You are not logged in.
There is Asbestos, but the precautions required to use it before it is soaked with resin make it impractical in my view. It does, however, occur naturally on Earth and possible elsewhere so if the objections can be overcome it might be a simpler answer than making fibres from basalt.
Offline
Pressure vessels as built for all sorts of purposes here on Earth are either cylindrical or they are spherical. Deviations from that induce huge bending stresses, no matter what happens to the tensile stresses. In bending there is tension on one surface and compression on the other. It is quite easy to get very large values of bending stresses.
For pressurized cylindrical vessels, the ends must be rounded. The lowest-stress solution is the hemispherical end, although other considerations drive the typical pressurized tank car on the railroad to elliptical ends. Even hemispherical ends have a mismatched strain rate versus pressure with the cylindrical portion, so the joint has to be locally thicker to take the local bending. The higher the pressurization, the worse the stresses, including the strain-mismatch bending.
The end can be inverted (convex inward instead of convex outward), and still serve. You see this on the bottoms of welding gas bottles. The tops are sort-of hemispherical. Anywhere there is a local mismatch effect, there is necessarily considerable local thickening. You can see that on the welding gas bottle around the boss where the tank valve screws in. It's not visible, but there's a lot of thickening where the bottom attaches to the cylinder (those are usually monolithic spin castings, with the boss machined to a flat face and threaded).
Doing vessels out of composite material requires that you orient the fibers very carefully. Every direction there is tensile stress, the fibers must orient. Crossed directions reverts to matrix strength only. You have to play God to anticipate all the directions, which is why composite rocket motor cases had such a checkered history getting started (initially, nobody knew where all the load directions were coming from). It's still basically true of all sorts of composite structures; these require huge amounts of load analysis for all sorts of cases to find out where all the fibers need to go. So there is a giant up-front engineering labor cost for getting the higher strength-to-weight structure. There always will be.
Pressurized cylindrical structures, far from the ends, are dominated by tensile hoop stress at stress = net pressure x ID / 2 x wall thickness. For a spherical shape, the 2 goes away (lower stresses), and this is numerically equal to the longitudinal stress in the cylinder portion. So the cylinder wall is in a biaxial state of stress, and must resist both tensile directions (and all the associated shears) simultaneously. The motor guys get this with spiral wraps of yarn in two opposite directions, interleaved with straight hoop wraps of yarn. Many, many layers are required, with multiple spiral angles. But it’s easier than forming woven cloth onto those curved ends.
You can do a hoop-oriented-only composite overwrap on a metal shell otherwise too thin by no more than a factor of 2, and thus get a lighter structure that way. Could be yarn, could be woven cloth. As long as the composite overwrap stays intact, you're fine. If it gets damaged while you are at full pressurization -- blooie! And you still have to locally thicken the joints with the ends because of local bending from the strain mismatch.
Kevlar fiber has higher tensile strength for its weight than steel. That's why kevlar-based composite structures are lighter for the same strength. They are not as stiff, because the modulus of elasticity in the fiber direction is far lower. You are limited in the structural strain you can allow, because the resin matrix often has limited elongation to failure, and so does the kevlar fiber. But within those strain limits, it is very tough against fracture. Exceed them, and --- blooie! again. Happens quite easily with a rock impact, actually. Carbon epoxy is far, far worse about vulnerability to rock impact. It’s hard to see the damage, too. You must have X-rays (or other imaging) of the material at its production to compare with.
To make asbestos or basalt fibers into useful composite materials requires a spinning operation to make threads and yarns, and a weaving operation to make cloth out of those threads and yarns. These must be done with the dry materials, which will create a fiber dusting problem in the space where these things are done. It is an inherent risk.
It's known to be severe with asbestos (lung cancer), we have ignored the issue with glass (so who knows yet what that does), and we already know about silicosis when silica fibers are used. Just something to think about. These materials and processes can be used with low risk, but you don't just sit down on the living room floor in your underwear and do it with your bare hands and face.
I would think resins would be a premium import from Earth for folks building things on Mars. At least for a long time to come. Seems to me like these structures should be built here and transported there. I think the waste during construction would likely offset the weight saved by using composites instead of metals. You'll need more cloth and resin than is contained within the finished article. By far.
GW
GW Johnson
McGregor, Texas
"There is nothing as expensive as a dead crew, especially one dead from a bad management decision"
Offline
Thanks for a good intro to hoop stresses, GW.
Apart from resins, it has been found that basalt fibers need to be sized (coated with a film) to reduce fiber breakage which adversely affects the eventual laminate strength. I believe that a silicone compound is currently used for this. Quantities are quite small so import from earth might be practical.
I still haven't come up with a resin that could be made easily and in large quantities using available Martian materials, though.
Offline