Zhu Hong-ping

Detection of structural damage through changes in frequency

Among all the structural vibration characteristics, natural frequencies are relatively simple and accurate to measure, and provide the structural global damage information. In this paper, the feasibility of using only natural frequencies to identify structural damage is exploited by adopting two usual approaches, namely, sensitivity analysis and neural networks. Some aspects of damage detection such as the problem of incomplete modal test data and robustness of detection are considered. A laboratory tested 3-storey frame is used to demonstrate the possibility of frequency-based damage detection techniques. The numerical results show that the damaged element can be correctly localized and the content of damage can be identified with relatively high degree of accuracy by using the changes in frequencies.

Noise analysis for sensitivity-based structural damage detection

As vibration-based structural damage detection methods are easily affected by environmental noise, a new statistic-based noise analysis method is proposed together with the Monte Carlo technique to investigate the influence of experimental noise of modal data on sensitivity-based damage detection methods. Different from the commonly used random perturbation technique, the proposed technique is deduced directly by Moore-Penrose generalized inverse of the sensitivity matrix, which does not only make the analysis process more efficient but also can analyze the influence of noise on both frequencies and mode shapes for three commonly used sensitivity-based damage detection methods in a similar way. A one-story portal frame is adopted to evaluate the efficiency of the proposed noise analysis technique.

An impedance analysis for crack detection in the Timoshenko beam based on the anti-resonance technique

An alternative technique for crack detection in a Timoshenko beam based on the first anti-resonant frequency is presented in this paper. Unlike the natural frequency, the anti-resonant frequency is a local parameter rather than a global parameter of structures, thus the proposed technique can be used to locate the structural defects. An impedance analysis of a cracked beam stimulated by a harmonic force based on the Timoshenko beam formulation is investigated. In order to characterize the local discontinuity due to cracks, a rotational spring model based on fracture mechanics is proposed to model the crack. Subsequently, the proposed method is verified by a numerical example of a simply-supported beam with a crack. The effect of the crack size on the anti-resonant frequency is investigated. The position of the crack of the simply-supported beam is also determined by the anti-resonance technique. The proposed technique is further applied to the “contaminated” anti-resonant frequency to detect crack damage, which is obtained by adding 1–3% noise to the calculated data. It is found that the proposed technique is effective and free from the environment noise. Finally, an experimental study is performed, which further verifies the validity of the proposed crack identification technique.

Failure Behavior of Axially Loaded Tubular Y-Joints under Fire

The failure behavior of axially loaded circular hollow section (CHS) tubular Y-joints under fire is investigated. As the plastic yielding at the intersection of the chord and the brace is the most possible failure mode, the yield line theory is adopted to evaluate the joint ultimate capacity. A commercial finite element (FE) package ABAQUS is used to simulate the joint behavior. A sequentially coupled thermal-stress analysis is activated to analyze the stresses of the joint at higher temperatures. The FE model of the joint has been verified against the experimental results of the CHS tubular T-joints under fire. The numerical analysis showed that the joint failed at the location of the intersection between the chord and the brace. The type of failure modes is dependent upon the geometric parameters (θ, β and γ) and the load ratios (n) of the joint. The critical temperature and the fire resistance limit of the joint under various brace loads are also investigated. It is found that the axial reaction of the chord due to thermal expansion of steel material is greatly larger than that caused by applied load, which seriously affects the fire resistance of the joint.

Finite element model updating of bridge structures based on sensitivity analysis and optimization algorithm

The dynamic characteristics of bridge structures, such as the natural frequencies, mode shapes and model damping ratio, are the basis of structural dynamic computation, seismic analysis, vibration control and structural health condition monitoring. In this paper, a three-dimensional finite-element model is established for a highway bridge over a railway on No.312 National Highway and the ambient test is carried out in site, the dynamic characteristics of the bridge are studied using the finite-element analysis and ambient vibration measurements. Comparison between the theoretical and experimental results shows that the frequency differences of the modes range between 0.44% and 8.77%. If the measurement is more reliable, the finite element model updating is necessary. Thus, a set of design variables is selected based on sensitivity analysis, then the finite element model of the bridge is updated based on optimization algorithm. The results of model updating show that the proposed updating method in this paper is more simple and effective, the updated finite element model can reflect the dynamic characteristics of the bridge better, the analytical results can provide the theoretical basis for damage identification and health condition monitoring of the bridge.

Study on Construction of Objective Function for Damage Identification Using Improved Genetic Algorithm

An improved genetic algorithm-based damage identification method is presented that accurately identifies both the location and severity of damage in a simulated simply supported beam. Damage identification based on finite element model updating is often used and damage is identified by minimizing the error between measured and analytical results. But whether model updating can be carried out successfully mainly depends on accuracy of model, quality of vibration testing, definition of optimization problem and calculation performance of optimization algorithm. In the paper, an improved genetic algorithm (IGA) is presented firstly, the construction technique of objective function based on different residuals and their weight coefficients is introduced secondly, in the end through the simulation of a simply supported beam, and damage identification is studied comparatively. From the results it can be seen that the damage identification method developed using IGA provides greater accuracy in identifying the location and severity of damage, and the performance is better when frequency residual has a larger weight coefficient.

Multi-dynamic displacement measurement based on digital image processing technology

The multi-point dynamic-displacement data of structure is an important parameter for structural health monitoring. The difficulties of traditional multi-dynamic displacement measurement are reference point selecting and massive data processing. A novel multi-dynamic displacement measurement system was developed based on digital image processing technology. The calibration method based on surface fitting and window-centroid tracking algorithm was applied to determine the relationship between the object space and pixel coordinate system. The displacement data was acquired by comparing the change of the centroid pixel. The expectations based on wavelet coefficients were used for sub-pixel edge detection, measurement accuracy can reach 0.02 pixels. The dynamic displacement of four-layer steel frame structure was monitored on the shaking table. The results show that the system can achieve the monitoring of multi-channel 2-D dynamic displacements, the accuracy of displacement survey can reach 0.1mm.

Experimental and Numerical Study on Damage Detection in I-type Steel Beam Based on PZT Admittance Signals

In recent decade, the electromechanical impedance (EMI) technique for structural nondestructive detection has been developed rapidly, and many researchers have carried out some experimental studies on simple structures. However, the numerical and experimental studies on relative complex structures are implemented less. In this study, firstly, a numerical EM admittance analysis of a I-type steel beam is operated by the use of the finite element method. Subsequently, the numerical results are validated by an experimental study. In this experiment, a PZT active sensor is bonded to the surface of the I-type steel beam, and the damage is detected by monitoring the variations of EM admittance measurements. Besides, a damage index, the root-mean-square deviation of real admittance (RMSDR) is defined, by which the extent of damage of the I-type beam is also evaluated. From this study it is found that the high order modes of the I-type steel beam are very dense, and the detection effect of damage extent is not ideal only depending on the EM admittance spectroscopy. By the use of the RMSDR index not only the occurrence of damage is detected clearly, but also can the damage extent of the I-type beam correctly reflect qualitatively in different frequency ranges.

Notice of Retraction Full-Scale Experimental Research on PE Jacket of Stay-Cable

Its easily damaged for stay-cable PE jacket during the setup process of stay-cable, due to incorrect choice of the sling clamp, or not proper rubber blanket of clamp, or incorrect clamping force. The full-scale experiment was carried out to study the compressive and tensile performance of PE jacket clamping by the clamp. And different clamp rubber blankets were compared. From the experiment, the proper clamping force, length of the clamp and the clamp rubber blanket were determined for Sutong Yangtze River Bridge, which is the largest-span cable-stayed bridge in service in the world. During the whole setup process of stay-cable of Sutong Yangtze River Bridge, the situation of cable clamp sliding or PE jacket crushing was never happened, which ensured the construction security, quality and operation life of the stay-cable.

Vehicle Velocity Recognition by Bridge Responses with a Pattern Search Algorithm

The vehicle velocity of an operational system is recognized by the bridge responses with a pattern search algorithm in this paper, which provides an approach to monitor the vehicle velocity in civil engineering. The Pattern Search algorithm is represented by a preliminary numerical example. The operational bridge system is simplified to a moving mass and a simple supported beam, and the vibration differential equations are obtained to get an objective function. The vehicle velocity is identified after 34 iterations while the mesh size is little than the threshold value. The mesh sizes, the best function value, and the evaluation of the iterations are illustrated by figures. The recognition results are compared to the other two algorithms, genetic algorithm and simulated annealing, respectively. It is concluded that the vehicle velocity could be identified by the pattern search algorithm in a convergent and efficient way.