Micromechanical modeling of viscoelastic voided composites in the low-frequency approximation.

The self-consistent model of Cherkaoui et al. [J. Eng. Mater. Technol. 116, 274-278 (1994)] is used to compute the effective material moduli of a viscoelastic material containing coated spherical inclusions. Losses are taken into account by introducing the frequency-dependent, complex shear modulus of the viscoelastic matrix. Mode conversion appears through the localization tensors that govern the micromechanical behavior near the inclusions. The results are compared with the scattering model and the data of Baird et al. [J. Acoust. Soc. Am. 105, 1527-1538 (1999)]. The two models are in good agreement. The advantage of the self-consistent model is that it is applicable to the case of nonspherical inclusions embedded in anisotropic materials.

Time-frequency representations of Lamb waves.

The objective of this study is to establish the effectiveness of four different time-frequency representations (TFRs)--the reassigned spectrogram, the reassigned scalogram, the smoothed Wigner-Ville distribution, and the Hilbert spectrum--by comparing their ability to resolve the dispersion relationships for Lamb waves generated and detected with optical techniques. This paper illustrates the utility of using TFRs to quantitatively resolve changes in the frequency content of these nonstationary signals, as a function of time. While each technique has certain strengths and weaknesses, the reassigned spectrogram appears to be the best choice to characterize multimode Lamb waves.

A new method for the absolute measurement of piezoelectric coefficients on thin polymer films

A new quasistatic method to measure piezoelectric coefficients on thin polymer films is presented. This method is based on a combined experimental/analytical approach, where small polymer samples (6 mm x 3 mm x 110 microm) are encapsulated in a soft silicone rubber and an electric field is applied across their thickness (3-direction). Strains are measured optically along three perpendicular directions using a laser Doppler vibrometer, and the experimental measurements are used in a Rayleigh-Ritz energy minimization procedure implemented symbolically in MATHCAD, which yields the absolute piezoelectric coefficients d(3ii). These measured coefficients are material properties of the polymer and do not depend on the specific boundary conditions of the problem. The validity of the method is established using the ATILA finite element code. Experimental values of d(311), d(322), and d(333) obtained for polyvinylidene fluoride (PVDF) at room temperature, in the frequency range 500-2000 Hz, are presented and compared with existing data; excellent agreement is found. The extension of the method to the determination of electrostrictive coefficients on soft polyurethane materials is introduced.

Dynamic thermal response of single-mode optical fiber for interferometric sensors.

The dynamic temperature phase sensitivity of a three-layer optical fiber is calculated for unjacketed as well as Al- and Hytrel-coated fibers. The calculations include both the variation of the refractive index with temperature and the thermally induced axial and radial strains. The calculated phase sensitivity indicates that it is currently possible to measure a 1-microdegree C temperature change at frequencies exceeding 50 kHz with 1 cm of a metal coated optical fiber.

Magneto-optic coupling coefficient for fiber interferometric sensors.

The magneto-optic sensitivity (coupling coefficient) has been measured as a function of frequency from 15 Hz to 3 kHz for a fiber interferometer incorporating a magnetostrictive nickel toroid. The frequency response of the coupling coefficient was found to be essentially flat from 15 Hz to approximately 600 Hz, where eddy-current losses become significant for the sample tested.

Temperature-induced optical phase shifts in fibers.

The static thermal sensitivity of the optical phase in bare and jacketed fibers has been studied both analytically and experimentally. Taking into account the exact fiber composition and geometry, the strains have been determined from the thermally induced stresses using the appropriate boundary conditions, and the resulting phase shift has been calculated. The results of this analysis are found to be in agreement with experimental results obtained from measurements employing a Mach-Zehnder fiber interferometer.

Static pressure sensitivity amplification in interferometric fiber-optic hydrophones.

The induced optical phase change produced when a static pressure is applied to the test arm of an interferometric single-mode fiber optic hydrophone is examined in terms of hydrostatic and radial mechanical models. The expressions for the models are given in terms of a 3-D solution to the equations of elastostatics for multilayered cylinders. The induced phase change is calculated using both models for various values of the diameter and elastic properties of fiber jacket materials. It is shown that the phase change predicted from the 3-D approach for each model can be adequately described in terms of much simpler 2-D plane strain models. Calculations show that the hydrophone sensitivity of a jacketed fiber is amplified compared with a bare fiber. The largest increase in sensitivity is predicted with the radial model. Calculated sensitivities for the hydrostatic model are shown to correspond closely in value with static pressure sensitivity measurements for the experimental arrangement used here.

Magnetic field sensitivity of an optical fiber with magnetostrictive jacket.

Expressions are developed for the strains induced by a weak axial magnetic field in an optical fiber with a magnetostrictive jacket. The magnetic field sensitivity of the optical fiber is calculated as a function of jacket thickness for a variety of magnetostrictive jacket materials. The pressure response for various jacket thicknesses is also considered to minimize this effect as a noise source.