Frequency-domain ultrasonic NDE of three-layered media : the inverse problem

This paper presents a frequency-domain ultrasonic technique for a simultaneous determination of the thickness (h) and wavespeed (c) of the individual layers comprising a multilayered medium using the entire complex spectrum. Each of the layers may be "thin" ; by thin we mean that the successive reflections of an ultrasonic pulse from the two faces of a layer are non-separable in the time domain. Plane longitudinal waves which are normally incident upon the medium are considered. A systematic analysis of the sensitivity of the complex-valued transfer function to the acoustical parameters of each of the layers has been carried out. Successful inversion results are presented for several three-layer specimens : aluminum/water/aluminum, aluminum/water/titanium, and titanium/water/titanium