Ultrasonic monitoring of friction contacts during shear vibration cycles

Complex and high-value jointed structures such as aero-engines are carefully designed and optimized to prevent failure and maximize their life. In the design process, physically based numerical models are employed to predict the nonlinear dynamic response of the structure. However, the reliability of these models is limited due to the lack of accurate validation data from the metallic contact interfaces subjected to high-frequency vibration cycles. In this study, ultrasonic shear waves are used to characterize metallic contact interfaces during vibration cycles, providing validation data for an understanding of the state of the friction contact. Work supported by numerical simulations of wave propagation within the material, a novel experimental method is developed to simultaneously acquire ultrasonic measurements and friction hysteresis loops within the same test on an in-house fretting rig. Large variability in the ultrasound reflection/transmission is observed within each hysteresis loop and is associated with stick/slip transitions. The measurement results reveal that the ultrasound technique can be used to detect stick and slip states in contact interfaces subjected to high-frequency shear vibration. This observation paves the way toward real-time monitoring of vibrating contact interfaces in jointed structures, leading to a new physical understanding of the contact states and new validation data needed for improved nonlinear dynamic analyses.