Interaction of acoustic beam with elastic structures
This thesis describes experiments and calculations performed on the interaction of acoustic beams in water and air with planar and cylindrical elastic structures. Ultrasonic reflection measurements have been used to elucidate the phenomena of guided wave generation and reradiation by selecting beam incidence at, or near, phase-matching conditions. Under these circumstances reasonant mode conversion of accoustic wave to guided wave mode energy can occur. This interaction has been studied in rubber-coated steel, aluminum, plexiglas, and graphite-epoxy composite. The acoustic coupling media used in these experiments has been either water or air;Some theoretical modeling has also been undertaken to explain these results. The calculations performed here exploit an efficient analytical tool that simplifies the construction of finite acoustic beams. The method relies on the interesting mathematical fact that displacing a real point source into the couplex plane, converts the source into a quasi Gaussian beam. The free-space Green's function, which satisfies the inhomogeneous Helmholtz equation, is converted to a complex Green's function that describes the interaction of two beams, one from the source and the other at the observation point;The interaction with elastic structures is treated by spectral decomposition of the incident and reflected beams weighted by the plane wave reflection or transmission coefficient. The resulting spectral integral is evaluated either asymptotically along a steepest descent path, keeping track of the reflection/transmission coefficient pole contributions or numerically;In the first problem the interaction of acoustic beams with steel layered cylindrical shells is studied. The difficulty introduced by the high damping in the rubber is resolved and its influence on the signal is analyzed. The bond rigidity between the rubber and steel are accounted for in the model calculation by the so-called spring model. It is found that disbonds in the layered cylinder can be detected by monitoring the leaky wave amplitude. Where low bond rigidity exists, the leaky wave is only weakly excited;In the second problem the effects of transducer misalignment in guided plate wave measurement are studied, where the receiver incident angle is misaligned with the transmitter angle. It is found that misalignment leads to changes in the relative amplitudes of the various contributions. In addition, the more highly collimated the beam, the more pronounced are the effects. It is shown that the signal maximum is not a; reliable indicator of receiver alignment;In the third problem the complex transducer point is applied to generate 3-D rotationally symmetric Gaussian beams in transmission to model air-coupled ultrasonic beam interaction with plates of plexiglas and composite. Studying the transmission coefficient permits characterization both isotropic and anisotropic material from narrowband experimental data. A comparison between a 2-D and 3-D analysis is shown. At all but the lowest incident angles, the differences are reasonably small;In the fourth problem the complex transducer point is applied to wideband transducer operation and employed to study the Lamb wave reflection frequency spectrum, which allows efficient and precision materials characterization and inversion. It is found that a full 3-D calculation is necessary to model the experiments and thereby infer the imaginary elastic stiffnesses accurately.