bidirectional or quasi-isotropic. Maximum strength is parallel to the fibers, and loads at right angles to the fibers tend to break only the matrix. See figure 14-21. Metals and composites respond differently when subjected to loads. See figure 14-22. The advantages of composites over metals are higher specific strengths, flexibility in design, ease of manufacturing, lighter weight materials, ease of repair (compared to metals), and excellent fatigue and corrosion resistance. The disadvantages are limited previous repair information, high start-up costs, difficulty of inspection, expense of materials, limited in-work times, poor impact resistance, sensitivity to chemicals and solvents, environmental attacks, and the low conductivity of the materials. Advanced composites are made up of fibers and the matrix. Fibers are a single homogeneous strand of material, rolled or formed in one direction, and used as the principal constituent in composites. They carry the physical loads and provide most of the strength of composites. Composite materials are made up of many thousands of fibers arranged geometrically, woven or collimated (in columns). The various types of fibrous materials used today are discussed in the following paragraphs. Boron Fibers Boron was developed in 1959. Boron fibers are made by using a 0.0005-inch tungsten filament heated to about 2200°F and drawn through a gaseous mixture of hydrogen and boron trichloride. A coating of black boron is deposited over the tungsten filament. The resulting fiber is about 0.004 inch in diameter, has excellent compressive strength and stiffness, and is extremely hard. Graphite Fibers High-strength graphite fibers were not developed until the early 1970’s. Fibers of graphite are produced by “graphitizing”   filaments of rayon or other polymers in a high-temperature furnace. The fibers are stretched to a high tension while slowly being heated through a stabilization process at 475°F in ambient air. The fibers are carbonized at 2,700°F in an inert oxygen rich atmosphere, and the graphitization process takes place at 5,400°F in an inert atmosphere. Then the graphite fibers are subjected to a treatment process that involves cooling and cleaning of the carbon dust particles to improve the interlaminar shear properties. These shear properties relate to the shear strength between adjacent plies of laminate. The resulting fibers are black in color and only a few microns in diameter. They are strong, stiff, and brittle; through control of the process, graphite of higher tensile strength can be produced at the cost of lower stiffness. Aircraft parts are generally produced with fibers of intermediate strength and stiffness. Kevlar® Fibers Kevlar® fibers are a registered trademark of E. I. DuPont de Nemours & Company Inc, which maintains exclusive production rights for the fibers. The structural grade Kevlar® fiber, known as Kevlar®, is characterized by excellent tensile strength and toughness but inferior compressive strength compared to graphite.    The stiffness, density, and cost of Kevlar® are all lower than graphite; hence, Kevlar® may be found in many secondary structures replacing fiber glass or as a hybrid with fiber glass. The fibers are golden yellow in color and measure .00047 inch in diameter. Matrix Although the fibers are the principal load- carrying material, no structure could be made without the matrix. The matrix is a homogeneous resin that, when cured, forms the binder that holds the fibers together and transfers the load to the fibers. The most common matrix material in current use is epoxy. Epoxies provide high mechanical and fatigue strength; excellent dimensional stability, corrosion resistance, and interlaminar (between two or more plies) bond; good electrical properties; and very low water absorption. The changing of the matrix properties (hardening) by a chemical reaction is called the “cure.”    Curing is the changing of the matrix properties (hardening) by a chemical reaction. Curing is usually accomplished with heat and vacuum pressure. The finished product may be a single-ply (lamina) or a multiply product called a “laminate.” Laminate A lamina is a single-ply arrangement of uni- directional or woven fibers in a matrix. A lamina is usually referred to as a “ply.” A laminate is a stack of lamina, or plies, with various in-plane angular orientations bonded together to form a structure. 14-20