Railway freight wagons are often braked using mechanical friction brakes in the form of tread brakes that act directly on the tread of the wheels which at the same time are in rolling contact with the rail. The tread brakes should provide sufficient braking capacity of the train for both normal service braking and extreme braking conditions. This braking system requires a minimum of components and is therefore a low-cost and low-maintenance choice for the industry. However, the utilisation of the wheel as a friction-heated component, also worn by the brake, comes at a cost in form of complex loading situations in which elevated temperatures resulting from braking are interacting with the wheel-rail rolling contact loads. To safely employ the brakes in all situations, accurate knowledge about how the materials interact during these loading situations is required. Studies have shown that temperatures above 500 °C are to be expected, and cases with temperatures more than 600 °C may occur. At such high temperatures, the normally employed ER7 wheel steel is significantly weakened and shows sign of rapid material breakdown by, e g, spheroidization of the pearlitic material structure. To account for these effects, computational models capable of simulation of the complex thermomechanical behaviour are a must. As part of our recent research, a novel viscoplastic material model has been calibrated against isothermal low cycle fatigue tests and against thermomechanical experiments based upon actual in-service scenarios for a range of temperatures, showing good results both for wheel rim material and for wheel web material. The novel material model constitutes a further enhancement of previously developed models that were calibrated solely by use of isothermal materials testing. The objective of the present study is to further investigate and examine the capabilities and accuracy of the novel material model when employed in detailed braking simulation. To achieve this, an axisymmetric finite element model of a standard freight wheel during tread braking is used to assess the performance of the material model. The finite element model accounts, in a simplified fashion, for residual stresses introduced by the rim hardening process at wheel manufacturing and for variations in material properties based on typical hardness values on a wheel cross section. A range of braking situations are assessed to achieve different loads and temperatures, mainly by mimicking downhill braking at constant speed for a prolonged time period. The numerical results are then compared to known experimental quantities, including residual stresses in the wheel rim as well as rim deflections. The results are also compared to the pertinent European standard on technical approval for forged wheels. Additionally, the same exercise is repeated for previous material models, calibrated merely by isothermal data, as a point of comparison with the thermomechanically calibrated one. The results show that the material model predicts realistic material behaviour for a wide range of braking situations. Compared with previous models, a general improvement is seen, suggesting that the newer features of the material model contribute substantially to more accurate modelling of the processes occurring in the wheel during high temperature tread braking.
Chalmers Railway Mechanics: Mr. Eric Voortman Landström, Dr. Tore Vernersson, Prof. Roger Lundén