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.  [Figure 7-7] Boron fibers are very stiff and have a high tensile and  compressive strength. The fibers have a relatively large  diameter and do not flex well; therefore, they are available  only as a prepreg tape product. An epoxy matrix is often used  with the boron fiber. Boron fibers are used to repair cracked  aluminum aircraft skins, because the thermal expansion of  boron is close to aluminum and there is no galvanic corrosion  potential. The boron fiber is difficult to use if the parent  material surface has a contoured shape. The boron fibers are  very expensive and can be hazardous for personnel. Boron  fibers are used primarily in military aviation applications. Ceramic fibers are used for high-temperature applications,  such as turbine blades in a gas turbine engine. The ceramic  fibers can be used to temperatures up to 2,200 °F.  An aluminum airplane is quite conductive and is able to  dissipate the high currents resulting from a lightning strike.  Carbon fibers are 1,000 times more resistive than aluminum  to current flow, and epoxy resin is 1,000,000 times more  resistive (i.e., perpendicular to the skin). The surface of an  external composite component often consists of a ply or layer  of conductive material for lightning strike protection because  composite materials are less conductive than aluminum.  Many different types of conductive materials are used  ranging from nickel-coated graphite cloth to metal meshes  to aluminized fiberglass to conductive paints. The materials  are available for wet layup and as prepreg.  In addition to a normal structural repair, the technician must  also recreate the electrical conductivity designed into the  part. These types of repair generally require a conductivity  test to be performed with an ohmmeter to verify minimum  electrical resistance across the structure. When repairing  these types of structures, it is extremely important to use only  the approved materials from authorized vendors, including  such items as potting compounds, sealants, adhesives, and  so forth.  [Figures 7-8  and  7-9] Resin is a generic term used to designate the polymer. The  resin, its chemical composition, and physical properties  fundamentally affect the processing, fabrication, and  Ceramic Fibers Lightning Protection Fibers Boron Thermosetting Resins Matrix Materials Figure 7-8.  Copper mesh lightning protection material. Figure 7-7.  Fiberglass (left), Kevlar® (middle), and carbon fiber  material (right). 7-6