Re: Questions about the DH Comet

Date:         17 Sep 97 02:49:25 
From:         J& (John Wright)
Organization: Us at home with our cat
References:   1 2 3 4 5
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On 11 Sep 97 03:35:30 , in <airliners.1997.2156@ohare.Chicago.COM>,
David Lesher wrote:
>J& (Janet and John) writes:
>Any strength of materials textbook.....
>>You should say "any strength of materials textbook written since the
>>Comet crash investigations". Fatigue behaviour was not well known at the
>>time, and the Comet crashes aided the development of the science
>By some measures, the entire field of fatigue behaviour started
>only a few years before, when the Allies tried to figure out why
>brand new Liberty Ships in the North Atlantic were suddenly
>cracking in half and sinking.

But that was more to do with the stresses induced in steel due to fusion
welding - then a relatively new technique - than fatigue. These welds
were not tempered (as they would be today) to relieve stresses, and the
ships were operated in Arctic waters where the steel was below its
brittle - ductile transition temperature. Stress in non tempered welds
can approach the UTS of the material, so the cracks would easily start
from stress raisers like the sharp corners of holds or hatchways (not
dissimilar to that in the Comet windows, though a different mechanism).
Since the entire structure was welded, there were no discontinuities to
arrest the crack, hence the ship split into two pieces and sank :-(

John Wright