Interface - The Journal of Wheel/Rail Interaction

RAIL STRESSES

Understanding Stresses in Rails
(Part 2 of 2)
By Jude Igwemezie, Ph.D., P.Eng • April, 2007

Part 1 of this article examined the contact and dynamic stresses generated by the wheel/rail contact loading environment. Part 2 examines the stress cycle at any point on the running surface of the rail as the wheel approaches it.

Stress in the railhead changes as the wheel traverses any point on the running surface. As the wheel approaches, the stress at point A (see Figure 1) is tensile, due to track uplift. At about 41 inches from this point, the stress reverses and becomes compressive. Both of these stresses arise from rail bending, alone. When the wheel is about 1/4 inch from this point, the stress reverses to become tensile, again, and exceeds the yield strength of the railhead material. When the wheel is directly over point A, the stress is reversed and becomes compressive, again, exceeding the yield strength of the railhead material. As the wheel passes point A, the stress cycle that was just described happens again, in reverse.

This means that the rail at point A will have undergone a three-stage stress cycle — ranging from tension-compression / tension-compression-tension (in which the yield strength of the railhead is exceeded) / compression-tension — under the action of one wheel (see Figure 2). One-quarter inch to the side of point A (beside the contact band), the stress value is less severe. Nevertheless, two reversing stress cycles shown with the dashed lines — tension-compression / tension (in which the yield strength of the railhead is exceeded) / compression-tension — are generated.

Residual Stress
Problems associated with internal stresses (also referred to as residual stresses), which are introduced during the making of rails, have been evident for the past two decades. The fact that few derailments have been attributed to them stems more from a lack of understanding of the phenomenon than from its absence in the cause of derailments. The primary interest in residual stresses, thus far, has been related to heat-affected zones, such as weld locations and areas within the railhead. Manufacturing and heat-treating processes that are used to produce head-hardened rail with improved properties (while maintaining geometrical tolerances) often produce residual stress in excess of acceptable values.

Rail manufacturers, today, are required to produce rail with a greater depth of hardness in the railhead, while maintaining a ductile web and base — all while keeping residual stresses low. This can be difficult to achieve, but it is essential in order to avoid residual stress-related rail failures and derailments. Research (1) into the manufacturing process and rail residual stress phenomena has shown that:

• Rail manufacturing processes control the residual stress pattern in the rail, but roller straightener settings can modify or completely alter these stress patterns.

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