Porosity in structural materials limits their ultimate strength and hence their utility. Recently several studies which discuss the porosity induced ultrasonic attenuation have appeared [1][2][3][4][5][6][7][8]. Porosity assessment by ultrasonic attenuation measurement involves two principal problems: first, how to relate the porosity induced ultrasonic attenuation to porosity parameters such as average pore radius and volume fraction, and second, how to separate the sought porosity induced attenuation from other components contributing to the actually measured total attenuation.

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The acoustoelastic method is a well-established way to obtain third-order elastic constants in bulk materials. Modifications are needed to extend this technique to obtain higher orders, e.g., fourth-order elastic constants, or to increase accuracy to adopt the technique for very thin layers, e.g., adhesive bonds. These higher-order nonlinearities are separated from the stronger first-order nonlinearities (associated with the third-order elastic constants) by the appropriate choice of polarization and harmonic analysis of dynamic acoustoelastic results. It has been shown that inclusion of the fourth-order elastic constants will lead to strength-related mechanical properties of the adhesive bond. [This work was supported by the Center for NDE operated by the Ames Laboratory, USDOE for the Air Force Aeronautical Laboratories/Materials Laboratory under Contract No. SC-89-168 with Iowa State University.]

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