non-round tubing on frames

I watched something on either the Discovery Channel or the Learning Channel about how some early
passenger planes were built with rectangular windows until they figured out the corners led to the
planes falling apart in the skies because of IIRC metal fatigue starting at the corners of the
windows. Is there anything potentially similar here with the rectangular bike frame tubing in some
bikes, or is the application just totally different, or is this plane story a myth?

 

"WooGoogle" <[email protected]> wrote in message
news:[email protected]...
> I watched something on either the Discovery Channel or the Learning Channel about how some early
> passenger planes were built with rectangular windows until they figured out the corners led to the
> planes falling apart in the skies because of IIRC metal fatigue starting at the corners of the
> windows. Is there anything potentially similar here with the rectangular bike frame tubing in some
> bikes, or is the application just totally different, or is this plane story a myth?

The metal fatigue you're referring to was exacerbated by the pressurization of the plane, for
passenger comfort. The pressure inside was higher than the pressure outside, plus the constant
changes in the pressure differential because of the airplane climbing to altitude and descending for
landing. That, and the normal aerodynamic stresses, caused the aircraft skin to rupture at the
corners of the windows. Slowly (relatively, but from start to finish was only seconds) at first, but
as the interior air escaped through the rupture, the huge volume of air going through a 'small'
opening at high pressure differential continued to rip the skin ever more rapidly causing
catastrophic structure failure and the aircraft fell out of the sky.

Bicycles are subject to entirely different and much milder stresses on their frames. The
applications are different.

The plane story is not a myth. The British Comet was the first passenger jet in service and suffered
these horrible failures. This tragedy led to better and safer aircraft.

 

WooGoogle wrote:
> I watched something on either the Discovery Channel or the Learning Channel about how some early
> passenger planes were built with rectangular windows until they figured out the corners led to the
> planes falling apart in the skies because of IIRC metal fatigue starting at the corners of the
> windows. Is there anything potentially similar here with the rectangular bike frame tubing in some
> bikes, or is the application just totally different, or is this plane story a myth?
The plane was the deHavilland Comet that had been built with rectagular windows rather than the
circular 'portholes' that engineers may have preferred. The cycles of pressurisation and
depressurisation that was new to this, the first passenger jet caused fatigue cracks to appear at
the corners of these window frames where stress concentrations ocurred. The crashes all happened
after just about the same number of pressurisation cycles.

So, the application is totally different, the similarity lies in the use of rectangular sections,
Stress concentrations occur at corners and abrupt changes in section. If the frame is going to
fail it will likely fail from a point such as these. The question is, have the designers taken
into account the loads that it is to subjected to and the resulting stress? The beauty of
circular sections is that they are much more forgiving in this respect.

 

In article <[email protected]>, davetspokane1 @attbi.com says...
>
> "WooGoogle" <[email protected]> wrote in message
> news:[email protected]...
> > I watched something on either the Discovery Channel or the Learning Channel about how some early
> > passenger planes were built with rectangular windows until they figured out the corners led to
> > the planes falling apart in the skies because of IIRC metal fatigue starting at the corners of
> > the windows. Is there anything potentially similar here with the rectangular bike frame tubing
> > in some bikes, or is the application just totally different, or is this plane story a myth?
>
> The metal fatigue you're referring to was exacerbated by the pressurization of the plane, for
> passenger comfort. The pressure inside was higher than the pressure outside, plus the constant
> changes in the pressure differential because of the airplane climbing to altitude and descending
> for landing. That, and the normal aerodynamic stresses, caused the aircraft skin to rupture at the
> corners of the windows. Slowly (relatively, but from start to finish was only seconds) at first,
> but as the interior air escaped through the rupture, the huge volume of air going through a
> 'small' opening at high pressure differential continued to rip the skin ever more rapidly causing
> catastrophic structure failure and the aircraft fell out of the sky.
>
The shape (rectangular voids with sharp corners in structural members) may have contributed to the
failures. Sharp corners and abrupt changes in size, shape, etc. of structural members can cause
stress concentrations that start fatigue cracks, which may ultimately result in catastrophic
failure. I don't know about the aircraft example, but I recall hearing in my Engineering Materials
course that in the early years of WWII, the rectangular corners on deck hatch openings on the early
Liberty class ships contributed to catastrophic hull failures.

Rick

 

On Mon, 02 Jun 2003 18:24:19 GMT, "Dave Thompson" <[email protected]> wrote:

>The plane story is not a myth. The British Comet was the first passenger jet in service and
>suffered these horrible failures. This tragedy led to better and safer aircraft.

It's too bad that this sort of thing usually kills the manufacturer along with, who usually had no
way at all of knowing in advance what would have happened (unlike, say, the Annan story).
McDonnell-Douglas was even more of a shame, though. DC10s are, as far as I can tell, very good
planes with a lot of bad press.

Jasper