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R E S E A R C H
INTRODUCTION
Developing an easy-to-use strain measurement system
Striving for a usable strain measurement
system
In addition to the aerodynamic forces (lift and drag),
thrust and weight, the structure of an aircraft is subjected
to large forces, such as pressurization loading. The aircraft
must have a lightweight construction while maintaining
the strength and rigidity to withstand these forces. In
order to realize this basic design, it is very important to
know how much the structure will be deformed when
any given force is applied. The ratio of this deformation
is called “strain” . Strain can be determined through
numerical analysis, but since testing is necessary to
check whether the numerical analysis is correct, the
measurement of strain is essential in aircraft development.
For strain measurement, common measurement
systems use a “strain sensor” (refer to page 5), which
measures strain as the amount of change in the electrical
resistance. (Fig. 1A) Since the strain for only the point
attached to the strain gauge can be measured with this
measurement system, an enormous number of wires
connecting to the strain gauges will be necessary to
measure every point of the aircraft, even for just a wing,
increasing the quantity of equipment needed and making
handling difficult. Preparations alone will take nearly one
week, and the mounting costs will also be a problem.
For these reasons, we concluded that “we would like to
create a more easy-to-use strain measurement system” .
Therefore, we continue to collaborate with Shibaura
Institute of Technology on research and development of
a new strain measurement system capable of transmitting
data wirelessly while maintaining precision and a reduced
mounting area. (Fig. 1B)
C on v en t ional s tr ain me asur emen t s y s t ems us e
Wheatstone bridge circuits and require amplifiers for
amplifying the minute measured voltage changes. The
new system uses a “CMOS-inverter oscillator circuit” ,
which had never before been used to measure strain.
Since this system can measure the pulse frequency of
voltage that changes according to the strain, rather than
the magnitude of the voltage change, amplifiers are no
longer necessary and the driving power can be reduced.
Because it can be powered adequately with a battery, the
system can be designed to be wireless and the equipment
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A : Conventional strain measurement system
B : New strain measurement system being developed
Fig. 1: Strain measurement systems
simplified.
Realizing high precision comparable to
conventional systems
“Precision” is required of measurement instruments.
No matter how good the concept may be, it cannot be
considered “usable” if measuring precision equal to or
better than the conventional system is not obtained.
Figure 2 shows the measurement results of the system
being developed. In fact, since the circuit contains
resistance everywhere, strain cannot be measured more
precisely if an appropriate correction is not applied ( ■ ).
By researching a method of making these corrections and
applying them to the system, we have obtained values
( ▲ ) close to those of the conventional system ( ◆ ).
Research and development of a “CMOS-inverter oscillator circuit strain
measurement system”
Looking closely at figure 2, there are subtle differences in
the values of the new and conventional systems. What we
must consider here is that, although conventional systems
are superior measurement systems with high precision
and reliability, an accurate value is not necessarily always
obtained. Since a perfect testing setup is quite difficult
to achieve and the results are affected by the testing
environment, the problem of measurement error already
exists. In other words, if there are subtle differences of this
degree, we must continue development of the system
while considering “what level of accuracy defines high
precision?” .
S tr ain me asur ement is also gr e atly a f f e c t e d b y
temperature. Noise (S/N), which is what devices with
electrical circuits always face, is another problem. Our
surroundings are flooded with various electronic devices
emitting radio waves, and an accurate
value cannot be obtained if the noise
of undesirable radio waves emitted by
them is measured. Therefore, we are
also continuing research in correction
technology for temperature and noise.
We will continue to make it more compact and integrate
such technologies as for increased precision and wireless
capabilities with the aim of commercialization in 2018.
Since the new system can also be used to measure
the strain of moving parts, such as engine and helicopter
blades, which had always been difficult to measure, it can
contribute to safety in aircraft development. This system
also has the advantage of being able to measure strain
for a long period of time while powered only by a battery.
Strain measurement is used not only in the aerospace
field, but also in many other fields of the manufacturing
industry, such as construction and other vehicles like
high-speed rail and cars. We believe that the new strain
measurement system utilizing these features can play
an active role in various fields, including, of course, the
aerospace field.
Technology with possible spinoffs
A patent application for the new strain
measurement system incorporating the
correction method has been filed, and
we are currently developing a prototype
in collaboration with a measurement
equipment manufacturer. The current
system is about as small as a compact
camera for research and development
and is driven by a 100 V power source;
however, by the end of the year, we expect
to be able to announce a battery-powered
prototype about the size of a fingertip.
Fig. 2: Sample of measurement results and results from the static tensile test of an
aluminum alloy
[ Airframes and Structures Group ]
Atsushi Kanda, Takao Utsunomiya
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R E S E A R C H
INTRODUCTION
What is the key technology for the next liquid hydrogen fuel tank?
occupies a large volume. Therefore, hydrogen is cooled
CFRP cryogenic tank for future aircraft
JAXA is currently carrying out studies of hydrogen fuel
aircraft, such as reusable launch vehicles, hypersonic
transports and hydrogen aircraft. Because hydrogen
is a comparatively lightweight fuel, it produces large
down below the extremely low temperature of minus
253 degrees Celsius and stored in a liquid state. In order
to make a no-leak CFRP cryogenic tank, it is required to
research the relationship between separation/cracks
and leakage at extremely low temperatures.
Strain measurement method for
understanding CFRP damage
amounts of energy with a small weight consumption.
In order to reduce the weight of aircraft, Carbon Fiber
Reinforced Plastics (CFRP) is studied for application
in the walls of hydrogen fuel tanks. CFRP has a light
weight as well as high stiffness and strength. CFRP
is made from multi-layered sheets which consist of
carbon fibers and a plastic resin system. Under load,
separation (sheets peeling away from each other) and/
or cracks in the resin will happen. If such separation or
cracks develop and penetrate the walls of the tanks,
hydrogen will leak out from the tanks.
At room temperature, hydrogen is in a gas state and
It is valid to measure strain in CFRP in order to
understand damage in CFRP when a load is applied.
For measuring the strain, a digital image processing
system is used (fig. 1). This image system is a noncontact surface measurement system, compared with
the conventional strain gauge measurement method,
with which it is difficult to measure large areas.
Two mechanical tests were conducted, a free-edge
separation test and a crack detection test, using the biaxial fatigue test frame (refer to page 5). The result of
the free-edge separation test is shown in figure 2. The
captured images show that
free-edge separation became
larger as the load was increased.
After the test, the specimen was
inspected using the ultrasonic
flaw detector system, and it was
confirmed that the separation
was detected correctly.
The result of the crack
detection test is shown in figure
3A. According to the absolute
strain distribution method,
it can be measured that the
strain increases as the load is
increased. However, it is difficult
to understand when the cracks
occur, because the change in
the strain that appears in the
surface after the cracks occur
is too small. It is necessary to
compare the images of just
Fig. 1: Damage detection system and detection method
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before and just after the cracks
Research of damage detection in composite materials by using an
image processing system
occur. The absolute strain distribution method
was switched to a relative method, which
compares two images next to each other. This
result is shown in figure 3B. Cracks #1 to #4
could be captured. In particular, crack #2 was
captured to start on the right side and move to
the left side.
In the future, it is planned to increase the
camera resolution and reduce the measuring
interval for further investigation of the
Fig. 2: Result of detection of free-edge separation
damage..
Integrating all related
research in a bi-axial fatigue
Study of not only the damage mechanism, but
also the leak mechanism has been conducted,
and a leakage test method under a cryogenic
environment is now being developed.
Development of the leakage test method
with one-axis loading is almost finished, and
development of the method with bi-axis
loading will start from next year.
Finally, a simulated test will be conducted,
covering the period from damage occurrence
to hydrogen leakage using a bi-axial fatigue
test frame under a cryogenic environment
A. Absolute strain distribution
for an understanding of damage and leakage
mechanisms of the CFRP material. If a no-leak
CFRP cryogenic tank is made, a reusable launch
vehicle and hypersonic transport will come one
step closer to realization.
B. Relative strain distribution
Fig. 3: Result of crack detection
[ Airframes and Structures Group ]
Takeshi Takatoya, Hisashi Kumazawa
04
Intermission
B reak
Strain measurement systems
■ Barometer for structural strength
Let’ s look at a cylindrical piece of rubber. Pulling the
top end of the rubber cylinder with the bottom end
fixed to the floor extends it vertically as well as reduces
the diameter of the column, making it thin (fig. 1).
When force is applied in this manner, causing changes
in an object, the ratio of this change is called “strain” .
When the applied force is released, the rubber cylinder
will return to its original column shape. However, if it
is pulled with a rather strong force, it will not return
to its original state, even after the force is released.
Continuing to pull the rubber cylinder further with a
strong force will finally cause it to break. Next, if we try
repeatedly pulling the rubber cylinder with enough
force that it returns to its original state when it is
released, fatigue will accumulate in it from the applied
force, eventually causing it to break.
In fact, this is a characteristic of not only rubber, but
also aircraft materials such as aluminum alloy and carbon
fiber reinforced plastic (CFRP). In order to construct
aircraft with optimum strength, strain measurement is
essential.
■ Conventional strain measurement systems
“Strain sensors” (refer fig.1A of page 1), made from
devices such as amplifiers (amps), Wheatstone bridge
If rubber is pulled, it stretches. The ratio that
it was extended is called “strain” . In contrast,
it shrinks in the direction perpendicular to the
pulling force. The ratio that it was compressed
is also called “strain” .
Fig. 1: What is strain?
circuits incorporating strain gauges, and a computer
for collecting data, have been used conventionally
as systems for measuring strain. With a weak current
running through the circuit, the resistance of the
connected strain gauges changes as the measured
object is deformed, changing the voltage of the circuit.
Because the correlation between the amount of strain
and the amount of change in the voltage has been
studied in advance, we can determine the strain by
measuring the amount of change in the voltage. Since
the amount of change in the circuit voltage is very
faint, it is necessary to amplify the voltage in order to
accurately capture the value. Considering amplifiers
are required for this purpose, it is very difficult to
measure strain simultaneously in numerous locations.
■ New measurement systems
If a high-precision strain measurement system
that is able to make measurements easily could be
established, we would be able to improve safety
and accelerate aircraft production. Therefore, we
are devising a variety of measurement systems and
advancing this research.
In this issue of “Sora to Sora” , we have introduced
our research on two types of strain measurement
systems (refer to Research and development 1 and 2).
In addition, we are continuing research on damage
If we can determine the load from the measured strain, we
can provide feedback on the design and operation of aircraft.
Fig. 2: Fiber-optic strain measurement testing for wing models
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detection through strain measurement using fiber
optic sensors as well as on load identification (fig. 2).