Residual Stress Evaluation of Railway Rails

Various stresses add to the railway rails and accumulate fatigue damage due to daily use. Fracture mechanics clarification concerning the growth behavior of cracks is needed, in order to decide the inspection period and the cycle [1], . Though the crack progression and influence of residual stress on fatigue strength has been researched [3] for many years, they are not clear now. The aims of this research are the assistance of the inspection period and the cycle decision of the rail maintenance. For these purposes, the residual stress distribution in the railway rails was measured by using the X-ray diffraction method, the strain gauge method and FEM analysis respectively. INTRODUCTION Rolling contact fatigue is one type of fatigue damage of railway rails. A lot of research has been done about rolling contact fatigue based on fracture mechanics. Rolling contact fatigue occurs by repeated contact stress at the railhead running wheels. Contact stress generally forms compressive stress. Tensile stress helps to progress the fatigue failure and it decreases rapidly under the contact surface, so a lot of cracks usually stop at the depth of several millimeters. However, the rails finally break when the crack propagation in the rail changes direction toward the bottom of the rail. This is because the crack receives the cyclic bending stress of the rail and the influence of the tensile residual stress. In order to obtain the crack growth rate, the measurement of the residual stress distribution is needed. Fracture mechanics clarification concerning the growth behavior of cracks is needed, in order to decide the inspection period and the cycle. The crack progression and influence of residual stress on fatigue strength has been researched [3] for many years. It is well-known the compressive residual stress improves the fatigue strength of the material. The method of stress measurement can be divided into two methods. One is destructive and the other is nondestructive. The former is done by calculating the residual stress from the strains released after cutting the blocks into small pieces upon which the strain gauges are attached beforehand. The latter is the nondestructive method which measures the change of the lattice spacing of the material’s structure using X-rays or neutrons. From 240 Copyright ©JCPDS-International Centre for Diffraction Data 2009 ISSN 1097-0002