Fibers aspect ratio

Figure 22 Distribution of fiber aspect ratio l/d and fiber number N versus fiber length class for skin and transition layer of the four groups of samples injection molded.

According to the composite theory, tensile modulus of fiber reinforced composites can be calculated by knowing the mechanical constants of the components, their volume fraction, the fiber aspect ratio, and orientation. But in the case of in situ composites injection molded, the TLCP fibrils are developed during the processing and are still embedded in the matrix. Their modulus cannot be directly measured. To overcome this problem, a calculation procedure was developed to estimate the tensile modulus of the dispersed fibers and droplets as following. 

Nardin, M. and Schultz, J. (1993). Effect of elastic moduli and interfacial adhesion energy on the critical fiber aspect ratio in single fiber composites. J. Mater. Sci. Lett. 12, 1245-1247. 

The size of damage depends on fiber Ff, fiber aspect ratio, types of fiber and matrix material, bonding at the fiber-matrix interface, layup sequence in multi-angle ply laminates, specimen geometry including laminate thickness, and loading... 

The effect of fiber diameter on the tensile strength of a glass-fiber-reinforced polystyrene composite is shown in Figure 5.100. Some reinforcements also have a distribution of fiber diameters that can affect properties. Recall from the previous section that the fiber aspect ratio (length/diameter) is an important parameter in some mechanical property correlations. 

Whiskers have been produced in a range of fiber sizes and in three forms grown wool, loose fiber, and felted paper. The wool has a fiber diameter of 1-30 p,m and aspect ratios of 500-5000. The wool bulk density is about 0.03 g/cm, which is less than 1% solids. This is a very open structure, suitable for a vapor-deposition coating on the whiskers or direct use as an insulation sheet. Loose fibers are produced by processing the larger diameter fiber to yield lightly interlocked clusters of fibers with aspect ratios of 10-200. In felt or paper, the whiskers are randomly oriented in the plane of the felt and have fiber aspect ratios ranging from 250 to 2500. The felted paper has approximately 97% void volume and a density of 0.06-0.13 g/cm. ... 

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).]...

Fig. 4.9 Effect of particle asperity on the relative viscosity of molten polymer suspensions. Particles studied were as follows , glass spheres , natural calcium carbonate A, precipitated calcium carbonate o, glass fibers—aspect ratio = 18 ...

A carbon fiber—aspect ratio = 23 + carbon fibers—aspect ratio = 27. Data are from Ref. 67. 

We first define some terms commonly used in the field of fibrous materials. We should add that some of these definitions are expanded upon later in this chapter. However, before one can define the term fiber, one needs to define the most important attribute of the fiber that serves to define a fiber, namely the fiber aspect ratio. The aspect ratio of a fiber is the ratio of its length to diameter (or thickness). A fiber can be defined as an elongated material having a more or less uniform diameter or thickness less than 250 p,m and an aspect ratio greater than 100. This is an operational definition of fiber. It is also a purely geometrical one in that it applies to any material. Having defined the basic unit, the fiber, we are now in a position to define some other commonly used terms related to fibers. These are given below in alphabetical order ... 

Since at steady state the angular distribution of fiber orientations is predicted to be symmetric about the flow direction in a shearing flow, Eq. (6-50) implies that the normal stresses (e.g., a oc [u uy) will be identically zero. However, nonzero positive values of N have frequently been reported for fiber suspensions (Zimsak et al. 1994). Figure 6-24 shows normalized as discussed below, as a function of shear rate for various suspensions of high fiber aspect ratio. These normal stress differences are linear in the shear rate and can be quite large, as high as 0.4 times the shear stress, which is dominated by the contribution of the solvent medium, cr Fig- 6-24, the N] data are normalized... 

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. 

The elastic moduli vs. temperature curves of the hypoeutectic Ti-Si7.5-All alloy and of the directionally solidified eutectic a-Ti-Ti5Si3 composite with discontinuous Ti5Si3 fibers -aspect ratio of lf/df 50- are depicted in fig. 14. In comparison the temperature-dependent Young s modulus of the polycrystalline Ti5Si3 compound is also shown in the diagram. A detailed description and discussion of the elastic moduli of the pure silicides is given in section 4.3. 

The longer the fiber (the higher is the fiber aspect ratio) and the more it is oriented longitudinally, along the deck board, the lower is the coefficient of thermal expansion-contraction. Overall, for different commercial WPC deck boards the coefficient is in the range of 2 X 10 to 5 X 10 1/°F. In other words, some commercial WPC boards can expand-contract by 250% higher than others. These overexpanded decks are very noticeable and sometimes cause complaints from the deck owners. 

Distribution of fibers Aspect ratio Filler Frequency of oscillation Coefficient of expansion of filler Shear modulus Gibbs free energy Heat of reaction, change Hildebrand unit, solubility parameter value High-density (linear) polyethylene Heat deflection temperature... 

Since most of the LCPs are immiscible with conventional polymers, the mechanical properties are less than those predicted in theory. This results from poor interfacial adhesion. Actually, this problem can be overcome by proper compatibilization. The same compatibilizers, which are common in other fields, can be used. The compatibilizers help to improve the dispersion of the fibers and increase the fiber aspect ratio. Compatibilizers are summarized in Table 16.5. 

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