Small cracks in wing spar web...

I have sent these photos to Vans but I thought I would post here as well to see if anyone out there has experienced this. I am attaching my wing ribs to my wing spar on my 9A. On the inboard 11 ribs there is a doubler that is a preassembled component on the wing spar. The rib attaches to the doubler then the wing spar and the whole thing is riveted together. On those ribs I did not have any cracking on the wing spar web. On the outboard 3 ribs (didn't attach the outboard rib yet per instructions) there is no doubler. It is just rib riveted to the wing spar. On those ribs there is a "spider web" looking series of cracks emanating from the hole. There is primer under the rivet (that is the gray stuff you see around the rivet), the holes have been deburred. Is this normal and why is it happening? I used a 3X rivet gun with an offset rivet set. Due to the offset set the gun was set at 55 psi.
The first two pictures are of the outboard 3 ribs without the doubler. The last picture is of one of the inboard 11 ribs that has the doubler.

Yep, normal. Build on!

These are cracks in the relatively brittle surface alodine....err, oops, anodizing layer (the yellow-gold color). The cracks do not go through to the aluminum below. As Jeremy said, it is normal.

minor correction

Stevea said:

These are cracks in the relatively brittle surface alodine layer (the yellow-gold color). The cracks do not go through to the aluminum below. As Jeremy said, it is normal.

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It is anodizing, not alodine. Anodizing does produce a fairly brittle aluminum oxide layer. The radial stress from expanding the rivet caused it to crack as the underlying ductile material stretched.

Stevea said:

These are cracks in the relatively brittle surface alodine layer (the yellow-gold color). The cracks do not go through to the aluminum below. As Jeremy said, it is normal.

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It is anodized, not alodined. The anodizing is very brittle and cracks when the rivets are set.

Edit: I was posting at the same time as Steve. His posts are always more intelligent than mine, so....what he said

Yup - normal - it's just the anodizing layer cracking a bit. Build on.

Official update from Vans

Thanks to all who replied. As it is, you are all correct. Vans Aircraft rep Joe Blank emailed back immediately this AM (Vans support and customer service are second to none).
His reply is as follows:
Aaron,
What you are seeing there is the anodizing spiderwebbing. Since it is a harder/brittle material, it fractures more easily. No cause for alarm as the structural material is just fine. The anodizing is merely a corrosion control agent. If you want, dab some primer on those spots if it concerns you.

Best Regards,
Joe Blank

Again, thanks to all who replied and to the great tech support department at Vans for prompt answers.

airguy said:

it's just the anodizing layer cracking a bit.

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Is it? The oxide layer is fused to the metal creating a continuum for the stress/strain. When it cracks due to tension/bending/compression it will create stress peeks in the metal along the cracks. These peeks will be way higher than the same material without a cracked oxide layer. A crack is a crack. When you breach the continuum of the stress/strain with a crack, you create a singularity where the stresses are only lowered by the material yielding.

I'm not saying it will do any harm in any practical sense, but it certainly is not just the anodizing layer cracking a bit.

SvingenB said:

Is it? The oxide layer is fused to the metal creating a continuum for the stress/strain. When it cracks due to tension/bending/compression it will create stress peeks in the metal along the cracks. These peeks will be way higher than the same material without a cracked oxide layer. A crack is a crack. When you breach the continuum of the stress/strain with a crack, you create a singularity where the stresses are only lowered by the material yielding.

I'm not saying it will do any harm in any practical sense, but it certainly is not just the anodizing layer cracking a bit.

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You're right - you got me. You should ground your aircraft immediately until you repair all these cracks on your spar.

Speaking from a metallurgical perspective - yes, it is just the anodizing layer cracking. The aluminum alloy beneath does not crack under this strain, it flows plastically. The anodizing layer on the surface is only microns deep and is much more brittle - it cracks under the plastic flow of the material beneath it. The separating distance between the anodized alloy and the virgin alloy is at best a micron or so, the two layer essentially act independently. In any case, the anodizing is nothing more than a corrosion protection layer, not a structural requirement. If the cracking of the anodized layer bothers you, prime with zinc chromate and you've covered all the bases.

SvingenB said:

Is it? The oxide layer is fused to the metal creating a continuum for the stress/strain. When it cracks due to tension/bending/compression it will create stress peeks in the metal along the cracks. These peeks will be way higher than the same material without a cracked oxide layer. A crack is a crack. When you breach the continuum of the stress/strain with a crack, you create a singularity where the stresses are only lowered by the material yielding.

I'm not saying it will do any harm in any practical sense, but it certainly is not just the anodizing layer cracking a bit.

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In the world of fatigue stress analysis, some companies may reduce the fatigue stress allowable of a part by up to 25% if it is anodized. The specific amount of reduction is very situationally dependent, and varies from company to company. So yes, anodizing has a negative impact on the lifespan of a part.

The anodizing is merely a corrosion control agent. If you want, dab some primer on those spots if it concerns you.

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I would.

Looks like a prime location for crevice corrosion to start. Prime it and it'll be protected at least.

CATPart said:

In the world of fatigue stress analysis, some companies may reduce the fatigue stress allowable of a part by up to 25% if it is anodized. The specific amount of reduction is very situationally dependent, and varies from company to company. So yes, anodizing has a negative impact on the lifespan of a part.

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This has been covered by Vans many years ago. It was determined that the increased corrosion protection FAR outweighs the negative impact of the anodizing.

Mel said:

This has been covered by Vans many years ago. It was determined that the increased corrosion protection FAR outweighs the negative impact of the anodizing.

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No argument from me Mel. I can think of one company that anodizes then applies a 15-25% life knockdown to every part, so they like it enough to make their parts heavier. It is fine if it was considered by vans like Mel states. I just wanted to point out, some big company testing has concluded that anodizing lowers fatigue life, and it is attributed to its contribution to crack initiation. There is probably more to it, but they don't go into molecular detail in the documents that I read.

I don't remember the specifics but I believe that while in theory the anodizing reduces fatigue life, in practicality, that lifespan is still well beyond that of most GA airplanes that don't fly 100 hours a month; and certainly well beyond their pilots' expected lifespans. I'm sure somebody with experience will chime in.

Chris