Damage Detection And Operational Modal Analysis Of Structure Based On Vibration Transmissibility

With the increase of cost of modern structure and the recognition of structural safety,structural health monitoring has become increasingly important,and is a significant research aspect of engineering structure.In the system of structural health monitoring,structural operational evaluation and damage detection based on test data are key processes to system health evaluation.Therefore,developing convenient damage detection methods and operational structure dynamic analysis have a deep engineering background and important practical significance.Vibration transmissibility is a physical parameter which describes the vibration transfer characteristics among the test points,which is easy to obtain in practice.Similar with the frequency response function,vibration transmissibility has a close relation with the structure dynamic.And,based on these properties,vibration transmissibility has been widely used in many fields.This thesis aimed to establish a theoretical framework of structural health monitoring based on vibration transmissibility,and applied the vibration transmissibility in structural damage detection and structure dynamic analysis.For this purpose,this thesis mainly focused on the following three aspects of research.Firstly,this thesis studied the definition the vibration transmissibility and analyzed its dynamic properties systematically.Based on the special properties,transmissibility has been applied in FRF evaluation,transfer path analysis,force identification,response estimation,etc.After the definition and classification of transmissibility,this article studied the characters of transmissibility under different excitation source,different number of excitation and different number of reference,including independence of transmissibility on the excitation,system poles and zeros,and connection of transmissibility with mode shape vectors at the system poles.All these study provided the theoretical basis for application of transmissibility in damage detection and operational modal analysis.Secondly,this thesis applied the vibration transmissibility in structural damage detection,and developed two kinds of new methods.Since the transmissibility is independent of poles of the system,and is influenced by the zeros,it is more sensitive to the local variation.Damage changes the values of transmissibility nearby,but the transmissibility far away from the damage is influenced slightly.Using this property,Damage detection methods were proposed.But,the classical methods based on transmissibility has two shortcomings: frequency band dependent and reference dependent.Concerning the first shortcoming,this article proposed two methods to decrease the dependent on the frequency band: methods based on the weighted coefficient transmissibility and wavelet transmissibility.By increasing the weight of transmissibility difference before and after damage at the resonant frequency and low frequency band,the method based on the weighted coefficient transmissibility can decrease the dependent on the frequency band.And,since the wavelet can filter the noise,the method based on wavelet transmissibility can improve the accuracy of damage location.The result of numerical simulation and experiment show that the two methods can locate the damage better than the classical transmissibility based method,and were more stable using different frequency bands.Concerning the second shortcoming,the article proposed two methods based on response of damaged structure: methods based on power spectral density transmissibility shape and transmissibility of operational deflection shape.Structural shape includes the local information of the damage.When the modes are discrete,power spectral density transmissibility converges to the same ratio of amplitudes of vibration modes at the system poles.A method was proposed based the power spectral density transmissibility shape.Using the combination of wavelet transform and Teager energy operator,this method can explore the characteristics of damage deeply.The result of numerical simulation and experiment show that this method can locate the damage correctly using the power spectral density transmissibility shape in the low frequency band.Excitation of a cracked structure at a single frequency will always generates higher harmonic components due to the nonlinearity of breathing crack.The transmissibility of higher harmonic operational deflection shape and basic harmonic operational deflection shape shows the energy transfer influenced by the crack.Using this characteristic,a method was developed based on the transmissibility of operational deflection shape.Numerical simulation and experiment show that this method can locate the single crack in cantilever beam.Thirdly,vibration transmissibility was applied in structure dynamic analysis,and two operational modal analysis methods were developed using transmissibility matrix.Classical operational modal analysis assumes that the excitation of the structure is white noise.When the input is white noise,the response only contain the dynamic properties of the system.When the input is colored noise,classical operational modal analysis methods can't distinguish whether the dynamic properties in the response is the dynamic of the system or were induced by the harmonic components in the colored input.Transmissibility is independent of the input spectrum,and converges to the same ratio of amplitudes of vibration modes at the system poles.Using these properties,two operational modal analysis methods were proposed to decrease the influence of harmonic components to the identification of modal parameters.Vibration transmissibility under multi-state excitation can constitute a pseudo-inverse matrix containing the same poles of the system.Applying the frequency-spatial domain decomposition to the matrix,a new method based transmissibility matrix was developed.The result of numerical simulation and experiment show that this method can identify the modal parameters correctly,and can remove the influence of harmonic component compared with the frequency-spatial domain decomposition method using power spectral density matrix.Power spectral density transmissibility under single state excitation can constitute a pseudo-inverse matrix containing the same poles of the system.Applying the least squares complex frequency domain method to the elements or matrix,a new method based power spectral density transmissibility matrix was developed.The result of numerical simulation and experiment show that this method can identify the natural frequencies and mode shape correctly.Compared with the other three operational modal analysis methods,this method can decrease the influence of harmonic component to the modal parameters.