Analysis of optimum Lamb wave tuning

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Civil and Environmental Engineering, 2002.Includes bibliographical references (p. 243-251).Guided waves are of enormous interest in the nondestructive evaluation of thin-walled structures and layered media. Due to their dispersive and multi-modal nature, it is desirable to tune the waves by discriminating one mode from the others. The objectives of this thesis are (1) to develop schemes and procedures for Lamb wave tuning, (2) to develop tools for understanding and analyzing the mechanism of various tuning techniques, and (3) to provide suggestions and guidelines for selecting optimum tuning parameters. In order to remedy the inherent problems of traditional tuning techniques using angle wedge and comb transducers (such as the inability to tune the modes with low phase velocities, and the inability to control the propagation direction of tuned waves), a novel dynamic phase tuning concept using phased arrays is proposed. In this approach, the constructive interference of desired modes is achieved by properly adjusting the time delays. As an extension to this concept, the synthetic phase tuning (SPT) scheme is introduced, in which the tuning effect is achieved by constructing virtual waves. The effectiveness of SPT against other techniques is experimentally demonstrated, which shows its feasibility. To understand the mechanism of tuning, an analytical model is developed to study transient waves, based on the Fourier integral transform method. The excitation conditions for both angle wedge and array transducers are taken into account. The surface displacements of individual modes and their temporal and spatial Fourier spectrum are derived and used to study the tuning behavior. The analytical results are compared with the experimental results as well as the numerical results obtained from the finite element simulation studies.(cont.) In dealing with broadband signals, laser generated Lamb waves are investigated. Both line and circular source loading models are developed to study the behavior in the ablation regime. The predicted waveforms and dispersion curves are in good agreement with the experimental results. Based on the same SPT scheme, virtually-tuned waves are constructed by processing a set of broadband signals. Finally, Lamb waves in a transversely isotropic composite plate are investigated. Although the analysis is limited only to the waves propagating in the principal directions, it could serve as the basis for future work on tuning of Lamb waves in composites. It is concluded from this thesis that the SPT method enjoys advantages over other methods including its low operation cost, ability to tune the. modes of low phase velocities, and capability to control the propagation direction of tuned waves. The analysis of transient waves allows us to examine various tuning scenarios. The investigation of the tuning effectiveness enables us to select optimum modes for the given conditions.by Yijun Shi.Ph.D

Modeling of acoustic waves for linear phased arrays

Thesis (M.S.)--Massachusetts Institute of Technology, Dept. of Civil and Environmental Engineering, 1998.Includes bibliographical references (leaves 112-115).by Yijun Shi.M.S