Study And Application On Some Methods Of Global-based Structural Damage Detection

The global-based structural damage detection techniques, which are different from the commonly used local based non-destructive evaluation (NDE) methods, can be generally classified into two kinds: one kind of methods depends on the structural dynamic model, while the other kind of methods is independent of the structural model and only the measured structural response signals are included to obtain the aim of damage examination. In this paper, model based methods and non-model based methods are all studied: firstly, the crack depth and position identification problem of a cracked shaft is studied based on its dynamic model in chapter 2; secondly, different non-model based methods, which compose main part of this dissertation, are studied to detect the structural damage existence, degree and position via simulation and experiment.In the second chapter, a structural dynamic model is established to study the model based method: the analytical continual model of a shaft with an open crack,which is based on the Euler-Bernoulli and local flexibility theories. Vibration analysis of the cracked shaft based on the continual model is carried out and relations between crack parameters and modal parameters are studied. A crack depth and position identification method is put forward based on the first two natural frequencies and the first mode shape of cracked shaft. Simulations show that the method, which is based on the finite element model or the analytical continual model, is effective and accurate in determining the crack position and depth of a shaft with an open crack.Singularities of structural response signal will result from damage occurrence, for example, loosed connecting components will induce structural impact and rub. In chapter 3 of this dissertation, wavelet singularity detection theory is used to analyze the structural responses when damage occurs, by which the time of damage occurrence can be obtained. The emphasis of this chapter focuses on the method separating the singularities due to the excitation and the singularities due to the system's changes when a damaged structure is subject to Gauss white noise excitation, and coefficients of the wavelet transform at a small scale corresponding to the system's change are separated from the output. Simulations on a three DOF system representing the support model and a FEM model of steel frame structure show that, the proposed method is more effective than frequency analysis and previously used wavelet singularity detection methods.Base on the characteristics of Empirical Mode Decomposition (EMD), two methods are studied to identify the incident of structural damage in chapter 4. The signal time-frequency representation method based on the combination of EMD and WVD is presented, which can reduce the interference effect while does not decrease the time and frequency resolution. The EMD-WVD combined method is effective in detecting multiple frequencies in the rotating machinery damage detection. The structural natural frequency varies little before and after damage for the non-rotating mechanical machinery, therefore, band-limited EMD method combined with the random decrement technique (RDT) is used to determine the response of each mode and obtain the free vibration modal response, which is utilized to identify natural frequencies and damping ratios of structures in various states. Simulation and experimental cases are studied, which show the validity of the proposed EMD-RDT method.The structural modal parameters such as the first natural frequency decreases little for the small incipient damage, which makes it essential to find sensitive damage patterns; on the other hand, there are many kinds of uncertainties in structural responses caused by many random disturbances, which make it necessary to use statistical methods for damage pattern analysis. In chapter 5, ARX and AR-ARX model damage pattern sequences are studied and compared through statistical analysis and parametric discussion. Simulation and experimental results show that the two damage pattern sequences are both sensitive to the early structural damage and able to identify different degree of early damage, moreover, the two damage pattern sequences are both insensitive to noise and sensor position.There are many global based methods studying structural damage position identification. As a method implementation and completeness of the whole dissertation, firstly a FEM model of a steel frame with loosed connecting joint is established in chapter 6; then joint looseness position identification method is put forward based on the combination of bispectrum and network. Simulation results show its validity under large noise conditions.