A constant-frequency ultrasonic phase method for monitoring imperfect adherent/adhesive interfaces

A primary mechanism of adhesive bond failure is a degradation of the adherent/adhesive interfacial stiffness from unwanted contamination or exposure to those environmental factors, which reduce adhesion quality. Substantial research has been conducted on the assessment of adhesively bonded structures and the detection of "kissing" bonds. Advanced ultrasonic assessment methods to interrogate bonded joints and measure interfacial stiffness using a distributed spring interface model have been developed. Amplitude-based ultrasonic methods have traditionally been used in adhesive bond quality assessment, but recent advancements in ultrasonic phase measurements allow for high measurement resolution with low-uncertainty. In this work, an ultrasonic phase technique for the monitoring of adhesively-bonded interfaces is demonstrated. Constant frequency measurements are obtained from the ultrasonic phase of the reflection coefficient from the adhesive bond with a glass adherent, where the degree of cure is controlled by exposure to ultraviolet light. A peak in the phase of the reflection coefficient, as predicted by the interfacial spring model, is measured experimentally. It is shown that the peak phase predicts the interfacial stiffness when some frequency dependent threshold value is crossed. With knowledge of the acoustic impedances of both materials at the interface, the interfacial stiffness is determined by an inverse algorithm involving measurements of ultrasonic phase shifts of bonded joint reflections. By monitoring the interface of bonded structures and coatings, this method permits a nondestructive inspection of bond strength from structural construction through its service life.