Normal Fiber Orientation

Due to the high normal and shear stresses, equivalent stresses, exceeding the yield limit of the matrix material, are produced near the surface and along the fiber/matrix interfacial regions. As a result, the matrix material becomes deformed and eventually can be sheared off in the form of thin wear debris layers. 

Comparing the failure mechanisms predicted, matrix shearing and fiber/matrix debonding seem to be the most dominant ones at the beginning of the wear process. They are followed by fiber cracking, as a further wear phenomenon, because the maximum compression stress at the top of the fibers is close to the compression strength limit. 

---

Figure 11 shows the contact pressure distribution in the cases of normal fiber orientation. It can be seen that mostly fibers transfer the load. The frictional force induces an asymmetric contact pressure distribution on the top of the loaded fibers. The local pressure peaks appear at the rear edges of the loaded fibers located within the contact area. 

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). 

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... 

The sheets of SMC are cut into a desirable shape (charge is weighed in case of BMC) and stacked on the lower half of the mold, and the movable upper half is brought down to close the mold. BMC material is normally consolidated by hand before they are placed in the mold in most cases the charge is placed centrally in the mold. The placement determines the quality of the part as it influences the length of the flow in the mold, fiber orientation, flow line, and other surface defects. 

In the low shear rate region, the first normal stress difference increases with the fiber content, which may be due to the hydrodynamic effect associated with fiber orientation in the flow direction. The effect of surface treatment was identical to that in the high shear rate region. The shear viscosity and the first normal stress difference as a function of shear rate for maleic anhydride modified PP (5wt.%) added composites (GF/mPP/PP compositese) are plotted in Fig. 2. Although the overall level of the both functions rj and Nj was higher than that in GF/PP composites (Fig. 1), the effect of surface treatment by lwt.% ASC on rj and NiofGF/PP composites is similar to that of GF/mPP / PP. 

A paper based on carbon fiber can be made using a conventional paper making plant, where chopped fiber is dispersed in a liquid carrier (normally water) containing wetting and binding agents. The dispersed fiber is removed from the slurry by vacuum deposition onto a perforated screen, washed and the carbon paper removed from the screen and dried. If a water flume is used to spread the fibers, then it is possible to give a product with more than 80% fiber orientation. Paper is used for specialist applications like loudspeaker cones. 

The orientation-efficiency factor was theoretically determined by Krenchel [26]. For in-plane uniformly distributed fiber orientations, the factor is 3/8 and for 3D uniform fiber orientation distribution is 1/5. In practice, the determination of fiber orientation distribution is normally done by image analysis of different sections of the composites [27]. In the case of all-cellulosic based composites this would be impossible, due to their thickness. In this case, the value of the orientatirai-effi-ciency factor was taken as the value of the order parameter, S, determined by SALS. 

The microindentation test is run on an individual selected fiber oriented normal to a polished cross-section of a high fiber volume fraction composite. Instead... 

Aside from normal fiber patterns with the fiber axis coinciding with the molecular axis, bi-oriented morphologies are frequent in a-iPP films due to selfepitaxy related to the quadritic morphology. Patterns may result evidencing two distinct crystallite orientations, for which the a ... 

Since xj/ is an even function, the average of products of odd number of components of p is zero. Also, since the distribution function is normalized (Eq. 5.2) and p is a unit vector, one has akk= 1- From the definition, one can also find the symmetry = Uji. Therefore, the second-order orientation Uy has only five independent components. Figure 5.1 shows some extreme cases of fiber orientation distributions and corresponding values of the second-order orientation tensor components. 

Figure 1. The studied fiber orientations with a hemispherical sliding asperity (a) normal (N), (b) parallel (P), and (c) anti-parallel (AP). ...

The sliding indentation problems for different fiber orientations were modeled, as shown in Figure 1. The normal and tangential forces, and Fj, produce a frictional contact state, associated with stresses and strains. In order to obtain accurate contact results, a larger segment of the bodies should be involved in the model. At this level, a macroscopic approach, assuming anisotropic material properties (see last column in Table 1), is used. The micro-model can describe the fiber/matrix structure in a more realistic way, but it requires much more elements. 

The debonding models use a linear elastic-plastic material law for the matrix material that allows to study the plastic deformation in the matrix material. Due to this fact, the smaller diamond indentor (R = 100 jam) with a normal load Fn= 1N was used in the debonding simulation. The friction coefficient, obtained experimentally for N-fiber orientation, was = 0.1. The results presented here are only for N-fiber orientation, although additional investigations were carried out also for P- and AP-fiber orientation. ... 

To simulate debonding phenomena that can occur during the scratch test described in Section 2, at first contact calculation must be carried out. At a normal load Ff) = 1 N, the measured coefficient of friction between the diamond indentor and the CF/PEEK specimen for N-fiber orientation was /x = 0.1. 

---