Friction Measurements for Aeroengine Applications

To achieve sustainability goals and reduced emissions, new aero-engine designs have to be lighter, at the cost of increased vibration levels. Uncontrolled, vibrations threaten the integrity of components that are already highly stressed by static loads, and this can result in early failures. Therefore, the correct analysis of the dynamic response of the engine components is of uttermost importance, but such analysis is limited by the lack of fully predictive modelling approaches. This lack is partly due to the presence of friction joints in the engine, which are difficult to model because of their complex underlying physics and a lack of experimental data from the contact interfaces. One of the ways to capture their physics, in state of art modelling approaches, is by means of friction hysteresis loops, which are the cyclic load-deflection curves that plot the friction force exchanged between two contacting components and their tangential relative displacement. From hysteresis loops, two important pieces of information can be extracted: the tangential contact stiffness and the friction coefficient. Hysteresis loops for nonlinear dynamic analysis are often provided by high-frequency friction rigs, whose dynamic response can, however, strongly affect the shape of the measured hysteresis loops. This influence from the dynamic behaviour of the friction rigs might lead to a significant amount of uncertainty when providing input contact parameters for nonlinear dynamic analysis. Furthermore, considering that different institutions use different in-house high-frequency friction rigs, hysteresis loops might include different dynamics effects too, making the contact parameters “rig-dependent”. The main goal of this study is to quantify the effect of the rig dynamics on the contact parameter estimations by measuring the contact parameters statically and comparing them with those extracted under high-frequency vibrations that are normally used. For this purpose, an existing high-frequency friction rig has been re-designed to accommodate a new mechanical system for performing static loadings by means of dead weights, and the Digital Image Correlation experimental technique has been used to measure the specimens’ motion. Simultaneously to static tests, ultrasound measurements have been also performed on the friction rig, with an existing and validated setup. Contact parameters have been extracted from the static loading measurements and compared to those from high-frequency and ultrasound measurements, in order to understand the physical origin of the recorded results. Lastly, static measurements have been employed to obtain the normal contact stiffness of specimens with different superficial roughness, to investigate the effect of the real contact area, and to compare the recorded results with the analytical solutions. These studies pave the way for a better and more accurate understanding of the underlying physics of contact interfaces in jointed structures, with the aim of creating predictive models for more accurate and reliable nonlinear dynamic analysis of those structures under vibration.