Question

Assessment 8.0: Wheel/rail interface Produce a report (500 words min) to describe the wheel/rail in relation to the following: • Gauge corner cracking • Rolling contact fatigue

Answer 1

Gage Corner Cracking (GCC) of rail and wheel from Rolling Contact Fatigue (RCF) is difficult to measure due to technology limitations in quantifying the length and depth of such cracks. These defects are essentially initiated on the surface or very close to the surface of rail. They develop due to excessive shear stresses at the wheel-rail contact surface and point towards the materials limited capacity to overcome the fatigue damages.

RCF is due to a ratcheting of each loading cycle (wheel pass), which exhausts the ductility of the steel and eventually generates an incipient crack. If a rail wears faster than RCF generates cracks, then there is no problem other than the fact that rails are prematurely replaced due to wear instead of flaws.

Due to RCF in 2000 Hatfield derailment in United Kingdom in which about 30 rail length shattered completely on the outer rail of 1500 m radius curve. According to Federal Railroad Administration’s statistics, in the eight years from 1995 to 2002, rolling contact fatigue was the main cause in 122 derailments, and it was suspected that RCF might have contributed to 160 more derailments.

Two main physical processes that govern the development of RCF defects are crack initiation and crack propagation in the rails. These are, in turn, governed by the factors like rail and wheel profiles, track curvatures, grades, lubrication practices, rail metallurgy, vehicle characteristics, track geometry errors, environmental conditions, and many others. All these factors play an important role in the formation and development of RCF and, therefore, could be optimized to control and minimize RCF defects.

Gauge Corner Crack develop on top corner of the gauge face in the form of a series of surface cracks spaced at about 2-5 mm intervals at a downward angle of 10-30 degrees and gradually spreading across the rail head. These defects are more prominent on outer rails of sharp curves and manifest themselves as fish scales.

Fig 1.) Gauge Corner Cracks – Initial Stage

Fig 2.) Gauge Corner Cracks – Intermediate Stage

Fig 3.) Gauge Corner Cracks – Severe Stage

Rolling contact fatigue of rails

Many observations have confirmed that cracks develop towards the direction of motion, initially inclined at a shallow angle of about 15^o to the head of the rail. When the cracks reach a depth of typically 10mm, the angle steepens to about 70^o and the cracks then propagate through the rail until failure. During the shallow angle growth, flakes of material may detach themselves from the head of the rail, but the rail danger of a broken rail is obviously a result of the turned-down crack, and, therefore, this phase must be avoided if possible.

The doubling of wheel load increases the contact stress by about 27%, a tripling increases contact stress by 44%. The effect of wheel diameter is limited to a 1/3rd -power function. The effect of transverse geometry of the wheel and rail profiles has, however, much stronger effect on contact stress. The factors that influence the actual position of the wheel on the railhead and therefore impact the contact stress are:

• Track Gauge – Changing the distance between the two rails usually modifies the position and geometry of the rail wheel contact. Tight gauge in straight track promotes gauge corner contact and RCF, whereas at nominal gauge more of the contacts will be carried towards the crown of the rail. In curves, controlling wide gauge is essential for mitigating low rail damage. Wide gauge curves are also more susceptible to dynamic rail rotation, which often contributes to unfavorable contact geometry.

• Welds and profile irregularities: The weld steel has hardness different from that of the parent rail steels. Where this difference is large (say 30 BHN), either softer or harder, the weld will deform greater or less, respectively. Softer welds produce a dip, known as cupping, which accelerates development of RCF. Harder welds produce high spots that increase the dynamic augment and are responsible for RCF damage that develops adjacent to the weld.

• Other factors that affect contact stress include cant excess and cant deficiency, hunting of wheel sets in tangent track and mild curves, track geometry errors, uneven wheel loads, and mismatched wheel diameters