Discontinuous Reinforcements

Over the last decade, considerable efforts have been committed to the toughening of sialons and substantial progress has been achieved using various reinforcements. According to the form of reinforcement, sialon composites can be classified as either particle reinforced, discontinuous fibre (whiskers/ short fibres) reinforced, or continuous fibre reinforced. 

General, the highest strength and stifihiess are obtained with continuous fibre reinforcement Discontinuous fibres are used only when manufacturing economics dictate the use of a process where the fibres must be in this form—for example injection moulding. 

Significant reinforcement is possible only if the matrix-fiber bond is strong. Because reinforcement discontinues at the fiber extremities, reinforcement efficiency depends... 

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. 

The interface region in a composite is important in determining the ultimate properties of the composite. At the interface a discontinuity occurs in one or more material parameters such as elastic moduli, thermodynamic parameters such as chemical potential, and the coefficient of thermal expansion. The importance of the interface region in composites stems from two main reasons the interface occupies a large area in composites, and in general, the reinforcement and the matrix form a system that is not in thermodynamic equiUbhum. 

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. 

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,... 

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

In order to understand the effect of discontinuous fibres in a polymer matrix it is important to understand the reinforcing mechanism of fibres. Fibres exert their effect by restraining the deformation of the matrix as shown in Fig. 3.28. The external loading applied through the matrix is transferred to the fibres by shear at the fibre/matrix interface. The resultant stress distributions in the fibre and matrix are complex. In short fibres the tensile stress increases from zero at the ends to a value ([Pg.226]

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... 

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. 

Silicone rubber Warrick et al. [29] reviewed the reinforcement of sUicone elastomers with various types of discontinuous fibers, some of which were generated in situ by graft polymerization. Polyester, cellulose and carbon fibers were also used to reinforce sUicone mbber [30,31]. 

One of the general understanding about the carbon reinforcement of rubbers, which is vaguely understood but widely accepted, is to consider such a system where well-dispersed and discontinuously connected carbon particles are strongly adhered to the matrix cross-linked rubber. 

Obtain specific properties using, for example, a self-lubricating composite insert. Discontinuous fibre reinforced thermoplastic composites can also be overmoulded onto GMT sheets. 

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. 

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