| The concrete filled steel tube(CFST)structure is one of widely used composite structures,which adequately plays a role in tensile properties of steel and compressive properties of concrete,and makes up for the shortcomings of the two materials.It is widely used in bridges,high-rises and super high-rise buildings as well as large gymnasiums,and has significant effects.However,due to various impacts such as poor construction,etc.,the interface peeling may be caused,which seriously affects the restraint effect of steel tube on concrete,and has a great influence on the bearing capacity and ductility,etc.,of the member,forming a potential safety hazard.Therefore,it is necessary to inspect defects in CFSTs.Ultrasonic guided wave(UGW)detection technology has characteristics of high efficiency,rapidity and wide detection range,etc.,and has been widely used in structural defect detections.The analysis of propagation characteristics of UGWs in CFSTs is one of the most critical scientific issues in this field,and it needs to be solved through systematic research.In this paper,the theoretical analysis,experimental verification and numerical simulation are combined together to study the propagation characteristics of UGWs in CFSTs.The main research contents include:Firstly,based on the published research results,the dispersion equations in the CFST are theoretically set up according to the ultrasonic guided wave theory.Under the given conditions of both material and geometric parameters,the transcendental dispersion equations are numerically solved by using a Matlab program.The dispersion curves of both group and phase velocities are drawn.By changing the material parameters and geometric parameters,the characteristics of the dispersion curves of UGWs are studied.The results showed that the UGW propagation in CFSCs has the frequency dispersion and multi-modes characters,which are not only related to the product of frequency and thickness,but also the material parameters of steel and concrete.The result lays a theoretical foundation for damage detections of CFST structures by using UGWs.Secondly,two kinds of test systems were designed according to the active wave method,and the frequency dispersion character verification test and damage identification test were carried out,respectively.By arranging a circular array of PZTs(Lead Zirconate Titanate),the L(0,2)modal UGWs for each frequency are excited for sensing signal analysis.The time of flight(ToF)method is used to calculate the group velocity test value of the UGW.The experimental value is compared with the theoretical value corresponding to the dispersion curve.After the error calculation,the maximum error does not exceed 7%.The test result and the theoretical value match well,indirectly verifying the correctness of the dispersion equation and the dispersion curve of the UGW in the CFST.Moreover,based on the dispersion curve,the interfacial peeling damage between the steel pipe and concrete was identified based on the energy method.Exciting L(0,2)modal UGWs of different frequencies to propagate in the CFST under both the healthy state and the damaged one,it is found that the UGW will have energy attenuation phenomenon which is more obvious in the damage state than in the healthy one.Therefore,the experiment shows that the damage of the CFST can be identified by UGWs based on the signal energy method,and the concept of damage coefficient is further proposed to characterize the level of the interface damage.Finally,the ABAQUS finite element software was used to simulate the propagation process of UGWs in the CFST.Finite element method(FEM)can effectively avoid the influences of environmental factors and the accuracy of the test instrument,etc.By simulating the received(sensing)signal combined with the ToF method,the guided wave velocity at different frequencies is calculated and compared with the theoretical value,and the maximum error is less than 3%.The correctness of the dispersion curve derived in this paper is verified. |
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