Can you use other materials instead of using fiberglass to cover your projects?

I remember my dad using silk. Now I have just learned about ripstop polypropylene. These materials seem to be very light weight but do they have the same strength as fiberglass?

From : Don Stackhouse

Yes and no. You have to watch out for the effects of elastic modulus (stiffness).

The fundamental idea behind fiber composites is that you have relatively stiff fibers (such as carbon, Kevlar or glass) being held in place by a relatively flexible (and weaker) matrix material (such as epoxy) holding them in position. The difference in stiffness between the fibers and matrix must be great enough that before the matrix has stretched enough to see much stress, the much stiffer fibers have already picked up the load.

If you don't meet this criteria, then the much lower strength of the matrix becomes the deciding factor in the strength of the part. For example, imagine a piece of concrete that is reinforced with steel rods. Concrete does very well in compression, but it's truly lousy in tension. However, since the steel rods are much stiifer, any tensile loads get picked up by the steel rods before the concrete has stretched far enough to suffer a tensile failure.

Now try replacing the steel rods with rubber rods. Before the rubber rods have stretched far enough to pick up any significant tensile load, the concrete has long since fractured and crumbled away.

Using silk or polyester fabric to reinforce epoxy runs into similar problems. The fabric is less stiff than the epoxy, so the epoxy does all the work. I think Burt Rutan said it best when he remarked (about Dynel fabric) that "it's a great way to hold the epoxy in place while it cures". The nylon tape that some companies sell for reinforcement suffers from the same problem. If you're going to use epoxy, better use a stiff fiber.

A way out of this is to use a much more flexible matrix material, such as butyrate dope. The problem here is that the density of butyrate is essentially the same as epoxy, but its physical properties are not as good (if they were, we wouldn't be bothering with epoxy!). It takes a lot more of this more flexible matrix plus these less strong and stiff fibers to get the same strength as a traditional epoxy composite with a good, stiff fiber. Despite the lower density of the cloth, you end up with a GREATER total weight required to achieve the same strength.

OTOH, if you're just looking for enough dent resistance, and just about anything that meets that criterion will also have enough strength in the other areas (for example, a wing skin that must have enough torsional stiffness to avoid flutter, but that will meet that requirement already if its dent resistance is sufficiently high), then low density (even at the cost of low modulus) for both the fiber and the matrix may be a valid approach. Things like dent resistance and resistance to buckling under compressive load are highly and non-linearly dependent on the thickness of the laminate. A low density combination of materials may end up with a greater thickness, and quite possibly a much greater strength in these parameters, even thoughit may not be as strong in its basic physical properties.

The success of Kevlar in fuselages for slopers is probably one example of this phenomenon. The strength of Kevlar in compression is truly atrocious, only a small fraction of the compressive strength of common E-glass. However, its stiffness is higher (which is a very important factor in buckling failures), and more importantly, its density is lower than glass. Therefore, it can be thicker for the same total weight, which helps it resist buckling better. A really well-designed fuselage that uses the full compressive strength of the shell (a crushing failure under compression instead of a buckling failure of the skin) would be stronger if made in E-glass, S-glass or graphite than in Kevlar. However, most failures of common fuselages are the result of compressive buckling, where the greater thickness of the Kevlar give it an advantage.

Composites in general give the designer a lot more freedom. Unfortunately, that also means there are more ways for the designer to get into trouble. It's essential to do your homework when designing with these materials!

Don Stackhouse
DJ Aerotech