Lightning Strike Protection For Composite Structure 4

[tapirg]

We are using a prepeg bronze mesh as 1st ply in the lay-up of some composite panels for lightning strike protection purposes.

My problem is that we ran out of this prepeg material and we need to lay-up a nose cone using a dry bronze mesh instead.
As there is not resin in the mesh I have been thinking in bulking it to a glass fibre ply so we can conform it to the awkward shape of the tool and keep it in place.
(The normal lay-up for this nose cone would be this prepeg mesh followed by 8 plies of carbon fibre)
Is there any reason not to do that?
Is there any normal practice for lightning strike protection?

Any comments or advice pointing me in the right direction would be really welcome

 

[kchan711]

I'm not a lightning protection expert, but I understand the underlying principles of deflecting the strike. Seen one on a house and seen a Gaussian cage, that should cover the basics.

I'm assuming a strike to the nose cone needs to be grounded to another section of the "object" that has a larger electrical sink. With that said, I imagine your intention is to protect valuable equipment within the nose cone.

Combining the mesh within a glass ply will still perform as a Gaussian cage; however, the glass and resin act as insulators. The carbon fiber lay-up is conductive, at least up until the gel coat. With the bronze mesh combined with the glass-ply (I'm assuming you do have enough resin to at least wet this ply), you shouldn't have a problem getting the original electrical protection you wanted. The same principles still work. But I must say again, I'm not a lightning protection expert and have never combined a lightning protective mesh into a composite body. I've only built race bodies with composites.

Hope that helps.

Kyle Chandler

http://www.chiefengineering.net

 

[Compositepro]

First I thing I would ask is - don't you have to use qualified materials for aircaft construction?

I doubt that the mesh is bronze. More likely it is copper. What you suggest doing could work but you could also run into probems. Lightning stike mesh is usually embedded in an adhesive film. Adhesive resins are more flexible and less brittle than matrix resins. Without the adhesive film you can get micro-cracks between the mesh and the resin due to CTE stresses. This can lead to poor surface finish and corrion of the exposed mesh. The micro-cracks will propogate through the applied paint.

 

[spudpeeler]

Compositepro, unlike expanded foils, the mesh products are often bronze. The mesh is also commonly used with matrix resins instead of adhesives. I don't recommend either of these situations but it is often the case - mostly in Europe. Boeing tends to use expanded foils as do most other US applications that I'm aware of.

I completely agree that if you are making your own impregnated LSP materials adhesives are much better for the job. Also note that when adhesives are used in this manner they are not unsupported. There is a polyester mesh in the product and this mesh should be between the laminate and the foil to avoid galvanic interaction between the copper, bronze, or aluminum and the carbon laminate - no issue here if the laminate is glass.

 

[tapirg]

First of all, many thanks all of you for your comments.

The main Idea was to use as first layer one ply of glass fibre (or adhesive) then the dry bronze mesh and then the carbon laminate as normal.

The reason I need this first layer of glass/adhesive is that I need something tacky to be able to laminate and keep it in place this bronze mesh onto the tool. (Otherwise the dry bronze mesh will create lost of wrinkles and bridging.

I am not worried about galvanic corrosion for the first one as it’s just a sample but I have to admit that best solution would be lay one ply of glass in between the bronze mesh and so carbon to avoid this effect.

My only concern was if an outer layer of glass covering the bronze mesh will stop my component performing as a Gaussian cage.

Thanks again

 

[RPstress]

Any material up to about a mm thick will be blown off local to the strike. So, it's ok to 'bury' the mesh, but don't go too deep (a mm (40 thou) is about the limit). It is still an effective Faraday cage.

 

[spudpeeler]

Correct, the Faraday cage effect not being influenced by it's location to the surface.

But, the depth of the conductive LSP material is very important in terms of how concentrated the strike is on the surface of the laminate.

As the conductive layer gets further from the laminate surface, the damage to the laminate during the strike increases. Even the paint thickness is important to success of the system.