Oriented fiber reinforcement effects

The fiber alignment is also a significant factor in composite properties. If the fibers in the foregoing example were randomly oriented their reinforcing effect would be less than 0.2 of the figure calculated above. 

J. P. Wu, T. C. Ovaert (1994) Effect of asperity-scale tensile stresses on the wear behavior of normally oriented fiber-reinforced polymer composites. Tribology Transactions 37,23. 

Note that no assumptions involve fiber-reinforced composite materials explicitly. Instead, only the restriction to orthotropic materials at various orientations is significant because we treat the macroscopic behavior of an individual orthotropic (easily extended to anisotropic) lamina. Therefore, what follows is essentially a classical plate theory for laminated materials. Actually, interlaminar stresses cannot be entirely disregarded in laminated plates, but this refinement will not be treated in this book other than what was studied in Section 4.6. Transverse shear effects away from the edges will be addressed briefly in Section 6.6. 

In the above, the variable R is the radius between center to center fiber spacing, while r is the fiber radius. The shear modulus (Gm) can be approximated as Em/3. The matrix modulus is effected by the level of crystallinity and it is important that the samples are fully crystallized to ensure reproducibility. The value of (> for 30wt% glass-fiber-reinforced PET has been calculated as 3.15 x 104. Using the mathematical analysis shown above, the orientation function of the glass fiber... 

Fu, S. and Lauke, B Effects of fiber length and fiber orientation distributions on the tensile strength of short-fiber-reinforced polymers, Composites Sci. Technol., 56, 1179 (1996). 

The effect of dispersoids on the mechanical properties of metals has already been described in Section 5.1.2.2. In effect, these materials are composites, since the dispersoids are a second phase relative to the primary, metallic matrix. There are, however, many other types of composite materials, as outlined in Section 1.4, including laminates, random-fiber composites, and oriented fiber composites. Since the chemical nature of the matrix and reinforcement phases, as well as the way in which the two are brought together (e.g., random versus oriented), vary tremendously, we shall deal with specific types of composites separately. We will not attempt to deal with all possible matrix-reinforcement combinations, but rather focus on the most common and industrially important composites from a mechanical design point of view. 

Fig. 11.24 The effects of the fiber aspect ratio, at constant 30% wt loading, on glass-filled polyamide-6. [Reprinted by permission from H. M. Latin, Orientation Effects and Rheology of Short Glass Fiber-reinforced Thermoplastics, Colloid Polym. Sci., 267, 257 (1984).]...

H. M. Laun, Orientation Effects and Rheology of Short Glass Fiber-reinforced Thermoplastics, Colloid Polym. Sci., 262, 257-269 (1984). 

In a properly foamed molding the foam cells are roughly spherical and the cell walls have no particular directional alignment. They are essentially randomly oriented. The local orientation and anisotropic reinforcement effect of the fibers around individual cell walls are integrated into, and become additional factors in, a general enhanced reinforcing effect that is isotropic in the molding considered as a whole (50). 

Material nonlinearity. This nonlinearity is present because of the fiber orientation in the composite. The effect of the Spectra fiber reinforcement on the composite structure was taken into account by a user subroutine which added the oriented fibers effect to the base material of the composite. 

Glass fibers are extensively used by industiy because of their reinforcing effect, and the improvements they produce in thermal properties such as a reduction in thermal expansion and an increase in heat deflection temperature. The most challenging tasks of fiber application include the incorporation process which must be designed to prevent breakage, improve matrix fiber adhesion, prevent fiber corrosion in some environments, and develop proper fiber orientation. 

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. 

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. 

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