Damage and thermally induced defects in railway materials
A major concern for the railway industry today is the problem of rolling contact fatigue (RCF) damage in wheels and rails as a result of increased traffic, accelerations, and loading conditions. The wheel/rail contact condition that occurs in railway applications is complex, and it is essential to understand the associated damage from combined mechanical and thermal loadings in order to predict component life and develop appropriate maintenance systems. The crack initiation and propagation processes of RCF have been extensively studied from both theoretical and experimental points of view; however, certain mechanisms for crack initiation and propagation are still not well understood. Thermally damaged surface layers, often called white etching layers (WELs), seem to contribute to crack initiation. One hypothesis is that such cracks can develop into so-called squats or studs in the rail, and to RCF clusters in wheels. The work presented in this thesis can be separated into two main objectives: the first was to properly describe the three-dimensional (3D) geometry of the network of RCF defects in rails using several characterization techniques, and the second was to study crack initiation from thermally damaged surface spots, which mimic a WEL, using low-cycle fatigue (LCF) tests.
The following methods were used to geometrically describe squat crack networks: high-intensity X-ray radiography complemented with geometrical reconstruction, serial sectioning and metallography in conjunction with microscopy, X-ray tomography, and topography measurements. The experiments were performed on squats from rail sections taken from the field. It was concluded that the different methods are complementary, and observations made using one method can supplement and explain the deficiencies of other methods. Secondly, investigation of crack initiation from thermally damaged surface spots was carried out via strain-controlled LCF experiments using a MTS 809 servo-hydraulic test machine. The effect of initial thermal damage, observed in the form of a small martensite spot (or WEL) on test bars, on fatigue crack initiation was examined and evaluated relative to smooth specimens. Comparisons between the WELs observed in field samples and those produced artificially on test bars and railheads were made regarding microstructure and residual stresses. The presence of initial thermal damage was found to slightly reduce fatigue life.