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Re: Carbon Fiber Composites -Do they age? [Re: Jeff Peterson] #258695
04/03/13 05:16 AM
04/03/13 05:16 AM
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Yes, Jake , but the surprising part for me was, that there's a whole new dimension in talking about the strenght of a laminate.

I started building windsurf boards about 35 years ago and had very much to cope with the tension-strenght because of the extreme loads on a board at sea and after jumps.

When carbon and kevlar were invented and on the market a few years later, we directly started using it in strips for enforcements. (It costed a fortune that days!).

But our expectations were soon belied, because the boards broke even so. Later on when fiber-clothes were cheaper and cheaper we could use full-board covering with carbon and the breaking point was shifted in time.
But in those time we never thought about fatigue in our design considerations.

I'm still reading in this e-book which Carl mentioned and, honestly, it scares the hell out of me.

I'm now sailing three years on my homebuilt cat which has hulls of 60-70% carboncomposite. With exception of the joints of the (carbon) beams with the hulls, the fiberclothing was laid in the lenght-direction of the hulls, so no 45 degree crossed layer. I would do that differently knowing what I now learned about fatigue.

One thing I did right: I have made the beams with a full wooden core and a lot of UD carbon! (ended with 10 kg for a 260 cm beam).

So, returning to the subject of this thread, when you count fatigue under aging, then fiber composites do definetly age. I'm still reading about the carbon ones.


ronald
RAIDER-15 (homebuilt)

hey boy, what did you do over there, alone far out at sea?..
"huh....., that's the only place where I'm happy, sir.
-- Have You Seen This? --
Re: Carbon Fiber Composites -Do they age? [Re: Jeff Peterson] #258730
04/04/13 06:31 AM
04/04/13 06:31 AM
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I'm thrown out of the site for reading this E-book: "Fatigue in composites". The computer says I reached my limit in reading for free. (book cost about 300 bugs).
So I have to live with what I found so far. Which is quite interesting.

In the book is stated that Hybrid composites ( so for instance glass layers mixed with carbon ones) are slightly more resistant to fatigue then pure mono composites.

I knew that already from my experience with carbon surfmasts. Pure 100% carbon masts break definitly earlier then f.e. 60% masts. I always thought that had to do with more vulnerability from the 100% carbon for impact accidents, but it seems to be fatigue!

So, you understand that I'm happy with my 60-70% carbon homemade cathulls.

Second remark is that they test fatigue in load-cycles with a minimum and a maximum load.
Now I've constructed my hulls (and beams) with a massive core , so the composite skins of the hulls are more fixed, they cannot move.
In a normal cathull the skin itself can bend a little when under load; hence creates a load cycle. (The old nacra's you could push in with your finger on the side)

I remember me the building of a hollow windsurfboard in the far past with only a few internal supports (like a cathull). It was made off 100% carbon and I jumped it in two halves after 3 months.

Last edited by northsea junkie; 04/04/13 11:32 AM.

ronald
RAIDER-15 (homebuilt)

hey boy, what did you do over there, alone far out at sea?..
"huh....., that's the only place where I'm happy, sir.
Re: Carbon Fiber Composites -Do they age? [Re: northsea junkie] #258734
04/04/13 12:42 PM
04/04/13 12:42 PM
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Houston
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Hybrid composites also resists crack propagation. Which I would also group with fatigue. The Gougeon Bro's book on wood boat building mentioned Kevlar added to a carbon layup would resist crack propagation. You can email DuPont and ask for more information. In the past they have been quite helpful. (Be sure to ask for composite layup information on all of their products) My carbon beams have 1 layer Kevlar for each 6 layers carbon. It was the Kevlar that held the beams together long enough to get to shore when the carbon failed.

Concerning your board failure issues, research resins. Epoxy resins may not all be equal. Resin suppliers will usually give you fatigue info.

In my opinion you have a right to be concerned about fatigue. It is rarely mentioned. Builders of commercial items will provide fatigue information if asked. Consumer product builders fall back on "We offer a lifetime or 20 year or ? warranty" which leads me to believe they expect you to sell or trash whatever they sold you before fatigue becomes a factor. I suspect one reason is if you had that information and repeatably used a product near it's limits and it wore out early, you could force them to replace it under warranty. Currently it is common for companies to refuse on the grounds that you abused their product.

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