The therm "composite material" denotes a wide range of material, some of which predate the Roman Empire and others that are still under development in materials research laboratories.
Composite material can be defined as an engineered material consisting of one or more reinforcing agents and a matrix binder acting together as a physical unit while retaining their identities. The reinforcing agent are generally in the form of fiber, whiskers, or particles. The matrix binder (at least in the aerospace environment) is usually some type of resin. Normally, the interface between the reinforcing material and the matrix binder is physically identifiable. in other words, the components of the composite material remain distinct.
Composites consist of mixtures of two or more materials that maintain their own identities but are attached together in such ways as to reinforce the properties of each by adhesive forces, by their respective positions, or frequently by both. Composites may be made up all of metals, combinations of metals and nonmetals or all nonmetals. The most typical reason for the development of composites has been light weight with high strength, tailored stress, and sometimes the additional feature or withstanding some unusual environmental condition.
An adhesive is most commonly considered to be a material with some "tackiness" or "stickines" and the animal glues used almost exclusively up to the current century met this requirements. Modern adhesives, however, have a wider range in this respect.
About the elements of an adhesive bond, Oxides usually remain on surface, so, resin solvents may provide some cleaning action.
Reinforcing agents are most noted for their ability to contribute to the strength, stiffness and impact resistance of the composite material. They have a wide range of form types: fibrous, whiskered, crystalline and spherical (in powders). Metals, ceramics, organics and inorganic are represented among the reinforcing agents.
In some situations, composites can be considered an enabling technology, in that they make possible designs or applications that are otherwise not feasible or economical. Through proper design of play orientations, it is possible to tailor filamentary composites to meet specific loading requirements involving stress. Five types of stress on structural members of aircraft components are as follows
- Tension
- Compression
- Torsion
- Shear
- Bending
Composite structures come in an almost infinite variety. In addition to this are "Sandwich panels" utilizing either a foam or honeycomb between two skins. A further level of complication is seen when all these elements are combined in hybrid reinforcement, such as the helicopter rotor blade. Thee can include carbon unidirectional tape, carbon woven fabric, a phenolic paper honeycomb core, fiberglass, adhesive, fillers and more. As can be seen, composite structures provide numerous difficulties for NDT inspections.
At least four mechanisms may be responsible for adherence. Electrostatic bonds and covalent bonds result from the sharing of electrons by different atoms and account for the formation of most common chemical compounds. Even after bonds of these types are stablished, the positive and negative charges of most atoms are not completely neutralized, and "van Der Waals "forces provide additional bonding between the atoms. While not strictly an adherence phenomenon, "mechanical interlocking" may take part in the action of some adhesives, although this action appears to be secondary to true adhesion.
As in welding of metals, the proper performance of an adhesive requires that intimate contact, in addition to adherent cleanliness, be established between the adhesive and the surfaces to be jointed. Important adhesives for the bonding of metals are "thermosetting compounds", the materials most used include epoxy, phenolic, polyester and urea resins.