Why aren't Spectra and S-glass used very much?

From : Don Stackhouse

Paul,

Actually S-glass is used quite a bit. It has been used for many years in helicopter rotor blades; in fact, in that application it was one of the first composites used for primary structure in an aircraft. It has impact strength rivaling Kevlar but has excellent compressive strength (unlike Kevlar, whose compressive strength is less than half that of ordinary E-glass!). We've used quite a bit of S-glass in Monarchs, particularly the 2-meter. The big reason you don't see more S-glass (besides its unfortunate lack of publicity) is that its density, like E-glass, is higher than Kevlar or carbon fiber. Its stiffness is also lower, although still better than E-glass.

Depending on the design, these properties may or may not result in a heavier structure. For example, in the case of helicopter rotors, one of the toughest design requirements is making them strong enough to support their own weight when they aren't turning (centrifugal force holds them out straight in flight). A Kevlar rotor blade would require a huge amount of material on the lower spar cap to carry the high compressive loads while parked, while an S-glass blade can use a much thinner (and lighter) lower spar cap.

The lower stiffness of S-glass can also be used to an advantage: it can make the laminate less sensitive to stress concentrations, and in the case of an impact (such as the loads on a fuselage in a "dork" landing), the lower stiffness allows the structure to deflect farther, which reduces the loads by reducing the severity of the decelleration.

Spectra has a lot of wonderful and amazing properties, but also some terrible shortcomings. The principle difficulty is its creep properties (also called "cold-flow" and "relaxation"). This means that if you put it under a sustained load it will gradually distort, stretching or compressing depending on the nature of the load. This isn't a problem for fishing line, you just have to turn the crank a little farther. On the other hand, an aircraft component that won't hold its shape is a serious problem. How would you like to have a fuselage with a tail boom that warped a few degrees due to gravity while sitting on the shelf through the winter?

The bottom line is that composites have much more complex behavior than conventional materials, which can create opportunities for the designer, but can also create more ways to get into trouble. You have to thoroughly understand the situation and do your homework when designing with composites!

Don Stackhouse @ DJ Aerotech