http://en.wikipedia.org/wiki/De_Havilland_Comet
"Engineers subjected an identical airframe, G-ALYU ("Yoke Uncle"), to
repeated re-pressurisation and over-pressurisation and after 3,057 flight
cycles (1,221 actual and 1,836 simulated), Yoke Uncle failed due to metal
fatigue near the front port-side escape hatch.[13] Investigators began
considering fatigue as the most likely cause of both accidents and initiated
further research into measurable strain on the skin. Stress around the
window corners was found to be much higher than expected, "probably over
40,000 psi," and stresses on the skin were generally more than previously
expected or tested. This was due to stress concentration, a consequence of
the window's square shape.
The problem was exacerbated by the punch rivet construction technique
employed. The windows had been engineered to be glued and riveted, but had
been punch riveted only. Unlike drill riveting, the imperfect nature of the
hole created by punch riveting may cause the start of fatigue cracks around
the rivet.
The principal investigator concluded, "In the light of known properties of
the aluminium alloy D.T.D. 546 or 746 of which the skin was made and in
accordance with the advice I received from my Assessors, I accept the
conclusion of RAE that this is a sufficient explanation of the failure of
the cabin skin of Yoke Uncle by fatigue after a small number, namely, 3,060
cycles of pressurisation."[14]
Before the Elba accident, G-ALYP had made 1,290 pressurised flights and at
the time of the Naples accident, and G-ALYY had made 900 pressurised
flights. Walker said he was not surprised by this, noting that the
difference was about 3 to 1 and previous experience with metal fatigue
suggested a total range of 9 to 1 between experiment and outcome in the
field could result in failure. Thus, if the tank test result was "typical,"
aircraft failures could be expected at anywhere from 1000 to 9000 cycles. By
then, the RAE had reconstructed about two-thirds of G-ALYP at Farnborough
and found fatigue crack growth from a rivet hole at the low-drag fiberglass
forward "window" around the Automatic Direction Finder, which had caused a
catastrophic breakup of the aircraft in high altitude flight."
-- Craig
Do not archive
-----Original Message-----
From: owner-zenith601-list-server@matronics.com
[mailto:owner-zenith601-list-server@matronics.com] On Behalf Of Jim Belcher
Sent: Friday, August 15, 2008 2:17 PM
To: zenith601-list@matronics.com
Subject: Re: Re: Corrosion protection
On Friday 15 August 2008 00:20, Thruster87 wrote:
<alania@optusnet.com.au>
>
> Early 50's models of the de Havilland Comet suffered from catastrophic
> metal fatigue, causing a string of well-publicised accidents. As they
found
> out how stress risers are a lot more critical on pressurized aircraft but
> do not under estimate there importance.
I could be mistaken, but I believe the metal fatigue in the case of the
Comet
was due to a structural aerodynamic problem. That is, there was structural
resonance within the airframe. The additional energy imparted by the (then)
much higher airspeeds caused the resonance to surface, which in turn flexed
the metal, causing the fatigue, and ultimate failure.
--
================================================
Jim B. Belcher
BS, MS Physics, math, Computer Science
A&P/IA
Instrument Rated Pilot
General Radio Telephone Certificate
================================================
