Numerical Investigation Of Nonlinear Ultrasonic Guided Wave In A Welded Joint

With the rapid development of welding technology,the welded structures have been widely used in many important areas.The failure of the welded structure may lead to serious losses,thus it is necessary to exploit various inspection methods to inspect and monitor the welded structures.In recent years,some scholars have discovered that there is a special guided wave in the welded joint called feature guided waves(FGW)whose energy is mainly concentrated in the welded joint and which have great promising application prospects for the detection of welded joint.However,the most current studies on FGW in welded joint are based on linear ultrasound,and there is little relevant research on nonlinear FGW in welds.Considering that nonlinear ultrasound is sensitive to microstructure changes of material and can detect damage in materials at the early stage,the aim of this thesis is to fully combine the advantages of high energy concentration and high detection efficiency of FGW with the high sensitivity of nonlinear ultrasound.This thesis investigates the characteristics of generation and propagation of nonlinear FGW and explore the feasibility of using nonlinear FGW to assess and locate localized damage in welded joints.This thesis provides a reference for the exploitation of detection technology on welded joint.The main content of this thesis includes:(1)The theory of second harmonic generation(SHG)is derived.The expression of amplitude of the second harmonic wave is obtained.The two conditions for generating the second harmonic wave with cumulative effect are discussed.Three mode pairs are selected according to the dispersion curves,and the characteristics of each mode pair are discussed.The relative amplitude of the second harmonic wave is calculated theoretically and firstly.Then,a three-dimensional model for time-domain finite element simulation is established to investigate the selected mode pairs.The simulation results are compared with the theoretical calculation results,and the generation and propagation characteristics of the SHG are discussed and summarized finally.(2)The frequency mixing response and the static component of FGW are derived.The calculation formulas which are to determine the mixing region of co-directional and counter-directional guided wave mixing are derived and analyzed.The static component generation of longitudinal guided wave(LG W)or shear horizontal guided wave(SHGW)is investigated respectively.The combined harmonic generation of frequency mixing response of two SHGW or a LGW and a SHGW propagating in the same direction or in the opposite direction are studied respectively.(3)The response of FGW interacting with closed crack in welded joint is investigated.The patterns of high order harmonic generated under three cases are explored.The cases include interactions between one SHGW,two co-directional SHGW or two counter-directional SHGW and closed crack in welded joint.The difference between the contact acoustic nonlinearity and the classical nonlinearity were analyzed.