Re: Questions about the DH Comet

Date:         17 Sep 97 02:49:24 
From: (Charles Crosby)
Organization: University of Pretoria
References:   1 2 3 4 5
Followups:    1
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Gerard Foley ( wrote:
: Janet and John (J& wrote:

: : On 29 Aug 97 08:10:43 , in <airliners.1997.2000@ohare.Chicago.COM>,
: : michael piersdorff wrote:
: : >My understanding of the problem was that most of the in-flight
: : >breakups were explosive decompressions of the fuselage resulting from
: : >fatigue failure ... Any strength of materials textbook will show that
: : 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
: : immeasurably,
: As a person working on endurance (fatigue) testing of materials (including
: uranium as well as duralumin) in the period 1940-1945 I take issue with
: the statement.  I believe poor calculation of stress raising, or
: complete neglect of pressurization cycles as a source of fatigue
: stress, was the reason for the fault.

>From a book on De Havilland (sorry, forget the title) the statement was
made that they were aware of the dangers of the window corners, so they
tested the design extensively.  However, they subjected it to a static
over pressure test first, and THEN tested for fatigue at relatively high
amplitude and reduced cycles.  The theory was that this method of testing
may have resulted in some stress-relieving plastic deformation at the
stress concentrations, and that it didn't accurately simulate the high
cycle, low amplitude stress situation in practice.  I'm not a stress or
fatigue analyst, but to my mind it sounds like a very realistic and
practical type of "Oh Shit" situation.

Charles Crosby