Jung-Ryul Lee

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.

Identification of the four orthotropic plate stiffnesses using a single open-hole tensile test

The identification of mechanical parameters for real structures is still a challenge. With the improvement of optical full-field measurement techniques, it has become easier, but in spite of many publications showing the feasibility of such methods, experimental results are still scarce. In this paper we present a first step towards a global approach of mechanical identification for composite materials. The chosen mechanical test is an open-hole tensile test according to standard recommendations. For the moment, experimental data are provided by a moiré interferometry setup. The global principle of the identification developed in this paper is to minimize a discrepancy between experimental and theoretical results, expressed as a cost function, using a Levenberg-Marquardt algorithm. This approach has the advantage of having high adaptability, largely because the optical system, the signal processing as well as the mechanical aspects, can be taken into consideration by the model. In this paper we consider different types of cost functions, which are tested using an identifiability criterion. Although mechanically based cost functions have been studied, a simpler mathematical form is finally more efficient. Two different models were tested. The first is an analytical model based on the Lekhnitskii approach. This approach has the advantage of being a meshless solution; however, the results appeared to be partially false due to boundary effects, leading to a second approach, a classical finite element analysis. The resulting identified values are similar to values from classical mechanical tests (within 6%). which, in practice, validates our approach.

In-flight health monitoring of a subscale wing using a fiber Bragg grating sensor system

In this paper, fiber Bragg gratings (FBGs) were applied to measure dynamic strains inside a subscale wing under real-time wind tunnel testing. Two re-coated FBGs were embedded in the wing skin. The FBG sensor system includes a wavelength swept fiber laser with a wavelength indicator and fast signal processing modules. The agreement among the three kinds of sensor inside the subscale wing (FBG, electric strain gauge and PZT sensor) was confirmed in the bench test. The optical fiber strain sensors had an excellent resolution (< 5μe) in the time domain and could detect a frequency response up to 100 Hz. Through the wind tunnel test of the subscale smart wing, the flutter was experimentally detected using FBG sensors and their usefulness as an in-flight health monitoring system was demonstrated.

Health monitoring of complex curved structures using an ultrasonic wavefield propagation imaging system

An ultrasonic wavefield propagation imaging system is introduced and then applied for ultrasonic wavefield imaging of complex curved surfaces. A Q-switched pulsed laser is utilized as a moving ultrasonic generator, and a PZT ultrasonic sensor is fixed during the laser beam scanning and detects the ultrasonic waves propagated from the points excited by the laser beam. The waveforms are allocated in the spatial domain of the scanned points and then manipulated in the form of a time versus wavefield movie. The visualized wavefields enable easy detection and interpretation of structural defects because anomalies during wavefield propagation can be visualized. Furthermore, this ultrasonic wavefield propagation imaging system enables reference-free inspection, complex curved surface scanning because it does not require control of focal length and incidence angle of the laser beam, and excellent adaptability with built-in structural health monitoring sensors, such as piezoelectric and fiber optic sensors. The system is demonstrated in the applications of wavefield visualization on a drill surface, detection of mass loss parts inside an elbow pipe joint, and detection and characterization of impact damage and stringer disbond in a composite skin–stringer structure.

Disbond monitoring at wing stringer tip based on built-in ultrasonic transducers and a pulsed laser

An active ultrasonic diagnostic method for monitoring the occurrence and growth of a disbond at a wing stringer tip was developed and verified using built-in piezoelectric lead-zirconate-titanate transducers. The diagnosis was based on the ultrasonic arrival time delay caused by an increase in the disbond-induced wave path and can provide quantitative information on the disbond length. The change in the ultrasonic amplitude, which is a conventional measure for disbond detection, was also included in the diagnostics. Finally a single transmission and multiple reception method was used to obtain the arrival time and amplitude distributions. Another active ultrasonic diagnostic tool based on a pulsed laser ultrasonic generation was proposed for a nondestructive evaluation to obtain more reliable results on the damage detected using the built-in transducers. The diagnostics were verified using a metal skin-stringer structure as an experimental model, and then applied to monitoring the disbond at a stringer tip in a composite wingbox.

Hot target inspection using a welded fibre acoustic wave piezoelectric sensor and a laser-ultrasonic mirror scanner

The direct attachment of piezoelectric transducers onto hot targets raises formidable challenges as piezoelectric transducers lose their piezoelectric characteristics at elevated temperatures or debond due to thermal expansion coefficient mismatches. We developed a welded fibre acoustic-wave PZT (FAWPZT) sensor to alleviate these temperature limitations. One end of the FAWPZT sensor, made from a stainless steel fibre, was welded onto a stainless steel target plate and the other end was bonded to a PZT sensor. An ultrasonic wave propagation imaging (UWPI) system consists of a laser mirror scanner and a Q-switched pulsed laser (QPL) acting as a non-contact ultrasonic generator was then used to scan a hot target surface with an artificial 2 mm-sized open crack. The result was presented in the form of an ultrasonic wave propagation movie. The damage was detected as a wavefield scattering from the damaged location and its size was evaluated from the plot of amplitude distribution along the propagating wavefront. Sensor performance was briefly discussed and the results confirmed that a FAWPZT sensor combined with a UWPI system has good potential for implementation in hot target integrated structural health management.

Simultaneous multipoint acoustic emission sensing using fibre acoustic wave grating sensors with identical spectrum

This paper introduces the development of a simultaneous multipoint acoustic emission (AE) sensing system using a narrowband tuneable laser with high power and fibre acoustic wave grating sensors (FAWGSs). The demodulation technique is the same as that used in existing methods where the narrowband laser peak is tuned to one mid-reflection point in the main lobe of a fibre Bragg grating (FBG) spectrum. However, the sensor head is changed to an FAWGS for which a FBG is installed in a strain-free configuration so that it can detect AE waves in a structure not directly but in the form of a fibre-guided acoustic wave. Therefore since the structural strain cannot make the Bragg wavelength change, multiple FBGs with identical spectrum can be connected with multiple optical paths realized by equal light intensity dividers. The possible temperature difference between the multiple FAWGSs is passively resolved by using short FBGs which provide a wider operating temperature region. Consequently, we can resolve the problem that the FBG spectrum is easily deviated from the lasing wavelength because of the strain. In addition, the simultaneous multipoint sensing capability based on a single laser improves the cost?performance ratio of the optical system as well as reducing the structural inspection time, and enabling in?situ health monitoring of real structures exposed to large and dynamic strains. The feasibility of the system is demonstrated in typical applications of in?situ structural health monitoring based on AE techniques.

Investigation of a fibre wave piezoelectric transducer

This paper is concerned with the development of an ultrasonic fibre wave piezoelectric transducer for the monitoring of local structural hot spots. A lead-zirconate-titanate (PZT) ceramic with electrodes is used as a wave generator or demodulator, and polyimide coated optical fibres installed on the PZT element function are used as ultrasonic carriers. It is demonstrated that the fibre wave PZT transducer offers major advantages over a conventional PZT transducer. These advantages include remote and multipoint ultrasonic transmission and reception, strong directivity, small size, and flexibility. The fibre wave PZT transducer possesses many advantages of the fibre optic ultrasonic sensors without the necessity of costly optical devices. The strong directivity, high spatial resolution, and good cost–performance ratio make the fibre wave piezoelectric transducer a useful tool for local structural hot spot monitoring.

Fiber optic liquid leak detection technique with an ultrasonic actuator and a fiber Bragg grating

We present a technique for liquid leak detection in which ultrasonic and optical waves are introduced into a fiber simultaneously. The system is based on an ultrasonic technique using an ultrasonic actuator and a fiber Bragg grating receiver. A fiber-guided ultrasonic wave is utilized to stress the fiber Bragg grating, which is remote from the ultrasonic transmitter. When the traveling ultrasonic wave encounters a liquid, part of the wave will leak out from the fiber, which results in an ultrasonic strain decrease in the fiber Bragg grating. The ultrasonic wave and its attenuation are detected by the light variation of a narrowband laser source reflected and transmitted from the fiber Bragg grating, and the amplitude variation of the ultrasound can eventually be correlated with the fiber area coupled with the liquid.

Fatigue crack propagation monitoring of stainless steel using fiber Bragg grating ultrasound sensors

Fatigue crack propagation of stainless steel was monitored using an ultrasonic inspection system consisting of a piezoelectric transmitter and a fiber Bragg grating (FBG) sensor. The fatigue crack developed perpendicularly to the line connecting the transmitter and the sensor, which were attached on the surface of the test specimen. The ultrasonic response signal was recorded at several fatigue crack lengths either in crack opening or in crack closure. In order to examine the influence of ultrasonic waveform on response behavior, the ultrasonic transmitter was driven by several kinds of excitation signal: a spike signal and a sinusoidal toneburst signal, whose frequency ranged from 200 to 800 kHz. The response signal delayed steadily with crack growth after the crack passed through the line connecting the transmitter and the sensor, although the delay in crack closure was smaller than that in crack opening. In crack closure, a significant response delay with crack growth was observed in the latter response to ultrasound excited by a 800 kHz toneburst signal, which was the highest frequency signal applied in the present study. The FBG-based ultrasonic inspection system proved to be effective for monitoring fatigue crack propagation even in crack closure.