No abstract

A better understanding of the thermomechanical loading of brake discs is important for controlling material fatigue and crack propagation in the disc. In the present study, full-scale drag braking experiments were performed on brake discs made from eight different grey cast iron alloys. The well-performing materials were also tested with an alternative brake pad material. A testing procedure with repeated drag brakings was used. The disc and pad temperatures were registered by thermocouples embedded at selected locations, and the disc surface temperatures by a thermocamera. Extensive analyses of the measured temperatures were performed. The results for the thermocouples at the mid-radius of the disc and at the end of brake applications indicatd that the two sides of the disc have opposite deviations from the mean temperature. The temperature deviations are generally temporally alternating, but also stationary variations can be found. The thermocamera gives the possibility of identifying the phenomena behind the temperature variations found from the thermocouples. Banding of the disc-pads contact with alternating one band and two bands of high temperatures is observed for the studied brake discs exposed to severe braking load cases. Moreover, it was found that hot-spot patterns develop on the disc surface, which are spatially fixed during each brake application. However, they may be either slowly migrating or fixed relative to the disc during consecutive brake applications. Thermal images show that small cracks do not affect hot-spot migration as a hot spot migrates over the crack. However, at a critical length of the crack, the heat becomes localized at the crack and increases its growth, thus limiting the life of the disc. The tests indicate that a combination of hot-spot migration, alternating bands and small temperature differences over the disc are significant factors to be considered when improving the lifespan of the discs.

A thermal model of railway tread braking is developed for use in routine calculations of wheel and brake block temperatures. Two-dimensional finite-element models of block(s) and wheel are coupled via a contact interface that controls the heat generation and also the heat partitioning between block and wheel through thermal contact resistances. The surface temperature variations around the wheel circumference as caused by frictional heating and intermediate cooling are accounted for in a mean sense, assuming high-speed sliding conditions. The thermal power generated at the block-wheel interface during braking is determined from train braking data. A model for heat transfer from the rolling wheel into the rail is developed where a film with thermal contact resistance is placed at the wheel-rail contact interface. The present model can be used to efficiently design tread braking systems for both freight and passenger trains. It can handle stop braking, drag braking at constant brake power, and also intermediate periods of cooling. The temperature history during a full train route can be calculated. The inclusion of heat transfer from wheel to rail means that the model is useful for comparing brake rig tests, where normally the chilling influence from the rail is not included, with in-field tests. Two companion papers with experimental results supplement the present numerical modelling. A brief numerical example demonstrates the heat partitioning and the influence of rail chill (about 30 per cent) for two braking configurations.

Tread braking generates high temperatures in railway wheels and brake blocks as the kinetic energy of the running train is transformed into heat. The temperatures induced in the components are here analysed with particular focus on the cooling influence from the rolling contact between the hot wheel and a cold rail. Controlled brake rig tests are reported, where the rolling contact is studied using a so-called rail-wheel in contact with the braked wheel, along with results from field tests. The data from these experimental studies are used for calibration of a simulation tool for calculation of wheel and block temperatures. The calibrated model analyses heat partitioning between block, wheel and rail and finds the resulting temperatures at braking. The rail chill is found to have a considerable influence on the wheel temperatures for long drag braking cycles. A successful calibration of the model using data from field tests is also reported.