Nondestructive Evaluation of Adhesive Bond Strength by Ultrasonic Phase Measurements

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 when first fabricated and also throughout its service life it is of utmost importance. Conventional nondestructive evaluation (NDE) methods have been used to detect gross bonding defects, such as delaminations and adhesive voids. However, conventional NDE has proved 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 parts and structures, careful control of the bonding procedure as well as pre-bonding surface preparation methods have been the primary methods of ensuring adequate adhesion quality. While several specialized NDE methods have been developed to inspect adhesive bonds, very 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 developed for the purpose of 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, yet compatible with standard commercial ultrasonic transducers and conventional ultrasonic NDE setups. Additionally, the CFPPLL method can maintain a given signal phase while changing frequency, so that the swept-frequency phase response of a material under test can be obtained. With a frequency-independent phase resolution of 0.022°, the instrument can detect thickness changes as small as 6.1×〖10〗^(-5) of an ultrasonic wavelength, which is equal to a thickness change of ~20 nm via reflection measurements in Al. Experimental results show the CFPPLL method has lower phase measurement uncertainty than other high-resolution ultrasonic time-delay measurement methods, such as cross-correlation and conventional broadband pulse ToF. An ultrasonic phase technique for the monitoring of adhesively-bonded interfaces is demonstrated. Constant frequency measurements are obtained from the ultrasonic phase of the reflection coefficient from the adhesive bond with a glass adherent, where the degree of cure is controlled by exposure to ultraviolet light. A peak in the phase of the reflection coefficient, as predicted by the interfacial spring model, is measured experimentally. It is shown that the peak phase predicts the interfacial stiffness when some frequency dependent threshold value is crossed. With knowledge of the acoustic impedances of both materials at the interface, the interfacial stiffness is determined by an inverse algorithm involving measurements of ultrasonic phase shifts of bonded joint reflections. In simulated kissing bonds -- where metal surfaces are held in dry-contact under compressive loads -- ultrasonic phase is able to assess interfacial stiffness regardless of the acoustic impedance values around the boundary. This work is the first demonstration of interfacial stiffness assessment on an adherent/adhesive single interface with ultrasonic phase and the first time the peak phase predicted by the interfacial spring model has been verified experimentally. By monitoring the interface of bonded structures and coatings this method permits a nondestructive inspection of bond strength from structural construction through its service life. Of more practical importance to the aerospace community than a single adhesive/adherent interface is the single lap joint, where an adhesive layer is sandwiched between two adherents. 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 can be observed from bond line reflections. Furthermore, the effect of weak bonding on the ultrasonic anti-resonance can be observed by modeling the ultrasonic interaction with adhesive/adherent interfaces by a distributed spring system. The interfacial stiffness constant, which has been shown directly proportional to interfacial adhesion strength, has been difficult-to-quantify with amplitude-based ultrasonic methods. The phase of ultrasonic reflections from the bond line, however, is extremely sensitive to material properties within the adhesive layer near the anti-resonance frequency and is able to detect small differences in interfacial bonding quality. The swept-frequency phase measurement method has been used to examine the degree-of-cure of ideal bonded joints cured with ultraviolet light. These results show ultrasonic phase near the bond line ultrasonic resonance frequency is sensitive to both cohesive and adhesive changes as a function of cure. By fitting measured the phase vs. frequency response of bond line reflections, the interfacial stiffness was extracted. In agreement with theoretical work, a strong linear correlation is found between interfacial stiffness and measured tensile bond strength. Additionally, the shift in anti-resonance frequency of the bond line is found to be a sufficient parameter to measure reduced bond strength when both interfaces can be assumed equally strong. Studies on real-world metal/epoxy joints prove ultrasonic phase measurements can identify kissing bonds and are sensitive to a single contaminated interface. By spray-coating the adherent prior to bonding with different concentrations of silicone or Teflon-based contamination, the resulting interfacial bond strength can be controlled. Ultrasonic phase evaluation of these joints and predictions of ultimate failure strength are made from the measured interfacial stiffness constants. Comparison with mechanical shear strength shows good correlation with theory. An additional investigation showed constant-frequency ultrasonic phase monitoring is capable of identifying precursors to bonded joint failure at around 80% of the failure load during quasi-static shear loading. Furthermore, the slope of phase shift is found to show a different characteristic response for interfacially contaminated joints in comparison to pristine joints. Early detection of bond failure would provide a significant impact to the SHM community, as in-service bonds could be monitored and weak joints could be identified prior to failure. Additionally, this method could be used as a tool for fracture mechanics experts to obtain better information about mechanisms in the bond line affecting bonded joint failure. The novel bond strength assessment method studied in this project uses commercial ultrasonic transducers with CFPPLL technology to obtain high-resolution phase measurements of ultrasonic tone-bursts over a range of frequencies. Narrowband filtering of the measured tone-burst promotes noise suppression and higher signal-to-noise ratio, resulting in lower phase measurement uncertainty. Furthermore, the CFPPLL method is compatible with existing ultrasonic inspection methods, including commercially available transducers and scanning systems. By nondestructively and quantitatively measuring adhesion quality in a practical system, 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 and can be used in many other applications in which sound velocity or ultrasonic phase monitoring can detect material property or system changes.