Dependence of fracture strain on flaw size in rail switches—Experiments and theoretical modelling

Abstract

Abstract With increasing requirements regarding wear resistance of rails especially in switch components, high strength materials have entered also this field of application. This poses new challenges for the manufacturing process, especially in bending the curvature radius. During the bending process of switch components a high strain is applied to the outer fiber of the rail foot, about ten times larger than in normal operation when a train passes through the rail. The current paper presents an investigation, whether and to which extent rails made of high strength materials have to be treated differently during the bending process if one takes into account potential flaws on the bending behaviour of rails. In the present contribution static tests on three-point bending specimens with surface cracks of varying lengths are performed for four materials with different microstructures from medium to high strength rail grades. For the prediction of static failure the failure assessment diagram (FAD) is widely used where the normalized crack tip loading is plotted against the degree of plastification. To provide a diagram more readily applicable to design purposes, the nominal strain of the outer fiber at failure—obtained by finite element (FE) simulations of each individual experiment—is plotted against the crack size. This diagram for the dependence of the static failure strain on the flaw size can be seen as the static equivalent to the Kitagawa-Takahashi diagram for fatigue.