In your technical analysis of Kevlar (R DuPont) vs. fiberglass, it's important to know that nothing sticks to Kevlar

In my 16 years as an applications engineer at DuPont Engineering Thermoplastics (retired three years ago), that was the problem with using it as a lighter-than-glass reenforcement in injection molding with nylon, Delrin (R DuPont) or Rynite (R DuPont) PET. DuPont sells it in a couple of injection-molded nylon applications mainly for wear as it lasts better in abrasive conditions (the end of the fiber, not the side). But no one has figured out a way to bond it to anything. Glass fibers have a coating on them to bond them to the resin so they can take the load. They break before the resin does as they are bonded to it. If they stick out like whiskers from the break, they were not bonded. So, with structures filled (not reenforced) with Kevlar (R DuPont)you get a not very well performing matrix under load. It is used in some applications for the sound of high tech for marketing hype. So it is just a filler in epoxy layups and ,as you describe, sort-of works but not as it should if it was bonded with the resin. This is why I think you have reservations about it from your experience.

From : Don Stackhouse

Thanks for the feedback!

Yes, I'm aware of that characteristic, but there's more to it than just that. It's my understanding that the Kevlar mlecule itself buckles under compressive load, transferring essentially the entire load to the matrix. The matrix crushes, leaving the fiber intact. However the details of the failure mechanism, the fact is that Kevlar composites have much lower compressive strength than even "E" type fiberglass.

The company I used to work for made Kevlar/epoxy propeller blades for full-scale aircraft propellers. In this situation the loading is usually purely tensile, because of the high centrifugal forces that usually dominate the overall load picture. Kevlar, with its excellent tensile strength (even higher than graphite/epoxy), good stiffness, excellent impact strength, high inherent vibrational damping characteristics and low density (about 10% lower than graphite/epoxy), is one of the best choices available for this application. It makes truly outstanding propeller blades that can accomplish naturally things that are simply impossible for other materials.

However, when we tried it for other applications such as prop spinners (those bullet-shaped fairings that cover the propeller hub, and that see a lot of compressive loading and flexing from vibration) it didn't do nearly so well. In the high-flexing areas such as around the blade cutouts, the epoxy matrix would disintegrate (much like you describe above), leaving a crack, but with the kevlar fibers across the crack nearly intact. Next, the kevlar fibers in the crack would rub on each other, eventually sawing through each other, but long after the original crack in the matrix had formed.

We also looked at using it for blades for very high horsepower applications, where the bending moments in the blade roots was so high that it overcame the tensile load from the centrifugal forces, resulting in a region on one side of the blade root that saw a compressive stress. Because of the extremely poor compressive strength of Kevlar, a Kevlar blade root had to be much fatter, so that its total weight was actually higher than graphite/epoxy. In addition, we were concerned with flutter on those blades, and the higher stiffness of the graphite/epoxy helped there as well.

The main reason I thinnk that Kevlar is popular on model sailplane fuselages (besides the marketing "hype" you mentioned) is its low density, and the indirect effect that has on buckling strength. Fuselages usually fail in impact by buckling, and there is a very non-linear relationship between buckling and skin thickness for most conventional shell-type fuselage structures and shapes. Buckling failure is also dependent on material stiffness. The stiffer and lower-density Kevlar/epoxy allows a much thicker shell wall for the same weight than a fiberglass/epoxy shell. This greater thickness and stiffness may allow a Kevlar shell with much poorer compressive strength to come out better in a buckling failure than a thinner fiberglass shell of the same weight. However, this would not be the case if the shell was designed from the outset to better utilize the material's compresive strength by avoiding buckling failure in the first place. The cross sections we use in many of our fuselages lean in that philosophical direction.

Another factor is the failure characteristic we have both pointed out, where the matrix fails leaving the fibers largely intact. The part looks like it's still intact, even though it has lost much of its stiffness and other structural qualities. Kevlar can "hide" a failure to a certain extent. Like some of the characters in the movie "The Sixth Sense", the part has died, but it doesn't know that yet.

The other main reason I don't favor Kevlar for some structures is because the available weights of Kevlar fabric are still too thick. This results in a too-heavy, overbuilt part. For things like RCHLG wings, I cna make a part from fiberglass that has adequate strength, but far less weight than what is possible with the lightest commercially available weights of Kevlar fabric. And of course it's crazy to use Kevlar for spar caps, because of the inevitable compression loads those see. It's just not the right material for those jobs.

Don Stackhouse
DJ Aerotech