Numerical Investigation of Friction Material Contact Mechanics in Automotive Clutches

A wet clutch model is required in automotive propulsion system simulations for enabling robust design and control development. It commonly assumes Coulomb friction for simplicity, even though it does not represent the physics of hydrodynamic torque transfer. In practice, the Coulomb friction coefficient is treated as a tuning parameter in simulations to match vehicle data for targeted conditions. The simulations tend to deviate from actual behaviors for different drive conditions unless the friction coefficient is adjusted repeatedly. Alternatively, a complex hydrodynamic model, coupled with a surface contact model, is utilized to enhance the fidelity of system simulations for broader conditions. The theory of elastic asperity deformation is conventionally employed to model clutch surface contact. However, recent examination of friction material shows that the elastic modulus of surface fibers significantly exceeds the contact load, implying no deformation of fibers. This article investigates the friction material contact mechanics through numerical simulations. A surface model is constructed based on microscopic examination of material topography and properties. An FEM simulation is conducted to examine the interactions between surface fibers and the surrounding medium under loaded conditions. The change in real contact area with respect to nominal surface pressure correlates qualitatively the simulations and experiments. The numerical study provides insight into frictional material contact mechanics that is not directly observable. It also supports the assumptions behind an empirical fiber contact model that was recently introduced to enhance hydrodynamic clutch models.