Friction, wear and tangential stiffness of metal surfaces under fretting conditions

Abstract

Bladed disk vibrations in turbomachinery can lead to failure due to High Cycle Fatigue. One way in which vibration may be reduced is by dry friction damping. Frictional damping originates from micro and macro slip in the contacting interfaces (“joints”) and is controlled by the relationship of the applied load and tangential displacement. In order to predict the dynamic response of the structure, knowledge of the coefficient of friction and the tangential contact stiffness of the contact are crucial. Vibration induced slip and the consequent damage in contacting surfaces has been widely studied and is usually called fretting. However, little is known about the effect of the changing interface during fretting on the coefficient of friction and the tangential contact stiffness, which is required when trying to predict these parameters. This study seeks an improved understanding of the effects of surface topography, surface chemistry, and elastic and plastic material properties on the friction and damping performance of joints under fretting conditions. In the present study experiments were conducted to measure the coefficient of friction and the tangential contact stiffness of different metals under different test conditions. Fretting damage mechanisms were investigated using metallography, SEM, EBSD, TEM and XRD techniques. The evolution of roughness and conformity was investigated by using interferometric profiling systems and image registration via cross correlation. An infrared radiation measuring system was employed to measure the dissipated radiation and frictional power in fretting which was then compared with calculated energy dissipation maps. Experimental results were used to validate models predicting contact stiffness which have been developed throughout the project by collaborating researchers. This study highlighted real contact conditions and their dependence on running time, which need to be taken into account when modelling friction contacts.