Nondestructive Evaluation of Adhesive Bonds by a Novel Ultrasonic Phase Measurement Method

As modern aerospace and automotive designs have endeavored for higher performance, reduced cost, and lower weight, the use of advanced composite structures has greatly advanced. Consequently, the need for adhesive bonding, a preferred method of joining composite materials, has also drastically increased. As such, ensuring the strength of an adhesively bonded joint upon fabrication and throughout its service-life is of utmost importance. Conventional nondestructive evaluation (NDE) methods have been used to detect gross bonding defects, such as delaminations and voids, but are unable to detect weak adhesion and “kissing” bonds, leaving only destructive testing for quantitative bond strength measurement. As destructive testing is impractical for in-service structures, careful control of the bonding procedure and surface preparation methods are the primary methods of ensuring adequate adhesion quality.

While several specialized NDE methods have been developed to inspect adhesive bonds, few have shown the sensitivity to quantify weak adhesive/adherent interfaces. The focus of most NDE research into bonded joints has used ultrasonic methods to mechanically interrogate the bonded joint on the atomic level. In this work, a high-resolution ultrasonic phase measurement system is investigated for quantifiably measuring adhesive bond strength. This method uses constant-frequency pulsed phase-locked-loop (CFPPLL) technology to obtain high-resolution ultrasonic phase measurements with unprecedented accuracy and precision, even when swept over a range of frequencies.

When the adhesive bond line thickness is equal to half of the ultrasonic wavelength, the adhesive layer acts as an ultrasonic resonance cavity. By proper choice of driving ultrasonic frequency, the ultrasonic anti-resonance effect is observed from bond line reflections. The effect of weak bonding on the ultrasonic anti-resonance is investigated by modeling the ultrasonic interactions with adhesive/adherent interfaces by a distributed spring system. The phase of ultrasonic reflection from the bond line is extremely sensitive to material properties within the adhesive layer and is able to detect small differences in interfacial bond quality.

The swept-frequency ultrasonic phase measurement method is used to examine interfacial bonding of ideal bonded joints cured with ultraviolet light. These results show ultrasonic phase around the resonance frequency of the bond line is sensitive to both cohesive and adhesive changes as a function of cure. By fitting the measured phase vs. frequency response of bond line reflections, the interfacial stiffness is extracted. Studies on real-world metal/epoxy joints also prove ultrasonic phase measurements can identify kissing bonds and are sensitive to interfaces contaminated with either silicone or Teflon. Ultrasonically measured interfacial stiffness constants correlate linearly with mechanically measured bond strengths, showing good agreement with theory and indicating ultrasonic phase measurements have the sensitivity to quantify interfacial adhesion quality.

By nondestructively and quantitatively measuring adhesion quality, this method has the potential to promote the use of more complex, lightweight, and safe aerospace and automotive designs utilizing advanced composite structures and adhesive bonding. The demonstrated ultrasonic phase method is applicable to a variety of bonding material systems, is compatible with standard commercial ultrasonic transducers and conventional ultrasonic NDE setups, and can be used in many other applications in which sound velocity or ultrasonic phase monitoring can detect material properties degradation and changes.