Interface - The Journal of Wheel/Rail Interaction

RAIL STRESSES

Understanding Stresses in Rails (continued)

Research into rail fracture resistance (2) and work to define the critical dynamic fracture load as a function of rail fracture toughness; defect size, location and orientation; rail ambient temperature; and residual stresses determined that:

• Defects such as vertical split heads, pipe web, head/web separations and horizontal split heads do not readily cause rail to fracture under impact loads. (This is due to a lack of crack opening forces.)

• Railhead transverse or detail-type defects, resulting from improper wheel/rail profiles and contact, readily produce rail fracture.

• Under impact load, a stiffer and heavier rail will attract a higher percentage of the dynamic loading, but will not fracture more or less readily than a smaller and flexible rail (3).

• The location, size and orientation of the defect, as well as tensile (thermal) forces and residual stress, are critical parameters in determining the rail fracture load. When a defect reaches a critical size, it will cause a pull-apart under tensile thermal stresses — regardless of whether the rail is new or worn.

Rail wear is a result of friction and stresses generated by wheel/rail contact. In tangent track, wear is produced by tractions, friction and vehicle hunting. On the high rail in curves, wear is produced by traction, creepage and flange forces. On the low-rail, wear is produced by lateral friction and traction and spin creepage from the wheels. Wear can be reduced with proper lubrication on curves and maintenance of gauge to reduce hunting.

Rail profile grinding generates "artificial" wear in which rail surface material that has plastically deformed and developed micro-cracks is removed. Rail grinding also controls the formation of transverse defects or contact fatigue damage.

Rail Stresses and Head Wear
As Part 1 of this article shows, the stresses in rails are concentrated in the area of the applied load (as shown in Figure 1). Figure 5 illustrates how stresses in the low rail of a curve vary with head wear under load. As illustrated, stresses are concentrated in the railhead; the stresses across the rest of the rail section, where bending stresses dominate, are minimal. In the example with most wear (extreme left), the "green" band has traversed the thickness of the head of the rail. At this level of wear, the stresses would be expected to lead to the formation of vertical splits in the railhead. The angled "purple" band running from the top left to bottom right of each contour in this illustration is the neutral axis under the combined loading.

Similarly, the variation of stress in the high rail with wear under combined loading is shown in Figure 6. Again, the stresses in the head of the rail change with head wear, but the rest of the rail section, where bending stresses dominate, is not affected. Contrary to expectations, there is not a significant change in the bending stresses at the base of the rail. The angled "purple" band running from the top right to the bottom left of each contour in this illustration is the neutral axis under the combined loading. Figures 5 and 6 show that the dominant stresses in rail under wheel loading is much more dependent on the amount of material left in the head of the rail than on the amount in the gross section of the rail.

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