Predicting the life of steel rails

Abstract

A model of plastic strain accumulation, wear, and rolling contact fatigue (RCF) crack initiation in rail steel has been developed. Local to the contact zone, material is subject to severe cyclic stresses taking it beyond yield and leading to incremental accumulation of plastic deformation (ratcheting). This model is based on a ratcheting law derived from twin-disc, rolling-sliding contact experiments and can simulate thousands of ratcheting cycles with corresponding strain hardening. The model is being further refined to account for detailed microstructural changes. Sections of worn and fatigued rail, removed from service, have been metallurgically analysed. To obtain further data on rail-steel deformation and RCF crack initiation, twin-disc tests have been performed using discs cut from across a railhead and wheel rim. Two heat treatments were applied to some rail discs to investigate the effect of pro-eutectoid ferrite phase distributions and volume fractions. Tests were run to failure (defined by an eddy current crack-detection system) and to percentages of fatigue lives. Micro- and nano-hardness tests, and microstructural observations, have been used to suggest a micromechanism of fatigue crack initiation for the model. Application of this model will contribute to reduced maintenance costs and an improved understanding of RCF development.

Visualization and Modelling to Understand Rail Rolling Contact Fatigue Cracks in Three Dimensions

“…Past work on rails removed from service [4, 14,15,21,22,23] has shown that vertical, angled sections normal to, and through the centre of, an RCF crack, i.e. cut in the direction of the maximum vector of composite creepage, give a clearer picture of crack severity and microstructural strain than seen with axial and transverse vertical sections, where crack branching and multi-crack interactions can confuse the analysis.…”

“…This has been modelled in 2D for various PE ferrite widths at the PA boundary by Franklin et al [14,16]. A 3D model of this very early crack formation and growth is being developed; the PA grain distribution has been represented by a volume (with periodic boundaries) of convex polyhedra (Figure 7a) with paths for potential crack growth along the representative PA grain boundaries mapped over time as the microstructure deforms plastically.…”

High-speed inspection of rails using ACFM techniques

“…The depth of the notches varied between 2 and 15 mm, while for more realistic representation of RCF cracking certain notches were angled with respect to the surface normal and the rail edge and the railhead surface. RCF head checks detected in rails are often inclined to the surface, penetrating initially at a shallow angle of approximately 15-301 until they reach a characteristic depth and turn down into the railhead at an angle of approximately 601 [24,25]. Fig.…”

<b>Rail Surface Fatigue Assessment Using Numerical Simulation of the Concurrence of Short Crack Propagation and Wear</b>

“…Where, H n is Nano-indentation hardness (compressive load: 30 mN), with different values applied depending on material phases. β is also a material-dependent constant (pro-eutectoid ferrite: 1.48, pearlite: 1.5 [4], and τ max : maximum value of orthogonally crossing shear stress within the cross section. γ c (critical value of γ) is taken here as 2.89 (= tan (90° -19.08°), by applying the head checking angle as 19.08° (mean value) according to [5].…”