The trend in railroad construction is to use very long rail segments that are welded seamlessly. These rail segments can be several miles in length and contain no expansion joints. During the summer months, the expansion of the rail causes a build up of stress. If the rail is not adequately bound to the cross ties, buckling can actually occur (sun kinks).
Does not the stress build up and compression weaken the strength of the steel rail, and thereby defeat its purpose? If not, why not?
Since the practice is wide spread, the rail service life is not dominated by cycles on compression and tensile stressed. I used to measure stress in rail years ago... I'll ask old colleagues about current practices and concerns.
Define what you mean by 'weaken the strength of the steel rail'. Are you talking about failure? Stress v. strain, or what?
The stress on the rail as the train passes over it is down.
The stress on the rail due to thermal expansion is along the rail.
There is a trivial amount of weakening due to heat.
I'll rephrase the question.
Is the ability of the rail to carry the load placed upon it (the train) affected by the state of compression?
Rail behaves similar to an I-beam as a train moves over it. Can the extra compression within continuous welded rail affect this behavior?
The rail is essentially a beam, or an I-beam. Under load the top is under compression and the bottom is under tension.
But rail is constructed, AFAIK, to bear more compression as the top portion contains more material.
Trivial? I would not think so.
During summer months, especially in the southern US, the stress increase is substantial and, as I mentioned, can cause the rail to literally rip itself out of the track bed by buckling. This is called a sun kink.
I don't know a whole lot about this (that's why I ask here).
There is also work hardening that happens because trains run over it.
New rail cuts like butter with machine tools, old, used rail is harder than hell.
There are two very obvious and simple solutions to the problem. The Victorians used the first, fish plates, the second is to build in a very long and gentle curve, allowing the rail to move sideways. The grass bends with the wind, the mighty oak is toppled. You know it will move, so let it. If you resist then the force will build up until catastrophic failure occurs. WTC collapsed when heat buckled the steel, not when a plane hit.
In the UK the rails are heated when laid and so pre-tensioned. The stress-free temperature is 27 degC.
It is trivial. You are getting nowhere near any sort of annealing temperature, hundreds of degrees. It takes little force mid span to deflect an "I-beam" a significant percentage over a long span.
It is a bad design, as Androcles hints at. You can see it in concrete slabs that are formed with insufficient clearance for thermal expansion. Not enough stress to spall the concrete, but they "tent up" really nicely.
Not quite. The entire rail is under compression... above the cross- ties.
You get plenty of work hardening, as essentially hot rolled material is "peened" into a cold-formed strength.
David A. Smith
Thanks for the responses.
I am always impressed when I see a long and heavy freight train traversing a section of continuous rail. The ride is both very smooth and very quiet. The train almost glides as if riding on air, and the only noise comes from the internal bangs of the freight cars and not the wheels.
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