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In this work, the normal contact stiffness of lubricated rough interface is evaluated theoretically by describing the lubricated rough interface as an equivalent thin layer. Layer parameters, including equivalent thickness and effective Young's modulus, are used to characterize the normal contact stiffness by incorporating the contributions of asperity contact and lubricant contact simultaneously. On the basis of layer parameters, the normal contact stiffness of lubricated rough interface is obtained as a function of interfacial separation, surface topography, and properties of contacting solids and lubricants. Effects of surface topographies and lubricant types on the normal contact stiffness are investigated at varying interfacial separations and contact area fractions. The proportion of solid stiffness and lubricant stiffness from the total normal stiffness is also discussed. Numerical solutions reveal that the normal contact stiffness depends sensitively on the lubricant property at initial contact, whereas the influences of surface topographies become obvious with the decrement of interfacial separation or increment of contact area fraction. Comparisons between the predicted values of normal contact stiffness and experimental data for both dry interface and lubricated interface are presented to validate the rationality of the developed model. [ABSTRACT FROM AUTHOR]

The nonlinear oscillatory motion of an elastic sphere in contact with a rigid flat surface to different loads is clearly dominated by the damping mechanism of the system. However, the damping power law is unable to be identified using the decrement method due to the almost identical free vibration responses. In this paper, a procedure which allows identification of the nonlinear viscous damping from the free response data is presented. The procedure is based on the figure relating the damping restoring force and the velocity which is distinguishable for different damping laws. A dynamic experiment is described, and from the free vibration data obtained the damping restoring forces are extracted for spheres with different materials being steel and low density polyethylene (LDPE). Subsequently, based on the models and observations the characteristics of the damping law are estimated. The results show that a linear viscous damping model is a good representation for the steel sphere contact, which confirms earlier studies; whilst the LDPE sphere contact follows a nonlinear power law for the viscous damping function, which is newly obtained. The validity and accuracy of the proposed method are supported by the good agreement between numerical results and experimental results. [ABSTRACT FROM PUBLISHER]