Railway wheel: thermal and residual stresses
When braking a railway carriage by pressing the brake shoes against the rolling surface of the wheel rims, then due to friction, the temperature of the rolling surface increases strongly.
Braking stresses in a railway wheel
During normal short braking for a normal train stop, the thermal stresses in the disc are completely insignificant because, that the thermal expansion of the surface layer of the rim is balanced by the lower, "Cold", layers of rim.
However, during prolonged braking on a long downhill or in case of emergency prolonged pressing of the brake pad to the rolling surface of the rim, the entire rim already warms up. In this case, the rim diameter increases and it "pulls" the disc, creating significant tensile radial thermal stresses in it.
Wheel rim stress: compressive and tensile
When these radial stresses in the disc exceed the material yield strength, irreversible plastic deformation of the disk occurs following the expanding, mostly elastic, rim. After braking stops, the rim will cool down and will try to return to its previous position, however, this will be prevented by an irreversibly expanded disc. As a result, the disc "pushes" the rim outward, creating tensile stresses in the rim. These stresses are residual and directed in a tangential (circular) direction.
The plastic deformation of the rim itself near the rolling surface also contributes to the formation of tensile residual tangential stresses in the rim., which occurs when the temperature difference between the rolling surface and the inner layers of the rim.
Residual stresses in the rim of a new wheel – compressing
When thermal hardening of the railway wheel rim in the vicinity of its rolling surface, compressive residual tangential stresses are created. However, after one or more of the aforementioned severe heating of the rim, these initial compressive residual stresses change to tensile. A graphic representation of this process is shown in the figure. 1.
After, How did it happen, small thermal cracks, which are always present on the rolling surface, begin to spread deep into the rim and in the worst case can lead to brittle destruction of the wheel.
Effect of disc shape
Back in the 1970s, it was established, that this change in residual stresses can be influenced by the shape of the disc. The key parameter is the offset of the rim relative to the hub, as shown in the picture 2. This offset can be either positive (picture 2), and negative, that is, the other way.
As seen from the graph in the figure 2, when this offset increases, temperature stresses in the disk, which occur during prolonged heating of the rim, decrease.
It means, that increasing the displacement of the rim increases the resistance of the disc to the occurrence of irreversible plastic deformations and prevents subsequent resistance of the disc to the rim when the wheel is cooled. As a result, after heating the rim, the sign of the residual stresses does not change., although a decrease in the level of compressive stresses in the rim is still possible. On the picture 3 shows a Japanese railroad wheel with a corrugated disc, which was designed according to this concept.