Volume 115, Issue 1, January 2004
Index of content:
- ULTRASONICS, QUANTUM ACOUSTICS, AND PHYSICAL EFFECTS OF SOUND 
115(2004); http://dx.doi.org/10.1121/1.1630999View Description Hide Description
This paper deals with the propagation of ultrasonicguided waves in adhesively bonded lap–shear joints. The topic is relevant to bond inspection by ultrasonic testing. Specifically, the propagation of the lowest-order, antisymmetric mode through the joint is examined. An important aspect is the mode conversion at the boundaries between the single-plate adherents and the multilayer overlap. The strength of transmission is studied for three different bond states in aluminum joints, namely a fully cured adhesive bond, a poorly cured adhesive bond, and a slip bond. Theoretical predictions indicate that the dispersive behavior of the guided waves in the multilayer overlap is highly dependent on bond state. Experimental tests are conducted in lap–shear joints by a hybrid, broadband laser/air-coupled ultrasonic setup in a through-transmission configuration. The Gabor wavelet transform is employed to extract energy transmission coefficients in the 100 kHz 1.4 MHz range for the three different bond states examined. The cross-sectional mode shapes of the guided waves are shown to have a substantial role in the energy transfer through the joint.
Ultrasonic interferometry for the measurement of shear velocity and attenuation in viscoelastic solids115(2004); http://dx.doi.org/10.1121/1.1631944View Description Hide Description
A method for the measurement of the shear properties of solid viscoelastic materials is presented. The viscoelastic material is cut into a cylindrical sample which is clamped between two rods. The transmission and reflection coefficientspectra of the fundamental torsional mode through the sample are measured by means of two pairs of piezoelectric transducers placed at the free ends of the rod-sample-rod system. Such spectra exhibit maxima and minima which occur approximately at the resonance frequencies of the free viscoelastic cylinder. Therefore, the shear velocity can be obtained by measuring the frequency interval between two consecutive maxima or minima. The shear attenuation is derived by best fitting the analytical expression of the reflection and transmission coefficients to the experimental spectra. The test is very quick to set up as the sample is simply clamped between the two rods.
115(2004); http://dx.doi.org/10.1121/1.1631938View Description Hide Description
Local variation in surface skimming longitudinal wave (SSLW) velocity has been measured using a scanning acoustic microscope. A very narrow width electrical impulse has been used to excite the transducer of the acoustic lens. This permits the separation of the SSLW signal from the direct reflected signal in the time domain. A simple method of measuring the time delay between the directly reflected signal and the SSLW signal at two defocuses has been utilized for the local measurement of SSLW velocity. The variation in the SSLW velocitymeasured over an area of the sample is scaled and presented as an image. The method has been implemented to image the variation of the SSLW velocity around a crack tip in a sample of Ti-6Al-4V. Since the SSLW velocity is known to change linearly with the stress, the SSLW velocityimage is considered as a representation of the image of stress around the crack tip. Local stress variation in the same region of the crack tip is directly measured using x-ray diffraction. The SSLW velocityimage is compared with the x-ray diffraction stress image. The contrast in the two images, spatial resolution, and the penetration depth into the sample of acoustic waves and x rays are discussed.
115(2004); http://dx.doi.org/10.1121/1.1630997View Description Hide Description
The scanning laser source (SLS) technique has been proposed recently as an effective way to investigate small surface-breaking cracks. By monitoring the amplitude and frequency changes of the ultrasound generated as the SLS scans over a defect, the SLS technique has provided enhanced signal-to-noise performance compared to the traditional pitch–catch or pulse–echo ultrasonic methods. In previous work, either a point source or a short line source was used for generation of ultrasound. The resulting Rayleigh wave was typically bipolar in nature. In this paper, a scanning laser line source (SLLS) technique using a true thermoelastic line source (which leads to generation of monopolar surface waves) is demonstrated experimentally and through numerical simulation. Experiments are performed using a line-focused Nd:YAG laser and interferometric detection. For the numerical simulation, a hybrid model combining a mass-spring lattice method (MSLM) and a finite difference method (FDM) is used. As the SLLS is scanned over a surface-breaking flaw, it is shown both experimentally and numerically that the monopolar Rayleigh wave becomes bipolar, dramatically indicating the presence of the flaw.