Damping of dynamic forces by materials used in the construction of ballast and ballastless railway tracks. Crushed stone

Abstract

Strength properties of mineral materials used in the track operation, which are crushed stone and sand, do not depend on the ambient temperature (up to 500 °C) and humidity, but gradually degrade when its pore space becomes clogged with fines, oil products and plant residues. Polymeric materials used in the construction of railway tracks for damping dynamic loads, when exposed to significant stresses, are gradually destroyed. The main operating conditions that determine the performance of polymer products include the ambient temperature, its humidity, the presence of a chemically aggressive external effect and a stress fields. Cuts or scratches — stress concentrators on polymer products — significantly change the thermal dependence of its mechanical properties. Therefore, hemispherical grooves, cylindrical cuts in the central areas of pads, made to reduce wear in the peripheral areas, lead to a significant narrowing of the temperature range of the effective operation of these products. Granular media have strong non-linear properties, the main causes of which are microcracks, boundaries and areas of contact of the granules a well as physic-mechanical properties of substances that fill the pore space of crushed stone layer. Coefficient of energy absorption in a granular medium is directly proportional to the viscosity of the polymer layer between the granules. Filling the pore space of the crushed stone layer with sand and viscous-plastic medium (polymers, organic binders and materials based on them, solid polyurethane foam, water or oil products) leads to decrease in the strength of crushed stone prism. An increase in the ambient temperature, especially above 30 °C, reduces the elastic properties and structural viscosity of viscousplastic materials that fill the pore space of a crushed stone prism, thereby reducing its strength. But at the same time, the energy absorption coefficient of vibration in a wide frequency range sharply increases and the dynamic loads on the track gauge profile are decreased. Most often, cheap organic-based compositions are used as dynamic load absorbers in the crushed stone layer: synthetic resins, acrylic acid amide polymers, latexes, lignin sulfonates and bitumen. When filling the pore space of crushed stone, an adhesive interaction occurs between the polymer molecules and the surfaces of the granules. Polymer molecules fill not only the pore space, but also penetrate into the contact area of the granules, reduce friction and decrease the angle of internal friction of crushed stone and the adhesion index of the bulk material. However the operation of railways is associated with permanent deformation and destruction of the surface layers of the polymer. Multiple repetition of the deformation process leads to an increase in mechanical hysteresis losses, causing friction and wear of the polymer in contact with the granules. Therefore, one should not expect a long service life when using a polymeric material for hardening the ballast layer.