Chih-Peng Yu

Remote monitoring and nondestructive evaluation of wind turbine towers

Wind turbine towers are in need of condition monitoring so as to lower the cost of unexpected maintenance. Wind loading from turbulence and gusts can cause damage in horizontal axis wind turbines even the supporting towers. Monitoring of wind turbines in service using embedded data sensor arrays usually is not targeted at the turbine-tower interaction from the perspective of structural dynamics. In this study the remote monitoring of the tower supporting a horizontal-axis wind turbine was attempted using a microwave interferometer. The dominant frequency of one tower was found to be decreased by more than 20% in 16 months. Numerical modeling using spectral finite elements is in progress and should provide further information regarding frequency shift due to stiffness variation and added mass. Expected outcome will contribute to remote monitoring procedures and nondestructive evaluation techniques for local wind turbine structures during operation.

Application Of Closed-Form Solution For Normal Surface Displacements On Impacted Half Space : Quantification Of Impact-Echo Signals

In order to investigate the feasibility of deducing a simulated transfer function based on the Rayleigh wave form in an Impact-Echo signal, the analytical solution for the normal sur- face displacement due to a heaviside force at the half-space was reviewed and used to compute the surface displacement responses resulted from various types of impulse forces. Based on a series of numerical studies on the characteristics of Rayleigh wave form in the surface displace- ment responses, this paper presents the idea of using an equivalent impact force to derive an in- tentionally scaled transfer function. The pseudo force can be obtained from using Rayleigh wave form as a pseudo force or by generating an equivalent half-sine impact force accordingly. The effect of using such pseudo and equivalent force functions was discussed in details. In the pro- posed method, the force amplitude was first estimated from an amplitude curve established from numerical simulations using half-sine force functions. The recovery of a simulated transfer func- tion was next achieved via the use of an estimated force amplitude and a selected force function. The proposed procedure also results in steady thickness amplitudes when measurements on two concrete plates were taken for various impacts associated with different steel balls and different impact locations. The success in recovering constant thickness amplitudes for plate-like struc- tural members proved that the derivation of simulated transfer function is a useful tool in ex- tending the Impact-Echo test. The quantitative evaluation of the interfacial property of the sub- strate layer will also benefit from this simulated transfer function.

Evaluation of bridge span by recovered stiffness data obtained with moving vehicle loadings

The stiffness of a bridge span is evaluated by the dynamic displacement response corresponding to a three-axial vehicle load moving with constant speed. The dynamic displacement influence line obtained from the dynamic displacement time history was filtered by window smoothing and empirical modal decomposition (EMD) methods to acquire the quasi-static displacement influence line. The beam stiffness was obtained by dividing the moment diagram corresponding to a concentrated load applying on the measuring position with the curvature of the quasi-influence line. The effects of three-axial moving load, moving speeds, and measuring positions on the stiffness estimation are explored. The results show the window smoothing method is a better technique to obtain the quasi-static influence line. The only discrepancies in curvature for single and three-axial load cases are near both ends of the beam. A larger range of correct stiffness can be recovered for load moving with lower speed. Similar stiffness diagram can be obtained from the influence lines at different measuring positions.

ASSESSMENT OF LOCAL STIFFNESS FOR SLENDER CONCRETE MEMBERS USING IMPULSE RESPONSE TEST

In a typical dynamic test of structural members, signals within low frequency range can normally reflect the overall stiffness of the member, while they seem not useful in assessing stiffness of a specific region. In this study, a simple back-calculation procedure was developed to potentially assess quasi-static stiffness for a specific portion of a linear member. The purpose of this study is to briefly introduce the theoretical background of such methodology and to verify the feasibility of such procedures with experimental data obtained from tests on concrete specimens. Based on the numerical and experimental data, it was found that the proposed method gives reasonable estimates of stiffness for the test portion of a symmetric reinforced concrete beam. Certain modifications are also required in extending such procedures to members with various boundary conditions.

Time frequency analyses of vibrations of wind turbine towers

Transient vibrations of the tower supporting a horizontal-axis wind turbine were recorded using a microwave interferometer. Variations in dominant frequencies have been reported in the previous study. Signal analyses aiming to uncouple different frequency components were performed using reassigned spectrogram, a time-frequency representation based on time-corrected short time Fourier transform. Optimal resolutions in both time and frequency domains were first investigated using synthetic signals. The goal was to seek out the favorable combinations of window size and overlapping portions of adjacent windows for a data sequence at a given sampling rate. The dominant frequency found in reassigned spectrogram agrees with that obtained using Fourier spectrum of the same transient measurements of the wind turbine tower under investigation.

Dynamic monitoring of stay cables by enhanced cable equations

In a typical vibration test of tensioned cables, tension forces are mostly estimated from theory of a vibrating string with the first natural frequency. To obtain slightly better estimations, formulas based on an axially loaded beam can be employed. However, uncertainty on both flexural rigidity and effective length of the vibrating cable raise difficulty in reliably determining the possible range of the tension value. From the previous work of the authors, an alternative approach for the calculation of tension forces without the need of rigidity data had been proposed, in which frequencies of high modes are instead required in recovering accurate results. This paper extends the previous work to also consider the discrepancy between the design length and effective length so as to further improve the results. Feasibility of the proposed methodology with enhanced equations was verified by actual cable forces measured in an extradosed bridge. Current study aims to apply the proposed approach to the dynamic monitoring of the in-situ stay cables so as to improve the traditional assessment results without increasing the testing costs.

On prediction of stiffness variation of slender members using impact responses

Vibration tests and elastic wave techniques have been applied to assess the integrity of slender members extensively. Impact tests incorporating simple calculation formulae have the ability to serve as front-line methods for quick diagnosis of structures with a large amount of potentially damaged members. This article proposes a methodology involving complex quotient of transfer functions, apparent segmental flexibility, and pseudo-profile of dynamic responses. The theoretical background is reviewed first, followed by a series of numerical simulations of impact tests. The accuracy of defect information extracted from each method is evaluated and compared. Procedures for recovering possible stiffness variations of slender members with these approaches are outlined and illustrated with examples of a simply supported beam. Results of these simulations have demonstrated that the proposed methodology is very promising in obtaining flexural stiffness and defect information on slender members.

A damage assessment model of slender bridge members based on 1D linear member theory with frequency dependent parameters

In this study, a linear model with frequency dependent structural property was used to generate the corresponding frequency response function and dynamic stiffness for selected dynamic problems where certain nonlinearity can be resulted from time/space varying characteristics of the bridge vibrations. Derivation of the proposed formula is based on the vibration theory of the elementary member with frequency dependent elastic properties, in which Modulus of Elasticity can be interpreted as serial and parallel connections of springs and dashpots. This paper first describes the use of the proposed formulation to reasonably depict the nonlinear cable vibration associated with the varying tension forces over time. The proposed formulation can also be used to simulate flexural vibration of damage beams in which the elastic property involves certain space varying or time varying characteristics. Simple numerical/experimental data were next used to demonstrate and confirm the potential application of such simulation idea. Consequently, it is concluded that such assessment model with frequency dependent parameters can be practically feasible and serve as a useful tool in the spectral analysis regarding dynamic problems of slender bridge members.