Verification of Strain Visualization Sheet with Aim of Field Application
M. OMACHI1, S. UMEMOTO1, T.TAKAKI2, K.MATSUO1, N. MIYAMOTO1,I. ISHII2, T. AOYAMA2.
(1) Keisoku Research Consultant CO., Hiroshima, Japan(2) Hiroshima University, Higashi-Hiroshima, Japan

Abstract
In recent years, aging and deterioration of bridges, tunnels, and other social infrastructure has
become a serious problem in Japan. Maintenance control of these structures is essential for securingsafety. Although the current main stream in operation and maintenance (O&M) of structures isperiodical inspection based on visual inspection, introduction of stress monitoring of structuralmembers, etc. is extremely effective for O&M of structures. However, due to financial constraints,
the structures where costly stress monitoring has been performed are limited. To solve this problem,the authors previously developed an inexpensive strain visualization sheet based on the principle ofMoiré fringes. As features of that device, absolutely no electrical elements such as amplifiers, strain
gauges, signal cables, etc. are necessary, large strain values can be read with the unaided eye, and it
is possible to obtain strain values with accuracy equal to that of conventional strain gauges byprocessing images photographed with a digital camera.
With the aim of application in the field, in the present research, some problems of the previouslydeveloped strain visualization sheet were solved, the materials and structure of the device wereimproved in order to achieve higher accuracy and simplify the installation method, and theperformance of the improved device was verified. As a result, it is possible to obtain strain values
with resolution of <100 με by visual observation and accuracy of approximately 10 με by imageprocessing. The temperature characteristics of the strain visualization sheet were also clarified, andsimplification of the installation method was possible. Thus, this study confirmed the applicabilityof the strain visualization sheet for use in the field.

In Japan, many huge structures were constructed from thepost-war economic recovery through the period of higheconomic growth up to the present. Due to aging anddeterioration of these large structures, numerous cases inwhich safety, utility, and functionality have declined can alsobe seen. Therefore, with appropriate operation andmanagement (O&M) of structures now becoming a social
requirement, rational and efficient O&M of structures has
become an important issue. At present, periodical inspection
based on visual inspection is the main stream in O&M ofstructures, but if information on the stress of members, etc.
can be obtained by monitoring utilizing sensors, morerational O&M is possible. However, due to recent problems
such as severe constraints on public finances and a declining
number of engineers, sensor-based stress monitoring has

Strain Visualization Sheet

1. Introduction

Figure1. Concept of a proposed
Strain Visualization Sheetbeen implemented for only a limited number of structures, and even visual inspections are notalways fully implemented. When stress monitoring is introduced in O&M of structures, it isnecessary to install strain gauges, strain meters, etc., wire the signal cables, and connect theseelectrical elements to a measuring device before monitoring can be performed. Various problems
are obstacles to implementation of these systems, including the fact that this installation work iscomplicated and the cost increases with the number of installation points, and in many cases, thiswork is not realized. Therefore, the authors are engaged in the development of a “strainvisualization sheet” based the concept shown in Figure 1. in order to enable simple, inexpensive
strain measurement by non-specialist personnel and establish an efficient strain measurement
method by a remote, non-contact technique. (Umemoto, S. et al. 2012) (Omachi, M. et al. 2013)
The strain visualization sheet is a novel type of sensor for strain measurement using the principle of
Moiré fringes, and has the following distinctive features.
i) Possible to read strain values directly with the unaided eye.
ii) Absolutely no electrical elements, such as strain gauges, amplifiers, and signal cables are used.
iii) Strain with high accuracy can be measured by a remote, non-contact technique by using anordinary digital camera, etc.
With the aim of application in the field, in the present work, several problems of the strain
visualization sheet developed to date were solved, and the materials and structure of the strain
visualization sheet were improved in order to further enhance the accuracy of the sensor andsimplify the installation method. This paper presents an outline of the newly improved strain
visualization sheet and reports the results of tests to verify its performance.
2. Outline of Measurement Principle of Strain Visualization SheetAs shown in Figure 2.(1), when a line grating 1 with a pitch p and a line grating 2 having a pitch
p+ p, which is p (<<p) larger than the pitch of line grating 1, are superimposed, a striped patterncalled Moiré fringes with a pitch W larger than that of these line gratings 1 and 2 appears. As
illustrated in Figure 2.(2), if line grating 1 is moved in the direction of (A) by a pitch p, the Moiré
fringes also move in the direction of (A) by pitch W. This means a displacement x can bedisplayed in a form visually enlarged by M times. Utilizing this principle, an enlarged display of a
microscopic displacement is possible. In the same, by using a grating in the character form(character grating) instead of line grating 1, as in Figure 3., the amount of movement when thecharacter grating is moved in the direction of (A) can be displayed as a change in the characters
displayed, as shown in (i’) – (iii’).
(1)pLine grating 1

p+Δp
(2)

Line grating 2

W

Line grating 1(A)

p(=Δx)

Line grating 1

Character gratingLine grating 2

Line grating 2

(A)

(A)

(ⅰ)

(ⅰ')

(ⅱ)

(ⅱ')

(ⅲ)

(ⅲ')

W(=Mp)

Figure2. Example of a moiré fringe

Figure3. A moiré fringe with characters

3. Structure of Newly Improved Strain Visualization Sheet
Figure 4. shows the structure of the newly improved strain visualization sheet. Although thecharacter grating, enlargement ratio of the Moiré fringes, and other features are the same as in theprevious version, the material of the newly improved strain visualization sheet has been changed
from the film used in the past to glass (thickness: 2.3 mm), which has a small coefficient of thermalexpansion and is not affected by humidity. In addition, jigs (width: 5 mm) which enable easy
installation on the object of measurement are attached to the both ends of the glass plates, and as aresult, the two glass plates form a single unit.
Figure 5. shows the appearance of the newly improved strain visualization sheet. The length, width,and thickness of the strain visualization sheet are 120 mm, 14 mm, and 4.6 mm, respectively, andthe reference length is 100 mm. The upper row of this sheet shows a character display with a scaleof 100 με pitch, and makes it possible to read strain values directly from the device with theunaided eye. The middle row is a simple Moiré fringe display for stain measurement by image
processing, and the lower row is a reference Moiré fringe display for improved image clipping
during image processing.

Glassline grating(front side)line grating(back side)
Glassadhesion

Mounting jigadhesion

mesurement
objectFigure4. Structure of Strain Visualization Sheet
100mm

14mm

120mm
2.3mm
2.3mm

8.5mm

Figure5. Prototype of Strain Visualization Sheet

4. Verification of Accuracy by Tensile Test
4.1 Test piece
The test piece (JIS Z2201, No. 14) for the tensile test is shown in Figure 6. The strain visualization
sheet was fixed at the center of the test piece with glue. For comparison, a strain gauge was also
attached adjacent to the strain visualization sheet.
4.2 Outline of apparatus for tensile test
As shown in Figure 7., the test piece was set in a testing machine (rated capacity: 1000 kN), and adigital video camera for photographing digital images of the strain visualization sheet was set up
facing the test piece at a distance of approximately 80 cm from the test piece. The digital video
camera was a commercially-available USB type with resolution of 2 x 106 pixels. The camera wasconnected to a real-time image processing device, and the strain gauge was connected to a strain
measurement device.
4.3 Test methodAs the test method, loading was applied in steps of 20 kN at a uniform speed so as not to cause
impact on the test piece. Loading was continued up to a maximum load of 140 kN. After reaching
this load, the test piece was unloaded by the same procedure. During the test, the strain visualization
sheet was photographed continuously with the digital video camera, and the amount of strain wascalculated in real time by image processing. The stain of the strain gauge was measured
continuously using the strain measurement device.

cameratest piece

Real-time ImageProcessing Device

Data Logger

unit : mm：Strain gauge：Strain Visualization Sheet

Figure6. Test Piece for Tensile Test

Figure7. Apparatus for TensileTest4.4 Verification results
Figure 8. shows the time-strain relationship. Figure 9. shows the stress-strain relationship. From thestart of loading, the measured values of the strain visualization sheet show substantially the samevalues as the strain gauge, the difference being generally within 5 με except for a partial differenceof approximately 10 με. A similar tendency was also observed during unloading. Likewise, in thestress-strain relationship, a linear relationship was obtained during both loading and unloading, andthe results of the strain visualization sheet and the strain gauge showed extremely good agreement.
These results verified the fact that strain can be measured with the newly improved strain
visualization sheet with the same accuracy as with a conventional strain gauge.
Next, Figure 10. shows images of the strain visualization sheet at intervals of 100 με. As can beclearly seen in the figure, when no load was applied, the scale around 0 μ appears dark, and the darkpart of the scale shifts to the right with each 100 με increment of strain. Moreover, the darkest partof the scale also coincides with the value of generated strain. Based on these results, the possibilityof reading generated strain with the naked eye with resolution of at least 100 με was verified.
900
Strain Visualization Sheet

800

Strain Gauge

700

Strain（µε）

600
500
400
300
200
10000-100

100

200

300

400Time（s）

Figure8. Time-strain relationship500600

1000
900
800

Strain（με）

700
600
500
400300Strain Visualization Sheet（loading）
200

Strain Visualization Sheet（unloading）
Strain Gauge（loading）

100

Strain Gauge（unloading）

0050

100150Stress（N/mm2）
Figure9. Stress-strain relationship

During loading

During unloading

0με

700με

100με600με

200με

500με

300με

400με

400με

300με

500με

200με

600με

100με

700με0με

Figure10. Change in character display with increasing strain200

5. Temperature Characteristics Test
5.1 Test method
In order to verify the temperature characteristics of the strain visualization sheet, a temperature
characteristics test was performed using a thermostatic chamber. The strain visualization sheet anda strain gauge were fixed with glue on a steel plate (SS400, coefficient of thermal expansion: 11.8 x
10-6/°C), as shown in Figure 11., and a thermometer was also installed to measure the temperatureof the steel plate. The steel plate was placed at rest in the thermostatic chamber so as to avoid theaction of constraining force, and a digital video camera was set up about 80 cm from the plate.
During the test, the temperature was changed in the range of -10°C to 50°C in steps of 10°C, andthe strain value was acquired when the plate reached the specified temperature. A humidity of 40%
was maintained during the test.

camerasteel plate

Figure11. Condition of temperature characteristics test

300

Apparent Strain（με）

200

y = 4.6114x - 94.581
R² = 0.999

1000-100

Strain Visualization Sheet-200

Strain Gauge（self-temperature-compensated type）-300-20-100

102030405060Temperature（℃）

Figure12. Relationship between temperature and apparent strain
5.2 Test result
Figure 12. shows the relationship between the temperatures of the strain gauge and the strain
visualization sheet and apparent strain. With the strain gauge, it can be understood that virtually noapparent strain is generated in the range of 10°C to 50°C because the strain gauge is a selftemperature-compensated type. In contrast, with the strain visualization sheet, apparent strain
increases in a linear manner as the temperature increases. The temperature characteristic of thestrain visualization sheet was 4.6 με/°C. Thus, for application in the field, restudy of the strain
visualization sheet will be necessary in order to realize a self-temperature-compensated type, likethe strain gauge, in which apparent strain is not generated.
REFERENCES
Umemoto, S. et al. (2012). Strain measurement of bridge members using Strain Visualization Sticker, Proceeding of theSixth International Conference on Bridge Maintenance, Safety and Management, Stresa, Lake Maggiore, Italy, 8-12
July 2012
Omachi, M. et al. (2013). Strain Visualization Sticker for structural health monitoring, Proceeding of the 6thInternational Conference on Structural Health Monitoring of Intelligent Infrastructure, Hong Kong, 9-11 December
2013