Do your own research

First let's consider why that high cruising altitude is beneficial. Over 7.5 miles up from the ground, the air is less than 1/5th the pressure at sea level. So, the Comet experienced far less air resistance in flight than other aircraft. It therefore consumed less fuel and was less costly to operate; a big win for operators. The cruising altitude also meant the Comet could fly above many clouds, making for a much less turbulent and more pleasant flying experience. Top marks!

Second, did you spot the issue? It was the windows! Look carefully - they are square, whilst in any modern plane you see, they are round. To people at the time, square windows were quite logical as they’d been used in houses for hundreds of years just fine! However, at the high altitude, the fuselage was under immense pressure, as the cabin was pressurised to a level much closer to that at ground level. These square windows acted as stress concentrators, with the highest stresses occurring at the corners.

Worse still, the fuselage was riveted which led to microscopic cracks developing during construction of the aircraft. In the highly stressed regions near the corners of the windows, the applied stress exceeded the critical value for the fuselage material and the cracks started to grow. With each successive flight cycle, the cracks grew slowly larger and larger until one day they reached a critical length.

Every material has a critical crack length which can be calculated using fracture mechanics laws. Lets save those for the degree! But the crucial thing is that, when you exceed this critical value, the crack will grow itself, even without applying any forces. Fast fracture results in a fraction of a second, destroying the aircraft!

Failure in this manner, caused by the growth of a crack over many cycles, is known as fatigue. To the average person, fatigue is the chronic tiredness you might experience after a long flight or a night out in Sheffield. But to materials scientists, it has another meaning too: fatigue is an important material property. In fact, in many instances it is just as important as other properties you might be more familiar with; lightness, corrosion resistance or yield strength. But engineers of the time didn’t fully understand fatigue failure, which unfortunately led to a number of disasters.

Luckily, nowadays, we have an extremely good understanding of fatigue failure and modern commercial aircraft are extremely safe. But hopefully you now appreciate that knowing about the stresses experienced by each and every part of your aircraft is very important. Choosing appropriate materials that are resistant to fatigue ensures you avoid crack growth and is one of the key reasons why modern commercial aircraft are so safe. Make sure you pay attention in those lectures when they come around!!

Lets try and think logically about what could be causing the steel to behave in this way. A ship is subjected to some very harsh environmental conditions; air and salty water. So, if you’re thinking it could be due to corrosion (rusting), that’s a nice thought. But, alas, not correct: this ship failed after just 24hrs, after all.

So perhaps it could be something to do with the grade of steel used in the ship? If you’re thinking along those lines, you’re getting closer. The liberty ships were actually made of all sorts of grades of steel; production needed to be so fast that the design just stipulated what strength the steel should be, not a particular steel composition. So variation in this shouldn’t have caused its failure...

Perhaps you think that there must have just been a mistake on this ship; a crack or some other defect that has caused it to break in half so spectacularly. Again, this is a good thought, but not the whole reason why many of the liberty ships failed in this way.

The truth is that it’s a combination of all three of these factors. That’s because steel has what is known as a ductile to brittle transition temperature (or DBTT for short). This is exactly as it says on the tin; a temperature below which the behaviour of steel transitions from being ductile, like copper, to being brittle, like glass. The exact temperature of this transition is dependent on the steel grade and how it was processed, which explains why only some ships broke apart. But in all instances of failure, the conditions were particularly cold that day. So, the steel was below its DBTT and behaving in a brittle manner.

But you still need some kind of force, or stress, to cause a crack in the hull to form and grow in the first place. This is where the third part comes in; defects in the welding were quite common. These acted to concentrate the stresses that the hull was subjected to and were the origin of crack formation.

So, a combination of temperature, material and dodgy welding were the reasons behind the failure of some of the Liberty ships. But luckily, their failure was in relatively small numbers, and many made dozens of crucial trips across the Atlantic and around the world to maintain supply lines and help the war effort.

You can’t really blame the engineers of the time for the failure of some of the ships either. That’s because the ductile to brittle transition temperature (DBTT) was not a known phenomenon in the 1940s, so only with subsequent research into the material have these failures been explained. So, proof, if ever you needed it, that an understanding of materials science is crucial to the success of any engineered product; be it a phone, ship, nuclear power plant or anything else for that matter!