Interface - The Journal of Wheel/Rail Interaction

RAIL STRESSES

Understanding Stresses in Rails (continued)

• Roller straighteners can cause weakening of the rail web and fracture of the rail base. The damage is such that the toughness that would otherwise arise through the use of alloys is mitigated.

• Rails that contain stress intensities (due to residual stress) that are greater than the rail fracture toughness will fracture readily along the web (see Figure 3), whereas rails with residual stress intensities that are less than the rail fracture toughness will not readily allow catastrophic crack propagation.

• Stresses that can cause rail to fail catastrophically are located in the rail head and base; they have a longitudinal orientation.

• A plastic zone of up to 5 millimeters in depth is formed in the railhead during roller straightening.

• As the rail wears, the danger of catastrophic failure from residual stresses is reduced.

• Work hardening of material in the railhead redistributes the residual stresses, generating lower stress intensities within the rail web.

The development of standards, tests and quality control methods to control residual stresses in rails has led to improved specifications.

Thermal Stresses
Thermal stresses are generated by the difference between the ambient temperature and the rail laying temperature. Stresses that arise from uniform temperature change are considered axial. They heat or cool the rail uniformly. Stresses that arise from temperature variations in the rail (as the sun heats one side of the rail, then the other) can cause rail lateral bending. When combined with wheel loads, thermal stresses can be high enough to cause a pull-apart rail fracture. In some instances, such as during cold snaps, thermal stresses, alone, can cause a rail pull-apart. Figure 4 shows the thermal force generated in different rails as a result of temperature change from the neutral (rail laying) temperature.

Rail defects that result from fatigue or originate at inclusions can propagate under cyclic loading from trains. These types of defects, in combination with residual, thermal and dynamic stresses, can cause brittle fracturing of rails. Rail fracture requires an energy input, which is typically delivered by defective wheels or a rapid temperature change (cold snap). In North America, rail fractures predominately occur during cold snaps at the beginning of the winter months when tensile stresses approach one-half of the yield stress of the rail.

The Association of American Railroads (AAR) incorporated standards for acceptable sizes of wheel defects in Interchange Rule No. 41. These standards are based on the geometric size and nature of the defect on the running surface of the wheel. There are, however, an infinite number of possible wheel defects, which change in size and shape as the train rolls along. The magnitude and duration of the impact load resulting from these wheel defects is speed-dependent. Within recent years, an extensive network of Wheel Impact Load Detectors has been used by the major freight (and passenger) railways to identify wheels that generate high-impact loads.

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