Thermomechanical behaviour of tread braked wheels

Licentiate thesis, 2022

Tread brakes, also known as block brakes, are a type of friction brakes commonly used in the railway industry. They are a low-cost and low-maintenance solution compared to more complicated systems such as disc brakes or electrodynamic brakes. Modern composite brake blocks designed to lower noise emission, however, cause increased thermal loading of the wheel since these blocks attract and store lower amounts of the generated frictional heat compared to older cast iron blocks. Long-term drag braking may raise the temperature of the wheel rim by several hundred degrees. After cooling the resulting residual tensile stresses in the wheel rim may lead to a broken wheel. Also a permanent degradation of the pearlitic steel material is induced by the high temperatures.

In the first part of the present work, a finite element material model is calibrated using isothermal and anisothermal experimental data from testing of railway wheel steel specimens, accounting for the macroscale material changes which occur during typical thermomechanical cycles at long-term braking. Results are presented with main focus on comparing finite element simulations to experimental measurements of the thermomechanical cycles. Material deterioration by spheroidisation of the pearlitic material is modelled using a time-temperature dependent law.

In the second part, the material model is further evaluated using axisymmetric simulations of tread braked train wheels, and the wheel performance is evaluated according to current European standards. Results are presented that show a substantial increase in the magnitude of the modelled residual tensile stresses as compared to other widely-used material models.

In the third part, experiments and measurements on tread braked wheels are conducted in a full-scale test rig. It is found that the temperature distribution in the axial and circumferential directions on the wheel tread exhibits substantial variations which can be of importance in the understanding of the full thermomechanical behaviour of a wheel. Numerical simulations show that there is an increase in residual tensile stress at locally hotter areas, a phenomenon which has a potential to compromise the safety of the system.