Acoustic microscopy for spherical inclusion characterization

In this paper a new nondestructive ultrasonic technique is developed to distinguish small spherical voids from small spherical elastic and rigid inclusions. In many materials, such as light metal casting alloys, ultrasonic scattered echoes from void-type defects and heavy metal inclusions used for strengthening purposes can give the ultrasonic signal of the same order of magnitude. It is shown in this paper how one can extract the phase information of the reflected signal to distinguish the voids from solid inclusions. When phase-frequency curves are extended to the zero frequency, then voids and solid inclusions meet at two distinct points on the phase axis separated by a distance of approximately 180°. Thus voids and solid inclusions can be clearly distinguished. Experimental results are verified by theoretical predictions.