An understanding of the generation of higher harmonics in Lamb waves is of critical importance for applications such as remaining life prediction of plate-like structural components. The objective of this work is to use nonlinear Lamb waves to experimentally investigate inherent material nonlinearities in aluminum plates. These nonlinearities, e.g. lattice anharmonicities, precipitates or vacancies, cause higher harmonics to form in propagating Lamb waves. The amplitudes of the higher harmonics increase with increasing
propagation distance due to the accumulation of nonlinearity while the Lamb wave travels along its path. Special focus is laid on the second harmonic, and a relative nonlinearity parameter is defined as a function of the fundamental and
second harmonic amplitude. The experimental setup uses an ultrasonic transducer and a wedge for the Lamb wave generation and laser interferometry for detection. The experimentally measured Lamb wave
signals are processed with a short-time Fourier transformation (STFT) and a chirplet transformation-based algorithm, which yield
the amplitudes of the frequency spectrum as functions of time,
allowing the observation of the nonlinear behavior of the material. The increase of the relative nonlinearity parameter with propagation distance as an indicator of cumulative second harmonic generation is shown in the results for two different aluminum alloys. The difference in
inherent nonlinearity between both alloys as determined from longitudinal wave measurements can be observed for the Lamb wave measurements, too.
| Identifer | oai:union.ndltd.org:GATECH/oai:smartech.gatech.edu:1853/13986 |
| Date | 25 August 2006 |
| Creators | Bermes, Christian |
| Publisher | Georgia Institute of Technology |
| Source Sets | Georgia Tech Electronic Thesis and Dissertation Archive |
| Language | en_US |
| Detected Language | English |
| Type | Thesis |
| Format | 792197 bytes, application/pdf |
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