The 8th International Conference on
Structural Health Monitoring of Intelligent Infrastructure
Brisbane, Australia | 5-8 December 2017

Visualization of Strain and New Strain Measurement Technique
M. Omachi 1, S. Umemoto 1, T. Takaki 2, K. Matsuo 1,
N. Miyamoto 1, I. Ishii 2, T. Aoyama 2
1

Keisoku Research Consultant Co., Japan, Email: umemoto@krcnet.co.jp
2
Hiroshima University, Japan

Abstract
A strain visualization device and a new strain measurement technique were developed in responseto the need for health monitoring of rapidly-aging structures. The device, which is called the “Strain
Visualization Sheet,” utilizes the principle of Moiré fringes. As a result of repeated improvements toenable application to field measurements, a self-temperature compensated strain visualization sensor
was realized. A revolutionary strain measurement technique was also established, which not only
enables quantitative visualization of strain, but also simple measurement with an ordinary digital
camera by personnel without special skills. Strain visualization resolution of 50μm/m andmeasurement error of <20μm/m were confirmed in a verification of accuracy in the laboratory witha calibration device. In a comparison with a conventional strain gauge in a tensile test of steel
specimens, the same <20μm/m level of error in the strain value was also obtained by thismeasurement technique, verifying the appropriateness of the measurement technique and the Strain
Visualization Sheet as a sensor. Based on these results, Strain Visualization Sheets were applied in
two fields. As a result, stable strain data were obtained and applicability to field measurement wasverified. With the application of the Strain Visualization Sheet to structural health monitoring,acquisition of strain information on structures are economical and simple. As a result, more advanced
regular visual inspections are possible, and strain information on structures can be obtained at low
cost.
1. Introduction
Health monitoring of structures is expected to have a variety of benefits, including a higher levelof inspection, labor-saving in inspections and early discovery of abnormalities, and also assist in
early recovery in disasters and other abnormal cases. In recent years, however, health monitoring
been limited to only some structures due to problems such as the condition of public finances, and isalso not performed adequately, even in visual inspections. The largest reason for the failure to
implement health monitoring of structures is the problem of cost. In addition to the cost of installation,
maintenance costs also increase as more sophisticated sensors are used. Moreover, if frequent
maintenance of the monitoring equipment itself is necessary, equipment maintenance costs will also
increase, and depending on the scale of the structure, this may result in reversing the logical order ofthings. Therefore, focusing on strain, which is one monitoring item, we developed a device called theStrain Visualization Sheet with the aim of establishing a remote, noncontact strain measurement
technique that enables simple measurement by personnel without special skills (Omachi, M. et al.
2015). The Strain Visualization Sheet is a new sensor for use in strain measurements utilizing theprinciple of Moiré fringes, and has the following distinctive features.
(1) Quantitative visualization of strain is possible.

(2) Because the Strain Visualization Sheet uses absolutely no electrical elements, a power source
is not necessary and device life is not determined by electrical failure.
(3) For the same reason, the Strain Visualization Sheet is not affected by electrical noise.
(4) Strain can be measured by a remote, noncontact method with higher accuracy than is possibleby visual inspection by using a general digital camera, etc.
(5) Maintenance is easy.
Recently, we developed a self-temperature compensated type of Strain Visualization Sheet for
application to field measurements, and after repeated improvement to solve several problems in fieldmeasurement, we succeeded in practical application of the new device. This paper presents an outlineof the Strain Visualization Sheet, which is a new strain measurement technology. The results of aperformance verification are reported, and examples of application of Strain Visualization Sheet in
the field are also introduced.
2. Measurement Principle of Strain Visualization SheetAs shown in Fig. 1(1), when line grating 1 with pitch p and line grating 2 with a pitch p+Δp whichis Δp (<<p) larger than the pitch of line grating 1 are superimposed, a striped pattern with a pitch W
larger than that of line gratings 1 and 2 appears. This pattern is called a Moiré fringe. The Strain
Visualization Sheet utilizes the principle of the Moiré fringe. The relationship between the pitch of
the line gratings and the pitch of the Moiré fringe can be expressed by Eq. (1), and pitch p can bedisplayed visually enlarged by (p+Δp) / Δp by the Moiré fringe. This magnification factor is M.
𝑊𝑊 =𝑝𝑝 + ⊿𝑝𝑝
・𝑝𝑝
⊿𝑝𝑝

(1)

In order to enhance visibility (legibility), a Moiré fringe with a high magnification ratio is
obtained by treating a number n of stripes with pitch p on line grating 2 as one unit, and arranging
this at intervals of np+Δp. The pitch of the Moiré fringe which is formed under this condition canbe expressed by Eq. (2), and the magnification factor is (np+Δp) / Δp.
𝑊𝑊 =𝑛𝑛𝑝𝑝 + ⊿𝑝𝑝
・𝑝𝑝
⊿𝑝𝑝

(2)

The amount of shift Δx m of the Moiré fringe in direction (A) when line grating 1 moves Δx in
direction (A), as shown in Fig. 1(2), is expressed by Eq. (3). This means that displacement Δx canbe displayed visually enlarged by M times. It is possible to measure the strain level by thisprinciple.
⊿𝑥𝑥𝑚𝑚 = 𝑀𝑀・⊿𝑥𝑥(3)

In the same manner, if a grating in character form (character grating) is used in place of line
grating 1, as shown in Fig. 2, the amount of shift when the character grating is moved in direction(A) can be displayed by changes in the characters, as in (i’) to (iii’). In visualization of strain,numbers corresponding to the amount of generated strain are displayed by arranging the character
grating in the form of a strain scale and adjusting the magnification factor.

Line grating 1

Moiré fringeLine grating 2
Line grating 1

ΔxLine grating 1

Character gratingLine grating 2

Line grating 2

Δxm(=M･Δx)
Fig. 1 Example of a moiré fringe

Fig. 2 A moiré fringe with characters

3. Strain Visualization Sheet
3.1 Structure of Strain Visualization Sheet
The structure and appearance of the Strain Visualization Sheet are shown in Fig. 3 and Fig. 4,
respectively. The device comprises the two glass plates that form the Moiré fringe pattern and a steel
temperature compensation plate for temperature compensation, and is assembled into a unit by using
two fixing rings. The length, width and thickness of the Strain Visualization Sheet are 120 mm, 14mm and 8 mm, respectively, and the gauge length is 105 mm. The Strain Visualization Sheet consistsof three fringe types. The Moiré fringes in the top and middle line change according to the strain.
The top line shows character patterns in the form of a scale with an interval of 100 μm/m, and allows
the user to read strain values directly with the unassisted eye. The middle line is a simple Moiré fringe
for strain measurement by image processing, and the bottom line is a reference Moiré fringe for
improvement of image cropping accuracy in image processing.
3.2 Specification of Grating Pattern
The pitch p, Δp and number of repetitions n of the grating pattern in the visualization part on the
top line are 0.1 mm, 0.01 mm and 73, respectively, and the magnification factor M = 730x. In theimage processing part on the middle line, the pitch and number of repetitions of the grating patternp, Δp and n are 0.07 mm, 0.00025 mm and 1, and M = 280x. Because grating pattern generation
technology has improved dramatically, it has become possible to set the pitch p and Δp on the image
processing part on the middle line to smaller values than were used until now, and it is also possible
to secure the same magnification factor M as in the past, even with the number of repetitions n = 1.
3.3 Structure of Pattern Glass
Fig. 5 shows the structure of the pattern glass. Photomask technology is used in generating thepattern on the glass. Grating pattern 1 is generated on the back side of the front glass. The back glassis first coated with a white undercoat, and grating pattern 2 is then generated. After the patterns are
generated a clear overcoat is applied to both line gratings.

Front plate
Back plate

Adhesion
Mounting jig

Fig. 3 Structure of Strain Visualization Sheet

120mm
105mm
14mm

17mm

8mm

1.1mm
1.1mm
1.6mm
Fig. 4 Appearance of Strain Visualization Sheet

Overcoat (Clear)

Line grating (Brack)

Undercoat (White)

Glass Plate (t=1.1mm)
Front
Plate

Back
Plate
Fig. 5 Structure of Pattern glass

4. Verification of Accuracy
4.1 Test Device
The appearance of the test device and the displacement control unit are shown in Fig. 6 and Fig. 7,
respectively. The test device consists of the displacement stage, laser displacement meter(repeatability: 0.05 μm) and their control PC, a USB camera (resolution: 2 x 106 pixels) for use inphotographing the Strain Visualization Sheet and a real-time image processing device. The USB
camera was set at a height of 90 cm above the Strain Visualization Sheet, and lighting was provided
to maintain a constant intensity of illumination.
4.2 Test Method
After the initial value was measured, displacement (strain ε = 45 μm/m equivalent) was appliedwith a pitch of 5 μm to the Strain Visualization Sheet by the test device, and the Strain Visualization
Sheet was photographed continuously with the USB camera. Strain was calculated from thecontinuous photographed images in real time by the real-time image processing device, and the strain
values were recorded at intervals of 5 μm.
4.3 Visual Measurement
Fig. 8 shows the change of the strain values in the visualization part. The strain values wereobtained by reading the central value in the numbers that could be recognized visually to a half
between indicated numbers. From the verification results, it can be understood that the strain values
can be visualized with accuracy of ±50 μm/m for the strain generated at each 90 μm/m.
4.4 Measurement by Image Processing
Fig. 9 shows the results of measurements of the strain generated by the displacement stage obtainedby image processing of the Strain Visualization Sheet. The results of measurements by image
processing showed extremely good agreement with the actual generated strain. Error was –8 μm/m
to +15 μm/m, confirming that strain can be measured with higher accuracy than by measurement by
visual inspection.

Illumination
Real-time image
processing device

Displacement control device

Strain Visualization
Sheet

Displacement
control unit
USB Camera

Fig. 6 Appearance of test setup

Displacement Stage

Laser displacement
sensorFig. 7 Displacement control unit

⑧ ⑦ ⑥

Reading
読取り値（μm/m）

Generated
Strain発生ひずみ（μm/m）

-450

-450

-350

-360

-250

-270

-150

-180

-50

-90

50

0

（με）

600

（με）

Measurement
Measurement ofof Strain
StrainVisualization
Sheet(μm/m)(ue)Visualization Sheet⑤ ④ ③ ② ①

400
200
0
-200
-400
-600
-600

-400

-200

0

200

400

600

Generatedstrainstrain(ue)
Generated(μm/m)
Fig. 8 Change in strain value of visualization partFig. 9 Image processing result

5. Comparative Verification by Tensile Test
5.1 Test Piece
Fig. 10 shows the tensile test piece (JIS Z2201, No. 14). A Strain Visualization Sheet and a strain
gauge as an object of comparison were arranged adjacent to each other on both sides of the test pieceat the test piece center. The gauge length of the strain gauge was 90 mm.
5.2 Test Method
The test piece was set in the test machine (rated capacity: 1 000 KN) as shown in Fig. 11, andloading was applied in steps of 20 kN at a constant rate so as not to apply impact to the test piece.
Loading was continued to a maximum of 140 kN, after which the load was removed in the samemanner. The Strain Visualization Sheets on both side of the test piece were photographed with ageneral digital camera (Fig. 12) while keeping the load at each 20 kN step. The values of the strain
gauges were also recorded at the same time.
The strain calculation application shown in Fig. 13 was used in calculations of the strain values.
This application, which was developed for use in field measurements, calculates strain from the static
images photographed by the digital camera. Strain is calculated by the procedure of (1) reading thestatic image, (2) extracting the image of the Moiré fringe, (3) fitting the measured value and thetheoretical value of the luminance distribution (sine approximately curve) of the Moiré fringe and (4)calculating the strain value from the phase difference with the original image. In actual operation, theoperator simply selects the black circles at the four corners of the Strain Visualization Sheet in orderand clicks the analysis button.
5.3 Verification Results
Fig. 14 shows the relationship of load and strain during the loading cycle, and Fig. 15 shows theresults during the unloading cycle. The average values of the strain on the two sides of the test piece
are shown for both the strain gauges and the Strain Visualization Sheets.

Fig. 11 Testing scene

unit : mm：Strain gauge：Strain Visualization Sheet

Fig. 10 Test Piece for Tensile Test

OLYMPUS STYLUS TG-3 ToughFig. 12 Digital camera

Moiré fringe extraction image
Top line:
Visualization part
Middle line: For image processing
Bottom line: Reference moiré forimage analysis
Fitting condition window for
Moiré fringe on middle line
Fitting condition window for
Moiré fringe on bottom line
Extraction
Visual image (static image)

Analysis button
Select black circles at 4 corners in order.

Fig. 13 Strain calculation application

During loading, the values of the Strain Visualization Sheet showed extremely good agreementwith the values of the stain gauge, the maximum difference being 14 μm/m. Although the differencewith the strain gauge during unloading was somewhat larger than during loading, the average
difference was 20 μm/m and the maximum difference was 32 μm/m. Thus, this verification, whichassumed field measurement, verified the fact that strain can generally be measured with highaccuracy by the Strain Visualization Sheet.

900

800

800

700

700

600

600

Strain （μm/m）

Strain （μm/m）

900

500
400
300

500
400
300
200

200

Strain gauge
100

Strain gauge

100

Strain Visualization Sheet

Strain Visualization Sheet

0

0

0

20

40

60

80
100
Load （kN）

120

140

Fig. 14 Relationship of load and strain (loading)

160

0

20

40

60

100
80
Load （kN）

120

140

160

Fig. 15 Relationship of load and strain (unloading)Welded type strain
gauge

No.2
No.1

Strain Visualization
Sheet

Fig. 16 Measurement positions

Fig. 17 Condition of measurement device installation

6. Application to Field Measurement
In order to verify the applicability of the Strain Visualization Sheet to field measurement, Strain
Visualization Sheets were applied to a steel power transmission tower in Saitama Prefecture, Japanand to a telecommunications tunnel in Tokyo, and strain measurements were performed.
6.1 Strain Measurement of Steel Power Transmission Tower
6.1.1 Outline of Measurement
As shown in Fig. 16, Strain Visualization Sheets were applied to two positions on the legs of thesteel transmission tower. In addition to installing these Strain Visualization Sheets on the tower legs,
welded type strain gauges (gauge length: 5 mm) were also installed adjacent to the Strain
Visualization Sheets for comparison. The condition of installation is shown in Fig. 17.
6.1.2 Measurement Results
The strain measurement results are shown in Fig. 18 (position No. 1) and Fig. 19 (position No. 2).
As the measured strain data for the Strain Visualization Sheet, static images were photographed byusing digital cameras, and strain was calculated by the strain calculation application software.
Although the amount of change in the strain is slight and the number of measurements is small,the measured values obtained by the Strain Visualization Sheet at the present point in time aretrending at a difference of 10-50 μm/m in comparison with the measured values by the welded typestrain gauge, and stable measured data are being obtained.

Strain （μm/m）

Tension →

300
250
200
150
100
50
0
-50
-100
-150
-200
-250
-300
2016/10Strain gauge
Strain Visualization Sheet
2016/122017/2

2017/42017/6

Fig. 18 Strain measurement results (position No. 1)← Compression

Tension →Strain （μm/m）← Compression

300
250
200
150
100
50
0
-50
-100
-150
-200
-250
-300
2016/10Strain gauge
Strain Visualization Sheet
2016/122017/2

2017/42017/6

Fig. 19 Strain measurement results (position No. 2)

6.2 Strain Measurement of Telecommunications Tunnel
6.2.1 Outline of Measurement
In this test, the object of measurement was the lining concrete of a telecommunications tunnel. As
the measurement locations, two locations with different environmental conditions ((1) Dry condition,(2) Wet condition) were selected.
A Strain Visualization Sheet was installed on the lining concrete, and a strain transducer (gauge
length: 100 mm) and a metal base strain gauge were installed near the Strain Visualization Sheet as
objects of comparison. The stainless steel type metal base strain gauge was used in order to prevent
insulation degradation due to moisture from the concrete. A thermocouple type thermometer was also
installed to measure the ambient temperature. Fig. 20 shows the condition of installation of thesemeasuring devices.
6.2.2 Measurement Results
Fig. 21 shows the strain measurement results. The strain data measured by the Strain Visualization
Sheet were obtained by photographing static images with a digital camera and calculating the strain
values with the strain calculation application software.
At the measurement point under the dry environment, the strain values were approximately inagreement from the start of measurement until about 4 months. However, after 4 months, a maximum
difference of approximately 200 μm/m was observed in comparison with the conventional
measurement method using the strain gauge and the strain transducer. Since we cannot rule out thepossibility that a malfunction occurred in the strain transducer, strain gauge or Strain Visualization
Sheet after the measurements in February 2017, we plan to continue checking the trend of themeasured data and investigate whether a sensor malfunction of other problem occurred.
At the measurement point under the wet environment, no changes in the strain data were observed
after the measurement in February 2017. Although a difference of approximately 100 μm/m occurred
between the Strain Visualization Sheet and the strain gauge/strain transducer, the cause is not knownat the present time. The strain gauge at the measurement point under the wet environment failed about
1 month after the start of measurements.

Strain Visualization Sheet

Strain transducer

Strain gauge

Thermocouple
thermometer

Tension →← Compression
Tension →Strain（μm/m）

500
400
300
200
100
0
-100
-200
-300
-400
-500
2016/9①Dry Condition
Strain Transducer

2016/10

2016/11Strain Gauge

2016/122017/1

Strain Visualization Sheet

2017/2

2017/32017/4

Thermocouple

2017/5

50
40
30
20
10
0
-10
-20
-30
-40
-50
2017/6②Wet Condition

Strain Transducer
2016/10

2016/11Strain Gauge
2016/122017/1

50
40
30
20
10
0
-10
-20
-30
-40
Strain Visualization Sheet
Thermocouple-50
2017/2
2017/32017/42017/52017/6

Temperature （℃）← Compression

500
400
300
200
100
0
-100
-200
-300
-400
-500
2016/9Temperature （℃）

Strain（μm/m）

Fig. 20 Condition of installation of measuring devices

Fig. 21 Strain measurement result

7. Conclusion
To respond to the need for health monitoring of structures, the authors developed and successfully
realized practical application of a device called the Strain Visualization Sheet as a new strain
measurement technology which enables quantitative visualization of strain. In a verification of
measurement accuracy in laboratory tests, highly accurate measurements were possible in bothmeasurement by visual inspection and measurement by image processing. The possibility ofmeasurements at a level that enables practical application was also demonstrated in a tensile test of
steel material assuming field measurement. Although there are a few examples of application toactual field measurement, this research showed the possibility of practical application in long-term
measurement. In the future, the authors plan to accumulate actual results of practical application in
the field and improve the completeness of the Strain Visualization Sheet.
References
Omachi, M. et al., 2015, ‘Verification of Strain Visualization Sheet with Aim of Field Application’, Proceeding of the
7th International Conference on Structural Health Monitoring of Intelligent Infrastructure (SHMII-7), Torino,Italy, 1-3
July.