The most common wheel defects encountered in the rail industry are wheel flats, shells and cracks. Failure of the wheel due to such defects can be avoided by careful inspection of the wheel sets during maintenance. Nevertheless, the existence of wheel flats and shells alone poses a severe problem to the rail industry. This is due to the fact that these types of wheel defects are known to cause abnormally high forces and stresses on the railway track and on the vehicle components, leading to significant damage on the rail network and reduced operational lifetime for the rail. Depending on the size and shape of the defect, axle load and speed, the stresses may be sufficient to initiate fatigue cracks, or even cause final failure, leading to derailment of the train. In addition to safety and economic considerations, these defects reduce passenger comfort and significantly increase the level of noise produced at the wheel-rail interface. It is believed that locking the wheels during braking causes wheel flats and therefore, they can occur without warning at any time. The flat spot, possibly starting as chord of the wheel circumference, tends to get longer and rounded after a few cycles of wheel rotation. Then the basic flat shape may stay on the wheel tread for a long time and repeatedly generate large impact forces. Wheel shells are mainly developed from micro-cracks, initiated by high strains in the wheel-rail interface due to high axle load and creep forces. Even though the appearances of wheel shells are different from those of wheel flats, the measured irregularity functions are similar and therefore the basic characteristics of the impact loads are analogous. Hence, the damage caused by wheel shells on rails is similar. Figure 3a) shows shelling on a train wheel and figure 3b) cracking on a train axle.
|Figure 3: a) Shelling on a train wheel and b) cracking on a train axle.|
The importance of detecting defects in wheel sets as early as possible is therefore of paramount importance for the rail industry in order to reduce maintenance costs and improve safety standards.
Wheel set defects have been associated with rail accidents, damage to the network, delays and unnecessary costs for many years. Their effect however has become more significant over the last few years as train speeds have increased and rail networks have become busier. The safety and economic implications of wheel set defects was underestimated by the rail industry until recently.