In the present contribution, a general framework for the completely consistent integration of nonlinear dissipative dynamics is proposed, that essentially relies on Finite Element methods in space and time. In this context, fully flexible structures as well as hybrid systems which consist of rigid bodies and inelastic flexible parts are considered. Thereby, special emphasis is placed on the resulting algorithmic fulfilment of fundamental balance equations, and the excellent performance of the presented concepts is demonstrated by means of several representative numerical examples, involving in particular finite elasto-plastic deformations.
Unlubricated sliding systems being economic and environmentally benign are already realized in bearings, where dry metal-plastic sliding pairs successfully replace lubricated metal-metal ones. Nowadays, a considerable part of the tribological research concentrates to realize unlubricated elastomer-metal sliding systems, and to extend the application field of lubrication-free slider elements. In this Thesis, characteristics of the dry sliding and friction are investigated for elastomer-metal sliding pairs. In this study ethylene-propylene-diene rubbers (EPDM) with and without carbon black (CB) filler were used. The filler content of the EPDMs was varied: EPDMs with 0-, 30-, 45- and 60 part per hundred rubber (phr) CB amount were investigated. Quasistatic tension and compression tests and dynamic mechanical thermal analysis (DMTA) were carried out to analyze the static a viscoelastic behavior of the EPDMs. The tribological properties of the EPDMs were investigated using dry roller (metal) – on – plate (rubber) type tests (ROP). During the ROP tests the normal load was varied. The coefficient of friction (COF) and the temperature were registered online during the tests, the loss volumes were determined after certain test durations. The worn surfaces of the rubbers and of the steel counterparts were analyzed using scanning electron microscope (SEM) to determine the wear mechanisms. Because possible chemical changes may take place during dry sliding due to the elevated contact temperature the chemical composition of the surfaces was also analyzed before and after the tribotests. For the latter investigations X-ray photoelectron spectroscopy (XPS), sessil drop tests and Raman spectroscopy were used. In addition, the dry sliding tribotests were simulated using finite element (FE) codes for the better understanding of the related wear mechanisms. Finally, as the internal damping effect of the elastomers plays a great role in the sliding wear process, their viscoelasticity has been taken into account. The effect of viscoelasticity was shown on example of rolling friction. To study the rolling COF for the EPDM with 30 phr CB (EPDM 30) an FE model was created which considered the viscoelastic behavior of the rubber during rolling. The results showed that the incorporated CB enhanced the mechanical and tribological properties (both COF and wear rate have been reduced) of the EPDMs. Further on, the CB content of the EPDM influences fundamentally the observed wear mechanisms. The wear characteristics changed also with the applied normal load. In case of the EPDM 30 a rubber tribofilm was found on the steel counterpart when tests were performed at high normal loads. Analysis of the chemical composition of the surfaces before and after the wear tests does not result in notable changes. It was demonstrated, that the FE method is powerful tool to model both, the dry sliding and rolling performances of elastomers.
The goal of this thesis is a physically motivated and thermodynamically consistent formulation of higher gradient inelastic material behavior. Thereby, the influence of the material microstructure is incorporated. Next to theoretical aspects, the thesis is complemented with the algorithmic treatment and numerical implementation of the derived model. Hereby, two major inelastic effects will be addressed: on the one hand elasto-plastic processes and on the other hand damage mechanisms, which will both be modeled within a continuum mechanics framework.
A general framework for the thermodynamics of open systems is developed in the spatial and the material setting. Special emphasis is placed on the balance of mass which is enhanced by additional source and flux terms. Different solution strategies within the finite element technique are derived and compared. A number of numerical examples illustrates the features of the proposed approach.