The Effect of Nonspherical Pores and Multiple Scattering on the Ultrasonic Characterization of Porosity

Abstract

Recently, several papers have dealt with the use of the frequency dependent ultrasonic attenuation to characterize gas porosity in structural solids such as cast aluminum [1–4]. The methods proposed provide an estimate of the volume fraction of pores and the average pore size. The estimate of the volume fraction for laboratory samples has been sufficiently accurate that it encourages thoughts of routine industrial use. In the past year, we have considered several corrections which may be of use in transferring the laboratory methods to industrial samples. Three corrections have been developed. The first is a simple way of accounting for the effects of as-cast surface roughness on the inference of ultrasonic attenuation from phase-sensitive scattering measurements. This topic will be dealt with in the companion paper by Nagy et al. [5]. The second correction deals with the effects of nonspherical pores on the inference of the volume fraction. This effect can be very substantial (e. g., a distribution of microcracks may lead to a large ultrasonic attenuation while having zero volume fraction). Finally, the third correction deals with the effects of multiple scattering. That is, the methods used up until now have relied on the scattering being uncorrelated (which is reasonable for low density gas porosity, i. e., volume fractions <2%). However, at higher volume fractions this assumption begins to breakdown. We propose a strategy based on the Kramers-Kronig relations for including multiple scattering effects. The last two corrections are the subject of this paper.