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Maximum Strain Detection of Structures Using Fiber Optic BOCDA Sensor with Aluminum Foil Packaged Optical Fiber



Smart structures are being developed with built-in sensors for long-term structural health monitoring. The maximum strain detection of structural parts is very important to evaluate overloading events. In general, fiber optic sensors are very applicable for smart structures because they are small in size, immune to electromagnetic interference, resistant to many hazardous chemicals and capable of distributed sensing through one optical fiber line. Especially, fiber optic Brillouin optical correlation domain analysis sensor can measure fully distributed strain through a sensing optical fiber line. We propose a distributed maximum strain detection sensor that uses an Alpackaged optical fiber for the first time. In order to measure strain, we constructed a fiber optic Brillouin optical correlation domain analysis sensor with 2 cm spatial resolution. We quantified the Brillouin frequency shifts in the Al-packaged optical fiber by the tensile strain and compared them for a varying number of Al layers in the optical fiber. The strain sensitivity, the ratio between Brillouin frequency shift and the applied strain of an optical fiber, with one Al layer had a slope of 0.038 MHz/me. After unloading, 87% of the strain remained as residual strain. When different tensile strains were randomly applied, the strain caused by the highest stress was the only one detected as residual strain. The residual strain was repeatedly measured for a time span of five months for the purpose of reliability testing, and there was no change in the strain except for a 4 % reduction, which is within the error tolerance of the experiment. We suggest that the Al-packaged optical fiber can be adapted as a distributed maximum strain detection sensor for smart structures, including aerospace structures.

doi: 10.12783/SHM2015/215

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