Composites discontinuous filament

Muki, R. and Sternberg, E. (1971). Load-absorption by a discontinuous filament in a fiber-reinforced composite. J. Appl. Math. Phys. (ZAMP) 22, 809-824. 

The morphological structure of the multicellular fibers makes them analogous to the modern-day fiber-reinforced, rigid-matrix composites materials in which (a) the fibers are all aligned in the same direction (unidirectional), (b) the volume fraction of fibers is very high, and (c) the fibers have discrete (discontinuous filaments) lengths. As in composites, the properties of multicellular fibers are determined by the physical, mechanical, and chemical properties of the morphological constituents. 

Fibrous Composites. These composites consist of fibers in a matrix. The fibers may be short or discontinuous and randomly arranged continuous filaments arranged parallel to each other in the form of woven rovings (coUections of bundles of continuous filaments) or braided (8). In the case of chopped strand mat the random arrangement is planar. In whisker (needle-shaped crystals or filaments of carbon and ceramics) reinforced materials the arrangement is usually three-dimensional and the resulting composites are macroscopically homogeneous. 

A composite material is defined as a material consisting of two or more distinct constituents or phases, which are insoluble in one another. The main types of reinforcement are particles, discontinuous fibers, continuous fibers (or filaments) and flakes. 

Dow, N.F. (1963). Study of stresses near a discontinuity in a filament-reinforced composite metal. In Space Sci. Lab. Missile and Space Div., General Electric Co. Tech. Report, No. R63SD61. 

Fiber-Matrix Composites. As shown in Figure 1.75, there are two main classifications of FMCs those with continuous fiber reinforcement and those with discontinuous fiber reinforcement. Continuous-flber-reinforced composites are made from fiber rovings (bundles of twisted filaments) that have been woven into two-dimensional sheets resembling a cloth fabric. These sheets can be cut and formed to a desired shape, or preform, that is then incorporated into a composite matrix, typically a thermosetting resin such as epoxy. Metallic, ceramic, and polymeric fibers of specific compositions can all be produced in continuous fashions, and the properties of the... 

The fibers are produced continuously (endless fibers, filaments) or discontinuously (short or staple fibers) depending upon the application. Most of the fibers are obtained by extruding a flowable form (melt or solution) of appropriate chemieal composition (Section 5.2.3 - 5.2.4). Other processes are based on deposition from the gas phase (Section 5.2.6.2) or thermal transformation (pyrolysis) of organic (Section 5.2.5) or organometallic polymers (Section 5.2.7). 

When the polymeric component forms the continuous phase, spheres, cylinders, or platelets may be added, as illustrated under reinforced polymers. The fiber composites are the most highly researched, as far as different modes of mixing are considered. The filaments may be continuous or discontinuous, or oriented or random in the matrix, with many subclasses of partial orientation possible (not shown). The tape composites are interesting since in some quarters these may be considered a two-dimensional analog of the highly oriented, continuous fibers embedded in a plastic matrix. The reinforced elastomers differ from the reinforced plastics in two ways the mechanical properties of the polymeric substrate, and the size of the reinforcing particles with respect to polymer chain dimensions. Because of the poor properties often obtained, it is rare to see a research paper on large particles dispersed in an elastomer. 

In addition to being available as continuous filaments and staple fibers in mats, fiberglass textiles are also available as biaxial, triaxial, knitted and three dimensional braided patterns. Many different resin matrices are in use but the emphasis in this chapter will be on unsaturated polyester and epoxy resins. While the strength and stiffness are controlled primarily by the reinforcements, the resinous matrix contributes to thermal conductivity and flexibility. The ultimate properties of these composites are based on a harmonious contribution of both the continuous and discontinuous phases. 

Advanced composite n. Polymer, resin, or other matrix-material system in which reinforcement is accomphshed via high-strength, high-modulus materials in continuous filament form or is discontinuous form such as staple fibers, filberts, and in-situ dispersions. Harper CA (2002) Handbook of plastics, elastomers, and composites, 4th edn. McGraw-Hill, New York. 

The same argument may be applied to tensile stresses. The composite structure will have tensile strength and tensile modulus which reflects the values for the glass plus the resin phase. The stress transfer around the discontinuities in the filaments will increase the average tensile strength to the glass phase. In addition, the yield in the resin phase will overcome the tendency to brittleness in the material. 

The reduction in tensile stress towards the ends of each fibre inevitably leads to a decrease in the tensile modulus compared with the continuous filament case. Consider a plane drawn perpendicular to the stress direction in an aligned discontinuous fibre composite (Figure 8.4), which must intercept individual filaments at random positions along their length. Hence the stress carried by the composite must be lower than that for the continuous filament case, and is dependent on the length of each fibre. Cox [9] predicted a correction factor rji for the tensile modulus in the axial direction that takes into account the finite length of the fibres so that Equation (8.3) is modified to... 

It is often the case that an old idea bears reexamination. In 1889, T. V. Hughes and C. R. Chambers patented a process for pyrolyzing marsh gas in iron crucibles to produce electric lamp filaments, one of many ideas whose time had not yet come. Today, General Motors Research (GMR) physicists are using hydrocarbon vapors to catalytically grow carbon fibers. The strong, stiff, discontinuous carbon fibers produced by this method have potential usefulness in many composite applications. 

Continuous fibers, such as those in filament winding, cross laminates, and cloth laminates, can transmit the applied load or stress from the point of application to the reaction via a continuous load path. If the fibers are not continuous between the load and the reaction, the matrix must transfer the load from one fiber to the next at the points of discontinuity. Fiber continuity also affects the type of failure of the composite. 

There are three basic types of engineered composites (1) laminates, (2) particle-reinforced composites, and (3) fiber-reinforced composites. In particle-reinforced composites, one can make the distinction between small (submicron) particle composites, where the particles are incorporated in the microstructure, vs. large particle composites, where the particles themselves actually do the work or carry the load. The reinforcing fibers can be discontinuous or continuous. The fibers in discontinuous fiber-reinforced composites can be randomly oriented to provide isotropic properties or aligned to enhance a specific property in a specific direction. Continuous fiber composites are generally designed for their unidirectional properties but can be crisscrossed to obtain multidirectional property enhancement such as in a filament-woimd pressure container. All possible permutations of metal, ceramic, and pol)uner are foimd in the laminated as well as in the reinforced composites. 

Another classification of composite materials is based on the morphology of the reinforcement introduced into the matrix. The composites therefore could be categorised into two main groups continuous reinforced system and discontinuous reinforced system. In the former, the matrix contains continuous fibres or filaments, which typically distribute in a uniaxial direction and enhance the strength and creep resistance of the matrix. In the latter, the matrix contains particulates, whiskers or chopped fibres, which distribute in the matrix uniformly in random orientations. Moderate toughening and reasonable levels of strengthening can be achieved. 

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