ULA Axes

Steve, I also wonder that. And this tool is tempting. Either way though, the boundary between the metal and the composite is a stress concentration area, and probable first failure point. I considered these, and bought the 210gram UIAA B rated Camp XLA, for days I might (but probably won't) need a piolet.

Brett, the reason I ask is that there was a high failure rate of early carbon fiber bikes with aluminum lugs. (Trek was one of the early carbon bikes that got it right.)

Carbon fiber and aluminum creates a small electric field (crude battery) and causes galvanic corrosion of the two materials and failure at the lugs.

What Trek did, and my 1988 Trek carbon fiber bike is proof it works for the long run, was to isolate the two materials with fiberglass or plastic sleeve. The sleeve was glued to the carbon fiber tube, and then the sleeve was glued into the lug.

So to get to my point, if galvanic corrosion hasn't been handled some way… well you get the idea.

> Carbon fiber and aluminum creates a small electric field (crude battery) and causes galvanic corrosion of the two materials and failure at the lugs
I agree that there is the potential for a battery effect there (awful pun), but there is a lot more to it than that.

For a start, with a bike you may be able to get a full electrical circuit, while with a PT there is not much chance of that happening. So the two cases are quite different.

The aluminium head on the PT is hard anodised before assembly. The anodised layer of aluminium oxide is actually quite a good insulator, so the aluminium and carbon may never contact. Also, the anodised layer may (will) effectively block any aluminium corrosion.

While raw aluminium metal can corrode, I don't think I know of any corrosion mechanisms for carbon. So if the anodised aluminium head can't corrode, there may not be anything else to corrode.

The inside of the PT shaft may be epoxy or polyester (or similar): the carbon fibers may not actually be significantly exposed. This is sensible design. In this case you may not get contact either, and the carbon may not even be contactable on the inside.

If the head is glued into the shaft the glue layer may also prevent contact.

Carbon fibre and aluminium are used in composite fighter planes and also in the new A380 Airbus plane afaik. I am not aware of any significant corrosion problems being encountered due to this effect.

I have been using anodised aluminium joiners on carbon fibre tube for tent poles for some years. (I make them.) I have never seen the slightest trace of corrosion.

The biggest problem with aluminium and carbon fibre is the difference in thermal expansion. Easton composite arrow shafts are known to have some problems here, although I suspect the extraordinary shock loading and presure waves up the length of the shaft may be significant here.

Roger, thanks your insight on this. How, normally, is the difference in thermal expansion between the two materials handled?

The aluminum is hard anodized before the trowel heads and end spike are epoxied and pressed into the carbon shaft. I am using a carbon/aluminum specific epoxy to join the materials together.

Brian

Hi Steve

>How, normally, is the difference in thermal expansion between the two materials handled?
Ah, well, good question. But you are not going to like the answer, although it may be what you expected.
It isn't handled.

For arrows (and similar), the manufacturers just hope that the temperature range experienced is not large. Make the arrows at a low temperature and hope for the best. When the aluminium is on the inside of the carbon fibre, an increase in temperature simple makes the aluminium expand a little more inside the CF around it: no harm done. Not life-critical.

For composite tail fin assemblies on planes …. well, they do delaminate sometimes. So they have to be scanned regularly, to detect any delamination. This is not easy to do, and is expensive. Big hassle for the industry, and tail fins do fall off sometimes.

What you can do is to use a slightly compliant adhesive rather than a brittle one. This allows for a small amount of differential stress – without causing any significant loss of strength. For instance, many superglues are quite brittle, but Loctite have made one called 'Black Max' which incorporates microscopic bits of rubber. It is very shock-resistant, and I have used it myself on the packframes I made. Good stuff!

I note Brian says he uses an epoxy designed for the joining of aluminium (oxide) and carbon fibre fabric. This may have similar properties.

Interesting topic, I hadn't thought of this with the ULA product, but I have seen it a couple times like you said with bikes, mostly on Kestrels.