FRICTIONAL DYNAMICS

The fundamental friction studies of rubber have generally dealt with single contact sliders or rollers. It has been demonstrated abundantly that the lubricated friction of rubber is mainly the ‘deformation loss’ component of friction. At moderate sliding speeds where thin film lubrication exists and the interface shear drag is small, the friction is the same as in rolling. The rubber substrate is continually deforming ahead of, and recovering behind, the contact in both rolling or sliding cases. Since the deformation of rubber is partially irreversible, energy is lost which is irreversible, energy is lost which is reflected as the ‘deformation loss’ component of friction at the contact. This deformation loss component of friction has been correlated with the “elastic hysteresis” or the “visco‐elastic losses”. The elastic hysteresis consideration alone does not fully explain rubber substrate deformation and friction behaviour. The assumptions used are incompatible. For example, the delayed or incomplete recovery of the rubber substrate behind the contact leads to residual strains which result in the contact area asymmetry as shown in Fig. 1. In contrast, the elastic hysteresis approach assumes Hertzian elastic contact which is symmetric. It may be noted that all ‘lossy’ materials whether plastic or visco‐elastic in nature must involve frictional contact area asymmetry. Various simplified visco‐elastic considerations of the rolling contact have been illustrated, only qualitatively, the contact deformation and frictional loss behaviour. Direct experimental and quantitive verifications have not been attempted, however. Some rigorous visco‐elastic, two dimensional, continuum analyses of the rolling contact are available in the literature and are very complex. It is difficult to use the results of these analyses to the problem of frictional loss evaluation, primarily because linear and simplified visco‐elastic models have been employed. Moreover, for the general friction problem of rubberlike elastomers which are nonlinear visco‐elastic solids of complex descriptions, physical quantification and interpretation of the parameters used in the above analyses are not possible. Employing the method of a visco‐elastic operator, a semi‐analytical technique has been used recently to express the asymmetry of the sliding contact area and the associated deformation loss component of friction. The results of the analyses agree reasonably with the experimental observations. Dynamic material property parameters used in the analyses are obtained from an indentation test arrangement under closely controlled conditions.