Nonlinear Vibration Characteristics Analysis And Debonding Identification Of Concrete Filled Steel Tube Columns

Concrete-filled steel tube columns consist of outer steel tubes and core concrete.The “hoop action” of the steel pipe puts the concrete in a three-direction compression state,and the plastic deformability is enhanced and the strength is greatly improved.At the same time,the support of the concrete to the steel pipe also compensates for the disadvantages of the easy-going instability and local buckling of the steel pipe.The advantages of the two materials have been fully realized,so they have been widely used in engineering.However,studies have found that debonding is common in concrete-filled steel tube columns,which has a great influence on their mechanical properties.At present,the debonding detection of concrete-filled steel tubular columns is mainly based on the traditional acoustics,optics,thermals,and mechanical methods based on the assumption of linear elasticity.These methods either need to pre-embed the sensors,the operation is more complex,or there are more restrictive conditions.The dynamic behavior of the structure shows strong or weak nonlinearity due to various nonlinear factors such as material,damping and geometry,and it is more accurate to use the nonlinear vibration theory to identify the structure debonding.In this paper,five specimens of concrete-filled steel tubular cantilever columns with different debonding states were designed and manufactured.The free vibration acceleration signals of six measuring points on the surface of the specimen were recorded under the excitation of hammer.The first frequency component of recorded signals was separated by the analytic mode decomposition,and then Hilbert transform was used to get frequency-amplitude curve.The frequency difference in the same amplitude section and the nonlinear coefficient obtained from the frequency-amplitude curve were used to examine the variation of the nonlinear vibration characteristics of the signals with the degree of debonding and the position of the measurement point,so as to explore the practical method of using the nonlinear vibration characteristics of concrete filled steel tube columns to identify the debonding.The main research contents and conclusions of this paper are as follows:(1)Introduced the composition and research methods of the concrete-filled steel tube cohesive force,in-depth analyzed the reasons for the debonding of concrete-filled steel tube,and conducted a comprehensive discussion on the effect of debonding on the structural mechanical properties.(2)Discussed examples of common nonlinear vibrations in engineering,introduced the classification of nonlinear equations and listed some typical differential equations.Contrasted experimental,geometric,analytical and numerical methods for studying nonlinear problems and introduced several typical analytical methods.(3)The type of vibration was analyzed and the classification was given,and the random process was emphatically expounded.The common signal denoising method is introduced by combining the test signals,and the advantages and disadvantages of the analytical mode decomposition and blind source separation are compared in dealing with the test signals,and the numerical simulation example has been used to verify the analytical model decomposition method.Several time-frequency analysis methods were compared for non-stationary and nonlinear signals in this last.(4)Hammer test was carried out on the specimens of concrete filled steel tube columns with different debonding States,and the free vibration acceleration signals were recorded on the surface of the specimen,and the nonlinear vibration characteristics of the signals were analyzed with the change of the debonding state and the position of the measuring point.The following conclusions were obtained: the specimen having no debonding exhibited noticeable nonlinear vibration characteristics,which is similar to weak Duffing system;the existence of debonding makes the nonlinear characteristics of the specimen weaken,and the greater the debonding area was,the smaller the nonlinearity will be;the nearer the test position was,the weaker the nonlinear characteristic will be.The nonlinear index is sensitive to the debonding,and is not easy to be affect by the component and boundary condition.So it’s a good indicator to identify the debonding of concrete filled steel tube columns.On the basis of previous studies,the nonlinear vibration theory was introduced into the debonding identification of concrete filled steel tube columns.The main innovation is: analytic mode decomposition and Hilbert transformation were used on the free vibration signals of non debonding specimens to get the frequency vs amplitude curves,which verified the nonlinear vibration characteristics of the concrete-filled steel tube columns due to the interface effect.The frequency vs amplitude curves were fitted by the least square method with the relation between the frequency and the amplitude of the Duffing equation.It is found that the free vibration of the specimen is similar to that of the weak Duffing system of the ? <0.The frequency difference in the same amplitude section and the nonlinear coefficient obtained from the frequency-amplitude curve were used to examine the variation of the nonlinear vibration characteristics of the signals with the degree of debonding and the position of the measurement point,which helps to obtain the practical method of using the nonlinear vibration characteristics of concrete filled steel tube columns to identify the degree and position of the debonding.