2017). Development of Self-Temperature
Compensated Strain Visualization Sheet, Proceeding of the 8thInternational Conference on Structural Health Monitoring of Intelligent
Infrastructure, Brisbane,Australia, 5-8 December 2017
Omachi, M. et al. (2017). Visualization of Strain and New Strain
Measurement Technique, Proceeding of the 8th International Conferenceon Structural Health Monitoring of Intelligent Infrastructure,
Brisbane,Australia, 5-8 December 2017