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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 01 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 02 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 03 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 05 detection through strain measurement using fiber optic sensors as well as on load identification (fig. 2).