Key Engineering Materials Vols. 413-414

Abstract: Many signals of wind turbine faults are non-stationary and have highly complex time-frequency characteristics. Traditional time-frequency analysis method, such as Windowed Fourier Transform method, has no noticeable effect in handing non-stationary signals. Hilbert-Huang Transform (HHT) is a new signal processing method for analyzing the non-stationary mechanical signals. Based on Empirical Mode Decomposition (EMD), the Intrinsic Mode Function (IMF) in HHT can reflect the intrinsic physical characteristics of original data. Moreover, it is a good way to identify the faults involving a breakdown change. First, the principles and advantages of the HHT are presented in detail in this paper. Then, three typical faults of wind turbine rotor, such as rotor imbalance, aerodynamic asymmetry due to blade surface roughness and yaw misalignment are discussed by the HHT. Last, reasonable conclusions are drawn by the comparison between this method and the Wavelet Transform (WT) method with the help of simulation fault signals. The results show the effectiveness of HHT method for diagnosing those faults of wind turbine rotor.

Abstract: Ensemble Empirical Mode Decomposition (EEMD) is a new signal processing technique aimed at solving the problem of mode mixing present in the original Empirical Mode Decomposition (EMD) algorithm. This paper investigates its utility for machine health monitoring and defect diagnosis. The mechanism of EEMD is first introduced. Parameters that affect effectiveness of the EEMD are then discussed with the assistance of a simulated signal in which the mode mixing exists. Experimental study on bearing vibration signal analysis verified its effectiveness of EEMD for machine health monitoring and defect diagnosis.

Abstract: In many cases, it is impractical to measure the vibrations directly at or close to their source. It is a common practice to measure the vibration at a location far from the source for condition monitoring purposes. The vibration measured in this way inevitably has high distortions from the vibrations due to the effect of the attenuation of signal paths and the interference from other sources. The suppression of the distortions is a key issue for the remote measurements based condition monitoring. In this paper, the influences of transducer locations are investigated on a typical gearbox transmission system for the detection of the faults induced to the gearbox. Several signal processing techniques’ analysis results show that the attenuation and interference cause high influences on the gear transmission signals. However, time synchronous average (TSA) is very effective to detect the local faults induced to the gear system.

Abstract: In this paper, a pendulum system is constructed to perform the fundamental research which is concentrated on the various frequency patterns of impact-object simulation tests. We confirmed the application possibility for the method of discriminable pattern recognition whether impact-object is human-like or not via frequency analysis using smart sensors. Therefore a specific experimental data are compared and analyzed with the obtained database within a short time period. The impact analysis system, which is based on the above processing, can be developed and then used to recognize the impact information measured by smart sensors.

Abstract: A stiffness reconstruction method is tested when rotational degrees of freedom are added to the dynamic model of the structure. The inverse problem is formulated as a minimization problem in terms of harmonic vibrations of the structure and its finite element model. An example of frame structure is analyzed by numerical simulations. The results of these numerical analyses show that the damage detection appeared to be much more effective when the angular amplitudes of harmonic vibrations are acquired. This makes very good prospects for the future applications of angular sensors in damage detection of structures.

Abstract: In the fault diagnosis of a machine, frequencies of its vibration are important indicators to show conditions of the machine. There are two main categories of methods to estimate frequency. One is based on the fast Fourier transform, and the other is on the signal subspace decomposition. Using FFT directly to estimate frequency may introduce larger estimation error, several approaches are proposed to correct or decrease the error, which comprise phase difference, energy centrobaric, interpolation and search method. The signal subspace decomposition method (SSDM) consists of Pisarenko harmonic decomposition, multiple signal classification. In order to assess the performance of these methods, the Cramer-Rao bound is used to compare with the error variance of difference frequency estimation methods, and simulations are based on Monte Carlo experiments for various record sizes and signal-to-noise ratios (SNR’s). The results show that there is a turning point about 25 dB for FFT based methods, above which FFT based methods are less sensitive to the noise, and SSDM achieves higher precision estimation at higher SNR and for the short time series, but produces poor accuracy at lower SNR’s.

Abstract: This contribution discusses the implementation of reliability-based criteria including target safety level, residual life time, economical constraints and inspected present condition of the structure in order to assess its structural integrity. Thereby global failure is emphasized and aspects such as structural robustness and progressive collapse are implemented. Recent requirements in standards are reviewed and practical recommendations are provided. Case studies regarding re-qualification of existing structures are briefly presented, especially with respect to the aforementioned aspects. The summarized studies deal with a) reanalysis of deteriorated platforms in the North Sea and in West Africa subjected to extreme environmental actions and b) reevaluation of subway structures in Central Europe after damage due to flooding. Conclusions and suggestions for the reassessment of existing structures are finally given.

Abstract: This paper is devoted to studying influences of matrix/particle interface debonding and particulate size in micromechanical predictions of the effective moduli of particulate reinforced polymer composites (PRPC). The PRPC is regarded as a three-phase composite that includes the matrix, particle and interphase. The formulation for the effective moduli of the interphase is derived by the cohesive zone model, and combined with the Mori-Tanaka method, the micromechanical model for the effective moduli of the PRPC is formulated with emphasis on the effects of the matrix/particle interface, particulate size and volume fraction. The numerical example shows that the interface debonding, the particulate size and volume fraction have significant influences on the effective moduli of PRPC. The effective moduli of the PRPC can be used to characterize its damage degree.

Abstract: This case study describes a structural integrity assessment of a 220 kV overhead power line. The line comprises 70 pylons over a distance of approximately 30 km, predominantly in a valley location. The pylons are spaced at intervals of approximately 400 m and each pylon is approximately 32 m in height. The line was originally constructed in the 1950’s, approximately 50 years prior to the requested structural integrity assessment. This paper describes the independent assessment that was carried out. The review established site-specific safety factors at the time of original design and construction; at the time of the review (2007), accounting for the possible presence of the “Thomasstahl” steel; and in the future, at the anticipated end of pylon life (in 2012).

Abstract: Various aspects of the effect of microstructural randomness exhibited by carbon fibre-reinforced cross-ply laminates on the delamination damage mechanism is investigated in this paper. In the first part, the matrix cracks with different spacings measured in experiments are simulated using finite elements in order to obtain the levels of degradation and effective properties for a composite beam loaded in bending. The results show significant levels of degradation of obtained effective properties depicting the importance of accounting for the inherent stochasticity in these laminates. In the second part of the paper, initiation of delamination at an interface between 0° and 90° layers due to stress concentrations at tips matrix cracks is simulated for a beam under tension. Stochastic cohesive zone elements with fracture parameters presented as random fields are used to model this interface in a composite. Different values of the axial stress are obtained for initiation of damage for a number of realisations based on this approach. The results emphasize the need to take into consideration the microstructural randomness in fibre-reinforced laminates for adequate predictions of damage and load carrying capacities.