Keng-Tsang Hsu

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.

EVALUATING BOND QUALITY AT STEEL/CONCRETE INTERFACES USING THE NORMALIZED IMPACT-ECHO SPECTRUM

The objective of this paper is to develop a nondestructive test technique based on stress wave propagation for evaluating the bond quality of concrete structures strengthened by externally bonded steel plates. In this paper, a newly developed technique based on the normalized impact-echo spectrum was used to quantitatively detect the size of air voids at the interfacial epoxy layer between steel plate and concrete. Experimental results show that in the normalized impact-echo spectra, the amplitude of the peak at the frequency corresponding to the wave reflections at the steel/epoxy interface increases with an increase in the air void size. The bond strength decreases with an increase in the peak amplitude due to the increasing size of the flaw.

Dynamic survey of wind turbine vibrations

Six wind turbines were blown to the ground by the wind gust during the attack of Typhoon Soudelor in August 2015. Survey using unmanned aerial vehicle, UAV, found the collapsed wind turbines had been broken at the lower section of the supporting towers. The dynamic behavior of wind turbine systems is thus in need of attention. The vibration of rotor blades and supporting towers of two wind turbine systems have been measured remotely using IBIS, a microwave interferometer. However the frequency of the rotor blade can be analyzed only if the microwave measurements are taken as the wind turbine is parked and secured. Time-frequency analyses such as continuous wavelet transform and reassigned spectrograms are applied to the displacement signals obtained. A frequency of 0.44Hz exists in both turbines B and C at various operating conditions. Possible links between dynamic characteristics and structural integrity of wind turbine –tower systems is discussed.

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.

Damage Assessment of a Prestressed Simply Supported Bridge Using Microwave Interferometer

The microwave interferometer was used for monitoring the behavior of a simply supported bridge under various loading cases. The instrument has a 1-D imaging capability for remotely measuring the displacements of multiple positions simultaneously. The loading cases include truck placed at the middle of the span, driven with variant speeds to generate ambient vibration of the span, driven through with a constant speed, and abruptly stopped at the middle to obtain the damping characteristics of the span. As the bridge has been in operation for 50 years, weak bearing stiffness for the support at abutment and possible fatigue fracture of a roller support above a pier was found from this investigation.

Evaluation of the Dynamic Characteristics of an Extradosed Bridge Using Microwave Interferometer

The microwave interferometer used in the present study is capable of multiple-point displacement measurements. In this paper, a general review of the principle and capacity of such an instrument is given. The vertical displacements of the Ai-Lan bridge corresponding to ambient vibration due to traffic loads are remotely monitored . Several modal shapes of the whole bridge are precisely identified by linking the mode shapes of seven sequential measurements.

Proper source-receiver distance to obtain surface wave group velocity profile for flaw detection inside a concrete plate-like structure

A technique leads to rapid flaw detection for concrete plate-like structure is realized by obtaining the group velocity dispersion profile of the fundamental antisymmetric mode of the plate (A0 mode). The depth of a delaminating crack, honeycomb or depth of weak surface layer on top of the sound concrete can all be evaluated by the change of velocity in the dispersion profile of A0 mode at the wavelength about twice of the depth. The testing method involves obtaining the A0 group slowness spectrogram produced by single test with one receiver placed away from the source of impact. The image of the spectrogram is obtained by Short-Time Fourier Transfer (STFT) and enhanced by reassigned method. The choice of window length in STFT and the ratio between impactor-receiver distance and plate thickness, d/T, is essential as the dominant surface wave response may simply a non-dispersive Rayleigh wave or following the A0 or S0 (fundamental symmetric mode) modal dispersion curve. In this study, the axisymmetric finite element model of a plate subject to transient load was constructed. The nodal vertical velocity waveforms for various distances were analyzed using various STFT window lengths. The results show, for certain d/T ratio, S0 mode would be dominant when longer window is used. The best window lengths for a d/T ratio as well as the corresponding largest wavelength which follows the A0 theoretical dispersion curve or Rayleigh wave were summarized. The information allows people to determine the proper impactor-receiver distance and analyzing window to successfully detect the depth of flaws inside a plate.A technique leads to rapid flaw detection for concrete plate-like structure is realized by obtaining the group velocity dispersion profile of the fundamental antisymmetric mode of the plate (A0 mode). The depth of a delaminating crack, honeycomb or depth of weak surface layer on top of the sound concrete can all be evaluated by the change of velocity in the dispersion profile of A0 mode at the wavelength about twice of the depth. The testing method involves obtaining the A0 group slowness spectrogram produced by single test with one receiver placed away from the source of impact. The image of the spectrogram is obtained by Short-Time Fourier Transfer (STFT) and enhanced by reassigned method. The choice of window length in STFT and the ratio between impactor-receiver distance and plate thickness, d/T, is essential as the dominant surface wave response may simply a non-dispersive Rayleigh wave or following the A0 or S0 (fundamental symmetric mode) modal dispersion curve. In this study, the axisymmetric fini...

Combining the 3D model generated from point clouds and thermography to identify the defects presented on the facades of a building

Defects presented on the facades of a building do have profound impacts on extending the life cycle of the building. How to identify the defects is a crucial issue; destructive and non-destructive methods are usually employed to identify the defects presented on a building. Destructive methods always cause the permanent damages for the examined objects; on the other hand, non-destructive testing (NDT) methods have been widely applied to detect those defects presented on exterior layers of a building. However, NDT methods cannot provide efficient and reliable information for identifying the defects because of the huge examination areas. Infrared thermography is often applied to quantitative energy performance measurements for building envelopes. Defects on the exterior layer of buildings may be caused by several factors: ventilation losses, conduction losses, thermal bridging, defective services, moisture condensation, moisture ingress, and structure defects. Analyzing the collected thermal images can be quite difficult when the spatial variations of surface temperature are small. In this paper the authors employ image segmentation to cluster those pixels with similar surface temperatures such that the processed thermal images can be composed of limited groups. The surface temperature distribution in each segmented group is homogenous. In doing so, the regional boundaries of the segmented regions can be identified and extracted. A terrestrial laser scanner (TLS) is widely used to collect the point clouds of a building, and those point clouds are applied to reconstruct the 3D model of the building. A mapping model is constructed such that the segmented thermal images can be projected onto the 2D image of the specified 3D building. In this paper, the administrative building in Chaoyang University campus is used as an example. The experimental results not only provide the defect information but also offer their corresponding spatial locations in the 3D model.

The effect of reinforcing bars to flaw detection in RC structure using group velocity profile generated by surface waves

A new flaw detection method for concrete plate-like structure is realized using the dispersion profile of the group velocity of surface waves obtained by a sensor with proper distance from the transient impacting load. The waveform obtained by the sensor is analyzed using STFT and reassigned method to obtain a group velocity spectrogram. The delaminating crack or honeycomb which locates underneath the test line between the impactor and the receiver as well as the low-density layer on top of sound concrete are proved to be detectable in both numerical and experimental studies. The velocity turning point in the wavelength-velocity profile is about 1.6 to 2.2 times of the depths of the flaws or the low-density layer wavelength. As the proposed method is easy to operate, inexpensive and effective on solving many problems of concrete deterioration, one essential question to be concerned is the effect of dense reinforcing rebar to the stress wave propagation. In this preliminary study, the theoretical modal dispersion curves for a plain concrete plate and a concrete plate containing a thin steel layer are compared. A 2D numerical model with concrete and steel layers was constructed. The images of slowness spectrograms obtained by placing impactor and receiver at variant distances are compared with theoretical modal dispersion curve. Experiments are performed on a heavy lattice arranged bridge pier. The results show that the response of the rebar layers is near 0.3 ms/m in slowness spectrogram instead of around 0.5 ms/m plain concrete. The steel rebar layer affects the results more severely when the test line is parallel to the direction of shallower rebars. For more clearly observing the condition of concrete, one can filter the response in the waveform with the time less than 0.4 ms/m multiplying the impactor-receiver distance.

Remote Measurements and Vibration Analyses of Existing Wind Turbines

The service life of a wind turbine may be threatened by the collapse of the rotor blades or the supporting tower due to extreme weather. The change of dominant frequency may indicate that the stiffness of the turbine-tower system is deviated from its design. Dynamic measurements of lateral displacement of four 2-MW wind turbines of the same model have been achieved using a ground-based microwave interferometer. The frequency contents of a turbine-tower system can be measured when the rotor is stopped or operates at low to medium speed. The rotor blade exhibits multiple frequencies higher than 1 Hz that can only be measured when the wind turbine is shut down. Possible links between dynamic characteristics and structural integrity of wind turbine—tower systems are discussed. The results of current research are promising in terms of combining non-contact dynamic measurements and vibration analyses towards the condition assessment of wind turbine-tower systems.