Orientation in composites

Usually, the fibres are not unidirectional oriented in composite materials. Fibres orientation varies in function of the pieces thickness, processing method, and mould geometry. Accepting a random orientation of fibres, or a known and measured orientation, the laminates theory was appealed in order to describe the behaviour of this type of composites. The method allows the determination of the weakest layers, which will be firstly fractured [1285]. The Figure 3.456 illustrates a good agreement between the theory and practice, considering, for comparison, the data furnished by P.E. Chen [1285] and J.K. Lees [1286]. 

Rybnikar F (1989) Orientation in composite of polypropylene and talc, J Appl Polym Sci 38 1479-1490. 

Laminated Strand Products. The most recent developments in the family of wood-based composites are a group of laminated strand products, made with strands oriented in the long direction of the product and marketed as stmctural composite lumber. One product is made with long, narrow strips of softwood veneer. The strips or strands are about 2.5 x 13 x 600 mm (0.1 x 0.5 x 24 in.), coated with a PRE adhesive, and pressed under heat and pressure into large blocks. After the resin is cured the blocks are resawn and planed into lumber dimension stock. 

Laminates ate a special form of composite material or reinforced plastic because the continuous reinforcing ply of fibrous material imparts significant strength in the x—j plane. The strength along the axis results from interlaminar bonding of resins. Very few fibers ate oriented in the direction, so it tends to be the weak link in this type of composite. 

As we have already seen, when an alloy contains more of the alloying element than the host metal can dissolve, it will split up into two phases. The two phases are "stuck" together by interphase boundaries which, again, have special properties of their own. We look first at two phases which have different chemical compositions but the same crystal structure (Fig. 2.5a). Provided they are oriented in the right way, the crystals can be made to match up at the boundary. Then, although there is a sharp change in... 

This is more than one-half of the strength of the continuous-fibre material (eqn. 25.3). Or it is if all the fibres are aligned along the loading direction. That, of course, will not be true in a chopped-fibre composite. In a car body, for instance, the fibres are randomly oriented in the plane of the panel. Then only a fraction of them - about - are aligned so that much tensile force is transferred to them, and the contributions of the fibres to the stiffness and strength are correspondingly reduced. 

Composites fabricated with fixed catalyst VGCF can be designed with fibers oriented in preferred directions to produce desired combinations of thermal conductivity and coefficient of thermal expansion. While such composites are not likely to be cost-competitive with metals in the near future, the ability to design for thermal conductivity in preferred directions, combined with lower density and lower coefficient of thermal expansion, could warrant the use of such VGCF composites in less price sensitive applications, such as electronics for aerospace vehicles. 

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. 

For scaly fillers the increase of relative viscosity with filler concentration is not as pronounced as in case of fibrous fillers [177,178]. Owing to filler orientation, the flow curves for systems with different concentrations of a fibrous and a scaly filler may merge together at high shear rates [181]. In composites with a dispersed filler the decrease of the effective viscosity of the melt with increasing strain rate is much weaker. 

FIGURE 4.15 Cross-section of multilevel interconnections for advanced bipolar devices. Fourteen separate layers are laid down in the fabrication of interconnections such as the one shown. The precise orientation and composition of these layers are controlled by chemical process steps. Copyright 1982 by the International Business Machines Corporation. Reprinted with permission. 

Different ways of the structural classification of deposits exist. In one system, the following structures are distinguished arbitrarily (1) fine-crystalline deposits lacking orientation, (2) coarse-crystalline deposits poorly oriented, (3) compact textured deposits oriented in field direction (prismatic deposits), and (4) isolated crystals with a predominant orientation in the field direction (friable deposits, dendrites). The structure of metal deposits depends on a large number of factors solution composition, the impurities present in the solntion, the current density, surface pretreatment, and so on. 

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