Fiber loading, role

Fibers are often regarded as the dominant constituents in a fiber-reinforced composite material. However, simple micromechanics analysis described in Section 7.3.5, Importance of Constituents, leads to the conclusion that fibers dominate only the fiber-direction modulus of a unidirectionally reinforced lamina. Of course, lamina properties in that direction have the potential to contribute the most to the strength and stiffness of a laminate. Thus, the fibers do play the dominant role in a properly designed laminate. Such a laminate must have fibers oriented in the various directions necessary to resist all possible loads. 

In contrast, when a stress acts in the 2-3 plane, as in Figure 5.86b, the matrix plays a crucial load-bearing role. Fibers and matrix now couple approximately in series, and the whole tensile force is assumed to be carried fully by both the fibers and matrix. The tensile forces in the fiber and matrix, a/2 and ct 2, are therefore equal to each other and to the overall stress in the composite, CT2I... 

Although the proteins in skin are also composed of about 5% elastic fibers, they do not appear to affect the mechanical properties of the tissue. The elastic fibers are believed to contribute to the recoil of the skin, which gives it the ability to be wrinkle-free when external loads are removed. As humans age, the elastic fiber network of the skin is lost, and wrinkles begin to appear. The mechanical role of the elastic fibers is very different in vascular tissue, however. 

One of the most important characteristics to consider in choosing a matrix is its adhesion with the fiber. The fiber/matrix interfacial adhesion plays a critical role in the mechanical properties of the composite. The loads are transferred from the matrix to the fiber through the interface, and the strength of the composite depends on the bond between fiber reinforcement and matrix. 

Orientational order plays an important role in solid polymers. It is often induced by industrial processing, for example in fibers and injection- or compression-modulated parts. In polymers with liquid-crystalline properties of the melt or solution, the anisotropies generated by the flow pattern are particularly pronounced. In order to improve the mechanical properties of polymer fibers or films, the degree of orientation is intentionally enhanced by drawing. At the same time, anisotropy of mechanical properties can result in low tolerance to unfavourably directed loads. In many liquid-crystalline polymers, in the mesophase near the transition to the isotropic phase, electric or magnetic fields can induce macroscopic orientational order [1]. Natural polymers such as silk protein fibers, which are biosynthesized and spun under biological condition, also have good mechanical properties because of their ordered structure [2]. 

Jin et al. [52] have deposited 5 wt% Pd on activated carbon fibers by alkaline hydrolysis of palladium chloride and obtained metal dispersions of 55 to 77%. Dispersions of 40 to 50% have been reported by Farkas et al. [53], who prepared Pd/C by fast addition of NaOH solution to a suspension of carbon in an aqueous solution of K2PdCl4. More highly loaded Pd and Pt catalysts (10 wt%) have been prepared by dropwise addition of the metal salt solution to the suspension of carbon in Na2C03 solution. In this case [54], a Pt particle size of 10 nm and a Pd particle size of 17 nm were reported. Ion adsorption led to much lower particle sizes. By quick addition of NaOH solution to a suspension of carbon support in PdCl2 solntion, Cabiac et al. [55] obtained 5- to 10-nm Pd particles at a loading of about 4 wt%. Clearly, details of support, metal loading, and the method of mixing of reactants all play a vital role in the dispersion and distribution of the metal in the finished catalyst. 

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