Friction modelling requirements and implementation in railway freight vehicle dynamic simulations

The highly nonlinear characteristics of dry friction damped suspension components used in railway freight vehicles increases the modelling challenges in Multibody Simulations (MBS) software packages such as VAMPIRE, SIMPACK and ADAMS VI-RAIL. These system nonlinearities then lead to lower confidence levels in modelling methodologies and therefore, more uncertainty in the simulation results produced by these models.

The main reasons for the low confidence levels associated with friction modelling methodologies in the context of railway freight vehicles, are due to the lack of reliable test data to validate the friction models used in the primary/secondary suspension. Therefore, there is an industry requirement to better understand the in service/maintenance requirements of these components, first from a testing perspective and then from a modelling one. Once this current gap in knowledge and understanding is bridged, the results generated from the simulation models could then be used in the future to replace on-track/laboratory testing, which would bring significant cost benefits to the rail freight industry.

The work presented in this thesis reviews the current friction testing and modelling methodologies as well as the simplifying assumptions that are implemented in commercial MBS software packages. The most common friction model that is used in railway vehicle MBS for example, is the Coulomb model of dry friction. This model however, is not capable of describing stick-slip behaviour, pre-sliding displacements or the viscoelastic effects of friction surfaces when they start to yield under extreme loading conditions. The Coulomb friction model also has a discontinuity in the vicinity of zero velocity, which can cause numerical issues in MBS as the friction force in this region, is non-differentiable. To handle the discontinuity, a variety of regularisation methods are generally implemented in the Coulomb model which are further discussed in the literature review.

The impact of these regularisation methods on the validity of MBS results however are currently not fully understood or justified, as these tend to be mathematical smoothing functions that are not actually related to the mechanical properties of the friction surfaces. Therefore, to better understand the limitations of the Coulomb friction model and its application, as well as the different regularisation methods that are implemented in freight vehicle secondary suspension friction models, several scale model benchmark tests have been carried out on a variety of rail industry standard secondary suspension wear liner components.

The critical modelling parameters that are required to develop a comprehensive/optimised freight vehicle secondary suspension friction model were then calculated from the benchmark test data. These parameters include the torsional stiffness and damping properties of the centre plate/wear liner components, as well as the Coefficient of Friction (COF) values and frequency signatures of the secondary suspension system.

From this research, three novel secondary suspension friction models are proposed that have been developed to work in conjunction with the SIMPACK software package via the co-simulation functionality. These models include the novel local/global friction models, which are used together in MBS to describe the hysteresis and frequency responses of the centre plate/wear liner components as well as increases in these parameters as a function of time and the number of running cycles.