Research On Spectral Element Method-based Characteristics Of Guided Wave Propagation And Damage Identification In Structures

Key structures in practical applications, such as aerospace industries, mechanicalengineering and civil engineering, run a high risk of suffering from damage, which canresult in catastrophic failure during their service lives. Structural health monitoringtechniques are attracting more and more attentions because they can provide in-timeinformation about structural health status.Guided-wave-based structural health monitoring techniques use embedded orsurface-bonded actuator/sensors to inspect integrity of structure. Health status of structureis evaluated by captured guided wave signals containing the information of defect.Therefore, it is essential to understand the physical mechanics of wave propagation instructures before performing guided-wave-based structural health monitoring techniques.This thesis presents the systemic application of spectral element method in field ofstructural health monitoring for characterizing of wave propagation in structures for thepurpose of damage detection. SEM models of structure, piezoelectric (PZT) disc anddamage were developed and characteristics of wave propagation, excitation and capture ofguided wave and wave interaction with damage in metallic and carbon fiber reinforcedplastic (CF/RP) composite structures were investigated based on proposed SEM model.Finally, a quantitative damage image reconstruction method based on a circular array wasproposed.First, the domestic and oversea research actualities of guided-wave-based structuralhealth method were summarized, methods for analysis of physical characteristics of wavepropagation were introduced and the advantages and disadvantages of them wereexpounded. Wave interaction with various types of damage in various structures andmodels for interaction between PZT disc and host structure were discussed, signalprocessing techniques and damage identification methods were introduced.SEM model for characterizing guided wave propagation in structures was developedby taking formulation of three-dimensional SEM as an example. Coupled PZTdisc-structure model was also developed considering electromechanical coupling. Stabilityand computational efficiency of SEM for simulating wave propagation in structures wereinvestigated; finally the computational process of SEM was introduced. Experimentalsetup was mentioned in brief and some parameters related to excitation were analyzed.A two-dimensional SEM model of isotropic beam and coupled PZT disc-structuremodel were developed. The dispersion characteristic of Lamb wave was investigated.Wave propagation properties under excitation of PZT disc was investigated in comparisonwith the single point force excitation model. Selection of Lamb wave mode and relationship between excitation frequency and magnitude of wave were investigated basedon PZT disc excitation model. Damage detection in an aluminum alloy beam based oncaptured Lamb wave signals in SEM model was implemented and validated byexperiment.A three-dimensional SEM model including PZT-structure coupling model forcharacterizing wave propagation in plate structures was developed and characteristics ofwave propagation in an isotropic aluminum alloy plate were investigated. Characteristicsof wave propagation under excitation of PZT disc were discussed. The damage in the platestructure was modeled by reducing the stiffness of the plate and wave interaction withdamage was discussed. Finally, the conclusions and the proposed coupled PZTdisc-structure model was validated by experiment and the results show that the proposedthree-dimensional SEM model can effectively and realistically simulate the excitation andcapture of the responses of the structures, as well as wave propagation in structures, incomparison with two-dimensional SEM model.Characteristics of wave propagation in thick beams with cross section of a large sizewere analyzed by using a three-dimensional SEM model, and a rule for selection ofexcitation frequency to obtain simple wave mode for damage detection in such thickbeams was derived. Simulation of wave propagation in beams with different thicknessunder excitation with different central frequencies reveals that when the wavelength ofshear wave is more than two times the beam thickness, local wave modes are suppressed,and the wave propagates in the length direction. The excitation frequency can be selectedto obtain a relative simple wave mode according to this conclusion when carrying out thestructural health monitoring strategy. Crack in the thick beams were modeled using nodeseparation method and the interaction between the guided wave and crack wasinvestigated. Finally, the conclusions were validated by experiment.Characteristics of wave propagation and interaction with delamination in multilayeredCF/RP beam structures were investigated based on a two-dimensional SEM model. Thefundamental symmetric (S0) mode and anti-symmetric (A0) mode interacting withdelamination were investigated, respectively, and some unique mechanisms of interactionbetween Lamb wave modes and delamination were revealed in detail. It is demonstratedthat the reflection at the far end of the delamination is much stronger in magnitude thanthat from the near end. The A0mode is more suitable for identification of delamination inmultilayered composite structures than the S0mode, especially when the delamination islocated at the mid-plane. The curves of reflection coefficient and transmission coefficientwere analyzed and they show an undulating shape depending on the ratio of thedelamination length to wavelength of Lamb wave. A quantitative identification method for delamination in composite beam structures was proposed and validated using anexperimental study.A three-dimensional SEM model was developed for concise analysis of Lamb wavepropagation in CF/EP composite laminate containing delamination. Three types oflaminate, namely unidirectional [08] laminate, symmetric cross-ply [02/902]slaminate andquasi-isotropic [+45/-45/0/90]slaminate were modeled using three-dimensional spectralfinite elements. Wave propagation characteristics in intact composite laminates were firstinvestigated. The results show that the characteristics of the fundamental wave modes (e.g.A0,S0,and SH0) are highly dependent on the layup configuration of the composite laminate.Scattered waves of symmetric and anti-symmetric modes by delamination located atdifferent interfaces were analyzed. The results demonstrated that the anti-symmetric modeis more sensitive to the delamination than symmetric mode under the same excitationfrequency. The transmitted waves are affected a lot by delamination.A method of quantitative damage image reconstruction using received signals from acircular PZT array was developed by considering the relationship between the magnitudeof Lamb wave and its propagating distance. Quantitative image reconstruction for thedamages with different sizes and distances away from the array were implemented,thereby the location and the degree of damage were evaluated.Results of this thesis are instructive for comprehensively understanding of thecharacteristics of wave propagation in structures and physical mechanism of waveinteraction with damage, thereby developing guided-wave-based structural healthmonitoring techniques.