Volume 115, Issue 5, May 2004
Index of content:
- NONLINEAR ACOUSTICS 
115(2004); http://dx.doi.org/10.1121/1.1695012View Description Hide Description
A quantitative study of the low-frequency parametric modulation of a pulsed surface acoustic wave(SAW) by a partially closed fatigue crack is described. In situultrasonic measurements were performed during a fatigue test for different crack lengths and static opening loads. The crack is initiated in the plastic-yielding zone induced by a surface cavity, and clamped due to the constraint of the surrounding elastic medium. Small periodic loading, superimposed on a static crack-opening load, changes the open crack segment length and/or the crack interfacial condition producing nonlinear modulation of the reflected ultrasonic pulses. The modulation spectrum is related quantitatively to the crack length and to the crack opening–closure behavior. It is demonstrated that the application of a small static crack-opening load with the modulation load could considerably enhance crack detectability. The increase of the second modulation harmonic is pronounced when the crack is nearly closed and when it is nearly open. Also, it is observed that the maximum modulation occurs at different static opening loads depending on the crack length relative to the plastic-yielding zone size. A low-frequency scattering model is presented based on the mechanism of modulation of the open/close segment length of the partially opened crack. The modeling results compare favorably with experiment.
Numerical and experimental analysis of second-order effects and loss mechanisms in axisymmetrical cavities115(2004); http://dx.doi.org/10.1121/1.1687734View Description Hide Description
This paper deals with the analysis of finite amplitude acoustic waves in three-dimensional resonantcavities. A specific finite element model is proposed which includes: (i) the pressure field nonlinearity using a perturbative method; (ii) the loss mechanisms using an experimentally determined effective bulk attenuation or modeling viscous and thermal losses at the walls and acoustic radiation through the aperture with complex impedances. An axisymmetrical cavity with transversal dimension larger than the wavelength has been experimentally studied. The acoustic field is generated by a high-power flexurally vibrating transducer which generates high pressures. Measurements are performed for the fundamental and second-order pressure components for several cavityresonance modes. An important standard deviation is observed. Experimental data are compared to predicted pressure field distributions. Numerical models describe the pressure distribution correctly but overestimate the amplitude.
Nonlinear waveform distortion and shock formation in the near field of a continuous wave piston source115(2004); http://dx.doi.org/10.1121/1.1695433View Description Hide Description
A classical effect of nonlinear acoustics is that a plane sinusoidal acoustic wave propagating in a nonlinear medium transforms to a sawtooth wave with one shock per cycle. However, the waveform evolution can be quite different in the near field of a plane source due to diffraction. Previous numerical simulations of nonlinear acoustic waves in the near field of a circular piston source predict the development of two shocks per wave cycle [Khokhlova et al., J. Acoust. Soc. Am. 110, 95–108 (2001)]. Moreover, at some locations the peak pressure may be up to 4 times the source amplitude. The motivation of this work was to experimentally verify and further explain the phenomena of the nonlinear waveform distortion. Measurements were conducted in water with a 47-mm-diameter unfocused transducer, working at 1-MHz frequency. For pressure amplitudes higher than 0.5 MPa, two shocks per cycle were observed in the waveform beyond the last minimum of the fundamental harmonic amplitude. With the increase of the observation distance, these two shocks collided and formed one shock (per cycle), i.e., the waveform developed into the classical sawtooth wave. The experimental results were in a very good agreement with the modeling based on the Khokhlov–Zabolotskaya–Kuznetsov (KZK) equation.