Time Frequency Energy Analysis Of Ultrasonic Guided Waves Propagating In Steel Strands

Steel strands are commonly used for prestressed concrete structures,cable stayed bridges,hoist elevators to bear tensile load.It is always in a high stress state for a long period and vulnerable to be corroded by water,air and corrosive media.With the constant cumulation of corrosion damage,the mechanical and material properties deteriorate gradually,which leads to the final break of wires and even the entire strand and pushing the structure in a high risk state.Therefore,it is one of the key issues for ensuring the safety and durability of structures to detect and evaluate the health status of steel strands timely and accurately.In this paper,the propagation law of ultrasonic guided waves(UGWs)in the steel strand was discussed from the time-frequency domain,and the strand tension was identified effectively based on the characteristics of UGWs.It provides an effective method for studying the propagation characteristics of guided waves in complex waveguide structures,and also provides theoretical support for the application of ultrasonic guided waves in non-destructive testing of steel strands.Firstly,wavelet transform was used to extract the time-frequency distribution of UGWs,and a modal analysis method based on the single-point fluctuation signal was proposed.The effects of corrosion and tension on the propagation mode modal and time frequency energy characteristics were analyzed.Then,the guided energy entropy spectrum with strong anti-noise ability was proposed by combining information entropy with time-frequency energy of UGWs.Finally,energy entropy spectra of UGWs were used as a feature vector to construct the tension identification index.The effects of propagation distance and sensor placement strategy was considered.The results show that:(1)Multimodal guided waves can be identified effectively based on the time frequency energy distribution of single point fluctuation signals.The multimodal distribution of UGWs can be identified by extracting the single node fluctuation signal of high-strength steel wire with wavelet time-frequency transform.The finite element simulation and experimental results are in good agreement with the theoretical modal curves.Modal identification of high frequency UGWs requires smaller grids.(2)The time-frequency energy distribution characteristics of UGWs vary with the increase of corrosion degree of steel wires.With the increase of corrosion degree,the energy of L(0,1)and L(0,2)gradually increase and decrease,respectively.When the cross section loss is 10.84%,the energy ratio of L(0,1)is increased by 52.1%.The total energy is gradually transferred to low frequency.(3)Guided wave energy entropy spectrum can effectively reflect the change of tension.With the change of tension force of steel strand,the energy entropy spectrum of guided wave varies obviously in specific scale based on wavelet time-frequency transformation.Even if the signal to noise ratio is 0d B,the energy entropy spectrum still shows good regularity.(4)The linear relationship can be found between the identification index and tension based on the distance between feature vectors and the coefficient of determination is more than 0.95.The finite element simulation is in good agreement with the experimental results.With the increase of the propagation distance,the tension identification index has a higher sensitivity.(5)It is better to use the non coupling arrangement strategy to identify tension in strands.Compared with the coupling layout strategy,the numerical and experimental K for uncoupled layout strategy are increased by 24% and 60.2%,respectively.No coupling layout strategy is recommended for site tests.