Porosity estimation using the frequency dependence of the ultrasonic attenuation

A new technique is reported for estimating the volume fraction of porosity in structural materials. The estimate for the volume fraction is proportional to the slope of the ultrasonic attenuation when plotted as a function of frequency. Both theory and experiment are considered. The theory, appropriate for dilute porosity, uses the uncorrelated, single-scatter approximation. An attenuation slope algorithm is derived within this approximation and its limits of validity are tested by computer simulation. Experimental tests consist of three parts. First, the method is compared with other existing techniques through estimates from the published data on gas porosity in aluminum casts. For the second test, cylindrical porosity is simulated by parallel through-holes drilled in aluminum blocks. Finally, the attenuation in porous graphite-epoxy samples is measured and compared with results predicted from theory.