with Application to. Rubber Pad Forming Processes. Deladi, E.L.. Advisor: Prof.dr.
ir. D.J. Schipper. Co-advisor: Dr.ir. M.B. de Rooijj. University of Twente, 2006.
Static Friction in Rubber-Metal Contacts with Application to Rubber Pad Forming Processes Deladi, E.L. Advisor: Prof.dr.ir. D.J. Schipper Co-advisor: Dr.ir. M.B. de Rooijj University of Twente, 2006 ISBN-10: 90-77172-22-X ISBN-13: 978-90-77172-22-3 EM research theme: Computational and Experimental Mechanics A static friction model suitable for rubber-metal contact is presented in this dissertation. In introduction, the motivation and the aims of the research are introduced together with the background regarding the related industrial application, which is the rubber pad forming process. Chapter 2 deals with definition, mechanisms and parameters which characterize static friction. The parameters required to describe the static friction regime are defined, starting with a short historical background of friction. The mechanisms responsible for static friction as well as for dynamic friction are presented. Then, the influence of several parameters such as pressure, tangential displacement, roughness, contact time, and temperature on this preliminary stage of friction is discussed. A literature survey is presented in this respect for the contact types which are of interest in rubber pad forming, namely: rubber/metal and metal/metal. In Chapter 3 the tribological system is reviewed. The viscoelastic properties of the rubber pad and the related measurement techniques are presented. Since adhesion is important in rubber friction, the surface free energy of materials has been investigated. Surface roughness plays a significant role in friction between the rubber pad and the metal sheet. Therefore, surface roughness parameters are introduced together with the measurement techniques. Depending on the relation between the real contact area and the apparent contact area, various approaches can be used to model the contact between the rubber pad and the metal sheet. These approaches are briefly discussed. Chapter 4 focuses on the single-asperity static friction model. First, the normal contact between a viscoelastic sphere and a rigid flat is modeled using a modified Hertz theory, in which the viscoelastic behavior is incorporated through a mechanical model. Then, when a tangential load is subsequently applied, a mechanism similar to that described by Mindlin's theory is assumed to take place in the contact area. At low loads adhesion plays an important role. Its effect has been modeled according to the JKR theory. A factor has been included which accounts for the work of adhesion of viscoelastic materials. Friction is attributed to the shear of the interfacial layer which separates the bodies in contact. The developed static friction model is based on the abovementioned contact models. Furthermore, a parametric study is presented regarding the influence of several parameters on the static friction force and limiting displacement. In Chapter 5 the single-asperity static friction model is extended to the multi-asperity case, first, by using a statistical approach. This multi-summit approach is usually suitable for cases where the real contact area is a small fraction of the apparent contact area. Then, a multi-asperity
approach is used further in modeling static friction between a rough viscoelastic surface and a smooth rigid plane. A parametric study is performed and the results obtained using these approaches are compared. The detailed microgeometry of the rubber surface is influencing the frictional behavior to a large extent. The experimental validation of the developed single-asperity and multi-asperity static friction models is presented in Chapter 6. Single-asperity friction measurements have been carried out on a nano-tribometer using a ball-on-flat configuration. The influence of several parameters such as normal load, radius of the ball and Shore hardness upon static friction was examined. Then, the multi-asperity static friction model is experimentally validated. The theoretical predictions are in general agreement with the experimental results. In Chapter 7 the developed friction model is used to obtain the curve-fits which are needed for the implementation of the static friction model in the finite element simulation of the rubber pad forming process. The results of the finite element simulations of the rubber pad forming process indicate that the static friction model has an effect on the radius of curvature of the formed strip. Finally, the conclusions and recommendations resulting from the theoretical and experimental investigation of the static friction in rubber/metal contact are presented..
