Mitsubishi Heavy Industries, Ltd. Technical Review Vol. 45 No. 4 (Dec. 2008) 5 Monitoring Aircraft Structural Health Using Optical Fiber Sensors TAKASHI YARI* 1 MASAHITO ISHIOKA* 1 KANEHIRO NAGAI* 1 MIKISHIGE IBARAGI* 1 KAZUO HOTATE* 2 YASUHIRO KOSHIOKA* 3 We have developed a system for monitoring aircraft structural health that uses optical fiber sensors. Our prototype device for Brillouin optical correlation domain analysis (BOCDA) uses an optical fiber as a sensor, and we verified that this device is sufficiently durable for installation on board aircraft. It functioned effectively as an aircraft structural health monitoring system in a flight demonstration test using a business jet. As the next development step, we plan to improve the reliability of this system for practical application as a product monitoring service. 1. Introduction It is necessary to reduce the operating costs of commercial aircraft. Reducing the cost of inspecting the airframe and the maintenance downtime this requires would further this goal. A structural health monitoring (SHM) system that can evaluate the health of the airframe structure while the plane is in flight and on the ground would provide a practical application that would satisfy this requirement. Mitsubishi Heavy Industries, Ltd. (MHI) is developing just such an aircraft SHM technology that applies Brillion optical correlation analysis (BOCDA), which can measure the strain distributed along the total length of an optical fiber sensor and the dynamic strain at arbitrary points along the optical fiber sensor. Consequently, it will be possible to measure the strain distribution, monitor deformation, and diagnose damage from the load history in a continuous section of the airframe structure during operation. This report introduces the development of an aircraft structural health monitoring technology that uses BOCDA. 2. Development of an onboard measuring device 2.1 Measuring technique Optical fiber is a suitable sensor for SHM because of its light weight, electromagnetic non-inductiveness, and durability. Of the measuring methods using optical fiber as a sensor, BOCDA, which can measure the distributed strain along the total length of an optical fiber sensor or the dynamic strain at arbitrary points on the optical fiber sensor, is an excellent technique for detecting a wide range and variety of damage. 1 BOCDA utilizes stimulated Brillouin scattering generated by the interaction between the pump light and the probe light launched from opposite ends of an optical fiber. Since the frequency shift of the stimulated Brillouin scattering light is proportional to the strain generated in the optical fiber, the axial strain along the optical fiber can be assessed by measuring this frequency. This shift is approximately 50 kHz/με for the optical fibers used for telecommunication. Figure 1 shows the BOCDA measurement principle. When the frequencies of the pump and probe lights are modulated at the same period, the point where the correlation between these lights is always constant (the correlation peak point) appears in the optical fiber. As the correlation between the pump and probe lights is not Stimulated Brillouin scattering Applied strain Pump light Pump light Probe light Probe light Distance Scattered light frequency Strain Scattered light intensity Correlation peak point Optical fiber sensor Optical fiber sensor Frequency 5 *1 Nagoya Aerospace Systems Works *2 Faculty of Engineering, The University of Tokyo *3 R&D Institute of Metals and Composites for Future Industries (RIMCOF) Fig. 1 BOCDA measurement principles