Sealing carbon fiber tubes for deep water application

[pbass]

Hello,

I'm looking for an effective way to seal a carbon fiber tube with a plastic (probably Delrin) end cap and o-ring. The tube houses electronics and is used in a deepwater application. I'm from a machine design background and would normally have a retaining feature (i.e.step or groove) machined in the tube ID if I were dealing with metal. I don't have much experience with composites and have concerns that machining will compromise the fiber layup and overall compression strength. Anyone know if this has been done with success, or if there are other effective options (I'm thinking adhesives/expansion insert/compression fit).

Thks

 

[Compositepro]

Why not put the O-ring in the cap or plug? Porosity can be a problem in fiber composites. You may need to use a sealant coating on the tube and seal surfaces to prevent leaks. Machining a groove in a carbon fiber tube is someting you should avoid, if possilbe.

 

[pbass]

Thanks for the feedback. The o-ring groove seal is something I've considered, however the tube is tethered to a moving object via the end cap so the cap-tube joint will need to support about 100 lb of axial loading. Not sure if the o-ring press fit alone would handle that. Although perhaps with the right adhesive...will have to investigate.

 

[GBor]

You are towing a carbon composite tube in "deepwater" and it is only going to see about 100 pounds of axial loading? Either your definition of "deepwater" isn't the same as mine, or you have a lot more to worry about than the 100 pound towing force.

There are some great adhesives out there these days: GE, 3M, and many others. If you can glue it, that is probably the way to go. Machining the end is likely a very bad option.

 

[pbass]

Gbor,

Obviously there are high external pressure loads to be resolved as well. That is a separate issue impacting the tube structural design.

Thanks for your input. I'll look into the adhesive options.

 

[RPstress]

To a degree machining stuff is metalhead thinking. (Why put stuff in just to remove it?) A better bet might be to have a pad up and to mould the groove in, perhaps with a thin metallic sheet or maybe just a thick gel coat to make the seal surface (if the outside surface is tooled).

If you're intent on machining stuff, then padding up the tube OD with sacrifical material should permit the machining of features (e.g. your O-ring groove) into the sacrificial plies. This could then be resin coated or metal plated to give a good seal surface. This might be appropriate if you're just tooling the inside.

Another option might be to shrink or bond on a metallic ring with the groove in it.

Depends a bit on what your tooling options are to make the tube. (Will you tool the inside or the outside or both?)

You will presumably be critical for circumferential E and strength; however, bear in mind that the groove could give a significant stress concentration for the axial compression stress, even if it's in a sacrificial ply build up.

 

[Joshmo]

How about bonding a fitting on the end and have it contain what ever cap seal you need? With a single lap bonded joint have the composite on the inside so that during a pressure load case the composite can't peel away from the fitting. Also Suggest using an aerospace grade structural adhesive...

 

[GBor]

Most aerospace grade structural adhesives don't like salt water...

Garland
Former marine engineer turned aero-engineering manager

 

[pbass]

Well, we haven't finalized the tube material or size yet, just working with a few preliminary quotes. I assumed that the tube would be manufactured in long continuous sections with a fiber layup to suit our loading, then cut to our specific length. I'm not set on machining by any means. I just wouldn't expect that adding in features to the tube section at a specific pitch (e.g. moulding in a groove) would be cost effective. BTW, the reason for adding a groove to the tube ID would not be for a o-ring; it would be for an internal retaining ring or collar to conceivably allow one end cap to be attached with a threaded fastener. Being able to remove one cap without too much difficulty is desirable for diagnostic purposes.

..But the route I'm leaning towards now is to cut o-ring grooves in our end cap then press/shrink fit this in the tube ID with a lap joint and recommended adhesive.

I am a bit concerned about the comments about CF porosity though. Are there any grades of CF which are impervious to sea water?

Paul
Mechanical Design Engineer
Marine Electronics Packaging

 

[bobbywvu]

If you are looking for a good adhesive, try plyogrip from Ashland. It is polyurethane based adhesive and I used that to test composites for tension testing.

 

[Compositepro]

Porosity can be caused by air bubbles in the resin during part fabrication and a number of other reasons. It can also be caused by microcracking of the resin due to internal stresses between the fiber and resin. This is controlled by material properties and fiber ply thickness. It is possible to make void-free composite parts but it costs more for tolling and processing. Often it is simpler to seal the part after it is made. Most composite pressure vessels have internal liners. Gas molecules can difuse through most polymers and carbon fiber. It is only a question of degree.

 

[RPstress]

Re "impervious to sea water," carbon and epoxy won't generally be degraded significantly. However, all polymers have transport properties, and water will diffuse into a polymer matrix laminate. This isn't usually an issue, and if you're trying to control the internal conditions such as humidity, then condensation has a much bigger effect. However, I'm not sure what effect elevated pressure has on moisture ingress.