Study on the lifecycle of a bogie's suspension elements, towards an optimised maintenance and safe dynamic behaviour

Transport industry and, more specifically, railway industry, is confronted with a permanent need of improvement of its products. The competitiveness of rolling stock does not come only from low-cost production, but also from well-calculated lifecycle costs. Nowadays, many contracts for railway operators include not only trains, but also maintenance services throughout its lifetime, which may reach 30% of global costs. Hence, deep knowledge about the system's ageing is a strong asset to assess a good performance, both on quality of service and financial costs. The aim of our project is to study an existent bogie platform from our industrial partner, to characterise the ageing of some critical elements and its effects on the system. Such elements mark the pace of overhaul operations, which are time and cost-expensive. Lifetime improvements require a thorough assessment of the train's safety and a good knowledge of the parts' characteristics during the lifecycle. The FMECA of the bogie is used to decide which degraded modes/elements will be studied on the project. Railway safety for dynamic behaviour is assessed with two main standard norms: UIC-518 and EN-14363. Both texts describe the basic compliance tests to be met by rolling stock and their implementation. Numerical simulation plays a key role on design and safety check, allowing virtual testing prior to full-scale trials. We work with three standard MBS simulations based on the norms' tests: derailment, roll coefficient and dynamic behaviour. Each simulation yields several safety factors that must be respected (for example, derailment ratio Y/Q must be smaller than 1.2). These models are used to test the impact of the degraded modes on the train's behaviour and safety compliance. A large experiment has been set up to test several levels of variation of mechanical characteristics and their effect on the performance indexes given by the norms. Its results will provide information about the system's operational boundaries (i.e. which stiffness changes on which elements might be critical for safety compliance) and, with an adequate treatment, to prepare metamodels which could be used to perform inverse analysis on the system, to find critical combinations of characteristics leading to non-compliant evaluations. A good knowledge of the system's operational boundaries is not enough to assess maintenance properly. The elements which have been chosen for the study are metallic-rubber parts, whose mechanical characteristics can vary strongly during a lifecycle. Such changes have two sources: ageing due to environmental conditions and mechanical loading. We propose a multi-level approach to characterise those evolutions: a test campaign on standard samples, new and aged, to determine the constitutive equation of the material during lifecycle conditions. Those data would then be transposed to a FE model of the suspension elements, which would allow to calculate the elements' stiffness under different lifetime scenarii, providing a predictive tool. However, the parts' geometry and rubber-exposed area make the ageing process progressive, with the outer parts more affected than the bulk of the piece. This gradient of properties should be taken into account. There are models that can calculate the change of the reticulation rate in rubber parts depending on exposure conditions and on the chemical reactions that might be present. Coupling both aspects, dynamics and rubber study, would provide a strong asset for maintenance optimisation. Multibody simulations would provide the safety boundaries for the system in terms of stiffness variation, while the rubber study would supply the necessary translation of those stiffness levels into a time reference. The use of both might allow for lifetime extensions, if safety assessment allows for it