Frame Structural Damage Identification Under Ambient Excitation

Various damages will appear in engineering structures or structure components during their service life due to degradation of material properties,initial design defect,and constant environmental erosion.And evolution of damages in a structure will lead to changes of structural physical properties such as stiffness,damping,or mass.The accumulation of damages will even lead to catastrophic failure of a structure.Thus,it is significant to determine structure damages timely and accurately,and much more attention should be paid to damage identification of structures to avoid severe life and economic losses.Firstly,in this paper virtual impulse responses of a structure are obtained through the inverse Fourier transform of frequency response function,and are decomposed into components by wavelet packet transform.Then the energy of these wavelet packet components is calculated to obtain the wavelet packet energy spectrum.And a novel damage index i.e.Energy Spectrum Anomaly Measure(ESAM)is developed to identify the existence of damage and give information about the location of the damage.Numerical results are presented to evaluate the effectiveness and reliability of the proposed index.It is shown that the proposed damage index works well in identifying structure damages and damage location.Secondly,in order to verify the validity and robustness of the proposed index in structural damage identification under ambient excitations,this paper applies the same method to a 3 degree-of-freedom numerical model.The difference from the above is that a new Gaussian white noise is re-generated randomly under different conditions to simulate ambient excitations of the model.The research process is as follows: First,the damage status of the structure is determined by the change of ESAM under different conditions.After that,the structural damage location is determined by comparing relative sizes of ESAM at different locations.Finally,the method is applied to a 4 degree-of-freedom structure at Los Alamos National Laboratory(LANL)to further verify the validity in damage identification.The results show that the damage identification method based on the proposed index can effectively determine the damage state and identify the structural damage location under ambient excitations.Then a new method based on modified Shannon entropy mutative-scale symbolic time series analysis is proposed to identify structural damage.This method combines virtual impulse response function and applies the principle that symbol sequences consisting of mutative-scale interval symbol contain the rich information of the overall structure frequency change rate to conduct structural damage identification.According to the calculated damage index ESAM,this method firstly determines the current damage state of a structure.After that,the location of the damage is determined by comparing relative sizes of ESAM in different locations.The proposed method is applied to a numerical model to verify the effectiveness.Simulation results show that mutative-scale symbolic time series analysis method based on modified Shannon entropy can effectively determine the occurrence of structural damage and locate the structural damage.Finally,in order to verify the validity of mutative-scale symbolic time series analysis method in structural damage identification under ambient excitations,this paper applies this method to the same numerical model mentioned above.The difference from the above is that a new Gaussian white noise is re-generated randomly under different conditions to simulate ambient excitations of the model.Combined with virtual impulse response function,this method applies the principle that symbol sequences consisting of mutative-scale interval symbol contain the rich information of the overall structure frequency change rate to conduct structural damage identification.This method firstly calculates virtual impulse response function of the structure under ambient excitations,and then works out the frequency of each symbol sequence based on the dynamic entropy mutative-scale symbolic time series analysis as the probability feature vector that will be used to calculate the damage index to determine the damage and locate the structural damage.Then to further evaluate the effectiveness and reliability the proposed method is applied to a frame structure tested at LANL.It is shown that the proposed method works well in identifying the occurrence of the structural damage and determining the structural damage location under ambient excitations.