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Re: [PSUBS-MAILIST] Fiberglass hulls
Hi Marc,
What I was getting at by mentioning the weave, was that apart from
what type of cloth, what weight, what fiber coating, biaxal, triaxal & a
multitude of
other configurations, there's the weave that figures in compressive strength
that needs
consideration.
In defence of the "metal heads"; a 6-10 ounce woven cloth requires 40-50
plies to acheive
a one inch thickness. The fibreglass sub I posted a referance to recently
was 2 inches thick.
This was reinforced with a steel band after several scale models colapsed
under pressure.
So as you've said, you have to build a mold first; then lay up 80-100 layers
of fiberglass
for a sub thats not overly big.
& of course you need reinforcement rings at various places, wich will be
bulkier, more
difficult to make & be of a larger diameter than their metal counterparts.
As I've said I'm making my sub as small as possible & my full scale model
will be my mold.
That's why I'm looking at fiberglass.
Regards Alan
----- Original Message -----
From: "Marc de Piolenc" <piolenc@archivale.com>
To: <personal_submersibles@psubs.org>
Sent: Friday, June 04, 2010 2:12 PM
Subject: Re: [PSUBS-MAILIST] Fiberglass hulls
On 6/4/2010 9:10 AM, Alan James wrote:
Thanks for your support Marc.
I'm printing that email out because it was a good summary of what I'd
read on the subject.
Additionally the weave is important because where the fibers cross each
other they bend
& create a point of weakness.
That's not the right way to look at it - there are no "points of
weakness." What any crinkle in the fibers signifies is that you have to
apply correction factors - available in the literature - to the strength
and stiffness of the resulting laminate. Similar correction factors have
been used for nearly a century to predict fabric strength from yarn
strength. Fabric being the thoroughly standardized material that it is,
those correction factors are highly reliable and can be used for design.
Many designers - especially amateurs - make the mistake of specifying
unidirectional tapes because of the very high strength and stiffness
achieved under ideal conditions, not realizing that minor errors in
workmanship carry a high penalty with uni that would not even be
noticeable with fabric. Better by far to specify fabric and accept the
lower allowables.
Fiberglass is the " F " word on the Psub site. Mainly because safety is
a big part of what the
group is all about. Ironically you need to know how really bad
fiberglass is & what the pitfalls
are before you should consider using it. I've learnt enough to know I
don't know enough &
that's why I want to talk to a composites expert; & if not too
prohibitively expensive, get my
design analyzed & a lay up designed. With some great programs out there
it should make fiberglass
safer & more viable.
There's not much to say to this. If a Psubs member or anyone else doesn't
want to use FRP, he's entitled to his preference, but in purely objective
terms composites make more sense for small submersibles than metals, for
the reasons that I have already given. As for learning the pitfalls of a
material, that is indisputable advice, but it applies to ALL materials,
not just composites. I have a strong feeling that people who assume that
metals are the only way to go are seeing them through rose-colored
glasses, simply because they are the established technology and there
doesn't seem to be any need for research.
Best,
Marc
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