Spider fracture around rivets

I was looking over my QB wing while deburring the holes for the bottom skin. I was using a flashlight and I noticed on the top of the spar where the tops skins attach there are spider fractures around most of the rivets, not so much the line of rivets closest to the spar web but the ones out on the flange. You can't see them if you look straight on to them, just when looking at an angle. The picture is taken in macro mode so it shows every detail on a magnified scale, click on the image to get the big view.



Is this normal? I can't believe so many of them would be this way if this was not normal I just want some reassurance. It appears to me to look like varnish that has cracked, just at the surface not into the metal. I will be sending this to Van's for there confirmation. Why does this happen, are the rivets set wrong? Thanks in advance.

Cheers

Perfectly normal. They all have em....

Yeah, that's normal. I think its the hard anodized coating cracking, not the underlying aluminum.

I recall reading several posts stating that Van's has said this is normal and not a problem.

Normal

It is normal as others have already stated.

Also....
Slow builders will hear a very disconcerting "squeaky crunching" sound as they squeeze a rivet in the spar. It gives me the "willies" even though I know it is just noise from the .001 thick aluminum oxide conversion coating.

This happens because the spars are anodized. When you set a rivet, you stretch the aluminum around the rivet slightly. However, the anodized layer doesn't stretch well since it's a ceramic; Aluminum Oxide, AKA Alumina.

One of the reasons that aluminum is such a great material for aircraft is that it forms a thin layer of alumina on its own as soon as it's exposed to the atmosphere. Anodizing basically just speeds this process up, so that the ceramic layer is much thicker. The reason our skins are AlClad is because aluminum looses this self protecting ability when alloyed with copper (2024 is aluminum alloyed with copper). Anyway, the thin layer of pure aluminum on our AlClad skins allows the sheet to protect itself with that thin alumina layer again.

Thanks and another ?

I kind of thought this was the case after I thought about it for a while. I have another question for anyone who has dealt with the QB wing. The spar holes come countersunk and there is a green substance for protection on there, looks like some type of lacquer, not primer. Should I remove this before riveting or just leave it as is?

Also I nicked some of the holes on the spar when drilling the skin, should I treat them with some thing? this only occurred on a couple of holes before I realize I did not need to drill through since the spar holes are already drilled out and countersunk.

In my book (Corrosion of aluminum and aluminum alloys by ASM International) this cracking is reported to be the problem with anodizing. In stressed materials anodizing does not prevent stress corrosion cracking, SCC. The reason is that the cheramic layer cracks up due to stress, and leave the material wide open for further SCC. It can even accelerate SCC compared with bare unprotected aluminum.

Tests done in seacoast environment with higly stressed samples showed that all the anodized samples had failed after 2 years. Ordinary Zinc Chromate primer had a survivability of 40% after 2 years, while epoxy paint had survivability of 75%. Shoot peened samples had a survivability of 85% and all samples that where shot peened and coated with epoxy survived.

When the cheramic layer cracks even before the spar is used (already when riveting), then why use anodizing? Pure epoxy paint, or rattle can zinc chromate primer is a much better alternative.

Yup

Agreed. Therein lies the reason for my "willies", Svingen.
The only thing that makes me sleep well is that there are a LOT of these anodized spars flying a lot of hrs. Stress levels in the spar must be low enough to prevent propagation.

This was researched back in the early '90s by Van himself and it was determined that the advantages of the anodized spar far outweighed the disadvantages.

Mel said:

This was researched back in the early '90s by Van himself and it was determined that the advantages of the anodized spar far outweighed the disadvantages.

Click to expand...

The disadvantage is that anodizing offers no protection against SCC, which is what you want to protect against in highly stressed structural parts. What are the advantages?