Takashi Yari

Aircraft Structural Health Monitoring using on-board BOCDA system

We developed the on-board BOCDA system for airplane and verified the flight environmental stability and durability through environmental test. The on-board BOCDA system adopted the polarization diversity technique and temporal gating technique to improve robustness of the BOCDA system. We successfully measured distribution of fiber Brillouin gain spectrum over 500m measurement range with 50mm spatial resolution, 60Hz sampling rate and ±13μ strain accuracy. Furthermore, we considered flight test to verify the validity of the BOCDA system. From these results, it was confirmed that BOCDA system has potential to be applied to an aircraft structure health monitoring system.

Aircraft structural-health monitoring using optical fiber distributed BOTDR sensors

We conducted theoretical and practical studies for applying the Brillouin optical timedomain reflectometer (BOTDR) to an aircraft structural-health-monitoring system. First, we measured strain and temperature independently and simultaneously using a combined system of the BOTDR and fiber Bragg grating (FBG) with wavelength division multiplexing (WDM). Second, we used Brillouin spectrum analysis and processing to enhance up to 0.5 m the spatial resolution of the distributed strain measured by the BOTDR. Finally, we verified the proposed improvement of the BOTDR measurement by manufacturing and testing a composite structure.

An application test using Brillouin optical correlation base analysis method for aircraft structural health monitoring

The authors developed a prototype Brillouin measurement system and carried out some application tests to verify the capability for aircraft structural health monitoring (SHM). The prototype Brillouin measurement system is adopted the Brillouin optical correlation domain analysis (BOCDA) method. This system is able to measure the distribute strain of full-length optical fiber sensor with 50mm of spatial resolution and 2.7Hz sampling of high-speed arbitrary point strain. Moreover, we conducted three application tests to evaluate the effectiveness of SHM using BOCDA system, such as the panel buckling test, the dynamic strain measurement test, and the demonstration flight test using the prototype BOCDA system. We verify the effectiveness of the BOCDA system for the aircraft SHM, and clarify the necessary development subject for the actual application.

Strain measurement using a Brillouin optical time domain reflectometer for development of aircraft structure health monitoring system

We conducted theoretical and experimental approaches for applying Brillouin optical time domain reflectometer (BOTDR) to aircraft and spacecraft structure health monitoring system. Firstly, distributed strain was measured by BOTDR under 3-point bending test and a spatial resolution was enhanced up to 0.5m using Brillouin spectrum analysis and processing though the device used in this experiment had a spatial resolution of 2m normally. Secondly, dynamic strain measurement was executed under cyclic loading conditions. Brillouin spectrum measured under dynamic conditions is equivalent to superposed spectrum using many spectra measured under static loading conditions. As the measured spectrum was decomposed into many spectra in static loading state, the strain amplitude and its ratio could be estimated. Thirdly, strain and temperature could be measured independently using combined system of BOTDR and fiber Bragg grating (FBG) with wavelength division multiplexing (WDM). Additionally, the application of BOTDR sensing system was shown for a prototype carbon fiber reinforced plastic (CFRP) liquid hydrogen (LH2) tank under cryogenic condition.

Delamination detection using embedded BOCDA optical fiber sensor

We conducted the delamination detect test for composites plate using the Brillouin optical correlation domain analysis (BOCDA) method. Firstly, a hole-assisted-fiber was chosen for embedding optical fiber sensor in order to avoid the increase of optical transmission loss induced by embedded into composite plate. The hole-assisted-fiber is functional better than comparing with the telecommunication optical fibers when optical fiber sensor was embedded into composite. Secondly, a delamination propagating was detected by the BOCDA from Brillouin peak frequency distribution and Brillouin gain spectrum shape changes.

Overview of damage detection and damage suppression demonstrator and strain distribution measurement using distributed BOTDR sensors

Structural health monitoring using optical fiber sensor is very attractive for aerospace structures, because of lightweight, durability and capability to be embedded in composites. Especially, distributed optical fiber sensing system, such as Brillouin Optical Time Domain Reflectometer (BOTDR), is hopeful method for large-scale composite structures. However, it is necessary to solve some problems for applying to aerospace structures. Low spatial resolution, strain/temperature effect, and long measuring period are the capital problems to be solved. For solutions of these problems, we have already reported these solutions. In this paper, we present practical application of the proposed techniques through the demonstrator test. Firstly, we measured mechanical strain and temperature simultaneously during CFRP panel curing process using a combined system of BOTDR and fiber Bragg grating (FBG) sensors with wavelength division multiplexing (WDM). Secondly, we measured the distributed strain in the whole structure and applied differential spectra method for improvement of a spatial resolution.

Development of the Damage Detection Method for CFRP Structures Using Distributed BOCDA Optical Fiber Sensor

The Brillouin optical correlation domain analysis (BOCDA) technology is one of the distributed optical sensing technologies utilizing the Brillouin scattering phenomena. The authors are developing, verifying and validating this technology. They are also developing the installation technique of optical fiber sensors, evaluating the durability of the BOCDA system and manufacturing the new device compatible to avionics bays for size. Furthermore, the authors are also improving the monitoring ability for composite damages. This paper reports the developmental status of the BOCDA monitoring ability for composite damages such as debonding at adhesive joints and micro-damages at bolted joints. First, debonding detection tests were conducted using carbon fiber reinforced plastics (CFRP) stiffened or repaired panels. In this test, test plates with a sheet stringer or a repair patch were applied tension load, and expanding of debonding areas was monitored by measuring strain distribution changes near these areas. Second, bearing damage detection tests were conducted using CFRP bolted joint specimens. In this test, multi-fastener single-lap CFRP specimens were applied tension load, and occurrences of micro-damages near bolt holes was detected. Micro-damages were detected by monitoring shapes changes of the Brillouin Gain Spectrum, which is one of the BOCDA measurement results.

Airplane structure health monitoring by the optical frequency modulation Brillouin distributed measuring method

The necessity for Airplane structural health-monitoring technology has been increasing because of improvement of reliability and cost saving. Optical fiber sensor system is an attractive method for structural health monitoring, because of its lightweight, non-electromagnetic interference, and to be embeddable to composite structures. Especially the distributed optical fiber sensor fits the health monitoring for large-sized structures. However, the distributed optical fiber measurement system using the pulse light represented by BOTDR has low spatial resolution and long measurement interval. These performances have been the obstacle of application to airplane structure health monitoring system. Then, the authors have proposed the Brillouin optical frequency modulation method for improvement of the spatial resolution and shortening of measurement intervals. In this work, we conducted basic approach in order to develop Brillouin Optical Frequency Domain Analysis (BOFDA) measurement system, such as pump power property and frequency modulation property for Brillouin stimulated light. We confirmed ability to measure stimulated Brillouin Scattering light in 50mm section. Moreover, We considered the optical fiber sensor installation issue on the airplane structure. The issue is optical fiber sensor birefringence under asymmetric load and durability of installation method. We conducted two confirmatory tests for the issues. The proposed installation method has adequate performance. From these results, it was confirmed that BOFDA system has potential to be applied to an airplane structure health monitoring system.