<p> Lamb waves based ultrasonic testing has been studiedfor many years. However, conventional methods of generatingand collecting of Lamb waves usually require bulky instruments and manual interference, thus can not be applieddirectly for in-situ or in-service monitoring of thestructural health. Especially, the method of interpretingthe Lamb waves in an active structural health monitoring(SHM) system with built-in piezoelectric sensors/actuatorsis not available yet. The objective of this study was to propose and validate, through numerical simulation and experimental studies, the feasibility of adopting the geophysical migration method to interpret the ultrasonic Lamb wave signals for the purpose of realizing quantitative damage identification. A homogeneous isotropic plate with a surface-mountedlinear piezoelectric ceramic (PZT) disk array is studied as an example. The piezoelectric disks act as actuators to excite Lamb waves and also as sensors to receive the waves reflected from the structural anomaly in the plate. The migration technique, which is an advanced technique in geophysics to reverse the reflection wave field and to image the Earth interior, is then used to back-propagate the recorded wave signals and to visually image the damage in the plate. Mindlin plate theory is adopted to model the propagating waves, and a two-dimensional 2-6 order explicit finite difference algorithm is used to synthesize the reflection waves and to implement the migration process. The stability and accuracy criteria of the finite differencealgorithm when used in plate problems is discussed. An analytical solution is derived for the transient Lamb waves of an infinite plate subject to a point loading. This solution is used to verify the accuracy of the finite difference calculation. Both poststack and prestack migration are studied to propagate the reflection energy back to the damages. For the poststack migration, a one-way version of flexural wave equation is derived and the data pre-processing procedures before migration, such as muting direct arrival, deconvolution and stacking, are discussed. For prestack migration, an excitation-time imaging condition specifically for the migration of waves in a plate is introduced based on ray-tracing concepts and the asymptotic properties of flexural wave velocities and the migration is proceeded through the full-way wave equation. The results of numerical simulation show that the migration method possesses the capability of identifying multiple discrete damages without a priori assumption on the distribution pattern of the damages. Thus not only the existence but also the shape and the dimensions of the damages can be visually identified. An experimental apparatus is then set up to validate the conclusions drawn from the synthetic data. For calibration of the system, an analytical model of the waves in a plate incorporated with PZT sensors/actuators is developed. The agreement between the model calculated data and the measured data in the experiment shows that A0 mode Lamb waves are accurately generated and collected. Finally, the migration results from the reflection waves of an artificial damage in an arc shape recorded in the experiment are presented. It is shown that the existence of the damage could be correctly imaged through the migration process as it was shown in the numerical simulation.<P>
| Identifer | oai:union.ndltd.org:NCSU/oai:NCSU:etd-20010324-152020 |
| Date | 26 March 2001 |
| Creators | Lin, Xiao |
| Contributors | F. G. Yuan, J. W. Eischen, Y. R. Kim, L. M. Silverberg |
| Publisher | NCSU |
| Source Sets | North Carolina State University |
| Language | English |
| Detected Language | English |
| Type | text |
| Format | application/pdf |
| Source | http://www.lib.ncsu.edu/theses/available/etd-20010324-152020 |
| Rights | unrestricted, I hereby certify that, if appropriate, I have obtained and attached hereto a written permission statement from the owner(s) of each third party copyrighted matter to be included in my thesis, dissertation, or project report, allowing distribution as specified below. I certify that the version I submitted is the same as that approved by my advisory committee. I hereby grant to NC State University or its agents the non-exclusive license to archive and make accessible, under the conditions specified below, my thesis, dissertation, or project report in whole or in part in all forms of media, now or hereafter known. I retain all other ownership rights to the copyright of the thesis, dissertation or project report. I also retain the right to use in future works (such as articles or books) all or part of this thesis, dissertation, or project report. |
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