The Rail brake systems and components session will take place on Wednesday May 18th and will be chaired by Raphael Pfaff of FH Aachen and co-chaired by Tim Hodges, Tenneco.
Topics and speakers for the session include:
Effects of high energy braking history on service use performance of railway disc brakes
Sylvain Delattre, Alstom
Disc brakes mounted on intercity trains can be exposed to severe thermomechanical loads during emergency braking, with conditions at the disc-pad interface exceeding 1.5MPa, 30m/s and 1000°C for several tens of seconds. This does not affect their integrity and performance under standard service use but may become an issue in the case of repeated loads during intensive testing. Recent experiments showed that repeated exposure to high energy braking may durably affect friction behaviour of disc brakes equipped with steel discs and sintered copper-iron pads. Such conditions can be met during UIC standard tests and could modify the outcome of the test programs performed afterward. In this study, friction and wear mechanisms are compared before and after repeatedly exposing a disc brake to high energy braking. The results should bring new highlights on how severe thermomechanical loads govern braking performance, and help analysing UIC test results regarding the brake’s thermomechanical history.
On a reduced scale pin on disc tribometer, a steel disc and a sintered copper-iron pad were exposed to repeated high energy braking sequences adapted from UIC 541-3 C2 test program. To understand how such conditions may durably affect performance during subsequent train service use, lower energy braking sequences, adapted from a field service route, were performed before and after high energy braking to characterize the evolution of friction and wear related to the brake’s thermomechanical history. To make sure this softer braking sequence does not alter performance, and to allow comparative analysis of rubbed surfaces, another pad was submitted to only service braking. During the test, the friction coefficient was calculated from recorded normal and tangential forces. Temperatures were measured by thermocouples embedded in the pad and disc, and thermal localizations were observed at the disc surface using a high speed infrared camera. An indicator of wear was given by a pad displacement measurement. After the test, post-mortem analysis of rubbed surfaces was achieved by a combination of Scanning Electron Microscopy (SEM) and Energy-dispersive X-ray spectroscopy (EDX), in order to better understand differences between friction and wear mechanisms after low and high energy braking.
First, friction behaviour was characterized after low energy braking. Results show that instantaneous friction is very dependent on the interface’s temperature, even under service use conditions. Different starting temperatures induce different thermal distributions in the contact and different friction levels over time. This concerns durations of one brake application but does not affect subsequent ones. Second, friction evolution was studied after high energy braking as compared to low energy one. Each high energy brake significantly modified friction mechanisms, and the disc’s surface exhibited thermal localization marks and uneven, deeply scratched surface. Afterwards, the contact needed several brake applications to recover stable friction levels and homogeneous disc surface, but friction levels were irreversibly lowered after each high energy braking sequence. These changes can be attributed to thermomechanical history, yet the induced evolution in friction and wear mechanisms is still to be determined
Improved modelling of tread braked wheels using an advanced material model
Eric Voortman, Chalmers Railway Mechanics
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.
Model based investigations of the NVH behaviour of bogie brakes
Andreas Krumm, TU Braunschweig
In the development of new train components, many development goals are in conflict with each other. For example, reducing the weight of the bogie and its components usually results in these components becoming more sensitive to vibrations. The bogie brake is a particularly critical component. The nonlinear frictional contact between the brake pad and the brake disc can lead to self-excited vibration phenomena caused by various excitation mechanisms that can result in NVH. These NVH phenomena can have a significant impact on component life and passenger comfort. To better assess the conflict between weight reduction and comfort or component lifetime due to NVH phenomena, simulations are often performed in addition to bench tests and field tests. By using appropriate simulation models and methods, problematic NVH phenomena can be addressed at an early stage of product development, which saves costs.
Experience shows that the simulation models used often do not describe the dynamic problems accurately enough and certain NVH phenomena cannot be reproduced by simulation. Previous work has already shown that the model boundary conditions used usually represent the boundary conditions of test benches and do not consider the installation situation in the train. However, these boundary conditions have a considerable influence on the simulation results of unstable vibrations with complex eigenvalue analysis (CEA). For example, by selectively extending the system limits of the brake disc in the simulation, it was possible to show that the system becomes significantly more sensitive to self-excited vibrations. The models developed for CEA are now to be extended to include a speed-dependent friction value to investigate the nonlinear influence of a falling friction characteristic on the system dynamics of the bogie brake. In addition, the influence of different brake components and different operating conditions on the dynamic behaviour of the entire brake unit will be investigated The main objective is to demonstrate the influence generated by lightweight construction on the brake's dynamic stability behaviour.