Integrated Computational Materials, Process and Product Engineering – IC-MPPE 2022

Modelling of squats
Sandra Baltic, Timna Gschwandl, Werner Daves

Squat-type rail cracks are characterised by a localised depression of the rail-wheel running band. A squat-type crack typically starts on a rail head surface and grows to depths of a few millimetres. Such cracks can grow within month or few years to a size, that rails have to be replaced. As this damage type is a worldwide problem it costs the railways billions of Euro per year.The problem of squats aroused mainly within the last 20 years and no solution to the problem is actually visible.
Possible initiation points are head checks, pittings but also scratches caused by grinding of rails.
Cross sectioning of rail samples revealed the crack initiation in planes perpendicular to the rail traffic direction. It is an aim to show the influence of factors that might trigger the squat initiation and formation.
The tendency of one defect to grow rather than another is analysed by calculating crack driving force value and directions.The results show that cracks are able to grow under particular wheel loading in the directions that are in accordance with observed squat growth. These results suggest lateral forces as the main suspected determinant for the squat initiation.
Residual stresses in rails have to be regarded, too. Stresses near the surface and especially near the plastified zone below the running band will strongly influence the crack path and growth rate of cracks. Such residual stresses are measured and are input for the simulation of squats.
Multiaxial cyclic experiments using tubular specimens are undertaken, to characterize the stress, plastic strain response of different rail materials and are used as input for squat simulations.
The lecture describes the actual efforts to accurately simulate squats from their initiation to a state the rail has to be maintained or replaced.