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Wear Model

The prediction of the wear phenomena at the wheel–rail interface is a fundamental issue in the railway field; in fact the consequent evolution of rail and wheel profiles involves serious effects on both dynamical and stability characteristics of vehicles. From a safety view point, modifications in wheel and rail profiles may compromise the vehicle stability and also increase the derailment risk due to wheels climbing over the rail.

Profile changes lead also to higher maintenance cost, mainly related to the periodical re-profiling operations of wheels and the undesirable replacements of rails, necessary to re-establish the original profiles. A reliable wear model can be used to optimize the original profiles of wheel and rail and to obtain a more uniform wear on the rolling surfaces. In such a way the overall amount of worn material can be reduced, the mean time between two maintenance interventions can be increased and, at the same time, the dynamical performance of the wheel–rail pair can be kept approximately constant between two succeeding repair interventions. It has been realized a procedure to estimate the evolution of the wheel and rail profiles due to wear specifically developed for complex railway networks.

The general layout of the model consists of two mutually interactive parts: the vehicle model (multibody model and 3D global contact model) and the wear model (local contact model, wear evaluation and profiles update). The general architecture of the model developed for studying the wear phenomena on complex railway lines is shown in the block diagram in which two different main parts are present: the vehicle model necessary to perform the dynamical analysis and the wear model. The vehicle model consists of the multibody model of the benchmark railway vehicle and the 3D global contact model that, during the dynamical simulation, interact directly on line creating a loop. At each time integration step the first one evaluates the kinematic variables (position, orientation and their derivatives) relative to the wheel sets and consequently to each wheel–rail contact pair. At this point, starting from the kinematic quantities the 3D global contact model calculates the global contact variables (contact points and contact forces, contact areas and global creepages), this model is based both on an innovative algorithm for the detection of the contact points and on Hertz's and Kalker's global theories for the evaluation of the contact forces. The global contact variables are then passed to the multibody model to carry on the vehicle dynamics simulation.

The main inputs of the vehicle model are the multibody model of the railway vehicle and the corresponding railway track, represented in this work by the ALSTOM DMU Aln 501 Minuetto. In wear estimation research activities the track description is a critical task due to the complexity of the railway nets to be studied: in fact the exhaustive simulation of vehicle dynamics and of wear evolution on all the railway net turns out to be too expensive both concerning computation times and memory consumption and concerning the availability and the collection of the experimental data needed for the model validation. To overcome these limitations, a statistical approach has been developed to achieve general significant results in areas on able time; in particular the entire considered railway net has been replaced with a discrete set of different curved tracks (classified by radius, super elevation and traveling speed) statistically equivalent to the original net. The wear model is the part of the procedure concerning the prediction of the amount of worn material to be removed from the wheel and rail surfaces and is made up of three distinct phases: the local contact model, the wear evaluation and the profile update.

The local contact model (based both on Hertz's local theory and on simplified Kalker's algorithm), starting from the global contact variables, estimates the local contact pressures and creepages inside the contact patch and detects the slip zone of the contact area. Subsequently the distribution of removed material is calculated both on the wheel and on the rail surface only within the slip area by using an experimental relationship between the removal material and the energy dissipated by friction at the contact interface. Finally wheel and rail worn profiles are derived from the original ones through an appropriate update strategy. The new updated wheel and rail profiles (one mean profile both for all the wheels of the vehicle and for all the considered tracks) are then fed back as inputs to the vehicle model and the whole model architecture can proceed with the following iterations.

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