Stuart Fields wrote:

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%.


Stuart,

Very interesting! A lot of things that you run
into in engineering are counter-intuitive, and
this is apparently one of them. I had not heard
of this phenomenon before now.

I'll certainly look into this some more when I get
some time.

Don W.