Carbon/Graphite
One of the first distinctions to be made among fibers is the
difference between carbon and graphite fibers, although
the terms are frequently used interchangeably. Carbon and
graphite fibers are based on graphene (hexagonal) layer
networks present in carbon. If the graphene layers, or planes,
are stacked with three dimensional order, the material is
defined as graphite. Usually extended time and temperature
processing is required to form this order, making graphite
fibers more expensive. Bonding between planes is weak.
Disorder frequently occurs such that only two-dimensional
ordering within the layers is present. This material is defined
as carbon.
Carbon fibers are very stiff and strong, 3 to 10 times stiffer
than glass fibers. Carbon fiber is used for structural aircraft
applications, such as floor beams, stabilizers, flight controls,
and primary fuselage and wing structure. Advantages include
its high strength and corrosion resistance. Disadvantages
include lower conductivity than aluminum; therefore, a
lightning protection mesh or coating is necessary for aircraft
parts that are prone to lightning strikes. Another disadvantage
of carbon fiber is its high cost. Carbon fiber is gray or black
in color and is available as dry fabric and prepreg material.
Carbon fibers have a high potential for causing galvanic
corrosion when used with metallic fasteners and structures.

Mechanical testing
Tensile tests on specimens measuring 25 by 250
mm were conducted according to ASTM 638-90 and
flexural tests on specimens measuring 11 by 140 mm
by ASTM 790-90 at the Forest Products Laboratory
Engineering Mechanics Laboratory with load applied
perpendicular to the surface of the fiber mat. A
crosshead speed of 5 mm/min. was used. The stressstrain
curves were analyzed using a three-parameter
hyperbolic tangent model that has been shown to
accurately represent the stress-strain behavior. The
composite specimens were all tested to failure. Information
collected in the tests included tensile strength
at failure, tensile modulus, flexural strength, and
flexural modulus. Notched and unnotched highspeed
impact performance of the composites (13 by
65 mm) was evaluated by Izod impact according to
ASTIM D 256.