Investigation of contact mechanics and friction of rubber compounds by experimental techniques and numerical simulations

Abstract

The contact of car tires with road tracks is a research field of high practical importance since grip properties during tire-road interaction have a direct impact on safety issues. To improve the tire properties it is necessary a deep understanding of the tire-road contact mechanics, as well as the underlying rubber friction physics. Despite this topic is widely investigated, there are no complete predictive models capable to describe the friction interaction for practical applications, for this reason, it still means a big challenge for the scientific community. Therefore, the main objective of the present research is to predict the performances of different rubber materials on dry and wet rough road tracks. The effect of many intrinsic and extrinsic factors such as substrate roughness, rubber-surface affinity, temperature, load, sliding speed, lubrication, geometrical parameters and viscoelastic material properties on contact properties and the resulting friction behavior is investigated. Hence, the complex friction process is split into different subtopics, which are analyzed and modeled. From one perspective, the contact mechanics through a 2D FEM tool is investigated on different substrates to determine the main friction contribution called hysteresis. This contribution is based on the energy losses in a broad frequency scale due to cyclic deformations of the rubber by asperities. An accompanying experimental tool based on the measurement of rubber indentation into substrate asperities is designed to provide a physical understanding of static and dynamic contact problems and to validate the FEM model. The tools developed also provide access to unavailable data, such as local pressures, dissipated energies, contact area and rubber indentation. It was also figured out the effect of rubber geometry on dynamic contact mechanics through experimental and 3D FEM simulations. On the other side, the adhesion is studied as the second main friction contribution, defined as a result of the peeling effects governed by viscoelastic crack opening mechanisms between rubber and substrate on dry and wet surfaces, by considering the physical-chemical nature of the contact. It is found that the hysteresis response is enhanced with the so-called dry patches effect in the water condition. Consequently, a partial adhesion occurs where the water is wiped away due to the dewetting phenomena. Furthermore, the thermal effect (heat build-up) which occurs during sliding friction on dry and wet rough surfaces is investigated with a thermographic camera. Dry friction heating arises differently between short/wide and long/narrow rubber blocks. By considering wet conditions, the presence of lubricant with high thermal diffusivity cools down the system generating less heat build-up in comparison to a dry surface. An analytical solution of lubricated heat build-up generated in the rubber during sliding due to the viscoelastic losses, is provided and shows qualitatively good agreement with the experiment. By recombining the modeled subtopics of the split friction process it was built-in a hybrid model composed of a FEM tool and an analytical adhesion model which shows a good prediction of dry/water grip at low macroscopic speeds where mainly adhesion and hysteresis occur. Finally, a continuous dry friction master curve is constructed by the combination of measured friction branches for different temperatures over a wide range of sliding speeds, even close to ABS range.