ULTRASONIC MEASUREMENT OF THE THICKNESS, PHASE-VELOCITY, DENSITY OR ATTENUATION OF A THIN-VISCOELASTIC PLATE .2. THE INVERSE PROBLEM

Part II of this two-part paper is concerned with the inverse problem: given the measured transfer function, deduce any one of the four acous tical properties of a thin, linear-viscoelastic plate (thickness, wave -speed, density or attenuation) given the remaining three, through a c omparison with the theoretically predicted transfer function. A system atic and detailed analysis of the sensitivity of the through-transmiss ion transfer function to the four acoustical parameters is presented. An inverse algorithm, which utilizes the well-known secant method in c onjunction with the method of least squares, is used to deduce the aco ustical parameters. The technique is fully automated and computer cont rolled and can be easily adapted for in situ NDE applications. The tec hnique is used to measure the acoustical properties of plates made up of aluminium (thickness = 100 mu m to 2 mm), steel (25 mu m-500 mu m), brass (1.625 mm), glass (1 mm), Plexiglas (500 mu m-2 mm) and bulk ep oxy (1.54 mm), using commercially available, low frequency transducers (0.5-10 MHz). The errors are frequency dependent and become progressi vely worse as frequency decreases. In the case of thickness, where the sensitivity is relatively large, excellent results were obtained down to (h/lambda) = 0(10(-2)), where h is the layer thickness and lambda, is the wavelength.