Fast & Accurate Composite Bond Strength Measurement

NASA's Method of Evaluating Adhesive Bond Strength is an accurate, robust, quantitative, and nondestructive bond strength measurement method that meets an immediate need in composite manufacturing. Even with careful control of the bonding procedure, destructive testing has shown that bonded joint strength shows substantial variation. Prior art in the field is insensitive to weak interfacial bonding, which leads to poor service life and potential catastrophic failure. Using NASA's method, phase measurements are acquired at a single frequency and then swept to obtain measurements at other frequencies. Narrowband filtering removes extraneous frequencies, which allows for much lower phase measurement uncertainty than other methods. Digital sine wave generation allows for better phase measurement resolution. The resultant system is a phase-based ultrasonic measurement tool for interrogating bonded joints and detecting weak adhesion with superior sensitivity than the state of the art. This new method models adhesive interfaces as a distributed spring system, where the interfacial stiffness constants of the joint can be determined from the zero- crossing frequency of the phase response and the "sharpness" or slope of the phase response. Previous theoretical research has shown that a linear correlation exists between interfacial stiffness constants and mechanically-measured interfacial bond strength. As such, the ultrasonic phase measurement method identifies intermediate bond strengths, rather than simply detecting good or bad bonds. This technique verifies bond quality in metal and composite joint systems, including those commonly found in aerospace, automotive, and many other transportation and infrastructure designs. The demonstrated ultrasonic phase method is applicable to a variety of bonding material systems.