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In 1965, Lloyd and Redwood predicted the existence of a new guided mode of wave propagation in solid-fluid-solid trilayers ͓Acustica 16, 169-173 ͑1965͔͒. In this so-called fluid mode, the two solid plates flex sideways in a symmetric way while the fluid in the thin gap between them is squeezed forward and backward parallel to the direction of wave propagation. This mode is slower than the lowest-order bending mode of the plates and asymptotically approaches the Stoneley-Scholte mode of the solid-fluid interface at high frequencies. While the other modes of the solidfluid-solid trilayer are very similar to the free vibrations of the free solid plates which are only slightly perturbed by the fluid coupling between the plates, the fluid mode cannot be related to any classical Lamb mode in the plates. It is shown in this paper that at low frequencies the fluid mode is truly a coupled vibration of the solid and the fluid, which contribute the stiffness and the inertia, respectively. The highly dispersive phase velocity of this mode was measured over a frequency range of one decade by using a laser interferometric technique. The experimental results were found to be in good quantitative agreement with the theoretical predictions. To the best of our knowledge, these measurements constitute the first conclusive experimental evidence of the existence of the dispersive fluid mode predicted by Lloyd and Redwood.

Recent research results indicate that eddy current conductivity measurements can be exploited for nondestructive evaluation of subsurface residual stresses in surfacetreated nickel-base superalloy components. According to this approach, first the depth-dependent electric conductivity profile is calculated from the measured frequency-dependent apparent eddy current conductivity spectrum. Then, the residual stress depth profile is calculated from the conductivity profile based on the piezoresistivity coefficient of the material, which is determined separately from calibration measurements using known external applied stresses. This paper presents new results that indicate that in some popular nickel-base superalloys the relationship between the electric conductivity profile and the sought residual stress profile is more tenuous than previously thought. It is shown that in delta-processed IN718 the relationship is very sensitive to the state of precipitation hardening and, if left uncorrected, could render the eddy current technique unsuitable for residual stress profiling in components of 36 HRC or harder, i.e.,