Numerical and experimental investigation of tyre compounds frictional properties
Creators: name: Missale, Elena
Publisher: name: Universit� degli studi di Trento
Year: 2022

description: The thesis aims to study the effects of the mixing of immiscible polymers on the frictional properties of rubber compounds. The novelty of this work is to consider the rubber as a heterogeneous material in which the microscopic inhomogeneities are the domains generated from the mixing of immiscible compounds. Systematic experimental tests are performed to investigate the frictional properties of two different groups of tyre compounds, both provided by Pirelli Tyre S.p.A. The first group is represented by the homogeneous compounds realized by using Natural Rubber (NR) or Styrene Butadiene Rubber (SBR) and different amounts of filler and Sulphur. The second group considers heterogeneous compounds, generated by mixing in different percentages the homogeneous compounds, to obtain compounds characterized by microscopic domains of NR and SBR. The experimental outcomes proved that the presence of domains increases the friction coefficients. A Dynamic Mechanical Analysis (DMA) is performed to correlate the dynamic properties of the specimens with the friction coefficient. The results of the DMA of the homogeneous compounds agree with the frictional properties while the heterogeneous compounds show intermediate dynamical properties in contrast with the frictional results. The DMA is not able to recognize the microscopic domains of the heterogeneous compounds interpreting the system as a uniform material, suggesting that more complex dynamics arise during the sliding. The dependence of friction on the composition of the material is also investigated by using a numerical approach. For this purpose, a straightforward model is chosen to investigate numerically the frictional behaviour of a rubber material starting from the microscopic properties. The model is discretized as a chain of blocks connected by springs and dampers. Playing with the microscopic parameters of the model, such as the elastic modulus and the damping coefficient, it is possible to link the macroscopic frictional response of the bulk to the microscopic characteristics that locally describes the interactions between the blocks. Firstly, the frictional properties of compounds characterized by i) uniformly distributed viscoelastic or elastic elements and ii) a combination of purely elastic and viscoelastic elements randomly distributed is compared. The numerical outcomes reveal an increase of the frictional properties for samples realized by mixing elastic and viscoelastic elements pointing out that the presence of different domains due to the mixing of two immiscible materials, affects the macroscopic frictional response. Secondly, a comparison between the experiments and the numerical simulations is performed to verify if the 1D model can correctly predict the observed experimental behaviour. The 1D model, in its simplicity, is unable to predict the increase of frictional properties observed experimentally testing the heterogeneous compounds, confirming that more complex interactions influence the friction as suggested by the DMA.