Stressing is a rail engineering process. It is used to prevent heat and cold tension after installation of

continuous welded rail Continuity or continuous may refer to: Mathematics * Continuity (mathematics), the opposing concept to discreteness; common examples include ** Continuous probability distribution or random variable in probability and statistics ** Continuous ...

(CWR). Environmental heat causes CWR to expand and therefore can cause the fixed track to

buckle A buckle or clasp is a device used for fastening two loose ends, with one end attached to it and the other held by a catch in a secure but adjustable manner. Often taken for granted, the invention of the buckle was indispensable in securing two ...

. Environmental cold can lead to the contraction of the fixed railway track causing brittleness and cracks. Before it is installed, the rail is altered by stretching with hydraulic tensors or heated to its stress-free temperature to make these dangerous problems less likely.

Background

Railway steel contracts at low temperatures and expands at high temperatures.Bibel G
''Train Wreck: The Forensics of Rail Disasters''
JHU Press, 2012 In extreme cold, a length of CWR suffers

tensile stress In continuum mechanics, stress is a physical quantity that describes forces present during deformation. For example, an object being pulled apart, such as a stretched elastic band, is subject to ''tensile'' stress and may undergo elongati ...

. This stress can cause railway steel to fracture. In extreme heat, a length of CWR suffers

compressive stress Compressive stresses are generated in objects when they are subjected to forces that push inward, causing the material to shorten or compress. These stresses occur when an object is squeezed or pressed from opposite directions. In everyday life, ...

. This type of stress can cause sun kink where a length of a railway buckles laterally (sideways). Other factors that affect CWR condition include the state of the

track ballast Track ballast is the material which forms the trackbed upon which railroad ties (UK: sleepers) are laid. It is packed between, below, and around the ties. It is used to bear the compression load of the railroad ties, rails, and rolling stock; ...

and its shoulders; the type and placement of the sleepers; places of increased shadow such as tunnels and bridges; track consolidation; and, to a lesser degree, the vertical curvature of the tracks. In the design and installation of CWR, a figure known as the "rail neutral temperature" (RNT) is calculated. The tensile and compressive longitudinal forces on the CWR are added. The RNT is the notional temperature when the total equals zero. A similar figure is the "stress free temperature" (SFT). It is the rail temperature at which the rail is the same length as in its unrestrained state.

Engineering solutions

Before laying new track, repairing track, or changing the sleepers, the rail is mechanically or thermally altered (stressed) so that its length equals the same at a chosen stress-free temperature; the rail can then be fixed in place with no thermal forces in effect. The stress-free temperature that is used is dependent on environmental extremes and thus varies with location. In the

United Kingdom The United Kingdom of Great Britain and Northern Ireland, commonly known as the United Kingdom (UK) or Britain, is a country in Northwestern Europe, off the coast of European mainland, the continental mainland. It comprises England, Scotlan ...

, CWR is stressed to , the mean summer rail temperature. In the US, standard stress free temperatures vary from . Despite stressing the CWR before installation, a rail may still reach its "Critical Rail Temperature" (CRT). This is the temperature of the rail above which buckling may occur. The CRT may be reached due to disturbance of the ballast, track components or track geometry. For example, the CRT may be reached due to a removal of a rail section or an Insulated Block Joint (IBJ). A stressing engineer measures the rail section to be removed and places indicators with marks on the foot of the rails. After the rail section is cut out, its first stress free temperature is determined. The new rail section is cut and replaced and then welded at one end. A "stressing kit" (hydraulic rail tensor), such as the Permaquip HSM70 Stressor, or other method is used to adjust the other end and make the join ready for welding. If a section of rail must be replaced urgently, a stress check is performed later on. A more recent engineering solution is "VERSE" testing equipment produced by the company, Vortok in 1998. It allows rail engineers to measure stress-free temperature without removing a section of rail.Railroad suppliers provide ways to prevent the track from bending and twisting
Progressive railroading March 2013. Accessed 24 February 2018. Vortok is Pandrol Vortok now.

External links

Volpe center website, US government

References

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