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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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