Nonlinear Stress-Modulated Ultrasonic Method for Bond Strength Characterization

Environmental degradation of adhesive bonds of composite or metallic structures affects predominantly the adhesive/adherent interface by decreasing the number of molecular bonds between the adhesive and the substrate. The objective of the proposed Phase I work is to perform a feasibility study of a novel non-linear ultrasonic method for quantitative characterization of molecular bond density with the objective of determining the local bond strength.The innovation of the proposed method is to perform an ultrasonic characterization of the interface enhanced by parametric dynamic vibrations. The density of molecular bonds, described by an equivalent interfacial spring density, is quantitatively determined using dual beam ultrasonic spectroscopy at different points of the dynamic loading curve. The method permits separating the linear and non-linear behavior of the bulk adhesive and adhesive/substrate interface. Comparison between the measured spring density at tensile and compressive loads leads to a characterization of the local interfacial strength.Based on the accomplishments of Phase I, Phase II developments will be proposed including extended study of the method applicability for assessment of adhesively bonded structures and the delivery to NASA of the dedicated Angle Beam Ultrasonic Spectroscopy acquisition system for evaluation of Adhesive Joints under Parametric Excitation.