Through-transmission ultrasonic attenuation

W. E. Woodmansee, Through-transmission ultrasonic attenuation measurements on adhesively-bonded structures, in Ultrasonic Materials Characterization, NBS SP 596, pp. 425-432, Proc. 1st International Symposium, Gaitherburg, MD (June 7-9, 1978). 

Since attenuation and porosity are related, traditional porosity determination in composites is performed as attenuation measurements using ultrasonic tone burst through-transmission. 

CSIRO Minerals has developed a particle size analyzer (UltraPS) based on ultrasonic attenuation and velocity spectrometry for particle size determination [269]. A gamma-ray transmission gauge corrects for variations in the density of the slurry. UltraPS is applicable to the measurement of particles in the size range 0.1 to 1000 pm in highly concentrated slurries without dilution. The method involves making measurements of the transit time (and hence velocity) and amplitude (attenuation) of pulsed multiple frequency ultrasonic waves that have passed through a concentrated slurry. From the measured ultrasonic velocity and attenuation particle size can be inferred either by using mathematical inversion techniques to provide a full size distribution or by correlation of the data with particle size cut points determined by laboratory analyses to provide a calibration equation. 

Attenuation i.s due to scattering of ultrasonic waves by the fibers and by absorption into isothermal resin. However, attenuation is reduced with the through-transmission method and by the use of low frequencies. Defects such as voids, resin rich or resin starved... 

In principle, the ultrasonic techniques described for solid-liquid flow measurement can be applied to measure air flow rate and particle velocity. Direct measurement of air flow rate by measuring upstream and downstream transit times has been demonstrated. But, the Doppler and cross-correlation techniques have never been applied to solid/gas flow because the attenuation of ultrasound in the air is high. Recent developments have shown that high-frequency (0.5-MHz) air-coupled transducers can be built and 0.5-MI Iz ultrasound can be transmitted through air for a distance of at least 1 in. Thus, the cross-correlation technique should be applicable to monitoring of solid/gas flow. Here, we present a new cross-correlation technique that does not require transmission of ultrasonic waves through the solid/gas flow. The new technique detects chiefly the noise that interacts with the acoustic field established within the pipe wall. Because noise may be related to particle concentration, as we discussed earlier, the noise-modulated sound field in the pipe wall may contain flow information that is related to the variation in particle concentration. Therefore, crosscorrelation of the noise modulation may yield a velocity-dependent correlation function. 

Conventional ultrasonic methods include the pulse-echo, the pulse-transmission and the pulse-resonance techniques [104]. Depending on the incidence of the piezo transducer with respect to the structural surface as well as on their design, P-waves, S-waves or a combination of both can be generated within the structure. P-waves are best suited for the inspection of thick components, for through-the-thickness damage detection, and are quite effective for the detection of anomalies along the sound path. By the pulse-echo method, detects are detected in form of additional echoes. In the pulse-transmission method wave dispersion and attenuation due to diffused damage in the material indicate possible defects [103]. 

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