Despite substantial advantages in material development and in periodic non-destructive inspection together with periodic grinding and other measures in order to guarantee safe service, fatigue crack propagation and fracture is still in great demand as emphasised by the present special issue. Rails, as the heart of the railway system, are subjected to very high service loads and harsh environmental conditions. Since any potential rail breakage includes the risk of catastrophic derailment of vehicles, it is of paramount interest to avoid such a scenario. The aim of the present paper is to introduce the most important questions regarding crack propagation and fracture of rails. These include the loading conditions: contact forces from the wheel and thermal stresses due to restrained elongation of continuously welded rails together with residual stresses from manufacturing and welding in the field, which is discussed in Section 2. Section 3 provides an overview of crack-type rail defects and potential failure scenarios. Finally the stages of crack propagation from initiation up to final breakage are discussed.

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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.

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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.

A numerical procedure to integrate simulation of high-frequency dynamic train-track interaction and prediction of rolling contact fatigue (RCF) impact is presented. Features of the included models and possibilities of applications are outlined. The influence of short-pitch rail corrugation and wheel out-of-roundness (OOR) on RCF of a high-speed passenger train is investigated. It is shown how the corrugation and the OOR will have a profound effect in that levels of wheel and rail irregularities that have been measured in the field may be sufficient to generate subsurface-initiated RCF. In particular, the high-frequency content of the contact forces is of importance. Errors induced by neglecting such high-frequency components in measurements and/or simulations are investigated by comparing RCF indices based on contact forces that have been low-pass filtered with various cut-off frequencies. To avoid cracking due to RCF, a maximum roughness level in the wavelength interval up to 10 cm is sought. To limit the effects of corrugation, grinding practices have been altered leading to a significant decrease in RCF.

Extensive field experiments (the ‘Silinge wheel flat experiments’) with a moving train have been designed, performed (at Silinge) and evaluated. More than 200 wheel flats were formed under controlled conditions involving different wheel loads, train speeds and sliding durations, and the friction coefficient between the wheel and the rail was also varied (and indirectly measured). Samples extracted from flats of the tested wheels have been metallographically examined with respect to phase transformations and cracks. A numerical model for wheel flat prediction has been qualitatively verified and quantitatively calibrated. In the experiments, martensite was found beneath all flats and cracks were observed in most cases. It is concluded that the risk for future spalling should be considered for all wheelsets with flats. A damaged wheelset should be taken out of service as quickly as possible. When reprofiling the wheels, all martensite and an additional layer of several millimetres should be machined off.