Volume 119, Issue 2, February 2006
Index of content:
- ULTRASONICS, QUANTUM ACOUSTICS, AND PHYSICAL EFFECTS OF SOUND 
119(2006); http://dx.doi.org/10.1121/1.2159291View Description Hide Description
A series of three papers by Sabatier and coauthors reported successive improvements to both the theory and measurement of excess attenuation due to the wall porosity of . We report an independent measurement of excess attenuation due to wall porosity of Celcor conducted to study the attenuation due to viscous and thermal effects separately for a thermoacoustic refrigeration application. A 38-mm-long sample was placed at the center of a 700-mm-long electrodynamically driven plane wave resonator. The location of the Celcor within the standing wave would select different contributions from viscosity and thermal conductivity. As per the technique described by Moldover et al. [Modern Acoustical Techniques for the Measurement of Mechanical Properties (Academic, San Diego, 2001)], the quality factors were measured between and in air at atmospheric pressure on a pristine sample. Wall pores were then blocked by an in situpolymerization sealing process. There was no observed decrease in attenuation of even modes (dominated by thermal loss) for the coated sample below . The volume exclusion due to pore sealing was inferred by the increase in even mode resonance frequencies in good agreement with other determinations of wall porosity. Thermoviscous losses were computed with DELTAE and compared to measured losses.
119(2006); http://dx.doi.org/10.1121/1.2146109View Description Hide Description
An oil-film thickness monitoring system capable of providing an early warning of lubrication failure in rolling element bearings has been developed. The system is used to measure the lubricant-film thickness in a conventional deep groove ball bearing (shaft diameter , ball diameter ). The measurement system comprises a broadband ultrasonic focused transducer mounted on the static outer raceway of the bearing. Typically the lubricant-films in rolling element bearings are between in thickness and so are significantly smaller than the ultrasonic wavelength. A quasistatic spring model is used to calculate oil-film thickness from the measuredreflection coefficient data. An accurate triggering system has been developed to enable multiple reflection coefficientmeasurements to be made as the contact ellipse sweeps over the measurement location. Experiments are described in which the loading conditions and rotational speed are varied. Lubricant-film thickness distributions measured ultrasonically are described and are shown to agree well with the predictions from classical elastohydrodynamic (EHD) lubrication theory, particularly at high radial loads and low rotary speeds. A range of parameters affecting the performance of the measurement are discussed and the limits of operation of the measurement technique defined.
A phase reconstruction algorithm for Lamb wave based structural health monitoring of anisotropic multilayered composite plates119(2006); http://dx.doi.org/10.1121/1.2149775View Description Hide Description
Platelike structures, made of composites, are being increasingly used for fabricating aircraft wings and other aircraft substructures. Continuous monitoring of the health of these structures would aid the reliable operation of aircrafts. This paper considers the use of a Lamb wave based structural health monitoring (SHM) system to identify and locate defects in large multilayered composite plates. The SHM system comprises of a single transmitter and multiple receivers, coupled to one side of the plate that send and receive Lamb waves. The proposed algorithm processes the data collected from the receivers and generates a reconstructed image of the material state of the composite plate. The algorithm is based on phased addition in the frequency domain to compensate for the dispersion of Lamb waves. In addition, small deviations from circularity of the slowness curves of Lamb wave modes, due to anisotropy, are corrected for by assuming that the phase and group velocity directions coincide locally. Experiments were performed on an anisotropic multilayered composite plate containing a single defect. Reconstruction of the defect is carried out using data for a weakly anisotropicLamb wave mode as a proof of concept of the proposed algorithm.