Jacek Jarzynski

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.

Immersion apparatus for ultrasonic measurements in polymers

An immersion apparatus for making ultrasonic measurements in polymers was constructed. A unique feature of this apparatus is that the specimen is held vertically, and the transducers are rotated in order to produce shear waves in the specimen. This arrangement allows measurements to be made through the melting point of crystalline polymers. Sound‐speed measurements, accurate to ±2%, were made on 25 polymers at room temperature and on five of these polymers as a function of temperature. On 15 of the polymers, both longitudinal and shear sound speeds were measured. The longitudinal wave speeds ranged from 2820 to 1020 m/sec, while shear wave speeds ranged from 1230 to 650 m/sec. Using these sound speeds and measured densities, the elastic constants of these polymers were calculated. For four polymers elastic constants as a function of temperature were calculated.

Time-frequency representation of Lamb waves using the reassigned spectrogram

This brief note reports on a study that applies the reassigned spectrogram (the reassigned energy density spectrum of the short-time Fourier transform [STFT]) to develop the dispersion curves for multimode Lamb waves propagating in an aluminum plate. The proposed procedure first uses the spectrogram to operate on a single, laser-generated and detected waveform to develop the dispersion relationship for this plate. Next, a reassignment procedure is used to refine the time-frequency resolution of the calculated dispersion curves. This reassignment operation clarifies the definition of the measured modes. This study demonstrates that the reassigned spectrogram is capable of distinguishing multiple, closely spaced Lamb modes in the ultrasonic frequency range.

The temperature, pressure and time dependence of lubricant viscosity

Abstract The general form of the pressure (Proc. Am. Acad. Arts Sci. 77 (1949) 117) and temperature (Physical properties of molecular crystals, liquids and glasses (1968) 350) dependence of viscosity has been known for at least 50 years. Viscosity varies with temperature in a greater than exponential manner and temperature–viscosity equations generally allow for an unbounded viscosity at some characteristic temperature. At high-pressures the pressure–viscosity response is likewise greater than exponential, often following a less than exponential response at low-pressures. In spite of this known behavior, tribologists working in EHL have generally assumed less than exponential pressure response as a means of applying the Eyring stress aided thermal activation theory to the viscous regime of EHL traction. As justification, time dependence of the lubricant properties in the response to a pressure transient has been advanced. We present acoustic, capillary and impact measurements for timescales less than EHL. While time dependence of properties may be important in the viscoelastic regime of traction, this paper will show that for the timescale of viscous response, a significant time dependence of viscosity is unlikely.

Miniature, high performance, low-cost fiber optic microphone

A small, high performance fiber optic microphone has been designed, fabricated, and tested. The device builds on a previous design utilizing a thin, seven-fiber optical probe, but now adds a micromachined 1.5μm thick silicon diaphragm active element. The resulting sensor head is thin (several millimeters) and light, and the overall microphone system is less expensive than conventional microphones with comparable performance. Measurements in the laboratory using a standard free-field technique at high frequencies, an enclosed calibrator at lower frequencies, and pseudostatic pressure changes demonstrate uniform broadband response from near dc (0.01 Hz) up to near 20 kHz. The measured microphone internal noise is nearly flat over this band and does not exhibit noticeable levels of 1∕f noise. Over the audible portion of this band, the minimum detectable pressure is determined to be 680μPa per root Hz with further reductions possible using lower noise∕higher power light sources and∕or improvements in the diap...

Geoacoustic parameters in a stratified sea bottom from shallow‐water acoustic propagation

Due to the difficulty of direct measurement, there is a need to develop inverse techniques for remote sensing bottom geoacoustic parameters in the lowan mode measurements are extended to extract acoustic attenuation and speed in a horizontally stratified bottom in shallow water as a function of frequency and depth. The computational and experimental results show that, for a limited frequency band, we can find an equivalent depth profile of sea‐bottom acoustic attenuation with a linear frequency dependence that simulates the effect of nonlinear frequency dependence (without depth structure) on some field characteristics, such as the attenuation rate of individual mode, the frequency response of long‐range sound propagation, and the amplitude ratio of mode 2 to mode 1. However, the resultant equivalent negative gradient for the sea‐bottom attenuation is too strong to be accepted in light of available data. The conclusion is that nonlinear frequency dependence of the acoustic attenuation in the upper sedimen...

The use of optical fibers to enhance the laser generation of ultrasonic waves

It is shown that optical fibers offer a convenient way to control, both spatially and temporally, the laser light used to generate ultrasound in a solid. Proper phasing between the acoustic signals received at the transducer can enhance the ultrasonic signal in a particular direction. The principle is validated experimentally, with two optical fibers guiding the light emitted by a cw argon‐ion laser.

Non-contact optical fibre phased array generation of ultrasound for non-destructive evaluation of materials and processes

Abstract The use of non-contact laser techniques for the generation of ultrasound has extended the limits of the application of traditional ultrasonic techniques. This paper focusses on the use of one such non-contact laser technique, known as ‘optical fibre array’, to generate shear and surface waves. The shear wave experimental directivity pattern results are presented and compared with the theoretical results of a single source and an array source. The experimental directivity results for the surface wave are also presented, and compared with the theoretical results. The data show that the array enhances signal generation in the forward direction for both shear and surface waves. The array gain is also discussed. The receiver for the directivity measurements was a contact piezoelectric transducer.

Laser detection of sound

A differential laser Doppler velocimeter (LDV) has been assembled and tested to provide noninvasive absolute measurements of acoustic particle displacements of standing waves generated in a water‐filled tube. The principle of the technique [see K. J. Taylor, J. Acoust. Soc. Am. 59, 691–694 (1976)] is to measure the Doppler shift of laser light scattered from colloidal microparticles oscillating under the action of an acoustic field. The system tested is capable of detecting particle displacements of the order of a few nanometers with a bandwidth of several kilohertz. The performances and limitations of the system are discussed. In particular, the effect of Brownian motion is shown to produce only negligible broadening of the spectral density of the signal of interest. The sensitivity of the present LDV system is estimated to be very close to the shot noise limit of the photomultiplier tube used to detect the Doppler shift of the scattered light. Experimental results are obtained under controlled laborator...

DEVELOPMENT OF DISPERSION CURVES FOR TWO-LAYERED CYLINDERS USING LASER ULTRASONICS

In this paper, laser-ultrasonic techniques are employed to develop a quantitative understanding of the underlying principles of the propagation of guided circumferential waves in two-layered cylindrical components. The high-fidelity, broad-bandwidth, point source/receiver and noncontact nature of these optical techniques are critical elements to the success of this work. The experimental procedure consists of measuring a series of transient, circumferentially propagating waves in a cylindrical waveguide and then operating on these transient waveforms with signal-processing techniques to develop the dispersion relationship for that waveguide; this procedure extracts the steady-state behavior from a series of transient measurements. These dispersion curves are compared to theoretical values. There is good agreement between the experimental and theoretical results, thus demonstrating the accuracy and effectiveness of using laser-ultrasonic techniques to study the propagation of guided circumferential waves.