Modeling of the Bonding Quality Within a Multilayer Structure Using the Ultrasonic Reflection Technique

Abstract

This theoretical study focuses on the verification of the possibility of using ultrasonic waves to characterize the bonding quality in an Aluminium/Adhesive/Aluminium multilayer structure where the two aluminium layers have the same thickness. This special case poses a problem in quality control because the same thickness implies identical resonance modes from the ultrasonic point of view, which means that the first aluminium layer will hide the second one. However, there is a mode doubling relative to the two aluminium layers under certain bonding conditions. This work is based on the monitoring of the behavior of this mode doubling from the evolution of the ultrasonic response modeled by the calculation of the theoretical reflection coefficient corresponding to the multilayer. This study has showed that this mode splitting depends on both the thickness of the aluminium layer and the adhesive layer, and it depends strongly on the quality of the bonding at the interfaces between aluminium and adhesive. From these results, it has been possible to deduce which multilayer structure can be characterized and in which frequency range this characterization is possible. The evolution of the modulus of the reflection coefficient as a function of the percentage of normal stiffnesses at the interfaces has showed the existence of a promoter parameter, called the resonance depth, which allows following accurately the quality of the bonding at these interfaces. This very interesting result makes it possible to express the quality of the bonding in terms of percentage, since it reflects the percentage of adhesion at the interfaces.