During pulse-echo ultrasonic testing (UT), inspectors
tend to consider the signals following the
primary return signal from a reflector to be irrelevant.
In the case where longitudinal waves are
introduced into the end or edge of a thin metal
plate, however, the noise signals following the
reflection of an interface do contain information
that is generally decipherable and quite useful for
thickness characterization. The objective of this
paper is to present a detailed physical understanding
of these secondary reflected longitudinal
waves and demonstrate their application for evaluating
the thickness of a sample. Both analytical
and finite element method (FEM) models are first
presented to provide the theoretical basis of this
phenomenon and its sensitivity to varying layer
thickness. Results from a series of experimental
studies for different structure configurations are
also presented to study the dependence of these
features with varying layer thickness and to understand
possible boundaries of the technique. In
addition to the primary goal of identifying the layer
thickness for a blind inspection, the potential for
the technique to quantify corrosion thickness loss
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