As the size of wind turbines increases, the early detection of structural instability becomes increasingly important for
safety. This paper introduces a fiber Bragg grating-based sensing system for use in multi-MW scale wind turbine health
monitoring, and describes the results of preliminary field tests of dynamic strain monitoring of the tower structure of an
onshore wind turbine. For this research, the Korea Institute of Energy Research (KIER) and the FiberPro, Inc. cooperated
on the development of a wavelength division multiplexing (WDM) Bragg grating sensing system for high-speed strain
sensing. The FBG interrogator thus developed can be used in the sensing of high-speed vibration as well as low-speed
dynamic strain. In the case of high-speed sensing, the interrogator allows a sampling ratio of over 40 kHz for six linearly
arrayed FBG sensors per channel. To monitor the dynamic strain behavior of the tower and substructure of onshore and
offshore wind turbines, 41 FBGs were installed on the supporting structures of the wind turbines. As a result, the Bragg
grating sensing system showed stable, accurate performance in the thermal chamber test and good dynamic strain
sensing performances during the strain monitoring of the tower structure at the Woljeong test-bed wind turbine in Jeju
In this study, a down-scaled wind turbine blade was designed and fabricated using glass and carbon fiber materials for
the skin and stiffener, respectively. In the course of its fabrication, an array of FBG (fiber Bragg grating) sensors was
embedded in the composite laminates. The embedded FBG sensor array was used to measure the residual strain in the
stiffener before and after the curing process, and after fabrication of the blade, the FBG array was used to monitor the
structural conditions, including structural dynamic behavior during the structural testing of the blade. The results of the
tests showed that the FBG sensor array effectively measured the residual strain distribution of the blade stiffener before
and after the curing process. And the measured natural frequencies and mode shapes by the FBG array matched the
results obtained from the FE analysis and conventional accelerometers.
Because of the higher attenuation of AE signal in composite materials, it is required to develop a damage assessment
technique less affected by the signal intensity in order to use AE signals for damage detection. In this research, we
investigated impact induced AE signals using FBG sensors on carbon epoxy composite panel. The frequency
characteristics of impact AE signals were examined with wavelet decomposition focused on the leading wave.
Consequently, we established a damage assessment technique using the sharing percentage of the wavelet detail
components of AE signal, and conducted a low-velocity impact test on composite laminates to confirm the feasibility of
the damage assessment method with FBG sensors.
Impact location monitoring is one of the major concerns of the smart health monitoring. For this application, multipoint ultrasonic sensors are to be employed. In this study, a multiplexed FBG sensor system with wide dynamic range was proposed and stabilization controlling system was also developed for the maintenance of maximum sensitivity of sensors. For the intensity demodulation system of FBG sensors, Fabry-Perot tunable filter (FP-TF) with 23.8 nm FSR (free spectral range) was used, which behaves as two separate filters between 1530 ~ 1560 nm range. Two FBG sensors were attached on the bottom side of the graphite/epoxy composite beam specimen, and low velocity impact tests were performed to detect the one-dimensional impact locations. Impact locations were calculated by the arrival time differences of the impact longitudinal waves acquired by the two FBGs. As a result, multiplexed in-line FBG sensors could detect the moment of impact precisely and found the impact locations with the average error of 1.32 mm.
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 types of the coherent sources used for fracture signal detection classify the systems--EDFA with FBG and EDFA with Fabry-Perot filter, and these systems were applied to extrinsic Fabry-Perot interferometer sensors imbedded in composite specimens to monitor the tensile tests. 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, measured strain agreed with the value of electric strain gage and the fracture detection system could detect the moment of damage with high sensitivity to recognize the onset of micro-crack fracture signals.