1. Introduction
With the development of high-speed and heavy-load railway, much higher requirements are put forward for the comprehensive properties of steel rails [1]. Since fracture accidents of steel rail threaten the safety of railway transportations directly, more and more attention is paid to the quality of rail steel. The fatigue fracture is the main failure form of steel rail. Despite substantial advantages in design, materials and non-destructive inspection, fatigue propagation in and failure of railway components remains an important issue for safety engineering which is also emphasized by a number of accidents over the last decades [2,3]. At the background of an increased volume of traffic, higher traffic speeds and higher axle loads, reliable damage tolerance design and effective maintenance methods have to be established. Therefore, failure analysis of the steel rail is very critical.
In addition to the fatigue load, rails are also subjected to other high mechanical loads and harsh environmental conditions. The main loading components are rolling contact pressure, shear and bending forces from the vehicle weight, thermal stresses due to restrained elongation of continuously welded rails and residual stresses from manufacturing (roller straightening) and welding in the field (Fig. 1) [2]. Welding is indispensable in the railway rail. Today, the most common rail
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