Stick-slip behaviour in sliding rubber contacts
Budi Setiyana is a PhD student in the department Elastomer Technology and Engineering. Supervisors are prof.dr.ir. D.J. Schipper from the faculty of Engineering Technology and prof.dr. J. Jamari from the University of Diponegoro, Indonesia.
Rubber is a compliant material which is often modelled as a viscoelastic or hyper-elastic material. Due to its very elastic behaviour with low viscous damping, a rubber surface tends to provide dynamic responses when friction forces are present during sliding. Such dynamic responses commonly occur in the form of stick-slip behaviour and are indicated by oscillation and fluctuating contact forces. In rubber abrasion, the effects of the stick-slip are indicated by the formation of a periodic wear pattern or wavy wear track of an abraded rubber surface. Therefore, the stick-slip phenomena play an important role in sliding contacts.
Numerous rubber friction tests have been reported in literature in which stick-slip phenomena have been detected, but the description of these phenomena on rubber friction is rarely discussed in literature. The presence of dynamic phenomena indicates that a sliding system can be regarded as a dynamic system consisting of masses, dampers and springs. To describe stick-slip behavior of a sliding system, modelling is needed.
This research focuses on investigating the stick-slip occurrence by modelling the rubber sliding system. The sliding system is designed as a moving rubber surface which experiences a friction force from a single rigid indenter. Three sliding system models are proposed here. The first model is a tangential compliance system. It considers the rubber specimen and indenter move or oscillate in the tangential direction only. The second model is a fixed load system or full compliance system. It considers the rubber specimen to be able to oscillate in both tangential and normal direction. In this case, besides moving relatively in the tangential direction, the indenter and its frame can also oscillate in the normal direction. Most rubber abrasion testers are constructed as such a system. The third model is a fixed depth system. It considers the rubber specimen to be able to oscillate in the tangential as well as in the normal direction, but the indenter moves relatively in the tangential direction only.
The analysis is performed for the first and the second model using the rubber as a viscoelastic material, while for the third model the analysis is performed numerically using the rubber as a hyper-elastic material. The results show that the stick-slip frequency for the first model occurs as a tangential oscillation and depends greatly on the tangential stiffness of the rubber surface. For the second model, the stick-slip frequency occurs more dominantly as a normal oscillation and depends greatly on the normal stiffness of the rubber surface, indenter load and its frame properties. However, for a low sliding velocity, the stick-slip phenomena can be described by the first model, i.e. the tangential compliance system. The numerical model shows that the stick-slip phenomena are also close to the first model for the low sliding velocity; however, at a high sliding velocity, the stick-slip phenomena clearly occur both in the tangential and normal direction as indicated by the fluctuation of the contact forces during sliding.
Experimentally, rubber abrasion tests are conducted using the pin on disc tribometer with a specified or fixed indenter load (deadweight). This study investigates the stick-slip frequency and periodic wear pattern of an abraded surface. The results show that the wear pattern spacing of the wavy wear track correlates to the stick-slip frequency. Analytically, the stick-slip frequency is the natural frequency of the normal oscillation of the indenter system. The indenter system includes the loaded indenter and its frame which oscillate together in the normal direction during sliding. Analysis shows that the load during abrasion consists of a static load due to the applied deadweight and a dynamic load due to the inertia effect of the indenter system. Therefore, the stick-slip frequency and wear pattern spacing depend not only on the deadweight but also on the indenter system properties. Accordingly, the proposed sliding system model is applied to understand and to describe the occurrence of stick-slip.