DSK wrote:

The navigator© wrote:


So you are saying it's Hookian until failure ?
If you look at a mast, CFRP has far more flex than Al before failure.


Nope, sorry, wrong again. Aluminum can bend & stretch a lot further before breaking than
carbon fiber can. You either must be thinking of load cycles to fatigue, or else you're
just making up malarkey to sound impressive.

Must I rub your nose in your ignorance? Why do you think windsufer masts
are made of carbon (before that GRP) and not Al? Have a look at the flex
of some unstayed carbon masts!



Carbon fiber also requires an order
of magnitude (or thereabouts) greater skill & preparation & tooling to make good
secondary bonds.

The bond in all FRP is plastic to plastic. There is no difference
whatsoever in that regarrd for CFRP.


You're saying that one can wet out and lay up carbon fiber just like fiberglass?

You can (and I have for some special local reinforcing) but most use
prepreg these days for big jobs.

Cheers MC

DSK wrote:

Martin Baxter wrote:


I was under the impression that CFRP parts had to be baked in a kiln. I am
curious
as to why you can't just wet it out and let the resin/epoxy cure like we do with
glass?


There are a couple of issues, one is that because of the surface properties of the carbon,
resins don't like to adhere to it and will not penetrate the weave unless forced through by
pressure. Some resins will not work at all with carbon fiber.

More BS. The key is that you really should use epoxy for it's superior
adhesion peoperties to the fiber so that it will transfer more load to
the fibers before bond failure but polyester type resins will work and
are used. The result will be a lower ultimate strength FRP but with a
greater deformation before failure. The failure mode is then cracking
between fibers and bonding rather than fiber breakage. Thgis allows more
slippage and energy dissipation before complete failuree. Given the
expense of carbon over glass, it makes sense to exploit it's strength
fully by using epoxy.


Another is that the relative density of the resin and the cloth is out of whack, and the cloth
will "float" itself right out of the resin if allowed to.

Good lord.


Cheers MC

Noooo Doug says you can't do that!

Bwhahhahahahaahah

Cheers MC

Capetanios Oz wrote:

On Wed, 19 Nov 2003 12:09:20 -0500, DSK
wrote:

There are a couple of issues, one is that because of the surface properties of the carbon,
resins don't like to adhere to it and will not penetrate the weave unless forced through by
pressure. Some resins will not work at all with carbon fiber.


We used a lot of CF mat, laid it up same as fg mat.


Oz1...of the 3 twins.

I welcome you to crackerbox palace,We've been expecting you.

The navigator© wrote:

Must I rub your nose in your ignorance?

You can try, but you better check your upper lip first.

DSK

Capetanios Oz wrote:


We used a lot of CF mat, laid it up same as fg mat.

Well, I only know 1- what I've been told 2- what I've read 3- what I've seen with my own eyes.
I have not ever built anything from carbon fiber, so that rules out #3. From what seem to be
credible sources, you can do it but will risk a higher rate of voids & poor bonding.

Now, if you were talking about resin-infused molding, then perhaps it's OK to "lay it up just
the same" as FG. The guys who are doing quite good work in CF say you should at least vacuum bag
it to get any of the benefits of using CF in the first place.

Maybe they've been doing it all wrong?

Fresh Breezes- Doug King

The purpose of vacuum bagging is to increase the ratio of fiber to
polymer. As far as voids go, the best possible layup is still by hand
-assuming the men doing it can be bothered to wet out and roll it out
well enough (and that is usually a big problem in FRP!). Vacuum bagging
does not prevent voids -in some cases it can make them much worse.

Cheers MC

DSK wrote:


Now, if you were talking about resin-infused molding, then perhaps it's OK to "lay it up just
the same" as FG. The guys who are doing quite good work in CF say you should at least vacuum bag
it to get any of the benefits of using CF in the first place.

Capetanios Oz wrote:


Poor technique will lead to poor bond and voids in any layup.

True, but with CF you waste more money and risk more dramatic failure.


Carbon is worse because you can't see when it's been wet out properly
so much more care is required.
We tend to use a slower brew to give more working time especially in
more intricate shapes.

but... but... but... the greatest expert in the world assures me that it can be laid up just like
ordinary fiberglass, I'd have thought that meant with the same resins & technique!


Prepreg is the way to go BUT it's expensive, can't be stored for any
great period and works best when cooked.

I looked into prepreg for our dinghy, it would be scandalously expensive and only a few ounces
lighter. For bragging rights, it ain't worth it (in this instance).


Baggin will give a better result in any layup allowing use of less
resin BUT you still need to wet out completely or you'l have problems.

Sure, especially with any type of core. Much better core-skin adhesion with vacuum bagging.


Maybe they've been doing it all wrong?

Nope, but there are more ways to skin a cat.

Is that another bad pun?

The stuff I'd like to try working with is honeycomb core & carbon & kevlar. The strength to weight
figures are almost unbelievable.... but it has to be done just right.

Fresh Breezes- Doug King

Capetanios Oz wrote:

Same resins can be used but as you said before epoxy is best..harder
still to wet out though. Technique is identical, jut much more
thorough.

I think I see... you *could* do it exactly the same, but anybody with a lick of sense goes the extra
yard to get good results.


I looked into prepreg for our dinghy, it would be scandalously expensive and only a few ounces
lighter. For bragging rights, it ain't worth it (in this instance).

Yikes, great if you can get offcuts...for free.
Seriously though, for a dinghy the weight saving is just not worth he
cost, you won't skin it any thinner and the only place the carbon
would be marginally useful would be at towing and rowlocks.

Well, the weight savings can be noticable even in a 9' dink. According to figures, the hull of our dink
will weigh a tad under 11 lbs; 4.8kg. Now, adding seats, hardware, especially the rubrail, will add in
some weight, but it will still be at least 30 lbs lighter than a 'conventional' dinghy... and I have
some tricks up my sleeve to get the thwarts & gun'ls weight down. I expect the whole pkg to come in
around 25 lbs minus the oars & sailing rig. Splashing the dink or getting back aboard will be a very
quick & casual operation, which is the way we want it.


Sure, especially with any type of core. Much better core-skin adhesion with vacuum bagging.

Secret with any core is to prefill the surface to reduce resin use and
improve adhesion.

Do you use a bonding filler? I have used WEST epoxy mostly, and like their bonding filler very much,
IMHO it at least doubles peel strength.


Used kevlar in a couple of HP dinghys a while back.
First had an ally space frame to take the rig loads, 2nd carbon tubes.
They sounded like drums when tensioned up.

Snare drum or kettle?

Fresh Breezes- Doug King

I think I see... you *could* do it exactly the same, but anybody with a lick of sense goes the extra
yard to get good results.


Capetanios Oz wrote:
Yep, that's what I said.

So, I was wrong... a clear cut victory for Navvie©


........ I expect the whole pkg to come in
around 25 lbs minus the oars & sailing rig. Splashing the dink or getting back aboard will be a very
quick & casual operation, which is the way we want it.

Still don't know if that's worth the expense though.

To me it is, but then I'm weird. Long after the money would have been spent on something equally
useful/useless anyway, we'll still be using the dinghy.


Snare drum or kettle?

Snare of course.
Cold moldies sound like kettles.

Wood boats have a special sound, sort of a 'whoosh.' It's beautiful to hear but for the most relaxing effect
it should be enjoyed on OPBs.

Fresh Breezes- Doug King