Solid Friction from stick-slip to pinning and aging

We review the present state of understanding of solid friction at low velocities and for systems with negligibly small wear effects. We first analyze in detail the behavior of friction at interfaces between wacroscopic hard rough solids, whose main dynamical features are well described by the Rice-Ruina rate and state dependent constitutive law. We show that it results from two combined effects : (i) the threshold rheology of nanometer-thick junctions jammed under confinement into a soft glassy structure (ii) geometric aging, i.e. slow growth of the real arrea of contact via asperity creep interrupted by sliding. Closer analysis leads to identifying a second aging-rejuvenation process, at work within the junctions themselves. We compare the effects of structural aging at such multicontact, very highly confined, interfaces with those met under different confinement levels, namely boundary lubricated contacts and extended adhesive interfaces involving soft materials (hydrogels, elastomers). This leads us to propose a classification of frictional junctions in terms of the relative importance of jamming and adsoprtion-induced metastability.Comment: 28 page

Numerical analysis of the influence of an actively controlled spoiler on the handling of a sports car

Normal force distribution among tires significantly affects the performance of the vehicle, especially in terms of handling and safety. Consequently, evolving technologies for controlling and regulating normal loads have the potential of boosting the controllability of the vehicle. This paper illustrates the effectiveness of the actively controlled aerodynamic system to increase the performance and the safety margins of a sports car in cornering maneuvers; normal loads on the front and rear axles at high speed are controlled by changing the angle of attack of the front and rear spoilers in opposite directions to alter load distribution. The performance of the controlled system is analyzed through a nonlinear 14-degree-of-freedom vehicle model. Numerical simulations are carried out to understand how the control logic can affect oversteer/understeer response, improving both stability and performance of the vehicle. Robustness of the control systems toward the variation of friction coefficient is also assessed