Matrix phase fiber-reinforced composites

The fiber-reinforced composite materials include three phases surface of fiber side, the interface between fiber and matrix, and the interphase. These phases are collectively referred to as the interface [12]. The characteris-... 

Assume that the conductivity of a undirectional, continuous fiber-reinforced composite is a summation effect just like elastic modulus and tensile strength that is, an equation analogous to Eq. (5.88) can be used to describe the conductivity in the axial direction, and one analogous to (5.92) can be used for the transverse direction, where the modulus is replaced with the corresponding conductivity of the fiber and matrix phase. Perform the following calculations for an aluminum matrix composite reinforced with 40 vol% continuous, unidirectional AI2O3 fibers. Use average conductivity values from Appendix 8. 

Access to phase pure silicon nitride materials via processable precursors is limited to just three approaches. The first, shown in reaction 6, provides one of the first oligomers exploited as a preceramic polymer24,253. This simple polysilazane, containing only Si, N and H, is known to be relatively unstable and will crosslink on its own to give intractable gels. Furthermore, it does not offer the 3Si I4N stoichiometry required for Si3N4. Nonetheless, it is useful as a binder and for fiber-reinforced ceramic matrix composites (CMCs)31. 

For non-conducting fibers, such as glass, the matrix resin is the more conductive phase, at least early in cure, and one would expect some internal polarization effects to be visible in parallel-plate data. However, in spite of a large body of literature on glass fiber composites (see Sect. 5), we have found no clearly documented cases of Maxwell-Wagner effects in fiber-reinforced composites. We speculate that... 

In a recent study, the interphases for different fiber/polymer matrix systems were investigated. By using phase imaging the differences in local mechanical property variation in the interphase of glass fiber reinforced epoxy resin (EP) and glass fiber reinforced polypropylene matrix (PP) composites could be unraveled. As shown in Fig. 3.68, the glass fiber, the interphase and the PP matrix can be differentiated based on their surface mechanical properties as assessed qualitatively by TM phase imaging. 

Detailed microstructural investigation [12] revealed enhanced precipitation of Nd-rich phases at the SiC/matrix interfaces in the QE 22 -SiC composite after T6 heat treatment and during creep (Fig. 9). Such precipitation can detrimentally affect the creep behavior in a similar way as to fiber-reinforced composites. Further, Moll et al. [11] have proposed that poor creep resistance of the QE 22-SiC composite may be explained by taking into account interfacial sliding as an additional creep mechanism... 

Closed-form expressions from composite theory are also useful in correlating and predicting the transport properties (dielectric constant, electrical conductivity, magnetic susceptibility, thermal conductivity, gas diffusivity and gas permeability) of multiphase materials. The models lor these properties often utilize mathematical treatments [54,55] which are similar to those used for the thermoelastic properties, once the appropriate mathematical analogies [56,57] are made. Such analogies and the resulting composite models have been pursued quite extensively for both particulate-reinforced and fiber-reinforced composites where the filler phase consists of discrete entities dispersed within a continuous polymeric matrix. 

If the materials are anisotropic, they will present different properties in the different directions. Examples of these polymeric materials are polymer fibers, such as polyethylene terephthalate, PET, nylon fibers, injection-molded polymers, fiber-reinforced composites with a polymeric matrix, and crystalline polymers where the crystalline phase is not randomly oriented. A typical method for measuring the modulus in tension is the stress-strain test, in which the modulus corresponds to the initial slope of the stress-strain curve. Figure 21.4 shows typical stress-strain curves for different types of polymeric materials. 

At manufacturing level, chemical reactions between the matrix and the liber produce an interface zone of different mechanical properties from the two phases producing it [158]. The load of a composite is usually transferred through the interface between the matrix and the fiber, and the toughness of the composite is determined. Karpur et al. measured ultrasonically the shear stiffness coefficient of the interface in fiber reinforced metal matrix and ceramic matrix composites [158]. They claim that the significance of the quantification of the shear stiffness coefficient of the interface is that the clastic property of the interface can be used as a basis for composite life prediction. 

Tests on tin oxide fiber coatings in model composite systems indicated some crack deflection at the coating-fiber interface (Siadati et al., 1991 Venkatesh and Chawla, 1992). However, tensile tests of tin oxide coated alumina fiber-reinforced alumina matrix composites demonstrated a decrease in the extent of fiber pullout as the density of the matrix phase was increased. This led to increasingly brittle fracture behavior in these composites (Goettler, 1993). Tin oxide also has thermal stability problems at elevated temperatures (Norkitis and Hellmann, 1991). For example, in the presence of air at temperatures above 1300°C (2,372°F), tin oxide (solid) decomposes into SnO (gas) and Oj (gas). This decomposition occurs at even lower temperatures when the partial pressure of oxygen in the test environment is reduced. 

Tensile properties of unidirectional BN/SiC coated Hi-Nicalon fiber reinforced celsian matrix composites [36-37] from room temperature to 1200°C in air are shown in Table 6, The value of Young s modulus decreased with increase in test temperature indicating the presence of glassy phase in the matrix. The yield stress decreased from room temperature... 

A carbon-carbon composite is a carbon fiber reinforced earbon matrix material, where the earbon matrix phase is typically formed by the pyrolysis of a solid, liquid or gaseous organic precursor material. The matrix can be either a graphitizable or a non-graphitizable earbon and the carbonaceous reinforcement is in fibrous form. The composite may also eontain other eomponents in particulate or fibrous forms. 

The enhanced compressive mechanical properties in these composites were attributed to the development of better hydrophobic character, which results in increased interfacial adhesion between G. optiva fibers and hydrophobic matrix phase. The better performance of raw fibers-reinforced polymer composites at 20% of loading in comparison to 30% loading is because of hydrophilic nature of raw fibers which at high loading causes agglomeration of particle fibers and hence poor dispersability of reinforcement with matrix. 

Thus far, the oxidation/corrosion behavior of materials which are nominally single phase, and form scales which are nominally single phase oxides has been discussed. In practice, composite materials are often desired to enhance mechanical properties or to improve other properties of the materials such as thermal conductivity, electrical conductivity, as well as oxidation resistance. In these cases, multi-phase materials are developed to enhance the desired properties. When these materials are subjected to high temperature oxidizing environments, the resulting oxidation products are also more complex. In general oxidation results in three classes of scales mixed oxides, compound oxides, or solution oxides. Examples of each will be discussed below. In addition, fiber reinforced composites are a special class of composite materials in which an interphase material is required between the fiber and the matrix to provide the required fracture toughness. The unique issues with these materials will also be discussed. 

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