Interface - The Journal of Wheel/Rail Interaction

RAIL STRESSES

Understanding Stresses in Rails - Part 1 of 2 (continued)

Rail Stresses and Fatigue
Stresses and wear are directly related to the life of the rail and susceptibility of the rail to fatigue damage. If the stress is external, such as contact or dynamic stresses, the rail will wear or fracture. If the stress is internal, such as residual or thermal stresses, it can accelerate the growth of fatigue defects, again causing fracture. A three-dimensional Finite Element (FE) analysis can be used to rationalize these different stresses. Figure 4 depicts the stress distribution in rails from the one- and two-axle loading (as shown in Figure 2). As can be seen, there is a concentration of stresses at the wheel locations. This means that assessment of the stresses in rails and the capacity of the rail to carry load depends on what is happening in the immediate vicinity of the load. It also means that the use of simple bending stress computations will be inadequate in assessing the stresses in rails. A detailed look at the stresses in rail in the vicinity of the load, and how wear influences the distribution, is warranted.

Contact and Dynamic Stresses

As the name suggests, contact stresses arise as a result of the wheel contacting the rail. The magnitude of the stress depends on the applied forces, the material and the geometry of the contacting surfaces. If the forces are applied perpendicular to the contacting surfaces, the maximum contact stresses do not occur at the contact surface but occur in a region located inside the two bodies and just below the contact point. If a horizontal force, such as friction, is introduced, the maximum contact stress can be pulled to the contacting surfaces. Figure 5 shows the relationship between the maximum shear stresses, the coefficient of friction and the radius of the top of the low rail computed for the same wheel diameter and contact radius. As shown in Figure 5, increasing the coefficient of surface friction increases the shear stresses, as does decreasing the radius at the top of the rail. Some amount of friction is necessary, however, to ensure that damage starts on the top running surface of the rail, rather than starting sub-surface (as shown in Figure 6). Where there is high friction between the two surfaces, the damage will start at the surface of the low rail as shown in Figure 7.

What controls the magnitude of the contact stress? Wheel/Rail contact, of which there are two primary types: one-point and two-point contact.

One-point contact occurs when the wheels and rails wear themselves into a conformal shape, or when the false flange of the wheel contacts the top of the low rail. In reality, conformal contact is never found in ordinary revenue freight service with interchange of vehicles between roads. This is because the rail wears to the average of all the wheel profiles that run over it. Similarly, the wheels wear to the average of all the railhead profiles and the wide- or tight-gauge conditions that they run over. In some cases, such as with transit systems, mining railways or railroads with captive consists, conformal wheel/rail contact can be achieved. Nonetheless, conformal contact leads to hollowing of the wheels, vehicle hunting and the formation of corrugations.

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