Mitglied der Helmholtz-Gemeinschaft Rubber Friction and Tire Dynamics: A Comparison of Theory with Experimental Data 21. April 2015 Boris Lorenz Bo Persson
Motivation Tire models are often used to simulate the interactions between the tire and the road. Most tire models only use a very simple describtion of the friction at the interface! This is the weak spot in most tire models Despite of its importance, rubber friction is still not well understood!
Rubber Friction General Understanding Energy dissipationbecause of viscoelastic deformations of the rubber bulk material when sliding.
Rubber Friction On different lengthscales
Rubber Friction On different lengthscales
Rubber Friction On different lengthscales
Problem in Tribology Real surfaces have roughness over a wide range of lengthscales!! e.g. from nanometer to centimeter nm µm mm m Taking into account the whole range is impossible using standard numerical methods like FEM An analytical approach for our problem is needed
Basic Processes (Process a) Viscoelastic deformations of the rubber bulk (Process b) Shearing the contact area
Rubber Friction Approach We assume that the sum of a viscoelasticand a contact area contribution gives the total rubber friction coefficient.
Measuring Topography Power Spectrum Measuring surface roughness along a line using a Profilometer or other methods From the topography we calculate the power spectrum: h( q) = h( x) e iq x d 2 x C( q) = h( q) 2
Material Properties Viscoelastic Modulus Dynamic Mechanical Analysis (DMA) provides all necesarry information on the material properties. The complex viscoelastic Modulus: E(ω)
Results for unfilled SB rubber
Contact Area Contribution Shearing the contact area: attach to the substrate get stretched detach, dissipating stored up elastic energy reattach again
Tire Tread Rubber Rubber friction of tread compounds on road surfaces:
Wet Rubber Friction
Temperature Effects The dissipated energy at the interface and in the rubber bulk lead to a heating up of the whole rubber block. As rubber friction is very sensitive to temperature. Heat transfer to the substrate is taken into account which is of great impotance.
Temperature Effects The rubber undergoes heating effects when moving through the footprint. This can either be taken into account exactly or interpolated.
2D Tire Model We have developed a simple 2D tire model to embed the friction subroutine: Rubber blocks are connected by springs and dampers. The friction model is applied to the individual blocks.
ABS Breaking Simulations
Summary Rubber friction can only be studied using analytical approaches We presented a friction model by Persson including: Viscoelastic dissipation Contribution from shearing the real area of contact Experiments show good agreement between theory and measurements We showed that the friction model can be embedded in a tire model describing the frictional interactions between the tire and the road substrate Thank you for your attention! Find more information on our homepage:
Appendix: Modified Surface Layer SEM picture of the tread of a car tire after sliding. (a) Low magnification (b) High magnification Increased concentration of oxygen suggesting chemicall reactions with the environment and undergoing oxidation processes during sliding. The wear layer has very different properties compared to the bulk material! N.V. Rodriguez et al., J. Engineering Tribology 227, 398 (2013)
Appendix: Contact Area Contribution Friction mastercurve of an acrylonitrile-butadiene rubber sliding over a smooth glass substrate. Energy dissipation is attributed to adhesive interactions between the rubber and the glass in the contact area K.A. Grosch, Proc. R. Soc. Lond. A 274, 21 (1963)