Boron was developed in 1959. Boron fibers are made by using a 0.0005-inch tungsten filament heated to about 2200F 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.

High-strength graphite fibers were not developed until the early 1970s. 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 475F in ambient air. The fibers are carbonized at 2,700F in an inert oxygen rich atmosphere, and the graphitization process takes place at 5,400F 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 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.

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.

Figure 14-21.Design properties comparison.  

Figure 14-22.-Response to applied loads.

Figure 14-23.Laminae stacking.

See figure 14-23. Drawings specify ply stacking angles and the sequence of the lay-up. A standard laminate orientation code is used to ensure standardization in the industry. The orientation code denotes the angle, in degrees, between the fibers and the "X" axis of the part. The "X" axis is usually spanwise of the part, or in the direction of applied loads. See figure 14-24. The laminate ply orientation or stacking sequence is denoted in brackets, with the angle of each ply separated by a slash (/); for example, [+45/45/+45/-45]. Laminae are listed in sequence from the first lamina to the last. The brackets or parenthesis indicate the beginning and the end of a code. The plus (+) and minus () angles are relative to the "X" axis. Plus (+) signs are to the left of 0, and minus () signs are to the right of 0. Adjacent laminae of equal angles but opposite signs are identified as , (45 = +45, 45). The directional strengths and stiffness of the laminate can be altered by changing the ply orientation.