Revealing Dispersion Characteristics Of Guided Wave In Rod-like Structures Based On2D-FFT And Finite Element Analysis Method

Parallel wire cord, pre-stress steel strand, wire rope and etc., are widely used in civilconstruction and special equipments. Being affected by the surrounding environment, the useand safety of the strands structure will be threatened inevitably, owing to the effect ofgenerating fatigue, corrosion and etc. Owing to the capability of detecting long-rangestructural defects, Ultrasonic Guided Wave Technology has a very broad application prospectsin the above mentioned areas, establishing and solving the dispersion equation of theultrasonic guided wave’ propagation characteristics will be the theoretical foundation of theapplication. The complex geometrical structure of the metal structural bodies (Parallel wirecord, pre-stress steel strand, wire rope and etc.), along with the internal diversified contactconditions, make it difficult to establish a precise dispersion equation. In theory, studies maytreat the above mentioned metal structural bodies as rod-like structures with special crossprofiles, and ignore the internal contacts among the steel wires. Finite element simulationcomputing model is an ideal tool to analyze these dispersion characteristics, considering bothcomplex geometrical structures and contacts among wires; along with signal processingmethods, the model can extract the dispersion curves, and provide further guidance of theultrasonic inspection technologies on these structures.Based on basic theories of ultrasonic guided wave’s propagation characteristics, thestudy applies transient dynamics method from finite element simulation, to get ultrasonicguided wave’s signals under the straight-rod structure, and using STFT (Short Time FourierTransform) and2D-FFT (Fast Fourier Transform Algorithm) methods to obtain dispersioncurves under the straight-rod structure. The study further applies finite element simulation and2D-FFT together, researches on guided wave’s propagation characteristics under a first-orderhelical rod model, and sums up the variation effects of the dispersion characteristics, whichare caused by the parameter changes of rod diameters and helical angles. Finally, the studyexplores, considering the contact conditions, finite element analysis on the guided wave’sdispersion characteristics, which happen under the2-wire strand and7-wire strand model.Major achievements are as follows:(1) Straight-rod model established through the ABAQUS finite element simulationplatform, simulates the ultrasonic guided wave’s propagation under the model, analyzeseffects on accuracy of the simulation results, which are affected by mesh type, mesh size andtime step, and optimize the computing parameters of the simulation. (2) Respectively, through STFT and2D-FFT methods, this study processes thesimulation signals obtained from the finite element simulation software under the straight-rodmodel. After acquiring the information of guided wave’s dispersion characteristics, andextracting dispersion curve under1MHz frequency, the study compares the obtaineddispersion curve with the theoretical one, and further gets the comparison results ofeffectiveness and accuracy from STFT and2D-FFT method.(3) Studying first order helical rod, the study introduces helical coordinate system, andderives the conversion relationship between helical coordinate system and Cartesiancoordinate system. Through2D-FFT method, the study processes the acceleration signals ofthe guided wave’s propagation, and extracts the frequency dispersion curve. It also studies thegeometrical parameters’ impacts, which from both helical angle and rob radius, on thepropagation of the guided wave, and phenomena of frequency’s cutoff and mode’s separationof wave’s propagation.(4) Through2D-FFT method, the study explores, under certain contact conditions,dispersion characteristics of2-wire and7-wire structures, and obtains changes of bothstructures’ dispersion curves under1MHz frequency, given different contact forces.