anodize versus fatigue

[klyde]

Recently there was a non-fatal crash involving an amateur built helicopter. The cause of the crash was a fatigue failure in a 0.5ODx 0.325"ID 27.9" long aluminum control tube. The tube was anodized with color to assist in building an award winning ship. It won at Oshkosh. The question is does anodizing reduce the fatigue life of aluminum? I've seen gyrocopters and helicopters with liberal use of anodized aluminum tubing and pieces.

 

[MikeHalloran]

I've read that anodizing leaves the surface covered with microscopic cracks, which doesn't sound like a good way to improve fatigue resistance. Don't take my word for it.

A quick scan of the first page returned by Googling }anodize fatigue| suggests that some effect has been documented, and the documents are available.



Mike Halloran
Pembroke Pines, FL, USA

 

[klyde]

Mike: Thanks for the reply. I've not only looked thru a lot of the Google stuff but I bought a textbook on fatigue design. The mechanism by which anodizing creates conditions where lightly loaded aluminum pieces receive stresses that exceed the Endurance Limit has still eluded me. While the anodized layer is more brittle than the base metal, and has microscopic cracks, the helicopter failed control had forces in the 50-60# ballpark which are significantly below numbers that would induce the 8-18ksi reported as the Endurance Limit range for wrought aluminum. Elements stressed less than the Endurance Limit are supposed to enjoy an infinite fatigue life. Ergo somehow the anodizing contributed to the production of stresses that exceeded the Endurance Limit. I would like to know how that is done.

 

[MikeHalloran]

Did the tube fail at a place where the full cross section of the tube would be appropriate for evaluating the stress level?

Or did it fail at, e.g., a swaged end, where ex-mill cold working and the presence of threads for a rod end should also be considered?



Mike Halloran
Pembroke Pines, FL, USA

 

[jetmaker]

Is there a possiblity that the anodizing process induced residual stressees that were missed? Something like hydrogen embrittlement, which affects steels and aluminums?). I seem to remember something about limiting the size of alodine repairs due to hydrogen diffusion.

It is possible that the above makes no sense, and you should look for confirmation from someone with more specific knowledge in this area.

jetmaker

 

[klyde]

Jetmaker: Thanks for the response. I found an equation describing the stress increase due to a small crack. It is scary. I'm now afraid to use a metal fork for my salad without xray or dye penetrant for cracks. The stress concentration generated by a small crack that had to be induced after the anodizing had to take the stress from 460psi to something over 10ksi. I've seen the tube, calculated the stresses and now I'm a believer. Don't ever use a sulfuric acid anodize, or even be careful selecting cleaners on flight critical parts. Even those elements whose stresses are computed to be significantly lower than the Endurance Limit.

 

[jetmaker]

klyde,

Somewhere in my library I have information on the effect of nickel and cad plating on fatigue life. These effects are significant and it is often associated with the surface cracks in these coatings. I have limited information that hard anodizing (per MIL-A-8625, Type III) produces adverse effects on fatigue live, with a reduction of about 45 percent.

jetmaker

 

[SUPERCRUISE14]

Anodizing or Anodic Coating application requires the use of current. How this current is applied and it's relative strength has a lot to do with the surface coating. However, I understand from my past experiences with thin walled aluminum tubing, if the current is applied in such a away and value it disloactes the lattice structure of the aluminum some what and told it makes it more brittle. Now, I haven't researched it to depth, but I beleive what the lab tests indicated after watching just current applied to the tubing in various amounts greater than the anodic process and later under the scope showed last minute surface cracks. I suggest maybe finding more on the subject since we (USA) has gone to these ISO standards, things are not coming back like they should from overseas. Look at the recent LEAD issue with the Mattel and China Toys. I Read an article that said they met ISO standards but what standards are there for paint with lead in the USA that meets ISO standards?

 

[dimachorny]

I have the following information about hard anodize influence on fatigue failure:

Aluminum / fatigue strength reduction
2024 / 30%
6061 / 60%
7075 / 17%
7178 / 65%
320 / 0%
350 / 0%

 

[klyde]

DimaC1974: This is interesting data. Do you have the source? We publish the Experimental Helo Magazine and would like to list this information and cite the source.

thank you Klyde

 

[dimachorny]

The source is a hard-copy of technical information from subcontractor that performing plating for aerospace industry and it is in Hebrew.