Modelling the contact behaviour in the presence of viscoelasticity

This project addresses critical knowledge gaps in frictional viscoelastic contact mechanics, focusing on the partial slip phenomenon, where the contacting area is divided into stick and slip zones under insufficient tangential loads. The transient evolution of these zones and their influence on subsequent gross sliding behaviour remain underexplored for viscoelastic materials. To fill in these gaps, a numerical framework was developed, integrating time-dependent viscoelastic effects and coupling mechanisms arising from the frictional contact of dissimilar materials.
The research is structured around four objectives. As the foundation of the numerical framework, an elastic partial slip contact model was first developed to investigate effects of surface roughness on partial slip solutions. For rough contact of similar materials (nominally-flat against flat), the stick ratio is unaffected by roughness parameters but follows a linear relationship with applied tangential load. However, the relationship becomes nonlinear when dissimilar materials are involved due to the coupling effects.
The elastic models were then converted into transient viscoelastic partial slip contact models using the elastic-viscoelastic correspondence principle. Different from elastic materials, the separation pattern of stick and slip regions for viscoelastic materials varies with time when different phenomena (creep induced by a constant load or stress relaxation induced by a constant displacement) are encountered in lateral and normal directions, even though the stick ratio remains constant under constant contact inputs.
A transient viscoelastic sliding contact model was further developed to explore effects of early partial slip period on sliding contact solutions, including the delay of the attainment of steady-state sliding and slight decrease of the peak pressure compared to frictionless sliding. These findings justify the use of a frictionless assumption in viscoelastic sliding contact analysis, where qualitative predictions of steady-state behaviour are the primary research concern, such as the assessment of long-term tribofilm performance.
Finally, the framework was extended to model viscoelastic layered contact, focusing on zinc dialkyl dithiophosphate (ZDDP)-derived tribofilms. The layer-substrate system exhibits viscoelastic or elastic-dominated behaviour depending on loading and sliding conditions. In practical applications, the tribofilm performs primarily as a soft elastic layer. This study bridges nanoscale viscoelastic findings with large-scale observations, offering insights for optimizing additive performance in tribological systems.