Abstract
Complex high-value jointed structures such as aero-engines are carefully designed and optimized to prevent failure and maximise 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 metallic contact interfaces subjected to high-frequency vibration cycles. In this study, ultrasonic shear waves are used to characterise metallic contact interfaces during vibration cycles, hence providing new validation data for an understanding of the state of the friction contact. 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 a high-frequency friction 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 is the first observation of this type and paves the way towards 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.
Highlights
Jointed structures use a wide range of friction joints to facilitate their assembly
Varying levels of specimen alignments and/or velocity may be responsible for some of the variation, but further refinement of the test setup will be required to confirm these results. These results indicate that (a) the highlighted changes observed at the stick–slip transitions are independent of the rig kinematics/dynamics, and, more importantly, (b) the junction growth and ageing behaviours that characterise the stick regime is still detectable in gross slip loops, making it the most promising indicator for slip-stick transitions at the contact interface
This work presents the use of ultrasonic shear waves to investigate the state of a dry metallic friction contact while it undergoes high-frequency shear vibrations
Summary
Jointed structures use a wide range of friction joints to facilitate their assembly. Beyond their primary functions of joining two components, providing structural strength, and potentially sealing, friction joints are often the main source of damping in the assembly, and as such they have a large influence on the nonlinear dynamic response [1,2,3]. To overcome some of the restrictions of impermeable materials, other optical measurement techniques have been proposed, such as Laser Doppler vibrometer measurements [20,21,22,23] or digital image correlation (DIC) with high-speed cameras [24,25,26,27,28,29,30], both of which have been used to monitor the motion of the boundaries of friction joint interfaces during a vibration cycle These studies measured friction hysteresis loops, which are the typical load–deflection curves representing the sliding behaviour of contact interfaces under vibration, making it possible to observe several contact phenomena, such as stick–slip transition at the edges of the contact, opening and closing of the contact and identification of permanent gaps. The findings provide novel insights into the physics of the contact and form the basis for future real-time interface monitoring of vibrating jointed structures, needed to prevent failures and provide validation data for state-of-the-art numerical models
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