Nondestructive evaluation of interfacial bond strength in single lap joints via swept-frequency ultrasonic phase measurements

As adhesive bonding is a preferred method of joining advanced composite structures for modern aerospace and automotive designs, the need for a quantitative nondestructive evaluation (NDE) method for adhesive bond strength has sharply increased. For many years, adhesively bonded joints have been examined with conventional NDE methods, but none have proved able to detect weak or “kissing” bonds, except where major bonding defects occur. Here, the problem of weak interfacial bonding is investigated with a novel swept-frequency ultrasonic phase measurement method. High- resolution, swept-frequency ultrasonic phase measurements are obtained at each frequency through the use of a constant- frequency pulsed phase-lock-loop instrument. The method is tested on single lap joints fabricated with polished glass adherents and adhesive cured with ultraviolet (UV) light. The cohesive and adhesive bonding properties are modified by varying the adhesive’s exposure time to UV light and thereby changing the degree-of-cure. After fitting the measured phase response to a model of the ultrasonic response of the bonded joint, the stiffness constant of the adhesive/adherent interface is found. The interfacial stiffness constant is shown to have a strong linear correlation with measured mechanical tensile strength. As such, high-resolution swept-frequency phase measurements are shown to have a sensitivity to interfacial bond strength without the presence of gross bonding defects, allowing for bond strength quantification rather than a simple identification of “good” or “bad” bonding. This method has the potential to be a quality control and service life assessment method for both newly-fabricated and in-service bonded joints for the aerospace, automotive, and other industries.