Determination of elastic moduli, density, attenuation, and thickness of a layer using ultrasonic spectroscopy at two angles

Abstract

An ultrasonic method for determination of the complete set of acoustical and geometrical properties of an isotropic layer embedded between two known materials (similar or dissimilar) is described. The method allows simultaneous determination of the layer thickness, density, elastic moduli, and attenuation (longitudinal and shear) from normal and oblique incidence reflection (transmission) frequency spectra. Reflection (transmission) from the layer is represented as a function of six nondimensional parameters which are determined from the two experimentally measured spectra: one at normal and one at oblique incidence. The introduction of the set of nondimensional parameters allows one to transform the reconstruction process from one search in a six-dimensional space to two searches in three-dimensional spaces (one search for normal incidence and one for oblique). Thickness, density, and longitudinal and shear elastic moduli of the layer are calculated from the nondimensional parameters determined. The sensitivity of the method to individual properties and its stability against experimental noise are studied and the inversion algorithm is accordingly optimized. Experimental examples are given for a layer immersed in water and for a thermoplastic joint.