DEM modelling of adhesion in sanded wheel-rail contacts

In railways, in the contact between the wheel and the rail forces according to 10 tons wheel-load and higher are transferred on an area of a fingernail. This results in extremely high contact stresses, both perpendicular to the rail surface (normal direction) and tangentially to it (tangential direction), e.g. caused by braking or curving forces. The maximum tangential force that can be transmitted is limited by the maximal adhesion coefficient (AC), which is approx. 0.35 under dry conditions. Contamination of the wheel-rail contact, e.g. by rainwater or fallen leaves in autumn, can reduce the AC, sometimes below 0.1, causing massive problems during braking and, in extreme cases, safety issues. To prevent this and to increase the AC, sanding systems have been used for decades, whereby grains of sand are blown into the wheel-rail contact. Sanding of the wheel-rail contact has been well studied experimentally. Both in the laboratory and on the track, different contact conditions such as dry, wet or different contaminates have been investigated, as well as different types of sand. Under wet conditions, different types of sand can leave the AC in the worst case unchanged or increase it, some almost to the range of dry conditions. The physical mechanisms for this are still poorly understood. Entering the contact, the sand grains are partially crushed, and plastic deformation occurs on the surfaces of the wheel and rail. The AC is possibly increased by form closure effects, when parts of the sand grains are pressed into the metal surfaces, or the ground sand solidifies under the high pressure and thus increases the effective contact area in the wheel-rail interface. The interaction between sand and water in wet contacts is also unclear. This lack of understanding is caused by (todays) inability to experimentally monitor any of the before mentioned mechanisms in the contact zone during roll-over. In order to understand more precisely which mechanisms increase the AC, a simulation model will be developed in the project, using the so-called "Discrete Element Method". This model will take into account the different mechanisms that occur during rollover in the sanded wheel-rail contact. A detailed parameterisation and validation of the model with experimental data is planned. In this way, the model can contribute to a deeper understanding of the adhesion-increasing mechanisms in sanded wheel-rail contacts.

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