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Re: [PSUBS-MAILIST] Fiberglass hulls
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.
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.
Regards Alan
----- Original Message -----
From: "Marc de Piolenc" <piolenc@archivale.com>
To: <personal_submersibles@psubs.org>
Sent: Thursday, June 03, 2010 3:29 PM
Subject: Re: [PSUBS-MAILIST] Fiberglass hulls
On 6/3/2010 7:07 AM, greg cottrell wrote:
Tensile strength in fiberglass comes
/mostly/ from the fibers- compressive strength in fiberglass /mostly/
comes from the resin.
Untrue. If it were so, then the use of composites in aeronautics would be
impossible, because the compressive strength of the matrix is orders of
magnitude lower than that of the very weakest fiber. The fibers do take
compression as well as tension, because they are braced against buckling
by the resin matrix. This makes fiber orientation and saturation even more
critical for compression than for tension, but this is a known problem and
it has reliable solutions. Read up on the use of unidirectional carbon
fiber on the compression side of a wing spar if you don't believe this.
Getting back to composites vs. steel, let's not forget that the
compressive strength of a material has little to do with its ability to
resist actual compressive loads in practical structures. What matters most
is stability against buckling, which is more dependent on stiffness than
on strength. This point can not be over-emphasized - the compressive
strength numbers in the handbooks have absolutely no relevance to our
work, because in real structures the material itself never fails in
compression. Composites have an advantage here over steel because they are
low-density materials, requiring more thickness to resist a given load,
and thickness is an advantage in stiffening the laminate against
out-of-plane distortions, or in other words in resisting buckling.
This is the main justification for sandwich panels and shells. Splitting a
laminate in thickness and separating the two half-thicknesses with a
low-density, bonded core does not improve strength one iota - the core
contributes nothing to tensile or compressive strength. It is used because
the sandwich increases stiffness against out-of-plane bending, which
allows the panel/shell to take a higher compressive load without buckling.
That doesn't mean that great subs can't be built using glass- the
British LR series subs were all GRP and certified by Lloyd's. Hawke's
first Deepflight was too. But the engineering is critical and way beyond
us "little guys"
Respecfully disagree. Composites require discipline and patience, but they
are not "rocket science." The people posting to this list all have more
than enough smarts to work with composites. Equipment does not have to be
expensive. The autoclaves used on production aeronautical work are for
consistently getting the highest possible ratio of reinforcement to resin,
to achieve the highest possible strength- and stiffness-to-weight ratio.
That need not concern us for subs. A little extra resin will cost a few
extra bucks and increase weight, but it will not reduce strength or
stiffness. Accuracy in layup is important, but again in our application a
little precautionary overbuilding won't hurt anything, unlike in
aerospace. Shape accuracy also matters, so some kind of form - male or
female - is highly desirable if not essential. A correspondent in Thailand
unknowingly showed me the perfect solution; he was asking what clever
underwater applications I could find for a surplus 20-foot tank container
that he owned. That triggered a brainstorm in which the tank became a
mold, used for manufacturing composite pressure hulls. Of course, I was
thinking of my favorite INorganic composite, ferrocement, but the same
principle applies.
I won't be using FRP for any project here in the southern Philippines
because importers buy large lots of resin and split them for retail sale,
so it is impossible to trace batch numbers or to know how long a
particular can of resin has been sitting on the shelf, or what conditions
it was stored under. The discipline that I mentioned in connection with
composites earlier has to extend to the entire supply chain, and it just
doesn't exist here, but if you can get traceable and dated lots of resin,
and can get fabric, roving or uni from a reputable supplier, there's no
reason to fear FRP.
References: On the general topic of structural stability I like Buckling
Strength of Metal Structures by Bleich. K.D. Wood's Airplane Design (I
have the 10th Edition) also has a good compact discussion of buckling in
connection with thin-walled (metal) structures. Bruhn: Analysis and Design
of Airplane Structures goes into more detail. For general composites
knowledge I have relied on The Hexcel Manual, published by the Hexcel
Corporation. For sandwich structures, Analysis and Design of Structural
Sandwich Panels by Allen (Pergamon, 1969). Of course there are many
suitable books - these just happen to be the ones in my collection.
Regards to all and thanks for your patience,
Marc
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