The reduced availability of natural resources, the increasing costs of production, and the apparent limit to our ability to fabricate high strength-to-weight metallic components necessitated the development of new materials to meet the demands of aerospace technology. In the following text, you will be intro-duced to the materials that provide high-performance capability now, with great expectations for the future. These materials are called advanced composite materials and will be used to replace some of the metals currently used in aircraft construction. Advanced composites are materials consisting of a combination of high-strength stiff fibers embedded in a common matrix (binder) material, generally laminated with plies arranged in various directions to give the structure strength and stiffness.

The much stiffer fibers of boron, graphite, and Kevlar have given composite materials structural properties superior in strength to the metal alloys that they have replaced. Specific applications of advanced composite materials and approximate percentages of total aircraft structures for some of our modern-day aircraft are shown in table 14-1.

Figure 14-20.Trailing edge repair (sandwich construction).

Table 14-1.Aircraft Advanced Composite Application Usage

Composites are attractive structural materials because they provide a high strength-to-weight ratio and offer design flexibility. The function of a composite is to replace heavy/dense metals with stronger, lighter weight structural components, allowing lightweight aircraft to carry payloads farther distances using less fuel. In contrast to traditional materials of construction, these materials can be adjusted to more efficiently match the requirements of specific applications. 

These materials are highly susceptible to impact damage, with the extent of damage being visually difficult to determine. A nondestructive inspection (NDI) is required to analyze the extent of damage and effectiveness of repairs.

Composites are classed by the type of reinforcing elements. These elements may be fibers, particle, flake, or laminar materials. They are further classified by the composition of the reinforcing materials and by the type of matrix materials. The primary factors taken into consideration when designing composites are the costs (research and development, production, fuel economy), type of application (load requirements of the structure, adjoining materials, service-life requirements), mission and maintenance requirements, and operational environment (hot/cold weather, relative humidity, altitude, land/carrier based).

The comparative properties of composites and metals are that metals have almost the same physical and mechanical strengths equal in all directions. Stresses and strains are equally transmitted in all directions. Composites can have different physical and mechanical strengths in different directions, and are considered to be anisotropic or quasi-isotropic. These strengths are determined by the fiber orientation patterns. The patterns are unidirectional, 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.