Characterization of Cohesive and Adhesive Properties of Adhesive Bonds Using Transmitted Ultrasonic Waves

The present research attempts to characterize mechanical properties, which are representative of the adhesive and cohesive states of adhesively bonded assemblies, using through-transmission immersion ultrasonic method. Transmission coefficients of an ultrasonic wave incident from different angles are measured for an assembly made of 2 substrates bonded with an adhesive. Knowing thicknesses and mass densities of the components, as well as the substrates elastic moduli (previously measured), the stiffness layer method serves as a support for solving an inverse problem, which provides the desired adhesive and cohesive properties. Firstly, a study of the influence of the interphase properties on the characterization of the elastic moduli of the adhesive is presented. The inverse problem resolution is performed assuming that the adhesive is an anisotropic solid. This study demonstrates that an abnormal apparent anisotropy of the adhesive layer is observed in the case of imperfect interfaces, revealing their imperfection. This is observed when the experiments are fully simulated numerically (with a full control of the state of the interphases) and confirmed by real experiments carried out using specifically made samples. Then, assuming that the adhesive layer moduli are known, and taking into account two thin layers (h = 1 μm) to simulate the interphases, the assessment of these interphase characteristics is presented. These results show the strong potential of the method to distinguish between adhesive and cohesive weaknesses of bonded joints, and to quantify corresponding mechanical properties (elastic moduli or interfacial stiffnesses).