Modelling of cyclic and viscous behaviour of thermomechanically loaded pearlitic steels; Application to tread braked railway wheels
Doctoral thesis, 2018

In service, railway wheel and rail materials are subjected to high stresses and, in some cases, elevated temperatures. The high stresses are caused by the rolling contact between wheel and rail. Furthermore, heat generated from tread braking and/or sliding between wheel and rail gives additional stress due to constrained thermal expansion. The main goal of this thesis is to improve modelling of the temperature dependent cyclic and viscous behaviour of pearlitic wheel and rail steels subjected to thermomechanical loadings.
Finite element (FE) analyses are carried out of generic heavy haul wheel designs subjected to thermal loading from high power drag braking. In these analyses, the results from using a plasticity and a viscoplasticity model are compared. Both models are calibrated against results from cyclic strain controlled (low strain rate) experiments with hold-time of ER7 wheel steel at different elevated temperatures. The comparison shows an increasing influence of the choice of material model with power of the drag braking.
Also, a methodology to simulate full scale brake rig tests is developed. It includes an axisymmetric thermal analysis, a 3D structural wheel-rail contact analysis and a 3D structural analysis with a traversing contact load. The wheel material behaviour is modelled by a plasticity model calibrated against cyclic strain controlled (low strain rate) experiments of ER7 steel. In addition, the infuence of important operational parameters such as axle load, maximum vehicle speed and block material is investigated with respect to the ratchetting life of the wheel tread.
To improve the modelling of the behaviour of ER7 steel for a wider range of loading rates and multiaxial loading, a viscoplasticity model is adopted and calibrated against test data of ER7 steel at different temperatures for slow cyclic strain controlled tests with hold-time, ratchetting tests with rapid cycles and cyclic biaxial tests. A simulation of a brake rig experiment is used to highlight the importance of using the viscoplasticity model in the prediction of the ratchetting fatigue life.
Finally, a cyclic plasticity model incorporating phase transformations is developed to examine what phases and residual stresses that are obtained in a railway wheel after repeated short term local heating followed by rolling contact. This model can be used to study thermal damage mechanisms in rail and/or wheel steels that may lead to initiation of cracks (e.g. squats (studs) in rails and crack clusters in wheels).

Railway wheels

plasticity

phase transformations

finite element analyses

viscoplasticity

rolling contact fatigue

pearlitic steel

tread braking

ratchetting

full-scale brake rig testing

Virtual Development Laboratory (VDL-room), Chalmers Tvärgata 4C, Chalmers University of Technology, Gothenburg, Sweden
Opponent: Dr. David Fletcher, Department of Mechanical Engineering, The University of Sheffield, UK

Author

Ali Esmaeili

Chalmers, Industrial and Materials Science, Material and Computational Mechanics

Ali Esmaeili, Johan Ahlström, Magnus Ekh, Dimitrios Nikas, Tore Vernersson. Modelling of temperature and strain rate dependent behaviour of pearlitic steel in block braked railway wheels

Ali Esmaeili, Johan Ahlström, Magnus Ekh. Modelling of cyclic plasticity and phase transformations during repeated local heating events in rail and wheel steels

Thermomechanical capacity of wheel treads at stop braking: A parametric study

International Journal of Fatigue,; Vol. 113(2018)p. 407-415

Journal article

There are about 1 million kilometers of railway rail lines and about 25-50 million wheels in service in the world. Due to increasing demands (such as axle loads, running speeds of trains, etc.) the maintenance costs have increased in the recent years. Accounting for the enormous size of the railway network and the number of wheels, even 1% cost reductions translate to a huge amount of money for infrastructure managers and train operators. Furthermore, the performance of rails and wheels is important for the safety of railway operation. Hence, a grand challenge for the metallurgists, engineers, and railway managers is to minimize the causes of rail and wheel damage and failures.
Due to the geometry of the railway rails and wheels, a typical wheel-rail contact surface is merely the size of a coin. The material in the vicinity of this contact is subjected to very high loads. These loads are generated due to axle loads that can be from 10 tonnes up to 40 tonnes (for heavy duty railways) and also frictional forces in the rail-wheel interface caused by train acceleration, braking, and curving. Add to that the frictional heat generated between brake block and wheel during braking or between wheel and rail during braking and acceleration which might result in elevated temperatures up to 500℃. In severe cases, such as malfunctioning traction systems, temperatures might even reach 800-1000℃. These complex loading conditions might result in damage and failure of the rail and wheel material causing major maintenance costs in the railway industry.
To be able to understand load limits (e.g. the maximum allowed train axle loads), to plan an efficient maintenance schedule and also to improve the components’ design for obtaining sufficiently long life of the components, we need to have a good understanding of the material behaviour in the components under operational conditions.
Due to high strength, high wear resistance and relatively low cost, pearlitic steels are widely used for railway rails and wheels. In this thesis, an effort has been made to develop material models that are able to numerically simulate the behaviour of the pearlitic steel in railway wheels when subjected to mechanical and thermal loads. These models are used to simulate different scenarios of railway operational conditions and for study of possible damage mechanisms that might result in failure of the wheel material.

Ali Esmaeili

Research into enhanced tracks, switches and structures (In2Track)

European Commission (Horizon 2020), 2016-12-01 -- 2020-12-31.

Areas of Advance

Transport

Materials Science

Subject Categories

Applied Mechanics

Other Materials Engineering

Vehicle Engineering

ISBN

978-91-7597-843-7

Doktorsavhandlingar vid Chalmers tekniska högskola. Ny serie: 4524

Publisher

Chalmers University of Technology

Virtual Development Laboratory (VDL-room), Chalmers Tvärgata 4C, Chalmers University of Technology, Gothenburg, Sweden

Opponent: Dr. David Fletcher, Department of Mechanical Engineering, The University of Sheffield, UK

More information

Latest update

12/13/2018