The real heart of any composite material is what is called the reinforcement. While reinforcements can include mat, roving, fillers, etc., the primary structural element is fabric. Structural reinforcing fabrics currently are available in four materials; E-Glass, S-Glass, Kevlar, and Carbon Fiber.
This is the most common form of fiberglass reinforcing fabric available. It has traditionally been used in just about every kind of consumer product imaginable. It has an average tensile strength of about 500,000 psi and is relatively economical.
Though nearly identical in appearance to the more common E-Glass, S-Glass has considerably more tensile strength at an average of 625,000 psi compared to the 500,000 psi of E-Glass. Though the same in weight it has the highest impact resistance of all of the common glass construction fibers.
Everyone will recognize this name from its celebrity in bullet-proof vests and body armor. Kevlar® is a registered trademark of the DuPont Corporation for its family of high strength, high modulus aramid fibers first introduced in the mid-seventies. Invented by DuPont scientist, Stephanie Kwolec, “aramid” is an abbreviation for “aromatic polyamide”—a distant cousin of Nylon. It is light in weight, about five times stronger than steel, ten times stronger than aluminum at only about 43% of the weight of conventional fiberglass.
The chemical composition of Kevlar is poly para-phenyleneterephthalamide, and thus is more properly called a para-aramid. Aramids belong to the family of nylons. Common nylons, such as nylon 6,6, do not have very good structural properties, so the para-aramid distinction is important. The aramid ring gives Kevlar thermal stability, while the para structure gives it high strength and high modulus.
Like nylons, Kevlar filaments are made by extruding the precursor through a spinneret. The rod form of the para-aramid molecules and the extrusion process make Kevlar fibers anisotropic—they are stronger and stiffer in the axial direction than in the transverse direction. In comparison, graphite fibers are also anisotropic, but glass fibers are isotropic—equally strong in both directions.
Kevlar fibers are resistant to corrosion, are non-conductive, and most importantly, are highly resistant to impact damage. Unlike fiberglass and carbon fiber, Kevlar will not crack or shatter on impact. Instead, it dents like metals. This makes it ideal for reinforcement of panels which might incur penetration impacts such as in race cars, race boats or fighter planes.
Kevlar is an easy fabric to wet out with resin but does not become translucent, instead retaining its characteristic golden color. Kevlar is a fabric that was never intended to be bonded to either itself or to other materials. Fabrics are generally treated with finishes which are compatible with both polyester and epoxy resins to improve adhesion. For this reason, the vinylesters and epoxies are generally recommended as the resin systems of choice to allow full recognition of the mechanical properties of this material.
Kevlar has an additional weakness. Though these fabrics are much stronger than fiberglass in tension, they are not as good in compression or bending. To fully realize the benefits of this fiber, it is usually used in conjunction with another fiber in the form of a hybrid fabric.
Today, there are three grades of Kevlar available: Kevlar 29, Kevlar 49, and Kevlar 149. The table below shows the differences in material properties among the different grades. Commonly available Kevlar woven fabric, it is most likely the Kevlar 49 version. The three versions compare as follows:
- Kevlar 29 Tensile Modulus = 83 GPa, Tensile Strength = 3.6 GPA, Elongation = 4.0%
- Kevlar 49 Tensile Modulus = 131 GPa, Tensile Strength = 4.0 GPa, Elongation = 2.8%
- Kevlar 149 Tensile Modulus = 186 GPa, Tensile Strength = 3.4 GPa, Elongation = 2.0%
The tensile modulus and strength of Kevlar 29 is roughly comparable to that of glass (S or E), yet its density is almost half that of glass. Therefore, Kevlar can be substituted for glass anywhere lighter weight is desired. Kevlar 49 or 149 can cut the weight even farther if the higher strength is accounted for.
Kevlar has other advantages besides light weight and high strength. Like carbon fiber, it has a slightly negative axial coefficient of thermal expansion—it shrinks with increasing temperature. Thus Kevlar laminates can be made thermally stable. Unlike graphite, Kevlar is very resistant to impact and abrasion damage. It can be used as a protective layer on graphite laminates. Kevlar can also be mixed with graphite in hybrid fabrics to provide damage resistance, increased ultimate strains, and to prevent catastrophic failure modes.
Like all good things, Kevlar also has a few disadvantages. The fibers themselves absorb moisture, so Kevlar composites are more sensitive to the environment than glass or graphite composites. Although tensile strength and modulus are high, compressive properties are relatively poor. Additionally, Kevlar is sensitive to, and deteriorates when exposed to UV light. To successfully use it, it must be shielded.
Finally, Kevlar is also very difficult to cut. You will need special scissors for cutting dry fabric or prepreg, and special drill bits for drilling cured laminates. Cutting of cured laminates without fraying is very difficult.
As a final note, you will notice that each fabric has a specific way in which its designers describe its weight. For Kevlar fibers, the descriptive unit is the denier. A denier is a unit of fineness for rayon, nylon, and silk fibers, based on a standard mass per length of 1 gram per 9,000 meters of yarn. Therefore, a 1500 denier Kevlar fiber weighs 1500 grams per 9,000 meters of length.
Carbon Fibers contain up to 95% carbon and yield the highest tensile strength in the FRP industry. These fabrics offer higher strength and stiffness-to-weight ratios than any other commonly available reinforcements.
To maximize the fiber properties we recommend using only epoxy or vinylester resin, although polyesters will bond to the fabrics.
This is such a popular fiber, we have a entire white paper on it entitled, “An Introduction To Carbon Fiber.”
Spectra® fiber is one of the world’s strongest and lightest fibers. A bright white polyethylene, it is, pound-for-pound, ten times stronger than steel, more durable than polyester and has a specific strength that is 40 percent greater than aramid (Kevlar) fiber.
Spectra® fiber is made from ultra-high molecular weight polyethylene that is processed by a patented gel-spinning process. Polyethylene is a remarkably durable plastic, and scientists at Spectra Technologies have captured the tremendous natural strength in the molecular backbone of this everyday plastic to create one of the world’s strongest and lightest fibers. The gel-spinning process and subsequent drawing steps allow Spectra® fiber to have a much higher melting temperature (150°C or 300°F) than standard polyethylene.
With outstanding toughness and extraordinary visco-elastic properties, Spectra® fiber can withstand high-load strain-rate velocities. Light enough to float, it also exhibits high resistance to chemicals, water, and ultraviolet light. It has excellent vibration damping, flex fatigue and internal fiber-friction characteristics, and Spectra® fiber’s low dielectric constant makes it virtually transparent to radar.
Spectra® fiber is used in numerous high-performance applications, including police and military ballistic-resistant vests, helmets and armored vehicles, as well as sailcloth, fishing lines, marine cordage, lifting slings, and cut-resistant gloves and apparel. Honeywell also converts Spectra® fiber into the Spectra Shield® family of specialty composites for armor and other applications.
The latest generation of Spectra® fiber was developed to provide super-fine, super-strong and ultra-lightweight fibers for armor, aerospace and high-performance sporting goods applications. Spectra® fiber 2000, Honeywell’s premier ballistic fiber, is stronger and lighter than most commercial high-modulus fibers and has one of the highest strength to weight ratios of any manmade fiber. Spectra® fiber’s high tenacity makes it pound-for-pound 10 times stronger than steel, more durable than polyester and gives it a specific strength that is 40 percent greater than aramid fiber. Specific performance is dependent upon denier and filament counts.
Spectra Shield© is the strongest, lightest, composite ballistic material made today. Spectra Shield© isn't a woven fabric like most ballistic materials (e.g. Kevlar). But instead, it is a thin, flexible ballistic composite made from two layers of unidirectional fibers held in place by flexible resins. These fibers are arranged so they cross each other at 0 and 90 degree angles, then, both fiber and resin layers are sealed between two thin sheets of polyethylene film similar to Saran wrap.
Spectra Shield composite armor panels are readily cut and fabricated using commonly available tools (a saber saw with a knife blade will easily cut the panels). The panels can then be glued in place, or installed with screws, nuts and bolts, etc.