Damage and defects in railway materials: influence of mechanical and thermal damage on crack initiation and propagation
With rising societal concern for the environment, railways offer travelers a sustainable alternative to other forms of fossil-fueled transportation. In order to be competitive with airlines, for example, the railway industry must provide safe, efficient, and affordable service. In the current state, frequent delays on many networks and some rare accidents lead travelers to question the reliability and safety of the rail transport system. This problem is a complex mishmash of different matters which must be addressed from various perspectives. This study focuses on the materials in wheels and rails themselves.
Railway wheel and rail materials are subjected to extreme stress in the field; increasing speeds, loads, traffic, at times in harsh weather conditions aggravate this. The combination of loadings affects the materials in a multitude of ways, for example through mechanical and thermal damage. One of the most common types of mechanical damage in the railway industry is rolling contact fatigue (RCF), the effect of which is frequently manifested in the surface of railway components, as cracks. Thermal damage, on the other hand, can affect both the surface and bulk of the material.
The aim of this project is to properly characterize mechanical and thermal defects in railway components, and evaluate their effects on crack initiation and propagation, and on mechanical properties. This has been approached through extensive characterization of field samples with a certain type of cracks called squat cracks, which in some cases may lead to rail break. The squat crack networks were examined through a variety of methods, followed by the recreation of similar defects in the laboratory. Finally the effect of such defects on the microstructure, crack initiation and propagation in laboratory experiments was evaluated.
Squat crack networks were characterized using several methods; detection limits of each technique have been clarified, and it was concluded that using a combination of methods, the network can be accurately described on many scales. In a second part, well-defined thermal damage on rail surfaces called white etching layers (WELs) similar to those found in field were produced using laser welding equipment, and the effect of these WEL spots on crack initiation and fatigue life has been shown. The WELs reduce fatigue life by providing a crack initiation site; both by stress and strain concentration and by decreasing ductility. The effect of thermal damage on bulk properties was also investigated using microscopy techniques including electron backscatter diffraction (EBSD) and differential-aperture X-ray microscopy (DAXM). It was found that the variation in local misorientation and residual strains decrease with increasing annealing temperature. Additionally, a method to examine crack face friction has been identified, and using this method, similar crack face features to those observed in cracks from field are created in the lab. The results from the friction experiments can be used as input towards crack propagation experiments.