A NEW TECHNIQUE FOR DISTINGUlSIllNG INTERNAL VOIDS FROM SOLID INCLUSIONS K. I. Maslov, T. Kundu* and O. I. Lobkis Institute of Chemical Physics, Russian Academy of Science Kosygin Str.4, 117334 Moscow, Russia *Department of Civil Engineering and Engineering Mechanics University of Arizona, Tucson, AZ 85721, USA INTRODUCTION An acoustic microscope has been proven to be a very effective tool for visualization and characterization of small internal defects in solids[ 1]. The distinction of internal defects such as cracks and voids from solid inclusions is sometimes necessary for material evaluation. For example in case of light metal casting alloys ultrasonic scattered echo from pores and heavy metal inclusions used for strengthening purposes can give the ultrasonic signal of the same order of magnitude [2]. In this paper it is shown how the phase information of the reflected echo can be used to distinguish void signals from solid inclusion signals. Conventional acoustic imaging techniques that use only amplitude information and ignores the phase information can not distinguish between voids and inclusions. It is well known that the phase of the acoustic wave reflection coefficient of a boundary is determined by the ratio of the acoustic impedance of the two media adjacent to the boundary [3]. However in case of hidden defects in a material it is not easy to determine experimentally the exact value of the phase of reflection coefficient due to the unknown distance of the defect. The phase shift is also affected by the defect geometry and by the transmission of the wave at the liquid-solid interface. Application of the time-frequency domain analysis [4] and phase spectral analysis [5] to characterize the object under inspection has been widely discussed in the literature. It has been theoretically shown that the absolute phase shift and the relative acoustic impedance of the obstacle can be determined using these techniques. Recently, combined acoustic microscopy and acoustic spectroscopy techniques have been used to determine the complex value of reflection coefficient of a bulk material with a flat surface [6]. In the present paper we develop a method to distinguish spherical voids from solid inclusions by the phase-frequency dependence analysis. Review of Progress in Quantitative Nondestructive Evaluation, Vol. 15 Edited by D.O. Thompson and D.E. Chimenti, Plenum Press, New York, 1996 883