Ultrasonic monitoring of adhesive curing process between adherends

An ultrasonic method to monitor the curing of a thin adhesive layer was developed theoretically and verified experimentally. The method was based on a model in which an interface spring represented an adhesive layer sandwiched between adherends of the same material. In this method, the interfacial stiffness in the spring model, which included the ultrasonic longitudinal wave velocity and the adhesive`s density and thickness, was obtained from the measured reflection or transmission coefficient for the wave in the adhesive layer. The ranges of interfacial stiffness for valid application of the method were theoretically clarified in terms of the stiffness`s sensitivity to the reflection and transmission coefficients and the stiffness error with respect to errors in the measured coefficients. The applicable ranges were verified by experimentally evaluating the curing process of an epoxy resin with adhesive several micrometers thick when joining aluminum alloy adherends. Specifically, the ranges were obtained as 3.1 ρ C < K / f < 62 ρ C [ Pa ⋅ s / m ] for the reflection coefficient and K / f < 6.3 ρ C [ Pa ⋅ s / m ] for the transmission coefficient, where ρC, K, and f are the acoustic impedance of the adherends, the interfacial stiffness of the adhesive, and the ultrasonic frequency. The measurable ranges of the adhesive`s interfacial stiffness were also determined for typical adherend materials such as alloys, polymers, composites, and ceramics.