What causes this pattern in stretch formed Aluminum skins?

[KirbyWan]

These stretch formed aluminum skins on a V2500 translating cowl inner panel have this odd pattern. It was first written up as corrosion, except we have seen the pattern on the same area of multiple panels and it doesn't seem to be corrosion. The primer is adhering well an where the primer was removed using a chemical stripper it has a nice golden color of a chemical conversion coated surface. Could this pattern have been caused by forming? I've never seen it before. See pictures in the attached word document.

Thanks,

-Kirby

Kirby Wilkerson

Remember, first define the problem, then solve it.

 

[MOHR1951]

I have seen and noticed this before,also.When I first saw it,I to thought it to be some form of corrosion,too.But as you point out,no other signs of corrosion exist.It has to be associated with the conditions it's exposed to while in service,possibly a combination of heat or temperature changes along with vibration,creating a tempering affect.I had seen it on the dove tails aft of the exhaust pipe,on the upper side of the wing,on Navy Convair 240,340 series aircraft that we were doing IRAN services on.The Navy and the company inspectors both looked at it,and all signed off on it,so I'm assuming it was nothing of consequence,and just cosmetic in nature,but it sure got a persons attention.

 

[KirbyWan]

If there was any change to the surface after the panel was primed, I would expect to see paint adhesion problems, or at least paint separation where the surface of the aluminum changed over time, something that would look like paint crazing. This makes me suspect that this condition was present when the panel was primed. But I could be wrong. This skin is internal. the solid back skin to the inner acoustic panel and the area is covered by the composite outer panel. And the TR sleeve is not normally exposed to high heat in this area.

Thanks for your response,

-Kirby

Kirby Wilkerson

Remember, first define the problem, then solve it.

 

[Compositepro]

I've seen a similar surface texture develop when bending 6061 aluminum tubing. It is caused by the grain structure of the aluminum being revealed when the aluminum is stretched. The aluminum between the grains yields and thins, while the grains do not yield.