DETERMINATION OF ELASTIC-MODULI, DENSITY, ATTENUATION, AND THICKNESS OF A LAYER USING ULTRASONIC SPECTROSCOPY AT 2 ANGLES

An ultrasonic method for determination of the complete set of acoustic al and geometrical properties of an isotropic layer embedded between t wo known materials (similar or dissimilar) is described. The method al lows simultaneous determination of the layer thickness, density, elast ic moduli, and attenuation (longitudinal and shear) from normal and ob lique incidence reflection (transmission) frequency spectra. Reflectio n (transmission) from the layer is represented as a function of six no ndimensional parameters which are determined from the two experimental ly measured spectra: one at normal and one at oblique incidence. The i ntroduction of the set of nondimensional parameters allows one to tran sform the reconstruction process from one search in a six-dimensional space to two searches in three-dimensional spaces (one search for norm al incidence and one for oblique). Thickness, density, and longitudina l and shear elastic moduli of the layer are calculated from the nondim ensional parameters determined. The sensitivity of the method to indiv idual properties and its stability against experimental noise are stud ied and the inversion algorithm is accordingly optimized. Experimental examples are given for a layer immersed in water and for a thermoplas tic joint. (C) 1997 Acoustical Society of America. [S0001-4966(97)0101 2-6].