Experimental Research On Fatigue Damage Detection Of Wires Based On Magnetostrictive Guided Waves

As the basic load-bearing unit of pre-stressing strands and bridge cables,wires inevitably generate fatigue damage during a long-term use,and may even suddenly break,resulting in catastrophic accidents.Therefore,it is of great significance to detect fatigue damage on wires.With the advantages of non-contact,no coupling agent,the entire cross section detection,the magnetostrictive guided wave testing technology has been widely applied in the detection of macroscopic defects such as broken wires and corrosion in wires,pre-stressing strands and bridge cables.However,there are few studies on the detection of microscopic defects such as fatigue damage.In this thesis,the experimental research on the fatigue damage detection of wires is carried out by using the magnetostrictive guided wave testing technology,which provides support for the fatigue detection of pre-stressing strands and bridge cables.Firstly,based on the dislocation theory and magnetostrictive effect,the influence of fatigue damage on the magnetostrictive guided wave coupling process is analyzed and verified by experiments.The results show that the fatigue damage leads to a decrease in the coupling efficiency of the magnetostrictive guided wave coupling process,and the bias magnetic field intensity corresponding to the maximum conversion efficiency increases.Secondly,the influence of fatigue damage on the propagation of longitudinal guided wave is studied.Based on the nonlinear stress-strain relationship,a finite element model of the fatigue-damaged wire is established,and the propagation characteristics of longitudinal guided waves are studied.The results show that longitudinal guided waves propagating in the fatigue-damaged wires generate second harmonics,which results in an increase of the nonlinear parameter.Considering the influence of the signal time-domain truncation on the nonlinear parameter when using the Fourier transform,a wavelet-based nonlinear parameter calculation method is proposed.Experiments verify the correctness of the simulation model by using this nonlinear parameter calculation method.Finally,the fatigue damage detection experiment based on magnetostrictive guided waves is carried out during the cyclic loading of the wire.The relationships among the wave speed,attenuation coefficient,coupling efficiency,nonlinear parameter and the cyclic loading times are studied.The results show that the wave speed decreases with the increase of cyclic loading times.The relationship between attenuation coefficient and cyclic loading times is complicated,and it shows different variation rules at different frequencies,so attenuation coefficient is not suitable as a parameter for fatigue damage detection.The coupling efficiency decreases with the increase of cyclic loading times.The nonlinear parameter increases with the increase of cyclic loading times.Therefore,the wave velocity,coupling efficiency and nonlinear parameter can be selected as the characteristic parameters to detect fatigue damage in wires.