#Rail101 - How rails die - Plastic Flow

This is the second part of looking at how rails die, or why they don't last forever. The first part is here. This is a short series of articles so follow #rail101 and me to make sure you get all the parts of the puzzle.

Plastic flow is an engineering/materials science term for material that has been deformed beyond the point it can return to its original shape, i.e. material is permanently moved. Despite being pretty hard, rail steels under the extreme pressure from the wheels passing over can deform and in some cases be smeared a little like butter! (Don't try putting steel on your toast though!)

Plastic Flow

This real-life rail above shows significant plastic flow. You can see both a mushrooming of the head on the lefthand (field) side of the rail as well as a huge lip/tongue of material on the gauge face (right hand side). You can also see the top of the rail is very flat. This is the sort of rail deformation that is typically seen when soft rails experience heavy axle loads from freight traffic.

What is the problem with plastic flow?

Aside from being unsightly there are several problems caused when this happens to rails

1. The large lips of material interfere with switch operation. The precision fit between rails at switches and crossings cannot cope with such material interfering with the gaps and close fit needed between rails leading to either a failure to open/close properly or worse still a potential safety hazard if the fit between rails is poor
2. The lips of material can become detached. The material that has almost detached from the rail on the righthand side may look fairly insignificant in cross section, however in 3D this piece can be many meters or even tens of meters long! A long spear/shard of rogue material detaching from a rail beneath a train is less than ideal - Think of where it could end up!
3. Wheel/Rail interface. The curvature of the top/crown of the rail, (together with the wheel shape), provides steering forces for the vehicle. The now flattened head of the rail compromises steering performance which can accelerate wear of rails and wheels. However the flat crown also does reduce the contact stresses which were the cause of the plastic flow in the first place which reduces further plastic flow, lipping and wear).

What can we do to reduce rail lipping/plastic flow?

In the short term, lips of material can be removed by grinding (or milling). It is relatively common for a significant amount of maintenance effort to be devoted to grinding switch and crossing rails to maintain their profile and keep the switch in serviceable/safe condition.

Switch and crossing rails also suffer more with plastic flow because of the higher contact forces as rail loads are transferred from one rail to another through a smaller cross section of rail than usual.

Below is a promo video from ROBEL for a grinding machine specifically designed for switches and crossings. (I couldn't find a good video of someone removing lipping - so if you know of any then let me know in the comments!)

A more permanent solution to lipping is to move to a harder grade of rail steel, which can then resist this plastic flow of the metal to a greater degree. (Hardness is a measure of resistance to plastic flow). You could of course reduce the axle load of the traffic to reduce the issue but that is usually not practical!

In Europe the rail steels are named after their hardness level. Two of the most common rail steels/grade are R260 and R350HT. The second rail grade is around 35% harder, and grades 50% harder also exist. Below is how the European rail hardness specifications compare.

Why not use the hardest rail for everything?

A good and recurring question. Aside from cost, factors other than plastic flow race to limit rails useful life too. There is no use having a rail with no plastic flow if its life is cut short shattering into a hundred different pieces due to fatigue cracking instead.

I'll return to this point many times in the coming articles - rail grade selection is about balance. One of the items to balance is resistance to plastic flow, but there are many more which we'll explore as the series progresses.

You'll have to stay tuned for my next article to find out about other degradation types that race to limit rail life, but feel free to list or guess them in the comments (so if I've missed any in my list I can include them in a later article!).

Thanks for reading. If you like this content then like, comment, share - you know the drill!

Remember to follow #rail101 for more posts like it. (I created the hashtag to help make our industry more accessible to both those in it, and those who are thinking of joining it), and I also created a Rail101 group too for ease of finding it in future - Join it here.