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Ultrasonic Technique Using Secondary Signals for Blind Thickness Characterization in Multilayer Structural Joints

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 is addressed.

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