A Fatigue Engineering Joke

There is a Cold War (remember, that was before 1989) engineering joke that goes as follows:

The Russians had built a brand new huge airplane, and it was the pride of Russia, but they had a continuous problem with fatigue fracturing at the wing to fuselage connection.

They tried everything and it just kept fracturing. One day there was a goodwill tour of Americans that came to Russia and one of the tour members was a Boeing engineer.

One of the Russian aerospace engineers discovered this and during a quiet moment approached the Boeing engineer to see if he had any suggestions on the fracturing problem.

The Boeing engineer quickly came up with a solution. He said: “just drill tiny evenly spaced little holes right along the path where the fracture appears and the problem will go away.”

The Russian thought he was kidding, but they were desperate and decided to try it. Surprisingly it worked, the wing never fractured at the wing root again. Years later the Russian met the Boeing engineer again, and asked how he came up with that idea. The Boeing engineer told him it was easy, he just made the wing look like the perforations in Russian toilet paper, since that never tore at the perforations either.

But is there a kernel of truth in the joke?

When a ship has a fracture it is often recommended, as a temporary measure, that a stopper hole be drilled at the end of the fracture.

The hole removes the sharp notch that exists at the end of the fracture and therefore reduces the tendency of the fracture to continue to grow from cyclic loads. In theory this is correct, but in practice there are all types of problems, and random stopper hole drilling in the marine environment is nice to add, but wrong to count on.

The most straightforward problem is that it is generally impossible to find the end of the fracture, because at the end it is tiny and difficult to actually locate. And if you drill the hole before the end of the fracture, it is useless, because the end of the fracture is still there and will continue to propagate.

Well at that stage the Boeing engineer’s idea is not so crazy. If you drill a series of holes along the expected path of the fracture, the fracture end will quickly grow into one of the holes, but at that stage will not grow beyond the hole it grew into. Therefore if you want to work the stopper hole theory you should drill a row of holes as long as you know where the fracture is going.

But wait a minute! American toilet paper does tear along the perforations! True, but it does so when subjected to a force that is in excess of the yield strength of the material (not in cyclic fatigue loading). At that stage the fracture will follow the path of the least material, which is from perforation hole to perforation hole. Fatigue is different, it grows fractures from continuous cycling at stresses less than the yield stress and therefore toilet paper will not necessarily fail at the perforations when cyclically loaded before use, it will fail where it can find a notch to start fracturing and will continue from there. And since round holes make bad notches, it is less likely to happen at the perforation.

Moral 1 of the story: If you want your toilet paper to rip at the perforation; use one firm pull, not many small pulls.

Moral 2 of the story: Don’t confuse fatigue failure with yield stress failure.