Orientation of fibers

Composites fabricated with the smaller floating catalyst fiber are most likely to be used for applications where near-isotropic orientation is favored. Such isotropic properties would be acceptable in carbon/carbon composites for pistons, brake pads, and heat sink applications, and the low cost of fiber synthesis could permit these price-sensitive apphcations to be developed economically. A random orientation of fibers will give a balance of thermal properties in all axes, which can be important in brake and electronic heat sink applications. 

Orientation of fibers relative to one another has a significant influence on the strength and other properties of fiber-reinforced composites. With respect to orientation three extremes are possible as shown in Fig. 5. Longitudinally aligned fibrous composites are inherently anisotropic, in that, maximum strength and reinforcement are... 

The eccentric rotation of filler in virtue of the variant shear rates along the extrudate cross-section has been proposed as an alternative to the plug flow mechanism to explain this orientation pattern [355]. In [357] it was noted that the cross-wise orientation of fibers at the core of molded specimens was established only after the entire mold had been filled the authors assumed this orientation to be due to the quasi-static stresses arising in the material under pressure. 

Molecular orientation results in increased stiffness, strength, and toughness (Table 8-12) as well as resistance to liquid and gas permeation, crazing, microcracks, and others in the direction or plane of the orientation. The orientation of fibers in reinforced plastics causes similar positive influences. Orientation in effect provides a means of tailoring and improving the properties of plastics. 

Mini-Static Air. These foregoing results indicated that the use of a scintillation vial as a static air vessel might facilitate the measurement of release rates for compounds which were difficult to recover from the more commonly used traps. With this in mind, several orientations of fibers in scintillation vials were experimentally tested until the one pictured in Figure 4 was finally adopted as being the most workable and repro-... 

Reinforced plastics are usually applied as laminates of several layers. Many variables are important in determining the performance of the finished product. Some of the important ones are orientation of plies of the laminate, type of resin, fiber-resin ratio, type or types of fibers, and orientation of fibers. 

Figure 3 Structures of mammal bone (a) Haversian systems and osteons, (b) Pl)rwood-like structure with twisted lamella, (c) Lamellar bone showing the different orientations of fibers in successive lamella, (d) Minerahzed collagen fibers...

In the present paper we are seeking for simple theoretical predictions of the relationship between crack length and tearing modulus. Since it is reasonable to assume that the material obeys the piece wise linear yield condition developed by R. H. Lance and D. N. Robinson [6]. Although the yield surface strongly depends on the orientation of fibers we confine our attention to the case of circumferential arrangement of fibers in the matrix material, provided ka is the yield stress in the circumferential direction. The yield condition for circumferentially reinforced cylindrical tubes is presented in Fig. 3. Here... 

As many as 5 or 6 vents may be cut into a large tool. Fan or weir-type gates like those used for urethanes are preferred, particularly when reinforced systems are used. This minimizes any tendency to part warping caused by radial orientation of fibers. 

Figure 7.3. Feed arrangements to produce orientation of fibers. [Adapted, by permission, from Allan P S, Bevis M J, Materials World, 2, No.l, 1994, 7-9.]...

Also, the proportion of fibers in the skin (or shell) and those in the core depends on the rates of fiow." Low injection rates and low temperatures causes an expansion of the shell (skin) region These relationships also affect the orientation of polymer chains in filled and unfilled polymers during proc-essing Orientation of fiber in blow molding of bottles filled with fibers caused anisotropy of properties. Tensile strength was increased in the machine direction. At the same time, talc filled bottles had more uniform tensile properties than unfilled bottles. ... 

Figure 7.9 shows that orientation of fiber affects the wear resistance of a mate-rial. The lowest wear occurs when fibers are perpendicular to the matting surface. If both matting surfaces are made out of fiber filled materials, the wear properties can be further optimized by the choice of fiber orientation in respect to both surfaces. 

To make practical use of fillers, a knowledge of filler orientation during the flow is needed The best description of orientation principles can be obtained from modelling Figure 9.10 gives the coordinate system used to describe orientation of fibers. 

Figure 9.11 shows that the average angle a of whiskers decreases with the elongational strain rate, Figure 9.12 shows the effect of the aspect ratio of ferrite on the apparent permeability of a composite. Both sets of experimental data are consistent with the model. The orientation of fiber increases with the elongational strain rate and fiber aspect ratio. 

In fiber filled systems, the elongation of a material correlates with the orientation of fibers (Figure 9.29). This phenomenon is frequently exploited in industrial processes to increase reinforcement and other properties which depend on fiber orientation. Hencky strain shown in Figure 9.29 is a parameter entering in the equation of elongational viscosity. ... 

Orientation of fiber is detrimental to performance of composites which contain carbon fibers (Figures 11.16 and 11.17). T ensile strength of composites containing fibers oriented in a direction parallel to the surface is not affected by moisture content. Composites which have fibers oriented in a direction perpendicular to the surface, lose tensile strength as moisture increases (exception — carbon fiber/PEEK composite). Similar effects on tensile modulus, compression modulus and elongation have been observed. 

The orientation of fibers discussed above has led to the use of uniaxially drawn polymeric material such as polyethylene or poly(vinyl) alcohol containing a guest molecule of interest to be used for fundamental studies on the optical transition moments of the guest. Molecules studied in this may Include pyrex, acenaphthylene, benzo(g.h.1.)perylene, benzo(g.h.l.)fluoranthene, naphtha-cene, perylene, and coronene, and the fluoranthenes (99-102). 

In addition, surface forces [240] and shear fields [%, 188, 241-243] have successfully been employed in orienting the LCPs. Solid state extrusion [10, 188] and mdt-spinning [96] produce fibers, with nearly perfect aligoonent of the director axes 0.9). This is demonstrated in Fig. 30. The H NMR spectra Ctop row) refer to melt-spun fibers of LCP 4 (a-CD ) and five different orientations of fiber axis and magnetic field. Drastical lineshape chan are observed when the sample is rotated. A detailed analysis, based on spectral simulations (bottom row), provides the parameters of micro- and macroorder, summarized in Table 9 [96]. 

As we have seen, the presence of fibers in the matrix has the effect of stiffening and strengthening it. The tensile deformation behavior of fiber-reinforced composites depends largely on the direction of the applied stress in relation to the orientation of the fibers, as illustrated in Figure 3.48. The maximum strength and modulus are achieved with unidirectional fiber reinforcement when the stress is aligned with the fibers (0°), but there is no enhancement of matrix properties when the stress is applied perpendicular to the fibers. With random orientation of fibers the properties of the composite are approximately the same in all directions, but the strength and modulus are somewhat less than for the continuous-fiber reinforcement. 

In flexure or shear, as in the previous case of compression, plastics reinforced with short fibers are probably better than those with continuous fibers, because in the former with random orientation of fibers at least some of the fibers will be correctly aligned to resist the shear deformation. However, with continuous-fiber reinforcement if the shear stresses are on planes perpendicular to the continuous fibers, then the fibers will offer resistance to shear deformation. Since high volume fraction (f>() can be achieved with continuous fibers, this resistance can be substantial. 

Angle of orientation of fibers in a ply relative to the laminate x-axis Laminate curvatures (units 1/length)... 

RPs are usually applied as laminates of several layers. Many variables are important in determining the performance of the finished product. Some of the important ones are orientation of plies of the laminate, type of plastic, fiber-plastic ratio, type or types of fibers, and directional orientation of fibers (Chapter 7). Nonwoven fabrics are fibrous sheets made without spinning, weaving, or knitting. They include felts, bonded short to long fiber febrics, and papers. The interlocking of fibers is achieved by a combination of mechanical work, chemical action, moisture, and heat by either textile or paper malting processes. 

Various processes are reported to provide a means to pulsate the melt to improve performance of the extruded product. An example is the Scorim process reviewed in the Injection Molding section in this chapter. A variation in the IM process has been applied to production of reinforced extruded TP pipe. This has been a center of interest, on the argument that a predictable orientation of fiber would considerably increase the pressure resistance of the pipe, without the need to increase wall-thickness (on the analogy of winding a TS resin pipe with continuous filament). The Scorim process combines the extrusion of a fiber-reinforced compound with pressure pulsing around the periphery of the die, which appears to have the effect of orientating the reinforcement. 

The description of the origin, effects and elimination of molecular orientations applies analogously to orientations of fibers (but not to their elimination). Glass fibers, for example, orient themselves in the direction of flow due to shearing forces in the plastic melt. 

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