Mechanical track deterioration due to lateral geometry irregularities

Licentiate thesis, 2012

This thesis deals with how a degraded track geometry influences further degradation of the geometry and the formation of rolling contact fatigue (RCF) and wear of rails. The overall objective is optimisation of railway maintenance. For this, further understanding and quantification of the deterioration of track components are needed. Dynamic multibody simulations have been performed featuring different wagons, and a track with different curve radii and different levels of track geometry degradation. With the resulting wheel/rail interaction degradation indicators are evaluated. Increased track shift forces are considered as an indicator of further track geometry degradation. Two different RCF indices (one shakedown map based and one wear number based) are used to quantify surface degradation of rails. Measured lateral track irregularities were found to follow a normal distribution reasonably well. Furthermore, the standard deviation of track shift forces was shown to have a linear relationship with the standard deviation of the lateral irregularities. For large curve radii an increase of the level of lateral irregularities increases the length of rail affected by RCF. The opposite is predicted for small curve radii where the length of rail affected by RCF decreases with increasing levels of lateral irregularities. For very small curve radii it was shown that the degradation mechanism shifts from pure wear for low levels of lateral irregularities to a mixed wear/RCF degradation for higher levels of lateral irregularities. Amplification of lateral irregularities in different wavelength spans revealed that an amplification of longer wavelengths (between 10--50 metres) had the largest influence on the length of rail affected by RCF in shallow curves. A correlation study between predicted tangential wheel/rail contact forces and lateral irregularities (amplitudes of the irregularities, first order derivatives and second order derivatives) was carried out. No significant correlation was found.