The strength of concrete near the surface would be greatly reduced subjected to high temperature, usually accompanied by surface cracking and spalling. In this study, two concrete slabs heated to 600°C and 800°C on one surface were tested using a small hammer and a displacement receiver placed on the two ends of a test line. The dispersion curve was obtained by performing Short-Time Fourier Transform and amplitude reassignment technique on a received displacement waveform. Combining 10 dispersion curves obtained from multiple grid lines, a three-dimensional surface wave velocity contour map is constructed. Concrete cores were taken after NDT testing and compared with the contour map of the location where the core was taken. The test results show that both the concrete strength and the crack depth affect the surface wave velocity. This technique allows rapid assessment of the cross-sectional wave velocity distribution of a surface, even on rough, spalled surfaces.
Thermal infrared images have been widely employed to detect defections. However, it is challenging to identify the defects from thermal infrared images covered with shadows and noise. For a series of thermal infrared images, principal component analysis (PCA) is often applied to transform the given series into a lower-dimensional linear subspace. In the lower-dimensional subspace, a low-rank matrix is generated to serve as an optimal estimation from the series of thermal images. However, the information stored in those thermal infrared images will be kept mostly during the transformation. Full recovery of the lost information is not possible. A template containing the major information from the given series can be extracted by employing PCA. Furthermore, PCA has difficulty finding the template should interferences occur while the thermal images are captured. The unnecessary information collected associated with the interferences causes some unfavorable characteristics of the template extracted by PCA. Robust PCA (RPCA) is less susceptible to the abovementioned constraints. In this study, RPCA is employed to extract a template from a series of thermal infrared images. Local binary functions are built to restore the image free of noise by keeping the local boundaries. The defects can be readily identified from the regional boundaries. The proposed approach combining RPCA and local binary functions to analyze the given images in conjunction with level set functions. The processed results demonstrate that the proposed scheme are more effective than PCA in analyzing a series of thermal infrared images containing interferences.
The present study is based on performing 3D vibrations studies of large structures using digital image correlation (DIC) technique. A 3D-DIC setup is developed for measuring in-plane and out-of-plane 3D displacements as well as natural frequencies of a vibrating structures (small to large scale). The DIC setup is first validated with laboratory experiments. The first indoor lab experiment is based on the application of the developed DIC setup in measuring pure in-plane and out-of-plane displacements. The in-plane and out-of-plane translations have been carried using a micrometer and a Vernier caliper, respectively. The developed DIC setup is able to accurately measure these in-plane and out-of-plane static displacements. With this success, in another indoor experiment, the developed DIC setup is used for performing vibration study of a PVC pipe (length 1m, inner diameter 56.6mm, outer diameter 60.6mm). The DIC setup is able to measure the dynamic displacements of the pipe in all three axes. Apart from this, the natural frequency measurement is also accurate. The first fundamental frequency of the pipe is 4.167 Hz. After successful validation and application in the indoor experiments, the developed DIC method is used for a field experiment. In the field experiment, the DIC technique is applied to perform vibration study of a light pole (length 3m, diameter 75.5mm, made of iron). The cameras are placed at a distance of 14.6m from the pole, whereas the distance between the cameras is 8.5m. A large size calibration board is fabricated and used for calibrating the stereovision system. The developed DIC method is able to produce 3D displacements of the vibrating pole as well as accurately measure its natural frequency. The fundamental natural frequency of the light pole is 4.9Hz. An accelerometer was also used during the field experiment for the validation of the developed DIC-based non-contact 3D vibration testing method.
Cracks do exist in a concrete building, and the reasons for cracks can be attributed to grouting, force actions, and concrete properties. Detecting existing cracks and monitoring their growth conditions is an important issue in structure health monitoring. This study proposed an integrated approach to fuse the thermal infrared images, high-resolution image, and acoustic tracking. Thermal infrared images can be used to record the surface temperatures. Those defect areas usually have their surface temperatures different from their neighborhoods. Those surface temperature differences can be an important clue to identify the defects. From the high-resolution images, cracks do occur at those discontinuities, and those discontinuities can be treated as the boundaries of the segmented regions. In this study, an approach based on considering the distributions of the segmented regions to segment the given high-resolution images to locate the boundaries of the segmented regions. Then, the defect map is generated by overlaying the thermal infrared image on the segmented regions such that the map can reveal the defect locations, and the surface temperatures can be another evidence to show the defects. Acoustic tracking is introduced to verify the results. All the defect information is stored in a 3D model such that a defect model can be established.
Wind and solar power generation have nearly become synonymous with green energy in recent years. Vibration analysis is crucial to the structural integrity of wind turbines, especially for those exposed to tropical storms and salt corrosion in the coastal environment. In this study, a one-dimensional linear continuous model is applied to analyze the spectral characteristics of the supporting tower of wind turbines. Simulated frequency-domain displacements were obtained for the vibration of the supporting tower subjected to the reduction in local stiffness, representing various cases of tower defects. To identify the defected section of the supporting tower, the unit angle of each section is used as the indicator of the defect that can be obtained using a signal processing technique developed in this study. The results of the numerical analysis show that the method can effectively expose the defect location Furthermore, the progression of the existing defect and the difference between the existing defect and the new one can also be observed. The results of this study demonstrate a great potential for future applications. In practice, the possible variation of the tower structure can be obtained by comparing the measured signal results at different periods. This would allow for maintenance to target precisely at the defected areas so as to achieve efficient allocation of maintenance funds.
Infrared thermography (IRT) is a matured tool, and it can be employed to monitor the health conditions of structures by measuring surface temperature information in real time and in a non-contact way. The surface temperature information provides an important clue to identifying the defects on the building exterior surfaces. According to the surface temperature measurements, for those parts covered by shadows, the surface temperature information is smaller than it is supposed to be. Similarly, glare effects in IRT can be defined as the excessive and uncontrolled brightness illustrated in IRT such that the surface temperature information is larger than it is supposed to be. In general, the shadow and glare effects are often introduced in the thermal images obtained using the passive IRT when the solar energy is the main heat source. The current study proposes an image model in a multiplicative way to evaluate the shadow and glare effects presented in IRT. The experimental results demonstrate that the proposed image model does efficiently remove the shadow or glare effects. A calibrated thermograph can be generated by introducing proper level set functions in the numerical model.
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.
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.
Effects of foundation stiffness on the linear vibrations of wind turbine systems are of concerns for both planning and construction of wind turbine systems. Current study performed numerical modeling for such a problem using linear spectral finite elements. The effects of foundation stiffness were investigated for various combinations of shear wave velocity of soil, size of tower base plate, and pile length. Multiple piles are also included in the models such that the foundation stiffness can be analyzed more realistically. The results indicate that the shear wave velocity of soil and the size of tower base plate have notable effects on the dominant frequency of the turbine-tower system. The larger the lateral dimension, the stiffer the foundation. Large pile cap and multiple spaced piles result in higher stiffness than small pile cap and a mono-pile. The lateral stiffness of a mono-pile mainly depends on the shear wave velocity of soil with the exception for a very short pile that the end constraints may affect the lateral vibration of the superstructure. Effective pile length may be determined by comparing the simulation results of the frictional pile to those of the end-bearing pile.
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.
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.
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.
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.
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.
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