The third-order elastic moduli of several isotropic polycrystalline metals have been determined from measurements of the velocities of both longitudinal and shear ultrasonic waves in uniaxially stressed specimens. In each case, the wave-propagation direction was chosen normal to the applied stress, and the shear waves were polarized either normal or parallel to the stress direction. Hence, a unique evaluation of all three thirdorder moduli was possible using the recent theory of Thurston and Brugger, specialized for isotropic symmetry. The measuring equipment is based on a new variation of the pulse-echo interferometric technique and is capable of resolving velocity changes of a few parts in 106. Results are presented for several steels, aluminum alloys, magnesium, tungsten, and molybdenum, and are shown to be in good agreement with alternative nonlinear elasticity data, including static measurement of the pressure derivatives of the bulk and shear moduli.

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Illustrations are given of information carried in the waveforms and frequency spectra of acoustic emissions. The effects of multiple reflections and resonances are discussed. A model of the acoustic-emission source wave is developed, and arguments are given why this wave should be a pulselike function, rather than an oscillatory function of stress. Further use of this model may allow more quantitative treatment of emission amplitudes, energies, and spectra. Experimental results show the use of two instruments for the evaluation of emission spectra. The feasibility of using frequency analysis to obtain information about source events is demonstrated.

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The moird fringe wave visualization technique produces instantaneous records of the wave field, from which the propagation characteristics of the surface may be deduced. This is particularly useful for surfaces whose properties are time variant. Although the wave damping properties of the polymer monolayers examined were considerably more complicated than those of monolayers of simpler surface active agents, existing theory was shown to be adequate in explaining the observed effects on the basis of the surface tension and surface dilational elasticity of the monolayer. This agreement of theoretical prediction with observation supports the evidence for the existence of longitudinal surface waves. Using the moird technique to investigate the time variation of solution-adsorbed monolayers of poly(ethylene oxide) on water, it was found that this polymer produces from solution the same type of stable monolayer as is produced by spreading the material in a monolayer on a pure water substrate.

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Stress waves generated internally by deforming materials provide information about the operative deformation mechanisms. These stress waves are known as acoustic emissions. Acoustic emission stands alongside conventional ultrasonic methods as a useful technique of nondestructive testing. The frequency spectrum of the emissions varies according to the nature and location of the deformation, and it may also provide information about the integrity of the structure under test. On fundamental grounds we conclude that the emission waveform at source is a stress pulse (displacement step) whose frequency spectrum extends from zero up to a frequency which depends on the duration of the source event. Between generation and detection, the spectrum is modulated by structural resonances which tend to obscure the information contained in the source waveform. These effects should be borne in mind during the design of practical emission detection systems. Experimental results from adhesive bonds confirm the validity of our theoretical approach and show how information about the source event may be gained from the spectrum by looking beyond the structural resonances.