Wave generation and propagation at tribological interfaces

Wave generation and propagation at contact interfaces is a relevant issue in mechanics because they affect directly a large number of mechanical systems involving either static or sliding frictional parts. Contact dynamics is at the origin of friction induced vibrations, acoustical instabilities, surface damage, wear and fatigue failures [IBRA 94][AKAY 02] Propagating waves at the interface can be exploited for controlling the global friction features of the contact. Beside classical experiments, numerical analysis on dry contacts in sliding state has been one of the mayor topic in recent literature to which a relevant number of important papers have been dedicated ([RENO 11] and reference therein), due also to the fact that it is a substantial subject of interest for researchers working on many different domains, from dynamics to tribology and geophysics. At the origin of the phenomena cited above there is the wave generation and propagation at the frictional contact interface. Moreover when the two materials in contact are different, the dynamics of the phenomenon increases in complexity due to the so-called "bimaterial" effect ([WEER 80][ADA 95][BEN 01] and reference therein). This thesis is addressed to the understanding of the mechanisms at the origin of the contact wave fields and its relationship with the local characteristics of the surfaces in contact, as well as with the global dynamics and macroscopic frictional behaviour of the system. The aim of this work is to provide insights on the generation and propagation of the waves through the contact both to avoid instabilities and to control their effect on friction. The work is organized in two main parts. The first part presents the development of a non-linear finite element analysis in large transformations of the dynamic rupture at the interface with contact friction separating two bodies (isotropic and elastic) without relative motion. A rupture is considered when an initially sticking zone shifts in sliding state. The properties of the obtained ruptures are analyzed for a flat interface between dissimilar materials in function of the nucleation energy; then the effect of the interface roughness is analyzed. The differentiated rupture inside the asperities and the conditions for coupling and uncoupling between the waves radiating in the two bodies have been also investigated. In the second part, the analysis deals with the sliding onset between two bodies in contact. The sliding between two bodies made of different isotropic elastic materials and separated by a frictional interface is simulated. The evolution along the time of the global normal and tangential forces is analyzed, relating it to the local phenomena occurring at the interface. This part tries to investigate how micro-slips at the interface, acting as distributed ruptures, trigger the macro-slips between the two bodies. The interaction between local and global dynamics is also studied. Finally a numerical parameter space study is carried out, as a function of several system parameters (contact law, friction coefficient, material damping, normal load, translational velocity and regularization time). The results show the key role of the micro-slips and precursors (detectable wave propagations that occur at tangential global force well below the critical value expected by the friction law) in triggering the macro-slip between the two bodies. Depending on their distribution and magnitude the evolution of the contact forces passes from stick-slip-like behaviour to continuous sliding. The local dynamics at the contact (wave and rupture propagation) is linked to the global behaviour of the system (stick-slip, continuous sliding, induced vibrations); the effect of the contact and system parameters on the transfer of vibrational energy between the sliding contact and the system is investigated. The numerical results obtained by the two parts of the work show a good agreement with experimental results in literature.