Mechanics and Novel Designs of Polymeric Railway Sleepers in Track Structure

Polymer sleepers can combine a stiffness behaviour comparable to timber sleepers with the consistency and lifespan of concrete sleepers, but their specific characteristics and potential advantages should be considered: material viscoelasticity, bending stiffness range, and freedom of shape.Due to viscoelasticity, laboratory tests on polymer sleepers should be performed dynamically, at strain rates applicable to in-track conditions.Cyclic loading should be performed intermittently, introducing pauses between cycling to reduce heating, creep effects and to give track representative results.In analyses, polymer (and timber) sleepers should be considered a beam on an elastic foundation, because of their limited bending stiffness.To accommodate analytical calculations, a simplified calculation method was derived, using a sleeper flexibility factor to account for sleeper bending effects on track deflections.As a result of repeated train loads, a gap arises between the rail seat location and ballast, and the sleeper shapes according to this gap (i.e.beds-in) on every train passage, causing equalization of the contact stresses over the sleeper length and ballast.Optimisation show that balancing the sleeper (prevention of centre-bound) can reduce resilient rail seat displacements by up to 40% without increasing material usage but only utilizing moulding processes allowing the freedom of sleeper shape.