| Metal stranded wire has the advantages of high strength,excellent electrical performance,strong bearing capacity,etc.It is widely used in high-speed railway,bridge,port,construction,marine equipment and other fields.Messenger cable is a bearing structure in the catenary system of high-speed railway,which belongs to a typical metal strand structure.Damage is more likely to occur in the cable when the cable is covered by the structure such as the center anchor clamp.The covered area is the current blind area of detection,and it is likely to cause a serious accident once the damage occurs.Therefore,the research on effective damage detection methods of messenger cable structure in covered area has become the frontier issue of structural health monitoring.Due to the strong penetration of ultrasonic guided wave,it can still propagate in a certain law in the covered area of messenger cable.Ultrasonic guided wave detection method is not limited to the strand material,and it is an effective new way to detect the damage of messenger cable in the covered area.However,the propagation law of ultrasonic guided wave in the strand is complex,and the damage scattering signal generated by the interaction between ultrasonic guided wave and inner damage is weak.These problems bring great challenges to damage detection and imaging.Therefore,in order to solve the problem of damage detection in blind area where the messenger cable structure in covered area is,a new method of ultrasonic guided wave spiral focusing detection of messenger cable structure damage in covered area was studied.The main research work of this paper is summarized as follows:(1)In order to solve the problem that is difficult to obtain the dispersion curve of multilayer spiral coupling structure of messenger cable by traditional analytical method,a semianalytical finite element method based on twisted coordinate system was proposed.By deducing the relation between stiffness matrix,mass matrix and wave equation of discrete element in waveguide cross section domain under twisted coordinate system,the calculation of mass matrix and stiffness matrix of spiral curved rod and straight rod with different rotation directions and different rotation radius is creatively unified.The Hertz contact is used to simplify the contact action between single wires to obtain the solution of dispersion curve of multilayer spiral coupling structure.In this paper,the propagation characteristics of ultrasonic guided wave in straight rod and spiral curved rod were analyzed.The effects of contact coupling between rods and helical geometry on ultrasonic guided wave propagation in coupling rods were studied,and the effects of cladding structure on ultrasonic guided wave propagation in messenger cable were analyzed.Finally,the ultrasonic guided wave detection mechanism suitable for the damage of messenger cable in the covered area was determined,which lays a theoretical foundation for the follow-up research.(2)In order to solve the problem of the weak damage signal received from inner layer of the messenger cable in the covered area which is difficult to be identified,a new mechanism of ultrasonic guided wave spiral focusing enhancement of the array of messenger cables in the covered area is constructed innovatively combined with the structural characteristics of the messenger cable.Through expanding the complex three-dimensional messenger cable structure in a plane,the multi-path propagation of array ultrasonic guided waves in spiral coupled structure was studied.The distance difference between the target point in the inner layer of the spiral structure and each array element in the array was obtained,and the spiral focusing adjustment matrix was obtained by the propagation velocity of ultrasonic guided wave.The full matrix capture data was obtained according to the arrangement of ultrasonic guided wave array,and the spiral focusing enhancement signal was obtained through the matrix.The damage index was proposed to evaluate the focusing enhancement effect.The finite element simulation models of the messenger cable were established,and the focusing enhancement effect of the array ultrasonic guided wave signal on the single line of each layer is analyzed through spiral focusing,as well as the enhancement effect of the spiral focusing method on the damage scattering signal of the central layer and sub outer layer of the messenger cable in the covered area.(3)In order to overcome the difficulty in distinguishing the damage signal of messenger cable in the cladding area including each modal direct wave,sidelobe signal and noise signal in time and frequency domain,a cross sparse representation recognition method of spiral focusing damage signal based on dispersion dictionary was proposed.According to the propagation characteristics of ultrasonic guided wave in messenger cable,a dispersion dictionary which can characterize the mode and propagation distance of ultrasonic guided wave was established.Signal without the first wave and the original signal was represented by cross sparse representation,and the guided wave modes were selectively screened to reduce the sparsity of cross sparse representation and improve the recognition rate of damage signal.A virtual bidirectional time reversal imaging method was proposed,in which the time reversal signals of damage reflected wave and transmitted wave are propagated in a virtual direction and converged at the damage,to realize the damage imaging.The simulation models of different damage states of the outermost single line of the catenary in the cladding area were established.Firstly,the correlation between section damage rate and damage index was analyzed.Secondly,with the inner layer damage models of the messenger cable in the covered area,the accuracy and relative positioning error of the damage signal extracted by the cross sparse representation method were comprehensively analyzed.Finally,the virtual bi-directional time reversal method was used to image the inner and outer damage of the messenger cable in the covered area.(4)In order to realize the spiral focusing detection of ultrasonic guided wave for the damage of messenger cable structure in the covered area,an array ultrasonic guided wave sensor with adjustable contact force was developed for the first time.The experimental platform for damage detection of messenger cable structure in the cladding area was built,and a series of physical damage experiments of messenger cable in the covered area were carried out according to the maintenance standard of messenger cable.The experimental results show that,compared with the sparse representation method using traditional dictionary,the new damage identification method combining cross sparse representation and dispersion dictionary proposed in this paper improved the average accuracy of damage signal recognition in a wide threshold range from about 20% to 99.4%,and greatly reduces the sensitivity of the result to the threshold while improving the accuracy.For inner layer damage detection,spiral focusing can significantly enhance the damage signals.Compared with the superimposed signal,the reflection damage index of spiral focusing signal increased from 51.4% to 83.6%.At the same time,the new method has good anti-interference ability and robustness.Under the condition of vibration interference or superposition of Gaussian white noise to SNR=0 d B,the inner damage signal can still be accurately extracted from the spiral focusing signal through the cross sparse representation method based on the dispersion dictionary.And the method is still effective even though the effective data in the full matrix data is not less than 75%.The single line damage of the outermost layer,damage of the whole inner layer and the single line damage of the inner layer can all be accurately detected and can be imaged by virtual bi-directional time reversal imaging.It shows that the new ultrasonic guided wave spiral focusing detection method proposed in this paper can effectively detect the structural damage of the messenger cable in the covered area. |
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