Don: I've got numerous phone calls to anodizing firms who have confirmed
the decrease in fatigue life due to anodizing. I've got a photo of a
fatigue failed anodized control tube, none of the non anodized control tubes
in any of the other similar helicopters even those with more hours have
failed. Further if you consult the excellent text:
titled Fatigue Design of Aluminum Components & Structures, Sharp, Nordmark
and Menzemer, a chart, page 110, shows decrease in fatigue life due to
pre-cleaning as well as the affects of Alodine and a couple of different
thicknesses of anodic coatings.

Further:

In a report authored by Thart, WGJ and Nederveen, the following was stated:

"Constant amplitude fatigue tests on anodized unnotched specimens reveal
that sulfuric acid and sealed chromic acid anodic layers cause the largest
decrease in fatigue strength. Phosphoric and unsealed chromic acid anodic
layers do not significantly affect fatigue life. Scanning electron
microscopy of fracture surfaces confirms that fatigue cracks initiate at
cracks in the anodic layer".



Mo

Shiozawa, Kazuaki; Kobayashi, Hirokazu; Terada, Masao; Matsui, Akira. Japan
Society of Mechanical Engineers, Transactions A. Vol. 66, no. 652, pp.
74-79. Dec. 2000

"The anodized film is fractured at an early stage of the repeated tensile
fatigue process, because it is too brittle to accommodate the substrate
metal."

Mo A P.E associated with the anodizing community said it even stronger.
"Never anodize flight critical components"

Van's of Van's RV aircraft and the subject was anodizing spars, said that
anodizing has been known to reduce fatigue life as much as 50%.



Boeing Aircraft has a special process whereby the ameliorate the effects of
anodizing on some parts.

I had a 36' McGregor catamaran with an anodized mast that I sailed in the
open ocean in the South Pacific. Even with the cracks in the anodized layer,
the frequency of vibration in the mast was much lower than the 17hz
associated with the helicopter. Looking back I would expect the mast on
the sail boat to have a much longer life than helicopter parts.

More data. The failed control tube was inspected by a laboratory in Canada
and they proved that there was no existing flaw prior to the anodizing. The
crack started after the anodizing and the control tube with a small load
applied, but subject to the vibrations produced by a helicopter, failed in
fatigue with very few hours.

Experience can be misleading. I've been in the amateur helicopte game since
97 and I'm a retired engineer but I had never heard that the fatigue life of
anodized parts could be reduced as much as 50%.



"Don W" wrote in message
t...
Stuart & Kathryn Fields wrote:

aluminum control tube failed causing helicopter crash. The helo had an
anodized aluminum control tube that failed in fatigue with less than
400hrs. The loading was low producing stresses in the area of 460psi.

It seems unlikely to me that anodizing was the real culprit here, since
the anodized layer is normally only .001" thick or so.

Discussions with anodizing folks say "Never anodize flight critical
components" Some research has provided data indicating that some
cleaners can also shorten the fatigue life of aluminum significantly.

I'm not an expert on anodizing, but I am an engineer, and I've studied
anodizing because I own two sailboats that have aluminum components
exposed to a salt-water environment, and anodizing is a great way to
passivate aluminum to prevent corrosion. The aluminum hydroxide layer
formed by anodizing is around an 8 on the hardness scale--roughly the same
hardness as rubys.

Do you have any links to the research you are citing? If this is really a
problem, I'd like to learn more about it.

I've seen gyrocopters with long
anodized aluminum control tubes shaking like your English teacher's No-No
finger. The May 07 issue of Experimental Helo has a more indepth article
on this with pictures of the helo before and after..

If there was an existing flaw in the aluminum tube _before_ it was
anodized, the anodizing might very well make the flaw worse, as it would
eat away at the edges.

Just my thoughts... I could be wrong. Have been before, and will be
again.

Don W.