If you aggregate the rail failure statistics in northern (or southern) latitudes you can almost always see a seasonal pattern, with increased numbers of broken rails occurring in the colder months of the year.
If you analyse the data still further, you can often see a spike in failures happening at the same time as or immediately after the first cold spell of the season.
What are the causes
Photo courtesy of Arie Overeem
This article aims to explain why rail failures are more common in the colder months. There are a few factors at play:
- Rail stress
- Pre-existing defects
- Material properties
I’ll try to cover each briefly. First, I must point out that rail failures are rare. In the UK there are around 10,000 miles of track (20,000 miles of rail), and currently fewer than 100 rail failures recorded per year (down from around 1,000 just 20 years ago). So there is certainly no need to panic. Let’s explore why rails are more likely to break in the winter than in the summer. If you think I’ve missed a cause, let me know in the comments.
Most new rail is continuously welded into long lengths many hundreds of metres long to provide smooth-running and low-maintenance track. However, being so long poses some engineering challenges. When temperatures change, objects change size very slightly (known as thermal expansion). For objects in our everyday life this change in size is imperceptible. However, when you have a rail 1 km long, the change in length becomes significant. A change in temperature of 20 degrees Celsius will alter the rail length by around 25 mm. As rails in direct sunlight can often be double the ambient temperature (think about how hot your car roof can get on a sunny day), there can be significant changes in rail temperature and therefore changes in rail length to manage. In order to deal with thermal expansion and contraction the rail is often stretched into position when it is fitted, leaving it under tension (stretched) most of the time. Rails under tension are far easier to manage than those under compression. Compression in a rail can result in track buckles or heat kinks which pose a real risk to safety.
Photo of a heat-induced rail buckle (heat kink) courtesy of Paul Fuller
The temperature at which rail stress changes from tension to compression is known as the ‘stress neutral temperature’. Other names for the same thing include the stress-free temperature and rail-neutral temperature. This temperature will vary depending on the climate, so each network may use a different value. Regions which experience very large seasonal temperature changes may actually have to re-stress their track between summer and winter.
So, why does this affect the failure rate of rails? Put simply, it is easier to break something that is under tension than something that isn’t. The more tension applied, the smaller the force or crack size needed to break the rail. Colder weather means the rails are under greater tensile stress and so are more likely to break. Hence failures are more likely in winter than in summer.
Defects happen in rails – it is a fact of life. With time and the passage of repeated traffic over the area, these defects can slowly grow. For example, a fatigue crack may start on the surface and slowly grow over thousands, if not millions, of loading cycles as each wheel passes over it. As the crack slowly penetrates the rail there is less and less material left to support the load, causing the growth of the crack to accelerate. Eventually there will not be enough material left to support the load and ‘BANG!’, the rail breaks.
Fatigue-related failure caused by damage/corrosion of the rail foot (Dark semi-circular area is the fatigue crack)
What does this have to do with the seasonal impact on failure rates? In the previous section we talked about the fact that in winter the rail is under greater tension. This increased tension makes cracks more likely to result in failure.
Imagine that a (fatigue) crack has the whole spring, summer and autumn to grow slowly. In the first cold snap the rail tension increases, and that increase in tensile stress can be enough to accelerate the crack’s growth so that the rail fails. This is the main reason that there tends to be a spike in rail failures around the first cold spell of the winter. The cracks that have been growing slowly all year (or indeed those that have survived the last winter or even the one before) are now placed under much bigger tensile stress as the temperature plunges. If the crack has grown large enough, the remaining rail will be unable to support the load, causing it to fail.
Material properties have a small effect, but I’ll include it for completeness. As steel (and indeed most materials) gets colder the energy required for a crack to grow through the material decreases. Due to their composition, rail steels generally have a low energy needed to grow cracks (a low impact energy), compared to construction steels. The colder it is, the lower the energy required to grow a crack. This means that it is more likely for an external factor such as a wheel flat impact to cause a rail to break. However, the difference in impact energy between normal and cold temperatures in common rail steels is a small effect compared to the others mentioned above.
I hope this helps explain why winter and, indeed, the first cold spell of winter is usually the worst time of year for broken rails. Often the key to success is understanding failures so they can be avoided in future.
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