aluminium fittings on carbon structures - Problems

[daiquiri]

Lausl, hope you don't mind if I reply - again with reference to aeronautical field.
Please check the Chapter 6 of the FAA's Advisory Circular 43.13-1B "Acceptable Methods, Techniques, and Practices - Aircraft Inspection and Repair", which is very explicit when it comes to use of graphite pencils on metallic surfaces:
http://rgl.faa.gov/Regulatory_and_Gu...apter%2006.pdf

In particular, check out the following items:
- Section 1, Article 6.2.a(11)
- Section 6, Article 6.91.c

I'm pretty sure there will be a similar document in the marine field, and perhaps Ad Hoc or Jehardiman could give us a hint.

[Ad Hoc]

Originally Posted by daiquiri

I'm pretty sure there will be a similar document in the marine field, and perhaps Ad Hoc or Jehardiman could give us a hint.

It is "common" knowledge, as landlubber has already noted, in the marine feild. Which is why when working/fabricating with ally, one does not use a pencil for markings, nor any metal for scoring too (fatigue cites). Correct method for marking etc is using a permenent marker type felt-pen. Surveyors like red coloured ones, funnily enough

[AndrewK]

[quote=CDK;412338]Very interesting.
Can you or any other member put 2 test pins on a carbon laminate and tell me the surface resistance and the distance between the pins?

30 ohms over 1000 mm, laminate is a 400g double bias.

[wardd]

on aircraft dissimilar metals have tape between them and have a bonding strap to bring them to similar electrical charge

[CDK]

[quote=AndrewK;412810]

Originally Posted by CDK

Very interesting.
Can you or any other member put 2 test pins on a carbon laminate and tell me the surface resistance and the distance between the pins?

30 ohms over 1000 mm, laminate is a 400g double bias.

Thank you AndrewK.

It means several things:

1.The surface layer is not a resin but exposed carbon.
2. Lots of products sold as carbon fiber, like fishing rods and motor cycle parts only have the texture but do not conduct at all, so they are probably something else.
3. The resistance is not low enough to provide a ground path in an electrical system, there would be a voltage drop of 30 volts @ 1 amp. But as material for a featherlight satellite dish it would be suitable.
4. At short distances the resistance is low enough to cause galvanic action on wet metal fixtures even when an insulating washer is used. The only way to prevent that is probably to seal the contact area both under, on top and around the screw or rivet with a flexible compound.
Or only use glass reinforced plastic fasteners.

[Herman]

It is common practice to insulate all metallics from carbon. This can be done in several ways:

-paint-like substances
-laminate glass over places where metal is placed
-instead of using fasteners, consider using glue. A very good glue for carbon, glass, aluminium, stainless, and many more, is Plexus (www.itwplexus.co.uk).

For more info on conductive rubber, I can suggest the following link: www.timcal.com
They make the carbon sooth that is used for instance to make plastics anti-static. we sometimes use it it make antistatic gelcoats, or flooring compound.

Super Conductive Furnace, SCF, N 294 (ASTM code), is the correct technical label for that stuff.

Richard

[Jimbo1490]

Originally Posted by PAR

I would think naval aircraft would need to address this issue. Living aboard an aircraft carrier can't be a pleasant experience for many of these material combination.

Almost all of their aircraft (certainly the jets) would be considered horrible 'maintenance hogs' in any commercial setting. The Navy address these problems with enormous amounts of hand labor, with constant inspection and meticulously re-working any flaw they find. Without this kind of 'beehive' level of attention, they'd probably last a couple of months at sea before becoming unairworthy.

Jimbo