Air-coupled ultrasonic NDE: experiments in the frequency range 750 kHz–2 MHz

Possibilities of air-coupled ultrasonic testing and imaging are examined by means of a measurement system designed for both through-transmission and reflection technique. For that purpose, B- and C-scans have been performed with respect to a wide variety of materials of different acoustic impedance. The observations reported in the paper deal with coating on textile, flaws in an aluminum plate, spot welds on metallic plates, tiny air inclusions in thin castings and ultrasonic reflection on an epoxy plate with a copper layer. The measurements have been performed in either continuous or pulse mode. In the former case, frequency swept sinusoidal signals are applied, in the latter modulated chirps. All results have been obtained at sound frequencies between 0.75 and 2 MHz, produced by air-coupled piezo-based transducers with matching layer.     

A Statistical Model for Determining the Through-Transmission Ultrasonic Amplitude Response for Imperfectly Contacting Interfaces during Thermoplastic Fusion Bonding

The focus of this work is to develop a system model for through-transmission ultrasonics (TTU) as a process monitoring technique for nonintrusive, online control of thermoplastic fusion bonding processes. A closed form model is developed to predict the TTU amplitude of a fusion bond using an ultrasonic wave propagation theory with a model for the evolution of intimate contact at the material interfaces. This new approach treats the attenuation of sensor response due to imperfect contact at the bond interface as a statistical averaging of the transmission coefficients for each of the possible sound paths between source and receiver. The results of experimental and model-based parametric studies show that the intimately contacting path dominates the overall signal behavior. An approximation of the full model for an arbitrary number of interfaces based on these results is derived.