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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