Parallel Fiber Orientation

With respect to the wear process, the most important wear mechanisms are fiber thinning, fiber/matrix debonding, fiber cracking, and shear events of the matrix material. Since the reinforcing fibers are oriented parallel to the contact plane, they are, first of all, subjected to bending. 

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Anderson (1978) recommended the rabbit detrusor muscle as an unique in vitro smooth muscle preparation. Rabbit detrusor muscles are thin and devoid of underlying submucosal tissue with parallel fiber orientation. The tissue exhibits autorhythmicity, characteristic of most single unit type smooth muscle preparations and can be employed in either isometric or isotonic organ bath recording systems. 

Analogous results have been found for stress relaxation. In fibers, orientation increases the stress relaxation modulus compared to the unoriented polymer (69,247,248,250). Orientation also appears in some cases to decrease the rate, as well as the absolute value, at which the stress relaxes, especially at long times. However, in other cases, the stress relaxes more rapidly in the direction parallel to the chain orientation despite the increase in modulus (247.248,250). It appears that orientation can in some cases increase the ease with which one chain can slip by another. This could result from elimination of some chain entanglements or from more than normal free volume due to the quench-cooling of oriented polymers. 

We will see in Section 5.4.2 that the elastic modulus of a unidirectional, continuous-fiber-reinforced composite depends on whether the composite is tested along the direction of fiber orientation (parallel) or normal to the fiber direction (transverse). In fact, the elastic modulus parallel to the fibers, Ei, is given by Eq. (1.62), whereas the transverse modulus, 2, is given by Eq. (1.63). Consider a composite material that consists of 40% (by volume) continuous, uniaxially aligned, glass fibers (Ef =16 GPa) in a polyester matrix (Em = 3 GPa). 

Real situations demand more uniformity in composite properties than can be provided by unidirectional composites. Therefore lamina stacking sequences are made where the fiber orientation is altered to provide good properties in all directions. Lamina composed of fiber and matrix in which the fibers are all parallel to each other are stacked on top of each other with a systematic variation in fiber direction. These lamina are then bonded together and the resulting material has more uniformity in properties. Likewise in short fiber or discontinuous fiber composites fiber orientation is random. Therefore properties in directions other than parallel to the fiber (i.e. off-axis) are important70>. 

Figure 17 illustrates the effect of orientation on the stress-strain properties of the rayon composite shown in Figures 10 and 11. The upper curve represents stress-strain behavior for stress applied parallel to the fiber orientation direction. In the lower curve the force is applied perpendicularly. Even a small degree of orientation has a large effect on the anisotropy of the composite. The differences in tensile strength, modulus and elongation at break in the two directions are considerable. 

Careful work is necessary to remove all preferred orientation from powder samples. Figure 1 shows results obtained with polyethylene terephthalate (PET) fibers. Curve is a typical azimuthal scan of the 010 peak (20 = 17,5°) for a bundle of parallel fibers placed perpendicularly to the x-ray beam. Curve b is the same scan carried out on a "powder" sample, showing that all preferred orientation is removed in our conditions of moulding (350 kg/ m2). For each kind of fiber, it is necessary to do preliminary trials to find the best experimental conditions. For PET fibers, we show on Figure 2 the relative crystallinity index and the residual orientation plotted against the cut-lengh. (5). 

Curve a bundle of parallel fibers put perpendicularly to the x-ray beam Curve b powder" made of cut fibers. It can be ascertained that all preferential orientation is practically removed when a powder sample made of regular small cross section is used. 

The 145.7 MHz proton noise decoupled 31p nmR spectra of the poly(dA-dT) duplex in 10 mM cacodylate buffer between 28° and 54°C are presented in Figure 9. A broad symmetrical unresolved resonance is observed at 28°C. By contrast, two resolved narrow resonances separated by 90.2 ppm have been observed for 150 base pair long (dA-dT)n (41). Thus, though the resolution of dT dA and dApdT phosphodiesters cannot be achieved at the synthetic DNA level in solution (18), it has been observed for the same sequence at a shorter well defined length (41). More recently, two resolved 31p resonances have also been reported in poly(dA-dT) fibers oriented parallel to the direction to the magnetic field by solid state 3lp nmr spectroscopy (42). 

Mineral fi ber Index of refraction of medi urn Color seen with fiber oriented parallel to polarizer Color seen with fiber oriented perpendicular to polarizer ... 

Fig. 2.45 (a) Schematic of a double tip (b) AFM height image of electrospun polymer fibers. The appearance of parallel fibers (i.e., a pair of fibers) is highly unlikely on the basis of the orientation dependence of the pairs... 

In injection molded composites of polypropylene containing short glass fibers, the fiber orientation depended on the flow pattern (which, in turn, is related to mold thickness, the position of the gate, and flow rate)." Substantial variation was detected along the thickness of the sample. Crystallites followed a pattern of fiber distribution but they grew in a direction perpendicular to the direction of the fiber and specimen surface. The direction of spherulite growth was different in neat resin where crystallites grew parallel to the surface of the mold (specimen). 

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. 

Figure 11.16. Tensile strength of laminates with fibers oriented parallel to the surface. [Adapted, by permission, from Selzer R, Friedrich K, Composites, Part A, 28A, 1997, 595-604.]...

Another limitation that is specific to the parallel plate electrospinning technique is the collection of extremely thin nanofibers, which have been observed to break because they were unable to sustain the forces of their own weight and of the repulsive charges from other fibers [62]. An electrically resistive substrate inserted into the gap between the plates can provide support to fibers suspended between the plates without influencing fiber quality [62], and may also help to shield any conductive materials below the air gap, which may attract unwanted non-aligned nanofibers. Substrates with bulk resistivity greater than 10 Q cm, such as quartz and polystyrene, are suitable for placement between parallel electrodes, while materials with bulk resistivity of less than 10 Q cm, such as glass, may result in random fiber orientations [67, 68]. 

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