Research On Damage Identification Method Of Nonlinear Structure Based On Time-Frequency Feature Fractal Theory

The operation and service life of the civil engineering structures have lasted for several decades even over a hundred years.But during its operation period,due to the effects of natural factors and human factors,all kinds of damage to civil engineering structure will happen,resulting in the decline of the bearing capacity and service life of the engineering structure,and even lead to major safety accidents.In order to avoid accidents,the identification of structural damage is very important.However,the excitation of the engineering structure is often nonstationary and its structure often presents nonlinear characteristics.The linear structural damage identification method can not accurately identify the damage of nonlinear structure.At present,many experts and scholars have studied many nonlinear time series analysis method,the nonlinear structural damage identification method based on fractal dimension of rapid development,which can directly reflect the complexity of the structure of the signal so as to reflect the structure of the state,and then identify the non linear structure damage degree and damage location.In this paper,the vibration response of nonlinear structures in civil engineering is taken as the object of analysis,and the box dimension method in fractal theory is applied to expand the research of nonlinear damage identification.The specific research contents are as follows:(1)first of all,this paper takes the cosine type time-varying stiffness of the cantilever beam as the numerical analysis object,and calculates the time frequency characteristic fractal dimension of the vibration response of the cantilever beam with different depth cracks under the sinusoidal excitation and the pulse excitation,and compares the time frequency characteristic box dimension of the response of different damage conditions.The relationship between the dimension of the box and the dynamic parameters is used to judge the effect of structural damage identification.(2)a single degree of freedom mass damping system with a bilinear oscillator is used as a numerical model.By changing the size of the structural stiffness reduction coefficient,the acceleration response is calculated to calculate the time frequency characteristic box dimension and analyze the effect of the non linear damage identification by comparing the change of the box dimension value under different working conditions.(3)based on the fractal theory analysis of the time frequency characteristic of the free vibration of the multi freedom cosine type respiratory crack simple supported beam,the acceleration response is transformed to the time frequency characteristic including the frequency information and the phase information after the free vibration acceleration response of the structure,and the modes of each order are intercepted according to the frequency band of the natural frequency.The time frequency characteristic of each order mode is calculated,and the results show the recognition effect of the damage degree and position of the nonlinear structure of the cosine type respiratory crack cantilever beam.