Temperature-sensitive Durability Analysis of Elastomeric Compounds

In a recent project funded by the Mechanical Engineering Research Federation (FKM) studies were carried out on an elastomeric compound based on natural rubber for characterising mechanical and thermal properties. Both were found to be significantly dependent on the temperature. At the same time studies with parallel measurement of the infrared spectrum of the sample surface showed that the self-heating of the material under dynamic loading was very strong, so that the material properties can change dramatically even without external temperature change during use. The durability analyses of the material were made on the basis of Hourglass samples as well as on various component samples. Again, the durability dependence on temperature was clearly visible. In order to make both predictable, the deterioration of the material properties as well as the durability deterioration upon increasing temperature, a thermo-viscoelastic material model was developed which bidirectionally coupled reproduces the mechanical and thermal material behaviour. Thus the heating of the material can be calculated by mechanical stresses using finite element simulation (FEM). This material model was integrated into a new method, which can predict local temperatures based on a sequential load-time-history. Result was a conceptual approach on a temperature-sensitive durability analysis which outlines the entire process from the experimental material characterisation on the finite-element analysis and computational temperature calculation up to the temperature-dependant accumulation of damage. Finally, the concept was validated against the hourglass-sample and exemplified for the fatigue analysis of an elastomeric bearing.

Research Institute: System Reliability and Machine Acoustics (SzM), TU Darmstadt and Institute of Mechanics, The Bundeswehr University Munich (UniBwM) Chairman: Dr.-Ing. Paul Heuler, Audi AG

figure 1

Hot-spot detection by finite-element analysis of web 2 of a four-web bearing (component sample 2)

© TU Darmstadt

Optimised FEM Reduction

The simultaneous calculation of different physical effects by ehd/mbs techniques is becoming increasingly important in the vehicle and drive technology and is carried out, inter alia, in the simulation of tribological systems. For this the calculation of the nonlinear motion behaviour, contact reactions, deformation and elastohydrodynamic (ehd) lubrication film reactions is essential. The reduction methods used so far are limited to the Component Mode Synthesis (CMS); the integration of modern trial functions in the contact calculation is not tested yet. As part of a FVV project the reduction software MatMorembs has now been extended to the specific problem of ehd bearing arrangements with many in- and outputs and contact events. Starting from various linear finite element models (FEM), model order reductions based on CMS, Krylov subspaces and Gramian matrices have been used. For the validation of methods, simulation models have been constructed to calculate the crankshaft dynamics, their components were reduced with classic and modern reduction methods and tested under identical conditions. The studies showed that, depending on the method and degree of reduction, the elastic deformations of the journal bearing vary resulting in different contact situations. It turns out that two-stage reduced structures based on the Krylov method in the first stage and a modal or tangential Krylov reduction in the second stage deliver the best results for ehd calculation with contact events. After all, the results show that modern reduction methods can also be used for ehd bearings with contact events. They offer the potential to further increase the degree of reduction and thus computation time in combination with similar accuracy.

Research Institute: Institute of Engineering and Computational Mechanics (ITM), University of Stuttgart and Institute of Machine Elements and Tribology (MT), University of Kassel Chairman: Dr Johannes Müllers, Robert Bosch GmbH