Fatigue crack growth in severely shear-deformed pearlite

The contact between railway wheels and rails may appear as an insignificant detail in the railway system, but it is of great importance for the safe and economic operation of railways. The wheel-rail contact plays a central role in carrying and guiding the railway vehicle as well as for transmitting forces in accelerating and braking. The wheel and rail materials in the contact area must withstand large loads. These loads cause permanent (plastic) deformation of the material near the surface that modify its properties. Microscopic cracks form at the surface that subsequently lead to wear (material removal) and the formation of deep cracks. Wear causes a gradual shape change of wheels and rails that needs to be rectified by periodic maintenance. Cracks with a certain depth also need to be removed by maintenance, because they can lead to fracture of wheels and rails and thereby to the derailment of railway vehicles in the worst case. Therefore, it is important to understand the formation of microscopic cracks on the surfaces of wheels and rails with respect to wear and crack formation. This requires addressing both the manufacturing process that determines the basic properties of the materials, and the operating conditions that modify the material properties due to the extreme loading. Changes of the microstructure of the materials and the formation of microscopic cracks will be described and investigated by computer simulation models in the project. The investigations focus on the effects of heat treatment during the manufacturing process and plastic deformation during railway operation on the formation of microscopic cracks in the modified material structure. In addition to the simulations, laboratory experiments will be carried out to obtain data about the plastic deformation behaviour and the crack growth behaviour of the materials. These data will be used to parameterize the simulation models and verify the simulation results. The models that are developed in the project should assist the further development of wheel and rail materials. This is done by systematically assessing the effect of different manufacturing conditions and operating conditions on the formation of microscopic cracks to subsequently minimize wear and crack formation in railway operation.

The contact between railway wheels and rails may appear as an insignificant detail in the railway system, but it is of great importance for the safe and economic operation of railways. The wheel-rail contact plays a central role in carrying and guiding the railway vehicle as well as for transmitting forces in accelerating and braking. The wheel and rail materials in the contact area must withstand large loads. These loads cause large permanent (plastic) shear deformation of the material near the surface that modify its properties. Microscopic cracks form in this deformed material near the surface that subsequently lead to wear (material removal) and potentially also to the formation of macroscopic cracks on railway wheels and rails. The growth of fatigue cracks in deformed pearlitic steel with varying microstructures was investigated in this project, both in simulations and in experiments. In the developed simulation framework the influence of the microstructure on the plastic cold deformation behaviour is modelled by a mesoscale dislocation model that was specifically adapted for pearlitic microstructures. To describe the fatigue crack growth behaviour, a simulation model based on the discrete element method was developed that allows to investigate the dependence of the fatigue crack growth rate as a function of the plastic deformation and as a function of the material microstructure. The model includes a hierarchical description of the material microstructure based on interfaces between the structural constituents of the microstructure. The near-surface plastic shear deformation observed in the wheel-rail contact is geometrically imposed on the model, thereby changing the density and orientation distribution of the interfaces. In the simulation, the same given microstructure can be systematically investigated in the model at different degrees of deformation. Accompanying fatigue crack growth experiments show that plastic deformation in pearlitic materials leads to an increase in fatigue crack growth rate parallel to aligned microstructure constituents. This behaviour is also observed in the simulation model. With the developed model, targeted investigations can now be carried out on the influence of individual aspects and properties with respect of microstructure and interfaces on the resulting fatigue crack growth rates and crack path geometry. The prediction of crack propagation in highly plastically deformed materials that takes microstructural aspects into account, is considered an important step towards a unified description of wear and rolling contact fatigue in this complex tribological system.