Discontinuous fibers

With continuous fibers, it can be assumed that the failure will ultimately occur by fracture of the fibers. Discontinuous fibers, on the other hand, can have three other types of failures (I) fracture of the resin at a weak net section (2) shear failure in the matrix at the points of discontinuity of the fiber and (3) failure of the bond between the fibers and the matrix. 

The main types of reinforcements used in composite materials include aligned continuous fibers, discontinuous fibers, whiskers (elongated single crystals), particles, and numer-... 

Monolithic metallic alloys are among the most widely used structural materials. By reinforcing them with continuous fibers, discontinuous fibers, whiskers, and particles, new materials are created with enhanced or modified properties, such as higher strength and... 

Ceramic matrix composites (CMCs) can be thought of as an improved form of carbon matrix composite in which the carbon matrix is replaced with ceramics that are stronger and much more resistant to oxidation. CMCs employ a variety of reinforcements including continuous fibers, discontinuous fibers, whiskers, and particles. Continuous fibers provide the best properties. There are many different types of CMCs, and they are at various stages of development. 

Reinforcements that have been used for CMCs include continuous fibers, discontinuous fibers, whiskers, and particles. Key continuous fibers used in CMCs include carbon, silicon carbide-based, alumina-based, alumina-boiia-sihca, quartz, and alkah-resistant glass. Steel wires are also used. Discontinuous CMC fibers are primarily silica based. Silicon carbide is the key whisker reinforcement. Particulate reinforcements include silicon carbide, zirconium carbide, hafnium carbide, hafnium diboiide, and zirconium diboride. 

Even though reinforcement efficiency is lower for discontinuous than for continuous fibers, discontinuous and aligned-fiber composites (Figure 16.8ii) are becoming increasingly more important in the commercial market. Chopped-glass fibers are used most extensively however, carbon and aramid discontinuous fibers are also used. These short-fiber composites can be produced with moduli of elasticity and tensile strengths that approach 90% and 50%, respectively, of their continuous-fiber counterparts. 

Triacetate offers better ease-of-care properties than secondary acetate ia many apparel appHcations. Of particular importance are surface-finished fabrics, eg, fleece, velour, and suede for robes and dresses. These fabrics offer superb aesthetic quaHties at reasonable cost. Triacetate is also deskable for print fabrics, where it produces bright, sharp colors. The recent discontinuance of triacetate fiber Hi the United States has led to the use of acetate with fibers such as polyester (47—50). 

Metal-Matrix Composites. A metal-matrix composite (MMC) is comprised of a metal ahoy, less than 50% by volume that is reinforced by one or more constituents with a significantly higher elastic modulus. Reinforcement materials include carbides, oxides, graphite, borides, intermetahics or even polymeric products. These materials can be used in the form of whiskers, continuous or discontinuous fibers, or particles. Matrices can be made from metal ahoys of Mg, Al, Ti, Cu, Ni or Fe. In addition, intermetahic compounds such as titanium and nickel aluminides, Ti Al and Ni Al, respectively, are also used as a matrix material (58,59). P/M MMC can be formed by a variety of full-density hot consolidation processes, including hot pressing, hot isostatic pressing, extmsion, or forging. 

Particle or discontinuously reinforced MMCs have become important because they are inexpensive compared to continuous fiber-reinforced composites and they have relatively isotropic properties compared to the fiber-reinforced composites. Figures la and b show typical microstmctures of continuous alumina fiber/Mg and siUcon carbide particle/Al composites, respectively. 

Laminated MMCs. There are three types of laminated MMCs (/) metallic matrix-containing fibers oriented at different angles in different layers, similar to that of polymeric laminates (2) two or more different metallic sheets bonded to each other and (3) laminated metal/discontinuously reinforced MMC. 

Crystalline polymers undergo a discontinuous decrease in volume when cooled through (Fig. 4). This can lead to nonuniform shrinkage and warping in molded objects. On the other hand, it also causes the polymer to "lock on" to reinforcing fibers, eg, glass (qv), so that crystalline thermoplastics benefit much more than amorphous thermoplastics from fiber reinforcement. 

Loose asbestos fibers, or formulations containing asbestos fibers for spray coatings, have been widely used in the building industry for fire protection and heat or sound insulation. Such applications used mainly chrysotile or amosite but, because of health concerns, this practice has been discontinued. 

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. 

The premise that discontinuous short fibers such as floating catalyst VGCF can provide structural reinforcements can be supported by theoretical models developed for the structural properties of paper Cox [36]. This work was recently extended by Baxter to include general fiber architecture [37]. This work predicts that modulus of a composite, E can be determined from the fiber and matrix moduli, Ef and E, respectively, and the fiber volume fraction, Vf, by a variation of the rule of mixtures,... 

In order for VGCF to be suecessfully incorporated into engineering composites, it must be available in forms which composite fabricators are equipped to handle. Since VGCF is bulky and discontinuous as produced, it is not amenable to the textile processing used for continuous carbon and glass fiber. Thus fiber... 

Baxter, W.J., The strength of metal matrix composites reinforced with randomly oriented discontinuous fibers, 1992, Metall Trans. 23A, 3045... 

Net-tension failures can be avoided or delayed by increased joint flexibility to spread the load transfer over several lines of bolts. Composite materials are generally more brittle than conventional metals, so loads are not easily redistributed around a stress concentration such as a bolt hole. Simultaneously, shear-lag effects caused by discontinuous fibers lead to difficult design problems around bolt holes. A possible solution is to put a relatively ductile composite material such as S-glass-epoxy in a strip of several times the bolt diameter in line with the bolt rows. This approach is called the softening-strip concept, and was addressed in Section 6.4. 

The processing of discontinuous fiber-filled thermoplastics owes much to the rheology of the system. Shear thinning and adiabatic heating associated with the broad... 

The rule of mixtures for discontinuous fiber composites may be expressed as ... 

A discontinuous fiber composite is one that contains a relatively short length of fibers dispersed within the matrix. When an external load is applied to the composite, the fibers are loaded as a result of stress transfer from the matrix to the fiber across the fiber-matrix interface. The degree of reinforcement that may be attained is a function of fiber fraction (V/), the fiber orientation distribution, the fiber length distribution, and efficiency of... 

With nondestructive ultrasonic test back and forth scanning of a specimen is accomplished with ultrasonics. This NDT can be used to find voids, delaminations, defects in fiber distribution, etc. In ultrasonic testing the sound waves from a high frequency ultrasonic transducer are beamed into a material. Discontinuities in the material interrupt the sound beam and reflect the energy back to the transducer, providing data that can be used to detect and characterize flaws. It can locate internal flaws or structural discontinuities by the use of high frequency reflection or attenuation (ultrasonic beam). 

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