A COMPOSITE MATERIAL can be defined as a macroscopic combination of two or more distinct materials, having a recognizable interface between them. However, because composites are usually used for their structural properties, the definition can be restricted to include only those materials that contain a reinforcement (such as fibers or particles) supported by a binder (matrix) material.
Thus, composites typically hive a discontinuous fiber or particle phase or particle phase that is stiffer and stronger than the continuous matrix phase. In order to provide reinforcement, there generally must be a substantial volume fraction (10% or more) of the discontinuous phase. There are, however, exceptions that may still be considered composites, such as rubber-modified polymers, where the discontinuous phase is more compliant and more ductile than the polymer, resulting in improved toughness.
Composites can be divided into classes in various manners. One simple classification scheme is to separate them according to reinforcement forms-particulate-reinforced, fiber reinforced, or laminar composites. Fiber-reinforced composites can be further divided into those containing discontinuous or continuous fibers.
A reinforcement is considered to be a "particle" if all of its dimensions are roughly equal. Thus, particulate-reinforced composites include those reinforced by spheres, rods, flakes, and many other shapes of roughly equal axes. There are also materials, usually polymers, that contain particles that extend rather than reinforce the material. These are generally referred to as "filled" systems. Because filler particles are included for the purpose of cost reduction rather than reinforcement, these composites are not generally considered to be particulate composites. Nonetheless, in some cases the filler will also reinforce the matrix material. The same may be true for particles added for nonstructural purposes such as fire resistance, control of shrinkage, and increased thermal conductivity.
Fiber-reinforced composites contain reinforcements having lengths much greater than their cross-sectional dimensions. Such a composite is considered to be a discontinuous fiber or short fiber composite if' its properties vary with fiber length. On the other hand, when the length of the fiber is such that any further increase in length does not, for example, further increase the elastic modulus of the composite, the composite is considered to be continuous fiber reinforced. Most continuous fiber (or continuous filament) composites, in fact, contain fibers that are comparable in length to the overall dimensions of the composite part.
Laminar composites are those composed of two (or more) layers with two of their dimensions being much larger than their third. Complicating the definition of a composite as having both continuous and discontinuous phases is the fact that in a laminar composite, neither of these phases may be regarded as truly continuous in three dimensions.
With some few specific exceptions, only "high-performance" composites will be considered in this Volume. These are composites that have superior performance compared to conventional structural materials such as steel and aluminum alloys. Thus, the emphasis will be on continuous fiber reinforced composites, although the principles will often be applicable to other types of composites as well. Furthermore, continuous fiber reinforced composites will generally be referred to as simply fiber reinforced composites, and, in some cases, as merely fiber composites or composites. In addition, composites with organic (resin) matrices will be emphasized throughout this volume, both because such composites are the most commonly used and because of the significant dissimilarities between organic-matrix composites and those made with metal ceramic, and carbon matrices.
Composite materials were developed because no single, homogeneous structural material could be found that had all of the desired attributes for a given application. Fiber-reinforced composites were developed in response to demands of the aerospace community, which is under constant pressure for materials development in order to achieve improved performance. Aluminum alloys, which provide high strength and fairly high stiffness at low weight, have provided good performance and have been the main materials used in aircraft structures over the years. However, both corrosion and fatigue in aluminum alloys have produced problems that have been very costly to .remedy. World War 11 'Promoted a need for materials with improved structural properties. in response, fiber-reinforced composites were developed, and by the end of the war, fiberglass reinforced plastics had been used successfully in filament-wound rocket motors and in various other structural applications. These materials were put into broader use in the 1950s, and initially seemed to be the only viable approach available for the elimination of corrosion and crack formation in high-performance structures. Although more recent developments in metallic materials have led to some solutions to these problems, fiber-reinforced composites still provide other substantial benefits to designers and manufacturers.
Inexpensive fiberglass composites are used today in a wide variety of applications, from consumer products, such as the fiberglass boat to aerospace. More advanced fiber-reinforced composites, however, have been limited in their commercial use because of high materials cost, lack of widely distributed property and processing data bases, and the absence of rapid and efficient manufacturing techniques. However, fiber-reinforced composites have been developed and widely applied in aerospace applications to satisfy requirements for enhanced performance and reduced maintenance costs. In large commercial aircraft they have found application because of the weight considerations that were highlighted by the energy crisis of the 1970s.
Fiber composites offer many superior properties. Almost all high-strength/high-stiffness materials fail because of the propagation of flaws. A fiber of such a material is inherently stronger than the bulk form because the size of a flaw is limited by the small diameter of the fiber. In addition, if equal volumes of fibrous and bulk material are compared, it is found that even if a flaw does produce failure in a fiber, it will not propagate to fail the entire assemblage of fibers, as would happen in the bulk material. Furthermore, preferred orientation may be used (as in aramid and carbon fibers) to increase the lengthwise modulus, and perhaps strength, well above isotropic values. When this material is also lightweight, there is a tremendous potential advantage in strength-to-weight and/or stiffness-to-weight ratios over conventional materials. These desirable fiber properties can be converted to practical application when the fibers are embedded in a matrix that binds them together, transfers load to and between the fibers, and protects them from environments and handling. In addition, fiber-reinforced composites are ideally suited to anisotropic loading situations where weight is critical. The high strengths and moduli of these composites can be tailored to the high load direction(s), with little material wasted on needless reinforcement.
Glass fiber reinforced organic composites are the most familiar and widely used and have had extensive application in industrial, consumer, military, and aerospace markets. Carbon fiber reinforced resin matrix composites are by far the most commonly applied advanced (non fiberglass), composites for a number of reasons. The extremely high specific properties, high materials that are readily available, reproducible material forms, increasingly favorable cost projections, and comparative ease of manufacture. Composites reinforced with aramid other organics, and boron fibers, and with carbide, alumina, and other ceramic fibers also used. Recent technology has provided a various reinforcing fibers and matrices that combined to form composites having range of very exceptional properties. In many instances the sheer number of available material combinations can make selection of materials for evaluation a difficult and almost overwhelming task. In addition, once a material is selected, the choice of an optimal fabrication, process can be very complex.
