Don: Be sure to share what you find. I think that I'm only scratching the
surface of some stuff. One thing I found and had never seen before was an
equation relating the increase in stress due to a crack. It scares the hell
out of me. I'm afraid to use a metal fork in my salad. The equation
basically says that the max stress is 2 times the load divided by the area
times the square root of the crack length divided by the radius of curvature
of the end of the crack!!! If the radius of curvature was equal to the
crack length, the max stress is already twice what you would calculate using
the applied load and the element cross section. Now put a reasonably sharp
crack and see what happens....as the radius approaches 0.001 times the
length of the crack......????


"Don W" wrote in message
et...
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.