Impact fatigue responses of pre-stressed concrete sleepers in railway track systems

Pre-stressed concrete sleepers are the main components of railway track systems. To carry and transfer the dynamic wheel loads from the rails to the ground, their current design and construction are limited by allowable flexural stress constraints under service conditions. In current design practice for such a component, the dynamic load effects due to wheel/rail interactions are treated as a quasi-static load using a dynamic impact factor. Then, the allowable stresses eliminate a crack initiation. In reality, the impact events are frequently recorded because of the uncertainties of wheel or rail irregularities such as flat wheels and dipped rails. These effects cause cracking in the concrete sleepers, resulting in excessive maintenance. Limit states design philosophy for the pre-stressed concrete sleepers, containing ultimate and fatigue limit states, has been recently proposed based on structural reliability concept to rationalise the design method and minimise the maintenance. On the basis of probabilistic approach, the high-magnitude low-cycle fatigue limit states, which are more significant in terms of damage evolution, have been addressed in this article. Series of repeated impact tests for the in-situ pre-stressed concrete sleepers were carried out using the Australian largest high-capacity drop weight impact testing machine at the University of Wollongong. The impact forces have been simulated in relation to the probabilistic track force distribution obtained from a heavy haul rail network. This article focuses on the impact responses of the cumulatively damaged sleepers. The effects on such responses of the track environments including soft and hard track supports are also highlighted in this article. It is found that a concrete sleeper damaged by the impact fatigue cycles could possess significant reserve capacity for resisting the axle load in a similar manner as the undamaged sleeper.