Study On Mechanism Of Nonlinear Vibration Characteristics Of CFST Affected By Concrete Debonding

In recent years,with the vigorously development of high-rise buildings and large span bridges,concrete filled steel tube(CFST)structures have been widely used because of the excellent mechanic properties.However during the construction of CFST structures,the quality of concrete pouring always can not be effectively controlled,and the quality defects of CFST are widespread,affecting the normal use of structures.Therefore,we need to monitor the interface state of CFST,so as to find out the damage in time and take appropriate measures to guarantee the joint work between steel tube and concrete.Nondestructive detecting technology of interfacial bond quality of CFST has become a hot issue among domestic and international scholars.Due to the complexity of the CFST interface bonding effect,the current detecting method has some limitations and cannot completely meet the needs of practical project,so the progress in theory and improvement of detection methods is needed.Based on CFST bond-slip theory and nonlinear vibration theory,the author conducted the push-out test on the three CFST specimens,measured the relative slippages and the free vibration acceleration signals of the specimens at different loading levels,and obtained the load-slip curves of each specimen.And by adopting the analytical mode decomposition method,numerical integration and Hilbert transformation etc.to the acceleration signals,the author also obtained the non-linear vibration characteristics of CFST.Finally,the author considered the load-slip curves and the corresponding nonlinear characteristics of the specimens,and analyzed the relationship and laws between the interfacial debonding damage of CFST and the nonlinear vibration characteristics,provided some reference for practical engineering damage detection of CFST.The main work and conclusions in this paper are as follows:(1)The author discussed the component of CFST interfacial bond stress and analyzed the changing process of bond stress during the course of CFST bond slip.Then summarized the cause of CFST interface debonding and the influence of interfacial debonding on the mechanical properties of the specimen.(2)The author analyzed the current common analytical methods of non-stationary signal processing and discussed the advantages and limitations of several analysis methods,and emphatically introduced the analytical mode decomposition and Hilbert transformation.According to the characteristics of CFST acceleration signal in this paper,the author chosen the analytical mode decomposition method to separate the first-order vibration component of acceleration signal and Hilbert transform method to get the amplitude-frequency curves,and analyzed nonlinear characteristics of CFST specimens.(3)By conducting the push-out test and dynamic signal testing of three CFST specimens,the author analyzed the nonlinear characteristics of the specimens under different loading conditions.The results are as follows: As the load level enhances unceasingly,the nonlinear characteristics index of specimen increased first,and then decreased when the specimen reach bonding limit load.Under the condition of debonding caused by load is not occurred,artificial debonding or poor quality of concrete pouring will increase the initial damage of the specimen,which results in the decrease of cement specimen ultimate load.The nonlinear characteristics of the specimens with initial damage is obviously lower than the specimens without initial damage.On the basis of previous research,this paper introduces the theory of nonlinear vibration to analyze the interfacial debonding damage of CFST,the main innovation points are as follows: Analytical mode decomposition method is applied to analyze the interfacial debonding damage of CFST.The first order vibration component of measured acceleration signal is separated by using the analytical mode decomposition method.The author test the acceleration signals under different working conditions in the process of push-out test,and analyze the influence of interfacial debond on nonlinear characteristics of CFST by comparing the load-slip curve.The secant modulus corresponding of the load-slip curve under the ultimate load is used as damage index to evaluate the defects of CFST specimens.The author analyzes the relationship between the nonlinear characteristics and initial damage of specimens,which lays a foundation for the quantitative relationship between initial damage and nonlinear behavior of CFST.