----- Original Message -----
Sent: Thursday, July 27, 2006 7:56
AM
Subject: [FlyRotary] Re: Inconel
I have a lot of experience with failure analysis. Used to do it with GM,
Ford, other warranty returns. One of my favorite experiences was solving a
million $ warranty issue on a tubing product. Although I'm not expert in this
area, I'd sure encourage considering other factors, other solutions.
In particular I'd encourage use of strobe lamp. Take cowl off, fire up
engine, use strobe. (yes, it's dangerous with prop on) You will suddenly have
appreciation for the forces involved. You should be able to predict failure
points with this test. Tack together the broken one to see what I mean.
The exhaust will turn into a sine shape at certain rpm. You will see huge
amount of movement on the busted one. Make a new design, compare movement to
predict how long it will last. A very minor design change can greatly improve
how much movement occurs.
I'm not sure this is applicable to you guys, but my exhaust design has
slip fit of two pipes instead of welding. It works phenomenal. I've had
conversation with other guys who welded instead and had failures. Same exact
application. So my exhaust floats in relation to the header. I just attach a
small bracket to the floating portion to prevent if from blowing off.
Truthfully, I never expected it to work, but it has quite well. I might have
pic if verbal description not adequate.
I say it is brittle because it has
failed twice and both times quite spectacularly. Not only did the inconel
fail in the area adjacent to the welds but spider cracks formed flowing out
of the main cracks and into the surrounding area. It looked like glass might
look if you punch a hole in it (i.e. not tempered glass!).
That is
interesting. Sounds like ‘stress corrosion’ cracking; which is the
reason to use inconel vs other SS alloys at these temps. Almost makes
me wonder whether the welder picked up the wrong material when he made the
parts; or at least didn’t give it any annealing heat treatment after
welding.
http://www.corrosion-doctors.org/Forms/scc.htm
Al
-al wick
Artificial intelligence in
cockpit, Cozy IV powered by stock Subaru 2.5
N9032U 200+ hours on
engine/airframe from Portland, Oregon
Prop construct, Subaru install, Risk
assessment, Glass panel design
info:
http://www.maddyhome.com/canardpages/pages/alwick/index.html