Rayleigh waves have unique characteristics that make them ideal for this application. The wave propagation mode is inherently non-dispersive. As surface waves, Rayleigh waves have a penetration depth of nearly one wavelength. Therefore, a depth profile of the changes in the elastic properties of a material can be determined by performing a number of velocity measurements at different frequencies. This approach has been used to map the depth profile of the Rayleigh wave as a function of depth in the aluminum-lithium alloy.

Additional features of Rayleigh wave propagation make them suitable for near surface materials characterization. The waves are bound to the surface of a material and are able to travel on curved surfaces. This includes rough surfaces that are typically encountered from machining processes. Rayleigh waves are twodimensional waves and, thus, attenuate as a function of I /r, where r is the distance of propagation. This enables them to travel much further than bulk waves, which attenuate as a function of I /r2. Rayleigh waves have the additional benefit of being able to eliminate the effect of anisotropy if the direction of propagation of the surface wave is held constant with respect to the anisotropy. Furthermore, Rayleigh waves are ideal for making measurements on all types of materials, including composites and polymers.

The measurements that have been performed on the near surface lithium depleted samples demonstrated the ability to detect the changes in the lithium concentration. The depth profile of the lithium depletion was established by performing Rayleigh wave measurements at frequencies between 1 and 30 MHz, which correspond to penetration depths of 3 to 0.1 mm, respectively. Additional measurements have demonstrated the ability to obtain the same depth profile by using toneburst generation and driving a broad band transducer at different frequencies to determine the velocity changes as a function of depth. The variation in the material properties of the aluminum-lithium alloy was subsequently confirmed by optical microscopy.

An alternative approach to completing the measurements was discussed. For this approach, a laser source was used, in the thermoelastic region, to generate a broad-based Rayleigh wave. By using a broad band detector, the received signal was filtered over different bandwidths to obtain the velocity change at different penetration depths. This approach demonstrated the ability to obtain the full depth profile in a single measurement.

The presentation concluded with a brief review of the potential areas for future growth of NDE. The current market for NDE, especially in flaw detection, is relatively mature, with projected growth rates of approximately 4% per year. The total size of the NDE market is also small, only $400 million per year in the United States. To achieve significant growth, NDE must expand beyond current defect detection applications and identify opportunities in process control. To realize this goal, the implementation of the NDE methodology must be performed voluntarily (not due to regulations) and realize a substantial amount of cost savings for the manufacturer. Eric would appreciate notifications of any such applications being forwarded to his E-mail address, lindgren@indqual.com.