The Propagation Characteristics of the Plate Modes of Acoustic Emission Waves in Thin Aluminum Plates and Thin Graphite/Epoxy Composite Plates and Tubes

Acoustic emission was interpreted as modes of vibration in finite aluminum and graphite/epoxy plates. The `thin plate'' case of classical plate theory was used to predict dispersion curves for the two fundamental modes described by the theory and to calculate the shapes of flexural waveforms produced by a vertical step function loading. There was good agreement between the theoretical and experimental results for the aluminum. Composite materials required the use of a higher order plate theory (Reissner-Mindlin) combined with lamination theory in order to get good agreement with the measured velocities. Plate modes were shown to be useful for determining the direction of motion of a source. Thus, with a knowledge of the material, it may be possible to ascertain the type of the source. For example, particle impact on a plate could be distinguished from a crack growing in the plate. A high fidelity transducer was needed to distinguish the plate modes. After evaluating several types of transducers, a broadband ultrasonic transducer was found which satisfied the fidelity requirement and had adequate sensitivity over the 0.1 to 1 MHz range. The waveforms were digitized with a 5 MHz transient recorder. The dispersion curves were determined from the phase spectra of the time dependent waveforms. The aluminum plates were loaded by breaking a 0.5 mm. pencil lead against the surface of the plate. By machining slots at various angles to the plane of a plate, the direction in which the force acted was varied. Changing the direction of the source motion produced regular variations in the measured waveforms. Four composite plates with different laminate stacking sequences were studied. To demonstrate applicability beyond simple plates, waveforms produced by lead breaks on a thin-walled composite tube were also shown to be interpretable as plate modes. The tube design was based on the type of struts proposed for Space Station Freedom''s trussed structures.

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