Composite Dash - Good Idea or Bad Idea

[theonlyred16v@hotmail.com (Brian Haygood)]

If it is built well, it would not crack.  By built well, I mean a VF of 55% 
or less should do it.  V.F. is volume fraction, and refers to the amount of 
resin in the fiber/resin mix.  Something made out of random chopped mat, 
like most composite parts (fiberglass boats, the reinforcements for the 
carbon hoods at Cincy this weekend) usually have a V.F. in the 80% or higher 
range.  A really smoothed out compoosite using a woven cloth (like the 
bidirectional (I think it was) cloth used to make the top of Josh's hood) 
will still have a high VF to get it that smooth, but that is only at the 
surface.  If its really thick this won't affect the overall deal as much, 
but we aren't talking about thick stuff.  The parts I made at my last job 
were running about 50% VF.  They did not have surfaces that were smooth as 
glass, but were textured a good bit, with texture depending on the 
production method.  Of course the mold sides were decently smooth, but not 
crazy smooth.

The reason this matters is that material properties for composites are 
related quite strongly to their constituents and their raitos.  In general, 
epoxy resin is really brittle stuff.  Composite fibers are really flexible 
(that's how they can be woven into cloths, afterall) and tough (and strong 
per weight, etc.).  So if you have something that is 95% resin, it cracks 
easily, and if it is 10% resin, it is basically still cloth.  Low VF 
(percent of resin, remember) composites are strong for their weight, but 
only in the directions of the fibers (just like the cloth of which they are 
made).

So I'll get to the point soon.

So high VF will cause the part ot be brittle, but this isn't just true of 
the overall VF, but if any areas are high in resin, they will be brittle.  
If you have a puddle of resin in a corner, cracks can start there easily, 
and even though the same amount of cloth is under it, the crack can spread 
from there.

So here's the point.  If your buddy can make composites of high structural 
quality, then your dash won't crack unless the car is totalled.  Any monkey 
with the right shop set up can make you a carbon dash that would crack under 
vibrations or during installation.

If you really don't want it to crack, make the dash with a vinyl (not 
vinylester) or ABS plastic material as the resin.  That resin isn't brittle 
at all.  You could use a clear ABS with two sheets of biderectional carbon 
cloth to make a dash you could flatten without breaking, but then clear ABS 
isn't really clear, is it.

Of course, whatever you make it from, be sure that it is UV protected out 
the wazoo (since dashes, like hoods, get lots of UV).  That means several 
clear coats, or preferably colored paint, of high quality stuff.  To keep 
that from cracking you might need a little flexitive (is that what they call 
it?) - the stuff they mix in paint that goes over bumper covers.  That's 
assuming you want a really thin, lightweight dash.  If it's thick enough to 
be as strong as a stock dash you might not need that (i.e. you need the UV 
protection, but not the flexitive).  While you are at it, mount it with some 
rubber washers or something.

HTH.

Brian Haygood

PS:
I'm a mechanical engineer, and my last job was at a place called "Automotive 
Design & Composites", where I designed, analyzed, and helped build prototype 
composite parts for structural applications in automobiles. I'm not an 
expert, but I picked up a few things.


>From: "Michael Abatzis" <abatzis2@hotmail.com>
>To: scirocco-l@scirocco.org
>Subject: Re: Good Idea or Bad Idea
>Date: Mon, 10 Jun 2002 21:09:47 +0000
>
>i'm thinking it would be too stiff--too much vibration, too hard to fit,
>still might crack/break. i do have an almost crack free dash though if yer
>interested off list.
>-m
>
>>From: "Adrian Culda" <adrculda@sympatico.ca>
>
>>I was just talking to a friend off mine at the local stereo shop and told
>>me
>>that he can pretty much make anything out of fiberglass or carbon fiber ..
>>that just gave me a idea ...
>>as it is im having a lil problem on trackin down a crack free dash ... (
>>see
>>where im going with this . :P  )
>>how does a CF Dash sound to u guys ( and because im his friend .. i only
>>have to pay the materials and a case of beer ) ??
>>Is it worth it ???
>>
>>
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