Why sausages split lengthways
A BBQ Some very cheap sausages (the more expensive ones are designed not to split) Willpower (you can't eat the sausages before the experiment is over!!)
1 - Light a BBQ - ask a sensible adult to make sure everything's safe
2 - Throw on a few sausages and let them cook - Don't make holes in them
3 - Stand well back as we are hoping the sausages will split and when they do this they will spit.
4- Keep an eye out for sausages with split skins. In which direction do they split?
Sausage skins split lengthways down the sausage much more often than they split around the sausages.
You can think of a sausage skin as being like a pressure vessel. It's actually holding in all the contents that are expanding with the heat. When the skin suddenly bursts, it's going to burst in the direction where the stress is highest. The stress is really just the amount of tension there is the skin. It turns out that for a pressure vessel, the stress going around circumferentially is twice as high as the stress going lengthwise. So that means that it tends to rip along the length because the circumferential stress is twice as high as the longitudinal stress. The same thing happens when you put a can of beer or a can of coke in the freezer: it will also burst along its length.
We can see this effect in other ways too. For example, you have gas pipe lines going thousands of miles across Alaska - a bit like a giant sausage. The one thing you have to be really careful about is that you don't get a burst pipe. If it starts to burst, the burst will go right along the pipe for hundreds of miles. The people in the know will say that the pipe is 'unzipping'. It's absolutely catastrophic because it releases gas over a large area.
But going back to our sausages - the best way to prevent them splitting is poking them, but that lets out all the delicious juices. So you really want to find a compromise, and this is to cook them at a nice gentle rate so they don't burst.