The paper provides a state of the art review of guided wave based structural health monitoring (SHM). First, the fundamental concepts of guided wave propagation and its implementation for SHM is explained. Following sections present the different modeling schemes adopted, developments in the area of transducers for generation, and sensing of wave, signal processing and imaging technique, statistical and machine learning schemes for feature extraction. Next, a section is presented on the recent advancements in nonlinear guided wave for SHM. This is followed by section on Rayleigh and SH waves. Next is a section on real-life implementation of guided wave for industrial problems. The paper, though briefly talks about the early development for completeness,. is primarily focussed on the recent progress made in the last decade. The paper ends by discussing and highlighting the future directions and open areas of research in guided wave based SHM.

https://iopscience.iop.org/article/10.1088/0964-1726/25/5/053001/pdf

Guided wave based structural health monitoring: A review

Structural Health Monitoring (SHM) can be understood as the integration of sensing and intelligence to enable the structure loading and damage-provoking conditions to be recorded, analyzed, localized, and predicted in such a way that nondestructive testing becomes an integral part of them. In addition, SHM systems can include actuation devices to take proper reaction or correction actions. SHM sensing requirements are very well suited for the application of optical fiber sensors (OFS), in particular, to provide integrated, quasi-distributed or fully distributed technologies. In this tutorial, after a brief introduction of the basic SHM concepts, the main fiber optic techniques available for this application are reviewed, emphasizing the four most successful ones. Then, several examples of the use of OFS in real structures are also addressed, including those from the renewable energy, transportation, civil engineering and the oil and gas industry sectors. Finally, the most relevant current technical challenges and the key sector markets are identified. This paper provides a tutorial introduction, a comprehensive background on this subject and also a forecast of the future of OFS for SHM. In addition, some of the challenges to be faced in the near future are addressed.

Fiber Optic Sensors in Structural Health Monitoring

As the demand for wind energy continues to grow at exponential rates, reducing operation and maintenance (OM) costs and improving reliability have become top priorities in wind turbine (WT) maintenance strategies. In addition to the development of more highly evolved WT designs intended to improve availability, the application of reliable and cost-effective condition-monitoring (CM) techniques offers an efficient approach to achieve this goal. This paper provides a general review and classification of wind turbine condition monitoring (WTCM) methods and techniques with a focus on trends and future challenges. After highlighting the relevant CM, diagnosis, and maintenance analysis, this work outlines the relationship between these concepts and related theories, and examines new trends and future challenges in the WTCM industry. Interesting insights from this research are used to point out strengths and weaknesses in today’s WTCM industry and define research priorities needed for the industry to meet the challenges in wind industry technological evolution and market growth.

/pdf/wind-turbine-condition-monitoring-state-of-the-art-review-29t23u97yb.pdf

Wind Turbine Condition Monitoring: State-of-the-Art Review, New Trends, and Future Challenges

Deployment of larger scale wind turbine systems, particularly offshore, requires more organized operation and maintenance strategies to ensure systems are safe, profitable and cost-effective. Among existing maintenance strategies, reliability centred maintenance is regarded as best for offshore wind turbines, delivering corrective and proactive (i.e. preventive and predictive) maintenance techniques enabling wind turbines to achieve high availability and low cost of energy. Reliability centred maintenance analysis may demonstrate that an accurate and reliable condition monitoring system is one method to increase availability and decrease the cost of energy from wind. In recent years, efforts have been made to develop efficient and cost-effective condition monitoring techniques for wind turbines. A number of commercial wind turbine monitoring systems are available in the market, most based on existing techniques from other rotating machine industries. Other wind turbine condition monitoring reviews have been published but have not addressed the technical and commercial challenges, in particular, reliability and value for money. The purpose of this paper is to fill this gap and present the wind industry with a detailed analysis of the current practical challenges with existing wind turbine condition monitoring technology.

/pdf/wind-turbine-condition-monitoring-technical-and-commercial-1nbg2ityh7.pdf

Wind turbine condition monitoring: technical and commercial challenges

Nowadays, smart composite materials embed miniaturized sensors for structural health monitoring (SHM) in order to mitigate the risk of failure due to an overload or to unwanted inhomogeneity resulting from the fabrication process. Optical fiber sensors, and more particularly fiber Bragg grating (FBG) sensors, outperform traditional sensor technologies, as they are lightweight, small in size and offer convenient multiplexing capabilities with remote operation. They have thus been extensively associated to composite materials to study their behavior for further SHM purposes. This paper reviews the main challenges arising from the use of FBGs in composite materials. The focus will be made on issues related to temperature-strain discrimination, demodulation of the amplitude spectrum during and after the curing process as well as connection between the embedded optical fibers and the surroundings. The main strategies developed in each of these three topics will be summarized and compared, demonstrating the large progress that has been made in this field in the past few years.

/pdf/fiber-bragg-grating-sensors-toward-structural-health-18jb9qghvo.pdf

Fiber Bragg grating sensors toward structural health monitoring in composite materials: challenges and solutions.

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.

https://iopscience.iop.org/article/10.1088/0957-0233/19/12/122001/pdf

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

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.

/pdf/cure-monitoring-of-composite-laminates-using-fiber-optic-3f6ayq85ax.pdf

Cure monitoring of composite laminates using fiber optic sensors

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.

https://iopscience.iop.org/article/10.1088/0957-0233/13/8/305/pdf

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

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.

/pdf/stabilized-interrogation-and-multiplexing-techniques-for-4z9kohezdl.pdf

Stabilized interrogation and multiplexing techniques for fibre Bragg grating vibration sensors

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.

Hyung-Joon Bang

Papers

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

Cure monitoring of composite laminates using fiber optic sensors

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

Stabilized interrogation and multiplexing techniques for fibre Bragg grating vibration sensors

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