The Liberty Ships
|Early in World War II, German U-Boats were sinking cargo ships destined to bring food and things to the people of Great Britain at three times the rate at which they could be replaced. Existing ship building methods still used rivets to clap the steel parts together, and that takes time.|
|The solution to the problem wasn't to let the
British starve but to built ships faster than our U-boat guys could sink them.
The "Liberty Ship" design, pioneered by the British, was just right
for that because those ships were made from widely available cheap steel that
was welded together and not riveted.
One could built a ship like that in about 50 days, less than 20 % of the time needed for traditional techniques.
|Well, you can't beat the first law of economics. Liberty ships could be build very quickly and were cheap, indeed - but they had this unfortunate tendency to break apart without help from German submarines. Here are some pictures:|
|Here is a bit of statistics (from 1)):
|What has happened? Well, with any major new technology you are going to pay some dues. Some things, including things that are rather obvious in retrospect, are just not known or appreciated in the early phase and cause havoc. In retrospect, people never understand how one could have been that stupid; witness the various crises' around money and all this innovative money-related products since the first collapse of the Internet bubble.|
|Not all that much was known around that time
about what we now call the "ductile to brittle transition"
that happens at low temperature for all steels, the question is only at what
temperatures. The fracture-initiating properties of
cracks that are often produced, for
example, at weld seams, weren't too clear either. The Liberty disaster actually
started serious research into crack formation and propagation, and thus gave
birth to what we now call "fracture
The links, by the way, will tell you far more about the topics than you ever wanted to know!
|In a nutshell, the fracture problems of the Liberty ships resulted from a mix of ingredients:|
|Shown is the probability that a specimen can be
fractured with a low energy of only 27 Joule (i.e. the steel is quite brittle)
vs. temperature for steel samples as indicated.
Several things become clear:
|Time changed, but problems remained. No matter if your sword or your ship breaks apart in cold weather, you are not going to enjoy it.|
|1)||J.D.G. Sumpter, J.S. Kent / Marine Structures 17 (2004) 575589|
3.2.2 The Charpy Impact Test
Fracture Mechanics I
Ductile to Brittle Transition or Cold Shortness
Heroes of Dislocation Science
6.2.3 Welding with Fire or Hammer
9.1.2 Problems with Alloying
Riveting, Soldering, Liquid Welding Plus Gluing and Screwing
5.4.2 Dislocation, Plastic Deformation and Hardness
© H. Föll (Iron, Steel and Swords script)