Volume 104, Issue 4, October 1998
Index of content:
- ULTRASONICS, QUANTUM ACOUSTICS, AND PHYSICAL EFFECTS OF SOUND 
On the use of acoustic contrast to distinguish between agglomerates of finely dispersed polymeric particles104(1998); http://dx.doi.org/10.1121/1.423731View Description Hide Description
This paper demonstrates how differences in the morphology of agglomerates of fine particles can be detected based on the response of the agglomerates to a low-intensity resonant acoustic field. The technique is based upon observing whether the agglomerates are collected in nodal or antinodal positions when suspended in different fluids spanning an appropriate range of acoustic properties. Experiments performed with agglomerates of acrylonitrile–butadiene–styrene copolymeric particles suspended in 2-propanol–water mixtures illustrate the technique.
104(1998); http://dx.doi.org/10.1121/1.423732View Description Hide Description
The performance of high-temperature composites can be significantly affected by the presence of residual stresses. These stresses arise due to the mismatch of thermal expansion coefficients between matrix and fibers during cooling from fabrication temperature to room temperature. This effect is especially pronounced in metal matrix and intermetalliccomposites. It can lead to plastic deformation, matrix cracking and fiber/matrix interface debonding. Elastic wave propagation in homogeneously stressed media has been frequently addressed in the literature. However, the effect of nonhomogeneous stress distribution has not been investigated. This is especially important since the average residual stresses are zero in equilibrium and thus their distribution is inherently nonhomogeneous. In this paper, the feasibility of using ultrasonic techniques for residual stress assessment in composites is addressed. A theoretical method is presented for determining the velocities of ultrasonic waves propagating through a composite material with a nonhomogeneous distribution of residual stresses. It is based on the generalized self-consistent multiple scattering model. Calculated results for longitudinal and shear ultrasonic wave velocities propagating perpendicular to the fiber direction in SCS-6/Ti composite with and without residual stresses are presented. They show that velocity changes due to the presence of stresses are on the order of 1%.
104(1998); http://dx.doi.org/10.1121/1.423733View Description Hide Description
Resonant ultrasound spectroscopy (RUS) is a powerful tool for determining the elastic properties of solids at high temperature. Before RUS can be used to measure the pressure derivatives of elasticity, however, effects of boundary conditions between the pressurizing gas and specimen must be understood. Data are presented that show effects of different pressurizing gases on the apparent or measured values of of fused silica spheres, where is the shear modulus, is pressure, and is temperature. The value of is found to depend on the molecular mass, of the pressurizing gas via when is in grams. Extrapolating to zero gas mass gives a value bracketed by results from previous plane-wave, ultrasonicmeasurements. An alternative analysis in which effects of the pressurizing gas are removed by theoretical concerns is also presented and suggests that is Our results quantify the effect of pressurizing gas on measuredpressure derivatives of when using RUS and indicate that systematic problems in transducer-specimen bonding from previous acoustic plane-wave measurements on fused silica at elevated pressure are not yet measurable.
104(1998); http://dx.doi.org/10.1121/1.423769View Description Hide Description
This paper describes the experimental confirmation of a numerical model of laser-generated ultrasound, where a 100-μs-long laser pulse was intensity modulated using a Bragg cell. Temporal modulation was used to produce narrow-band ultrasound signals, which can substantially improve signal-to-noise ratios over ultrasound generated by Q-switched laser pulses. A curve fit function that represents Bragg cell modulation is given. Experimental results were obtained using a piezoelectric transducer(PZT) on an aluminum test block, and good agreement between theory and experiment was obtained while the modulated pulse train was on. Though the simple PZTmodel used here results in some discrepancies between theory and experiment late in the signals, it is concluded that the numerical model reliably predicts Bragg cell modulated laser-generated displacements.