Carbon fibre resistivity

Search around matweb.com

(for example - not exactly what you want but for an idea: http://www.matweb.com/search/DataSheet.aspx?MatGUID=54e01b3246754b1191eccb73964c9449)

I suspect you'll find that carbon fiber will have a similar resistivity of a carbon resistor. So it will close a conductive path in a loop but instead of being the low resistivity of a metal, the current will dissipate power in the carbon fiber.

Having said that, I would instead be suspicious of the binding agent holding the carbon fibers together. You might find a hidden UV sensitivity breakdown of the polymer binding. It may even be organic (bacteria) having a meal. If you can, have the corrosion itself chemically analyzed. This will help identify if your problem is the fibers or the binder (resin).

This isn't going to help directly, but it may prove useful, nonetheless. The carbon fibers themselves are very conductive, but the bulk conductivity of the material is a function of the density of the fibers within the epoxy resin, which is a total insulator. The density is important because it determines what might be called the "mean free path" of how many fibers touch each other vs. total discontinuities between different "blobs" of touching fibers. You can see that if there are lots of discontinuities, the resistance will be higher.

Also, these sorts of materials are stacked in layers where the the "lay" of the fibers in one layer is at 90 degrees from those in the next layer. Conductivity will be greater in the direction of the lay of a given layer than in the opposite direction. Kind of like you work with the grain of the wood, not against it.

emc_c

But carbon is a semiconductor. It sits right above silicon on the periodic table. It can accept or give four electrons to complete its valence electron shell. Carbon fiber is nominally made of the allotrope of carbon known as graphite. The resistivity of graphite changes dramatically depending on the axis of conduction but the lowest resistivity, 1.2 μΩm, is an order of magnitude larger than the inverse of any metal's conductivity. (Copper conductivity ≈0.017 μΩm, Iron conductivity ≈0.095 μΩm) This is precisely what makes carbon a good material to make resistors from. So the electric power dissipation will be dominated by the carbon resistance and not the metal resistance.

I still think that this is a red herring though about how this composite material is failing. Carbon itself does not corrode. When carbon fiber material decomposes, the epoxy resin is falling apart from some unexpected agent. This is like claiming the reinforced concrete collapsed because the re-bar rusted inside the concrete.

My carbon fiber fishing pole hits me with large static sparks before thunder storms approach. A very unique experience (ouch!) and a useful early warning system if I'm not paying attention to the weather.

If I read your post correctly, your are concerned about corrosion of metallic parts/fittings due to the conductive carbon fiber structure interacting with the electrical system. As carbon fiber composites can vary a lot, I don't think you will find a single resistance value to work with. You obviously have a problem and a pretty wide range of resistance values will still be a problem.

An isolated electrical system might be the best option.

carbon fibers are conductive to varying degrees. You should ask the carbon composite supplier to get this data from his supplier. You can buy carbon fibers with insulated coatings to mitigate the conductivity problems.

The density of the fiber is also an aspect. To get the greatest density = greatest strength the fibers are wound tightly to minimize the % that is carbon free (the glue that keeps them together). The tight coupling provides paths for greater conduction

Some are not wound, they are made of woven mats that are stacked and wetted with glue, a vacuume drawn to eliminate air and a press used to compress the composite to the required density, while heat to cure the glue(epoxy or ??) is applied. The finished parts are then cut from this material or the flat plates used to make a larger composite, and sanded and glued.

The attachment of metal parts often uses an embedded force distribution plate, which might be metal, to which other parts are glued.

This is a complex task an the companies that are good at it have learned their skills over many years and want to sell you parts, not tell you how to make them. That said, there is a large body of knowledge available by searching.

http://www.google.ca/#hl=en&source=hp&q=%22carbon+composite%22+%2Bcorrosion&btnG=Google+Search&meta=&aq=f&oq=%22carbon+composite%22+%2Bcorrosion&fp=7cb5519368a1228f

http://www.google.ca/#hl=en&safe=off&q=%22carbon+composite%22+%2Bfastener+%2Battachment&meta=&aq=&oq=&fp=7cb5519368a1228f

http://www.google.ca/#hl=en&safe=off&q=%22carbon+composite%22+%2B%22electrical+conductivity%22&meta=&aq=&oq=%22carbon+composite%22+%2B%22electrical+conductivity%22&fp=7cb5519368a1228f

Carbon Fiber composites are not new to aerospace--where long term corrosion concerns are extreme. Ability to safely withstand lightening strikes are also a large concern in aircraft structures.

The proven method of achieving lightening strike survivability is to ember a conductive mesh (copper) in the composite to collect and distribute the lightening charge and not allow significant resistance heating.

Electrical system in composite structures must NOT depend on conductivity of the composite to complete any circuit. Resistance in composites varies significantly as section thickness, fiber density, and fiber specifics change for different strength plies.

Thanks all for the input.

The problem is with the metal (generally some sort of ally) fittings.

The surface of the laminate will almost certainly be insulating gel coat but, I suppose, the fittings are fixed through cut holes which will expose the ends of the carbon strands. As most of these fittings are sealed with mastic so I have no idea how much contact will be made.

I am certain that there is no intentional current paths over the laminate.

This link http://www.eng-tips.com/viewthread.cfm?qid=159174&page=8 refers to corrosion between aluminium and carbon fibres. Does anyone know anything on that score? It is a possibility I had not even considered. There is a major probability that an electrolyte, saline, will be present.

We will be doing the investigation on Tuesday and Wednesday. I will come back with the results

Chas

I've not had time to read through everything posted but could this be galvanic corrosion?

I work with the composite industry, mainly aerospace, and large scale carbon composite parts often have a "galvanic glass layer" either locally or over an entire surface to protect where metal parts will subsequesntly be bonded. Drilled holes are less easy to protect with metal fastener material selection being more important.

There is a lot of research going on in this area but it has enormous commercial significance in the aerospace industry so a lot of it is protected.