Towards enhanced mechanical braking systems for trains. Thermomechanical capacity of wheel treads at stop braking

Licentiate thesis, 2017

Modern trains are equipped with different braking systems, between which the braking effort can be distributed. Among these, tread braking is still the most common system for friction braking. Tread brake systems are cheap and robust. However, extensive usage of tread brakes demands knowledge of operational limits to ensure safety and decrease life cycle costs (LCC) of the running gear. In the present work, a state-of-the-art literature survey has been compiled which covers topics related to establishing operational limits such as: brake control and blended braking, braking temperatures, brake block materials, wear and rolling contact fatigue of wheels due to tread braking, and capacity of tread brakes and brake discs. A methodology to simulate full-scale brake rig tests, including wheel-rail contact, has been further developed. It includes an axisymmetric thermal analysis, a 3D mechanical wheel-rail contact analysis, and a 3D thermomechanical analysis of the braked wheel. The behaviour of ER7 grade railway wheel material is mimicked by use of a plasticity model calibrated against results from cyclic experiments on test specimens. The results from the simulations in terms of predicted fatigue lives show good agreement with full-scale test rig results for three combinations of initial velocity and brake block material. The developed methodology is employed in parametric studies. These consist of braking load cases characterised by operational parameters such as axle load, maximum vehicle speed, deceleration, block material, and initial wheel temperature. Damage evolution in the wheel tread is studied. A strong infuence of high temperatures on rolling contact fatigue formation in the wheel tread was observed. In particular, braking temperatures over 450 °C can result in a very short life up to crack initiation. However, for braking temperatures in the range of 300 - 350 °C, wheel tread material is more resistant to fatigue due to strain hardening.