Mechanical state of the rail underhead region under heavy haul operations

The occurrence of tensile stresses in the underhead region as a result of localised vertical and lateral head bending on the web is significant, and can potentially contribute to fatigue cracking. Field measurements under heavy haul conditions have shown that these stresses occur as a tension spike when the instrumented location is directly beneath the wheel. The aim of this study was to evaluate the stress state of the rail underhead region with respect to changes of the rail profile due to wear. The study was undertaken using the finite element method (FEM) involving static stress analysis. The analysis revealed that the longitudinal stress at the rail gauge corner of the outer rail in curved track is highly dependent on several operational parameters including the offset of the contact patch location from the rail centreline, the ratio of lateral (L) to vertical (V) loads (L/V ratio), the direction of lateral traction, foundation stiffness and the head wear (HW). The magnitude of the tension spike at the surface of the rail in the underhead region is increased, but the depth below the contact surface at which the stresses become tensile is reduced when increasing the offset of contact patch location, the L/V ratio, the foundation stiffness and the HW. Both residual (RS) and thermally induced stresses increase the value of the stress in this region. The tensile stresses could potentially cause fatigue crack initiation at the rail underhead region and, in particular, could cause rolling contact fatigue (RCF) cracks on the gauge corner of the rail to turn downwards into transverse defects (TDs)