Hyung-Joon Bang

Structural health monitoring for a wind turbine system: a review of damage detection methods

Renewable energy sources have gained much attention due to the recent energy crisis and the urge to get clean energy. Among the main options being studied, wind energy is a strong contender because of its reliability due to the maturity of the technology, good infrastructure and relative cost competitiveness. In order to harvest wind energy more efficiently, the size of wind turbines has become physically larger, making maintenance and repair works difficult. In order to improve safety considerations, to minimize down time, to lower the frequency of sudden breakdowns and associated huge maintenance and logistic costs and to provide reliable power generation, the wind turbines must be monitored from time to time to ensure that they are in good condition. Among all the monitoring systems, the structural health monitoring (SHM) system is of primary importance because it is the structure that provides the integrity of the system. SHM systems and the related non-destructive test and evaluation methods are discussed in this review. As many of the methods function on local damage, the types of damage that occur commonly in relation to wind turbines, as well as the damage hot spots, are also included in this review.

Simultaneous measurement of strain, temperature and vibration frequency using a fibre optic sensor

In this paper, a novel technique for the simultaneous measurement of strain, temperature and vibration for structural health monitoring is demonstrated using a fibre optic sensor system, which combines both a wavelength-swept fibre laser (WSFL) system and a laser diode system using a wavelength division multiplexer. An aluminium beam was placed in a thermal chamber for the simultaneous measurement of its strain, temperature and vibration characteristics. A fibre Bragg grating/extrinsic Fabry-Perot interferometer (EFPI) hybrid sensor system with WSFL was used for the strain and temperature measurements, while the EFPI sensor in the hybrid sensor operated by a laser diode simultaneously measured the vibration characteristics of the beam.

Cure monitoring of composite laminates using fiber optic sensors

In this paper, we present the simultaneous measurement of the strain and temperature during cures of various composite laminates using fiber Bragg grating/extrinsic Fabry-Perot interferometric (FBG/EFPI) hybrid sensors. The characteristic matrix of the hybrid sensor is derived analytically. For the fabrication of the three types of graphite/epoxy composite laminate, two FBG/EFPI hybrid sensors were embedded in each composite laminate in two mutually perpendicular directions. We performed the real-time measurement of fabrication strains and temperatures at two points within the composite laminates during the curing process in an autoclave. Through these experiments, FBG/EFPI sensors are proven to be a good choice for efficient smart monitoring of composite structures.

Stabilized interrogation and multiplexing techniques for fibre Bragg grating vibration sensors

We demonstrated a simple interrogation system for multiplexed fibre Bragg grating (FBG) sensors in a high-frequency range. A tunable fibre Fabry–Perot (FFP) filter with narrow free spectral range (FSR) was used to simplify the multiplexing demodulator for FBG vibration sensors. A stabilization-controlling unit was also developed for the maintenance of maximum sensitivity of the sensors. In order to verify the performance of the stabilization control unit, we measured the sensitivity of the FBG sensor by changing environmental temperature, and the system showed an average sensitivity of 2.5 neRMS Hz−1/2 for a stabilization-controlled case. Finally, multi-point vibration tests using in-line FBG sensors were conducted to validate the multiplexing performance of the FBG system.

Simultaneous measurement of strain and damage signal of composite structures using a fiber Bragg grating sensor

For the simultaneous measurement of strain and a damage signal, a fiber Bragg grating sensor system with a dual demodulator is proposed. The dual demodulator is composed of a demodulator with a tunable Fabry?Perot filter measuring the low-frequency signal with large magnitude such as strain and another with a passive Mach?Zehnder interferometer measuring the high-frequency signal with small amplitude such as a damage or impact signal. Using the proposed fiber Bragg grating sensor system, both the strain and the damage signal of a cross-ply laminated composite beam under tensile loading were simultaneously measured. The strain and the damage signal measured by the single fiber Bragg grating sensor showed that sudden strain shifts were accompanied with vibration at a maximum frequency of several hundred kilohertz at the instant of transverse crack propagation in the 90? layer of the composite beam.

Structural health monitoring of a composite wind turbine blade using fiber Bragg grating sensors

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.

Optical fiber sensor systems for simultaneous monitoring of strain and fractures in composites

In this paper, a fiber optic health monitoring system was proposed and applied to the sensing of strain and fracture behaviors of composites. In order to detect strain and fracture signals simultaneously, we combined a white light interferometer with an intensity demodulator in an EFPI (extrinsic Fabry–Perot interferometer) sensor. Both a broadband light source and a narrowband filter such as an FBG (fiber Bragg grating) and a Fabry–Perot filter were coupled in an EFPI sensor imbedded in composite specimens. This paper compares the performance of two fracture sensing units with differing intensity demodulating filters and describes the implementation of the time–frequency analysis of STFT (short time Fourier transform) for the detection of the fracture signals like matrix cracking. From the test of tensile load monitoring using the optical fiber sensor system, the measured strain agreed with the value of an electric strain gage, and the fracture sensing unit could detect matrix cracking with high sensitivity to recognize the onset of the micro-crack fracture signals.

Composite Structural Analysis of Flat-Back Shaped Blade for Multi-MW Class Wind Turbine

This paper provides an overview of failure mode estimation based on 3D structural finite element (FE) analysis of the flat-back shaped wind turbine blade. Buckling stability, fiber failure (FF), and inter-fiber failure (IFF) analyses were performed to account for delamination or matrix failure of composite materials and to predict the realistic behavior of the entire blade region. Puck’s fracture criteria were used for IFF evaluation. Blade design loads applicable to multi-megawatt (MW) wind turbine systems were calculated according to the Germanischer Lloyd (GL) guideline and the International Electrotechnical Commission (IEC) 61400-1 standard, under Class IIA wind conditions. After the post-processing of final load results, a number of principal load cases were selected and converted into applied forces at the each section along the blade’s radius of the FE model. Nonlinear static analyses were performed for laminate failure, FF, and IFF check. For buckling stability, linear eigenvalue analysis was performed. As a result, we were able to estimate the failure mode and locate the major weak point.

Shape estimation and health monitoring of wind turbine tower using a FBG sensor array

This paper introduces a high-speed fiber Bragg grating (FBG) sensor system for use in shape estimation of wind turbine tower under dynamic loads. A fiber Bragg grating interrogator was developed with a spectrometer-type demodulator based on a linear photo detector. In the high-speed demodulation mode, up to six arrayed FBGs can conduct 40 kHz sampling per each channel simultaneously, and in the low-speed demodulation mode, the system can be effectively used for the dynamic strain monitoring of large structures that require many sensors, by expansion of the sensing channels. Real-time shape estimation of the wind turbine tower structure was accomplished using strain data gathered by surface mounted fiber Bragg grating sensors. The finite element model of the wind turbine tower was created and a displacement-strain transformation based on a modal approach was applied to find the displacement-strain transformation (DST) matrix. The estimated shapes measured by ten Bragg grating sensors are analyzed for two operational load cases of wind turbine. The results show very close agreement showing the potentials of the proposed shape estimation technique based on displacement-strain transformation using Bragg grating sensors.

Structural health monitoring of wind turbines using fiber Bragg grating based sensing system

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 Island.