From the Jet Pilot’s Organization
Composite Materials: Kevlar
by Art Gajewski
This article will provide some insight into aramids
commonly known as Kevlar. As jet modelers, most of
us are familiar with the popular fabrics used in the
construction of our aircraft. Certainly, we have all
built or flown models made of fiberglass and even
some with carbon fiber and Kevlar. However, have you
ever wondered how these materials are made and what
are some of the tricks to use them properly?
Introduced commercially in the 1970s, Kevlar aramid
is an aromatic organic compound of carbon, hydrogen,
oxygen, and nitrogen. Kevlar fiber is produced by
spinning long-chain polyamide polymers using
standard textile techniques. The low-density,
high-tensile strength, low-cost fiber produces
tough, impact-resistant structures. The compressive
properties of Kevlar laminates are low (because of
poor coupling of resin matrixes to the aramid
fibers), so, applications are typically secondary
structures or tension-critical applications.
Kevlar fiber, originally developed to replace steel
in radial tires, has found increasing use in the
belts of radial car and truck tires, where it saves
weight and increases strength and durability
compared to steel belts.
Two Common Kevlar Alloys
Kevlar 29 is a low-density, high-strength aramid
fiber designed for ballistic protection,
slash-and-cut resistance, ropes, cables, and coated
fabrics for inflatable and architectural fabrics.
Kevlar 49 aramid fiber is characterized by
low-density and high-tensile strength and modulus.
These properties are the key to its successful use
as reinforcement for plastic composites in aircraft,
aerospace, marine, automotive, other industrial
applications, and in sports equipment. It is
available in continuous-filament yarns, chopped
fiber, woven and unidirectional fabrics, tissues or
veils, and tapes for reinforcement applications.
Kevlar 49 aramid is used in high-performance
composite applications where lightweight, high
strength and stiffness, vibration damping and
resistance to damage, fatigue, and stress rupture
are key properties. Reinforced composites can save
up to 40% of the weight of glass-fiber composites at
equivalent stiffness. The aramid composites resist
shattering upon impact, and the presence of the
fiber inhibits propagation of cracks. Depending upon
the selection of resin systems, aramid composites
have a useful temperature range from -320° to 400° F
(-196° to 204° C).
Kevlar 49 is not a carbonized or graphitized
material. Unlike other organic materials, its
stress-strain behavior is linear to ultimate failure
in tension at 340 kips/square inch (2344 MPa) and
1.8% elongation. Toughness of the fiber composites
is significantly higher than carbon graphite
composites. Furthermore, the very low density of the
fibers provides a higher specific strength than
glass or carbon reinforcing fibers. The specific
modulus is between four and five times higher than
that of glass fiber. The usable strength of Kevlar
49 reinforced epoxy is about four times that of
7075T6 aluminum at less than half the density.
Kevlar—Getting the Most Out of Yours
Kevlar is lighter than fiberglass (for a given
strength) and tougher than carbon fiber. Therefore,
it sounds like the ideal composite, right? Well, yes
and no. Let's see how to best use this aramid
material.
First, cutting it can be a real pain. Special shears
are required to cut Kevlar fabrics and tapes. These
shears are designed to hold the fabric securely as
the cutting blade does its job. If you look at these
shear blades closely, you'll notice that there are
serrations on the "holding" edge and a sharp edge on
the cutter. These shears are a specialty item and
are therefore somewhat expensive, but they are well
worth the price in reduced aggravation and improved
results. Don't try to cut Kevlar without them.
Second, use a compatible resin. Kevlar does not bond
well with polyester resins. Keep it simple and use
epoxy resins for the best results.
Last, use Kevlar for specific applications including
reinforcements as opposed to entire structures,
predominantly tensile loads, vibration damping, or
scuff resistance. Kevlar works well as reinforcement
in fiberglass structures. Cost may become
prohibitive when used as the only fabric in a
composite structure and its compressive strength
isn't as good as some other materials. I have seen
Kevlar canoes, but I don't know how well they
perform. Kevlar works really well as localized
reinforcement in vibration-prone applications (e.g.
engine-mount boxes in Giant Scale airplanes with
gasoline engines). Scuff resistance is another good
application—wing tips, fuselage bottoms, etc.
Always use high-quality, engineered resin. Some
hobby resins may not have all of the strength
properties we desire in our applications. I
personally use and recommend WEST Systems 105 resin
with fast or slow hardener. WEST Systems is
competitive on a cost-per-ounce basis. This resin
dries hard, is easy to sand, it's tough and not
easily damaged compared to some other hobby resins
intended for the same application.
Once again, a quick word about hybrid fabrics
(carbon fiber and Kevlar)—these hybrid fabrics are
popular because not only do they look attractive but
they also can provide the best of both worlds. They
provide the lightweight, high strength, and
stiffness of carbon fibers with the lightweight,
toughness, and abrasion-resistance of aramids. I
have built hybrid composite landing gear using
alternating layers of carbon fiber and Kevlar with
excellent results. One would need to understand the
application very well to select the right composite
properly (fiberglass, carbon fiber, aramid, or a
hybrid). Hybrids have their place.
Note: Information in the article is
adapted from Composite Materials Handbook,
M.M. Schwartz, McGraw-Hill Book Company, 1984.
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