Introduction to CompositesDaniel B. Miracle and Steven L. Donaldson, Air Force Research Laboratory

Fig. 1 Common forms of fiber reinforcement. In gen-eral, the reinforcements can be straight contin-

uous fibers, discontinuous or chopped fibers, particles orflakes, or continuous fibers that are woven, braided, or knit-ted. Source: Ref 1

A COMPOSITE MATERIAL is a macro-scopic combination of two or more distinct ma-terials, having a recognizable interface betweenthem. Composites are used not only for theirstructural properties, but also for electrical, ther-mal, tribological, and environmental applica-tions. Modern composite materials are usuallyoptimized to achieve a particular balance ofproperties for a given range of applications.Given the vast range of materials that may beconsidered as composites and the broad range ofuses for which composite materials may be de-signed, it is difficult to agree upon a single, sim-ple, and useful definition. However, as a com-mon practical definition, composite materialsmay be restricted to emphasize those materialsthat contain a continuous matrix constituent thatbinds together and provides form to an array ofa stronger, stiffer reinforcement constituent. Theresulting composite material has a balance ofstructural properties that is superior to eitherconstituent material alone. The improved struc-tural properties generally result from a load-shar-ing mechanism. Although composites optimizedfor other functional properties (besides highstructural efficiency) could be produced fromcompletely different constituent combinationsthan fit this structural definition, it has beenfound that composites developed for structuralapplications also provide attractive performancein these other functional areas as well. As a re-sult, this simple definition for structural com-posites provides a useful definition for most cur-rent functional composites.

Thus, composites typically have a fiber or par-ticle phase that is stiffer and stronger than thecontinuous matrix phase. Many types of rein-forcements also often have good thermal andelectrical conductivity, a coefficient of thermalexpansion (CTE) that is less than the matrix, and/or good wear resistance. There are, however, ex-ceptions that may still be considered composites,such as rubber-modified polymers, where thediscontinuous phase is more compliant and moreductile than the polymer, resulting in improvedtoughness. Similarly, steel wires have been usedto reinforce gray cast iron in truck and trailerbrake drums.

Composites are commonly classified at twodistinct levels. The first level of classification isusually made with respect to the matrix constit-

uent. The major composite classes include or-ganic-matrix composites (OMCs), metal-matrixcomposites (MMCs), and ceramic-matrix com-posites (CMCs). The term organic-matrix com-posite is generally assumed to include twoclasses of composites: polymer-matrix compos-ites (PMCs) and carbon-matrix composites(commonly referred to as carbon-carbon com-posites). Carbon-matrix composites are typicallyformed from PMCs by including the extra stepsof carbonizing and densifying the original poly-mer matrix. In the research and developmentcommunity, intermetallic-matrix composites(IMCs) are sometimes listed as a classificationthat is distinct from MMCs. However, significantcommercial applications of IMCs do not yet ex-ist, and in a practical sense these materials donot provide a radically different set of propertiesrelative to MMCs. In each of these systems, thematrix is typically a continuous phase through-out the component.

The second level of classification refers to thereinforcement formparticulate reinforcements,whisker reinforcements, continuous fiber lami-nated composites, and woven composites(braided and knitted fiber architectures are in-cluded in this category), as depicted in Fig. 1(Ref 1). In order to provide a useful increase inproperties, there generally must be a substantialvolume fraction (10% or more) of the rein-forcement. A reinforcement is considered to bea particle if all of its dimensions are roughlyequal. Thus, particulate-reinforced compositesinclude those reinforced by spheres, rods, flakes,and many other shapes of roughly equal axes.Whisker reinforcements, with an aspect ratiotypically between approximately 20 to 100, areoften considered together with particulates inMMCs. Together, these are classified as discon-tinuous reinforcements, because the reinforcingphase is discontinuous for the lower volumefractions typically used in MMCs. There are alsomaterials, usually polymers, that contain parti-cles that extend rather than reinforce the mate-rial. These are generally referred to as filledsystems. Because filler particles are included forthe purpose of cost reduction rather than rein-forcement, these composites are not generallyconsidered to be particulate composites. None-theless, in some cases the filler will also rein-force the matrix material. The same may be true

for particles added for nonstructural purposes,such as fire resistance, control of shrinkage, andincreased thermal or electrical conductivity.

Continuous fiber-reinforced composites con-tain reinforcements having lengths much greaterthan their cross-sectional dimensions. Such acomposite is considered to be a discontinuousfiber or short fiber composite if its propertiesvary with fiber length. On the other hand, whenthe length of the fiber is such that any furtherincrease in length does not, for example, furtherincrease the elastic modulus or strength of thecomposite, the composite is considered to becontinuous fiber reinforced. Most continuous fi-ber (or continuous filament) composites, in fact,contain fibers that are comparable in length tothe overall dimensions of the composite part. Asshown in Fig. 1, each layer or ply of a contin-uous fiber composite typically has a specific fiberorientation direction. These layers can bestacked such that each layer has a specified fiberorientation, thereby giving the entire laminatedstack (laminate) highly tailorable overall prop-erties. Complicating the definition of a compos-ite as having both continuous and discontinuousphases is the fact that in a laminated composite,neither of these phases may be regarded as trulycontinuous in three dimensions. Many applica-tions require isotropy in a plane, and this isachieved by controlling the fiber orientationwithin a laminated composite. Hybrid organic-metal laminates are also used, where, for exam-