Vassilios Kappatos

An experimental study on the applicability of acoustic emission for wind turbine gearbox health diagnosis

Condition monitoring of wind turbine gearboxes has mainly relied upon vibration, oil analysis and temperature monitoring. However, these techniques are not well suited for detecting early stage damage. Acoustic emission is gaining ground as a complementary condition monitoring technique as it offers earlier fault detection capability compared with other more established techniques. The objective of early fault detection in wind turbine gearboxes is to avoid unexpected catastrophic breakdowns, thereby reducing maintenance costs and increase safety. The aim of this investigation is to present an experimental study the impact of operational conditions (load and torque) in the acoustic emission activity generated within the wind turbine gearbox. The acoustic emission signature for a healthy wind turbine gearbox was obtained as a function of torque and power output, for the full range of operational conditions. Envelope analysis was applied to the acoustic emission signals to investigate repetitive patterns and correlate them with specific gearbox components. The analysis methodology presented herewith can be used for the reliable assessment of wind turbine gearbox subcomponents using acoustic emission.

Increased range of ultrasonic guided wave testing of overhead transmission line cables using dispersion compensation

Overhead Transmission Line (OVTL) cables can experience structural defects and are, therefore, inspected using Non-Destructive Testing (NDT) techniques. Ultrasonic Guided Waves (UGW) is one NDT technique that has been investigated for inspection of these cables. For practical use, it is desirable to be able to inspect as long a section of cable as possible from a single location. This paper investigates increasing the UGW inspection range on Aluminium Conductor Steel Reinforced (ACSR) cables by compensating for dispersion using dispersion curve data. For ACSR cables, it was considered to be difficult to obtain accurate dispersion curves using modelling due to the complex geometry and unknown coupling between wire strands. Group velocity dispersion curves were, therefore, measured experimentally on an untensioned, 26.5m long cable and a method of calculating theoretical dispersion curves was obtained. Attenuation and dispersion compensation were then performed for a broadband Maximum Length Sequence (MLS) excitation signal. An increase in the Signal to Noise Ratio (SNR) of about 4-8dB compared to that of the dispersed signal was obtained. However, the main benefit was the increased ability to resolve the individual echoes from the end of the cable and an introduced defect in the form of a cut, which was 7 to at least 13dB greater than that of the dispersed signal. Five echoes were able to be clearly detected using MLS excitation signal, indicating the potential for an inspection range of up to 130m in each direction. To the best of the authors knowledge, this is the longest inspection range for ACSR cables reported in the literature, where typically cables, which were only one or two meter long, have been investigated previously. Narrow band tone burst and Hann windowed tone burst excitation signal also showed increased SNR and ability to resolve closely spaced echoes.

Tensile properties of corroded embedded steel bars B500c in concrete

Purpose – The corrosion of reinforcing steel bars reduces significantly the life and durability of concrete structures. This critical concern causes great losses to the economy and industry. The purpose of this paper is to estimate the effects of corrosion on the tensile mechanical properties of embedded steel bars B500c in concrete.Design/methodology/approach – The concept is based on the curve fitting modelling, as well the mathematical correlation of the tensile mechanical properties between corroded bare and corroded embedded steel bars. In order to achieve this, extensive experiments were carried out on both bare (O8, 10, 12, 16 and 18 mm) and embedded (O8 mm) steel bars B500c, which were subjected to artificially accelerated corrosive conditions in a chloride‐rich atmosphere for several exposure times.Findings – The research results show that the estimation method is available and effective in simulating the tensile mechanical behaviour of corroded reinforcing steel bars B500c.Originality/value – As...

High Temperature Shear Horizontal Electromagnetic Acoustic Transducer for Guided Wave Inspection.

Guided Wave Testing (GWT) using novel Electromagnetic Acoustic Transducers (EMATs) is proposed for the inspection of large structures operating at high temperatures. To date, high temperature EMATs have been developed only for thickness measurements and they are not suitable for GWT. A pair of water-cooled EMATs capable of exciting and receiving Shear Horizontal (SH0) waves for GWT with optimal high temperature properties (up to 500 °C) has been developed. Thermal and Computational Fluid Dynamic (CFD) simulations of the EMAT design have been performed and experimentally validated. The optimal thermal EMAT design, material selection and operating conditions were calculated. The EMAT was successfully tested regarding its thermal and GWT performance from ambient temperature to 500 °C.

Coded Waveform Excitation for High-Resolution Ultrasonic Guided Wave Response

Ultrasonic guided wave-based nondestructive testing systems are widely used in various fields of industry where the structural integrity of components is of vital importance. Signal interpretation in these systems might become challenging due to multimodal and dispersive response of the interrogated structure. These phenomena degrade the signal-to-noise ratio and also lower the spatial/temporal resolution. This paper compares the use of Maximal Length Sequences and linear chirp excitation signals to develop a novel signal processing technique using dispersion compensation and cross-correlation. The technique is applied to both simulated and experimental multimodal signals from an aluminium rod for performance assessment. It is quantitatively validated that the technique noticeably improves the signal-to-noise ratio of the guided wave response and is able to acquire an accurate time of flight of the individual wave modes, and hence, the propagation distance. The technique is compared for both linear chirp and maximal length sequences excitation signals. Noise analysis for these excitation signals is also presented.

Crack detection in noisy environment including raining conditions

Purpose – In outside constructions (e.g. aircraft frames, bridges, tanks and ships) real‐life noises reduce significantly the capability of location and characterization of crack events. Among the most important types of noise is the rain, producing a signal similar to crack. This paper seeks to present a robust crack detection system with simultaneous raining conditions and additive white‐Gaussian noise at −20 to 20 dB signal‐to‐noise ratio (SNR).Design/methodology/approach – The proposed crack detection system consists of two sequentially, connected modules: the feature extraction module where 15 robust features are derived from the signal and a radial basis function neural network is built up in the pattern classification module to extract the crack events.Findings – The evaluation process is carried out in a database consisting of over 4,000 simulated cracks and drops signals. The analysis showed that the detection accuracy using the most robust 15 features ranges from 77.7 to 93 percent in noise‐free...

Airfoil morphing based on SMA actuation technology

Purpose – This study aims to develop an innovative actuator for improving the performance of future aircraft, by adapting the airfoil shape according to the flight conditions. The flap’s camber of a civil regional transportation aircraft’s trailing edge actuated and morphed with the use of shape memory alloys (SMA) actuator technology, instead of the conventional split flap mechanism is studied. Design/methodology/approach – For the flap’s members sizing an efficient methodology is utilised based on finite element (FE) stress analysis combined to analytically formulated design criteria. A mechanical simulation within an FE approach simulated the performance of the moving rib, integrating both aerodynamic loads and SMA phenomenology, implementing Lagouda’s constitutive model. Aim of this numerical simulation is to provide guidelines for further development of the flap. A three-dimensional assembly of the flap is constructed to produce manufacturing drawing and to ensure that during its morphing no interfer...

Feature Selection for Robust Classification of Crack and Drop Signals

This paper presents a study to the problem of features selection for accurate recognition of crack signals in raining conditions, using a multilayer perceptron and a radial basis function neural network. The features extraction process is accomplished for two time frames: in the first time frame the presence of the signal reflection is minimal; in the later a wider window include signal reflections at the specimen edges. An extensive set of 90 features (41 of them are novel), 67 in the time domain and 23 in the frequency domain, are extracted from the normalized signals and are sorted according to Fisher ratio (F-ratio). The signals database consists of over than 20,000 simulated cracks and drops signals. The NNs classification accuracy of a single crack signal in rain conditions using the most robust features ranges from 80.08% to 99.08%, which remains in the same levels when white-Gaussian noise up to 0 dB signal-to-noise ratio is added to the signal.

Pulse-compression based iterative time-of-flight extraction of dispersed Ultrasonic Guided Waves

Ultrasonic Guided Wave (UGW) based NonDestructive Testing (NDT) systems are widely used in numerous branches of industry, where the structural integrity of components carries vital importance. In those systems, signal interpretations might become challenging due to multi-modal and dispersive response of the structure under examination. This results in degradation of the signals in terms of Signal-to-Noise Ratio (SNR) and spatial/temporal resolution. This paper uses Maximal Length Sequences (MLS) to develop a novel signal processing technique by employing the Short-Time Fourier Transform (STFT), dispersion compensation and cross-correlation. The technique is applied to experimental multi-modal signals from an aluminum rod for performance verification. It is quantitatively validated that the technique noticeably improves the SNR of the guided wave response, and is able to derive an accurate time of flight of the individual wave modes and thus the propagation distance.

Developing a Novel Ice Protection System for Wind Turbine Blades Using Vibrations of Both Short and Long Wavelengths

Icing conditions in cold regions of the world may cause problems for wind turbine operations, since accreted ice can reduce the efficiency of power generation and create concerns regarding ice-shedding. This paper covers modelling studies and some experimental development for an ongoing ice protection system that provides both deicing and anti-icing actions for wind turbine blades. The modelling process contained two main sections. The first part involved simulation of vibrations with very short wavelength or ultrasonic guided waves (UGW) on the blade to determine optimal excitation frequency and transducer configuration. This excitation creates horizontal shear stress at the interface between ice and blade and focuses energy at the leading edge for de-bonding ice layers. The second modelling approach simulated the effects of vibrations with very long wavelength along with estimation of fatigue life due to harmonic forces to characterise the best parameters for shaker(s) mounted on blades. In parallel with this study, an empirical array of novel resonating shear transducers has been developed using a Design of Experiments (DoE) approach to demonstrate the practicability of inducing shear horizontal waves at the leading edge of wind turbine blades. This experimental verification also makes it possible to investigate the many parameters influencing ice-removal. In addition, piezo-electric and macro-fibre composite actuators have been investigated in place of conventional electromagnetic shakers, in order to save weight and simplify integration of the deicing system components. The ongoing research is intended to provide an active solution for icing prevention and deicing, enabling safe and reliable operation of wind turbines in adverse weather conditions.