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This paper introduces a FBG (fiber Bragg grating) based AE (acoustic emission) sensing system for use in health monitoring of composite wind turbine blades. In this study, a multiplexing high speed FBG sensor system was developed with a spectrometer-type demodulator based on a linear photo detector. Pencil break test was performed using an FBG sensor and the results were compared with the results of piezo-based AE sensor. For the performance test of fracture sensing in composite materials, a down-scaled wind turbine blade was fabricated and drop impact tests were performed. Arrayed 4 FBG sensors were installed in the skin of wind turbine blade and impacts of 15 J energy were applied by a drop weight. The frequency characteristics of impact induced AE signals were examined with short-time Fourier transform focused on the leading waves. Finally, the onset of fractures in composite structure was successfully assessed using arrayed FBG sensors.

To perform the real-time health monitoring of the smart composite structures, two fiber optic sensor systems are proposed, that can measure the strain and detect the moment of fracture simultaneously. The two types of the coherent sources were used for fracture signal detection -EDFA with FBG and EDFA with Fabry-Perot filter. These sources were coupled to extrinsic Fabry-Perot interferometer sensors imbedded in composite specimens to monitor the fracture behavior of composites. To understand the characteristics of matrix cracking signals, at first, we performed tensile tests using surface attached PZT sensors. This paper describes the implementation of time-frequency analysis such as short time Fourier-transform for the quantitative evaluation of the fracture signals like matrix cracking. From the test of tensile load monitoring using optical fiber sensor systems, the measured strain agreed with the value of electric strain gage and the fracture detection system could find out the moment of damage with high sensitivity to recognize the onset of micro-crack fracture signals.

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