Rolling contact fatigue (RCF) is a dominant cause of failure occurring in all types of railway systems, causing the initiation and propagation of surface or subsurface cracks. It greatly increases the cost of wheel and rail maintenance and can result in catastrophic accidents. The realistic working conditions of the wheel-rail system such as the contact profiles, track curvature, train speed, etc. make RCF more complicated than conventional fatigue problems. It is still a challenge to provide accurate predictions of the fatigue performance of wheel-rail systems. With the rapid growth of global train traffic, we urgently need to develop an efficient method for RCF assessment.
In RCF, cracking occurs due to high-stress wheel-rail contact. Potential cracking sites include wheel rims, rail joint welds, rail heads and rail undersides in contact with corroded metal. Crack characteristics at the nucleation stage have a significant impact on the progress of crack growth. However, crack nucleation criterions that apply to conventional fatigue, such as the critical plane approaches, are unable to give a reliable estimate of the fatigue life in rolling contact. The main reason is that the realistic working conditions of the wheel-rail system are not considered in the abovementioned methods. Thus, a unified model incorporating real data needs to be developed.
This project aims to develop a model for predicting the progression of crack nucleation and propagation in wheels and rails under rolling contact. The implementation scheme of the project is illustrated in Figure.1. The first step of this model is the stress analysis of the wheel-rail contact, including the surface and subsurface stress fields. This analysis is based on the railway vehicle dynamics which will take real axle load conditions into account. Stress analysis will provide a predictive fatigue model for the remaining lifetime of wheels and rails. This will be used to timely and cost-effective maintenance programs.
Both the crack initiation and propagation life are evaluated together. Rolling contact fatigue testing will be conducted to validate the developed model. New methodologies being investigated include:
- Evaluation of surface contact pressure and subsurface stress fields using the developed contact mechanics model.
- Testing of material tensile and fatigue properties of wheels and rails.
- Development of a fatigue damage model to predict crack nucleation and propagation in wheels and rails.
- Evaluation of wheel and rail profiles to prolong their lifetime.
A comprehensive application of the model considering the operating conditions of the East-West Line (EWL) of the SMRT investigated historical rolling contact fatigue of the wheel and rail there.
The variation of the contact forces on the EWL was studied using multibody dynamics considering the effect of track curvature. Sub-surface stresses and contact pressures were calculated using finite element methods based on contact forces. The fatigue damage was thus evaluated through the linear damage accumulation law. The distribution of the total fatigue damage in the wheel was calculated and the maximum damage beneath the surface of the wheel determined.
Effects of train velocity, track curvature and track cant on the fatigue behaviors of rails were investigated. It was found that when wheels traverse the rail, the accumulated maximum fatigue damage increses as track curvature increase (i.e. reducing radius). Furthermore, one-point contact becomes two-point contact as cant angle increases. When the cant angle is small, the maximum stress point is at the inner side of the rail. However, the maximum stress point shifts to the top as the cant angle increases (Figure.2).