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.
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.
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.
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.
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.
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.
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