Dynamic Properties And Applications Of Defect Modes In Periodic Beam Structures

In recent years, research about elastic wave propagations in periodic structures, such as phononic crystals or metamaterials, have been widely investigated. Due to the existence of the band gaps, periodic structures possess special elastic wave propagation behaviors. Theoretically, elastic wave propagations within the band gap are not allowed. With the presence of defects, elastic waves are localized or propagate only along the defects, which makes tuning of the elastic wave possible. This research focuses on elastic wave propagations in periodic structures with point defects. Influences of the defects on phononic crystal beams and elastic metamaterial beams are investigated through theoretical studies, FEM simulations, and experimental verifications. Specifically, we propose applications of the defect modes in phononic crystals to structural health monitoring (SHM). In addition, we realize one-way elastic wave transmission at a specific frequency using metamaterials.This research includes:(1) Band gap structures of the phononic crystal beams and metamaterial beams are theoretically and numerically calculated with the transfer matrix method (TMM). By changing the material properties of the scatters, various defect models are introduced and their effects on the band structure are analyzed.(2) Characteristics of the band gap structures in phononic crystals are thoroughly discussed. Various defect modes in phononic crystals are considered. Feasibility of applying the defect modes to SHM is investigated.(3) Mechanisms of the existence of the band gaps in elastic metamaterials are discussed. Influences of the defects, introduced by variations of lattice constants or parameters in local resonators, on the band structure, transmission characteristics, and displacement fields are studied. One-way transmission of elastic waves at a specific frequency are realized by designing local resonators with gradient stiffness.(4) Experiments are performed to investigate the periodic beam structures with point defects, including a phononic crystal beam and an elastic metamaterial beam. A high-sensitive fiber Bragg grating (FBG) displacement sensing system is set up to measure transient bending waves and obtain the frequency responses.To summarize, through theoretical analysis, FEM simulations and experimental verifications, influences of defect modes on dynamic behaviors of periodic beam structures are studied. This work will promote research on periodic structures with defects and relating applications.