Discontinuous reinforcement composites

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. 

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... 

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. 

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. 

Many naturally occurring materials such as wood are reinforced composites consisting of a resinous continuous phase and a discontinuous fibrous reinforcing phase. 

There are many ways to classify composites, including schemes based upon (1) materials combinations, such as metal-matrix, or glass-fiber-reinforced composites (2) bulk-form characteristics, such as laminar composites or matrix composites (3) distribution of constituents, such as continuous or discontinuous or (4) function, like structural or electrical composites. Scheme (2) is the most general, so we will utilize it here. We will see that other classification schemes will be useful in later sections of this chapter. 

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 second factor that affects performance in discontinuously reinforced FMCs is fiber length. This has an effect primarily on the ease with which the composite can be manufactured. Very long fibers can create difficulties with methods used to create discontinuously reinforced FMCs and can result in nonuniform mechanical properties. The third factor is also related to fiber geometry, namely, the fiber shape. Recall that the... 

Calculate axial and transverse modnli and strengths for nnidirectional, continuous and discontinuous, fiber-reinforced composites and laminate composites. Describe the structure of soft and hard biologies that give rise to their unique mechanical properties. 

A.2.3 Composite Moduli Halpin-Tsai Equations. Derivations of estimates for the effective moduli (tensile E, bulk K, and shear G) of discontinuous-fiber-reinforced composite materials are extremely complex. The basic difficulty lies in the complex, and often undefined, internal geometry of the composite. The problem has been approached in a number of ways, but there are three widely recognized... 

Figure 5.98 Schematic illustration of discontinuous, aligned fiber-reinforced composite.

The rule of mixtures generally provides a poor estimate of the bulk and shear moduli in both the axial and transverse directions, respectively, in unidirectional, discontinuous-fiber-reinforced composites. 

In this chapter, we have sought to provide a state-of-the-art review of the mechanics and micromechanisms of high temperature crack growth in ceramics and discontinuously reinforced ceramic composites. Because of the limited amount of experimental data available in the literature which pertains primarily to oxide cermics and SiC reinforcements, the discussions of crack growth rates and fracture mechanisms have centered around alumina ceramics, with and without SiC reinforcements. However, the generality of the mechan-... 

R. M. Aikin, Strengthening of Discontinuously Reinforced MoSi2 Composites at High Temperatures, Mater. Sci. Engr., A155, 121-133 (1992). 

Stress Distribution and High Temperature Creep Rate of Discontinuous Fiber Reinforced Metals, Acta Metallurgies et Materialia, 38, 1941-1953 (1990). 26. A. G. Evans, J. W. Hutchinson, and R. M. McMeeking, Stress-Strain Behavior of Metal Matrix Composites with Discontinuous Reinforcements, Scripta Metallurgica et Materialia, 25, 3-8 (1991). 

Lehmann, R. L. el al. (Ed.) 1995. Handbook on Discontinuously Reinforced Ceramic Matrix Composites. Am. Ceram. Soc., Westerville, Ohio. 

For this reason, considered at present are possibilities of creating essentially more cheap materials, namely discontinuously reinforced Ti-matrix composites - so called DRTi. According to Miracle [20], the reinforcement with TiB fibres is the most effective one. For example, incorporation of reinforced phase up to 40 vol. % allows a combination of properties (especially stiffness) to be essentially increased. 

Key words Titanium in-situ composites, discontinuous reinforcement, silicide reinforcement, boride reinforcement, mixed silicide-boride reinforcement, ductile reinforcement, microstructure, a -phase, (3-phase, mechanical properties, fracture mechanisms. 

Titanium-based composites discontinuously reinforced with silicides, borides and their mixtures are an attractive candidate to be a material with... 

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