VDL, Chalmers Tvärgata 4C
Opponent: Professor Odd Sture Hopperstad, NTNU, Norway
Have you ever noticed how hard it is to bike with soft tires? What about how different it feels after you inflate them? The bike may roll more easily, but the bumps on the road are also more noticeable. The same happens for railway wheels: A steel wheel rolling over a steel rail has a very low rolling resistance. This makes trains very eco-friendly. The drawback, however, is a very high contact loading equivalent to the weight of 100 bikers on an area the size of a coin. This, together with forces from acceleration and braking, cause the surface layer of the rail to deform. As the deformations increase, the properties of the rail material change, and cracks appear. When the cracks become too large they need to be removed by maintenance grinding. This is a costly and slow operation that requires planning well in advance to avoid delayed trains.
In this thesis, a new experimental method is presented. It is used to investigate how a material's properties are affected by large deformations. One important property is the yield limit. This is the maximum stress a material can withstand without permanently deforming. It is found that the yield limit is initially the same in all loading directions. After the deformations have accumulated, however, the yield limit depends on the loading direction. This effect is typically not accounted for when modeling the behavior of the rail material. Models that are capable of capturing this effect are therefore evaluated in this thesis. Such models can be used to optimize maintenance planning. Ultimately, our research should lead to smoother railway operations with fewer delayed trains.