Preferential fibre orientation

In a transversally isotropic material, there is a plane in which all properties are isotropic. Perpendicular to this plane, the properties differ. One example for such a material is a hexagonal crystal which is transversally isotropic with respect to its mechanical properties.Other technically important materials may also be transversally isotropic, for example directionally solidified metals in which the grains have a preferential orientation (see also section 2.5), or composites (chapter 9) with fibres oriented in one direction, but aligned arbitrarily (or hexagonally) in the perpendicular plane. 

Fibre alignments in nonwoven fabrics are usually anisotropic, ie, the number of fibres in each direction in nonwoven fabrics is not equal. The most significant difference of these properties is the differences between the fibre orientation in the fabric plane and in the direction perpendicular to the fabric plane (ie, transverse direction or fabric thickness direction) in most nonwoven webs except air-laid nonwovens, most of the fibres are preferentially aligned in fabric plane rather than in fabric thickness. There are also significant differences of fibre orientation, which are also found in the fabric MD and in the fabric CD in the nonwoven fabric plane. 

During extrusion processes, the shear forces tend to align the fibres parallel to the direction of flow, but relaxation, for example in a mould, tends to restore an isotropic distribution of fibre orientation. However, the shear forces in the shaping process impart further anisotropy, which may not relax out of the structure, particularly if the cooling rate is high. Among the consequences is a tendency for the fibre orientation near surfaces to be preferentially parallel to the surface. The complexity of... 

In structural studies, fibres are generally considered to be transversely isotropic. However, it is sometimes found that commercial fibres have a distinct radial differentiation of structure. This is most pronounced in fibres spun from solutions, which show a distinct skin-core effect. A radial nonuniformity has been also found in some melt-spun fibres, e.g. in polyester fibres produced at very high spinning speeds.It should also be noted that in some fibres e.g. aramids" ) there is a preferential radial orientation of certain crystal planes, in contrast to a random radial orientation normally exhibited by conventional commercial fibres. 

Preferential orientation of the fibres in a particular direction will tend, of course, to increase the conductivity in that direction at the expense of the others. 

British Handsheet Machine. Preparation of samples using this method involves filtering a fibre suspension through a fine screen. The newly formed sheet is then pressed and dried. The disadvantage of this method is the lack of preferential orientation of the fibers in the sheet. Except for this drawback sheets can be prepared with well defined characteristics and are suitable for any study in which the preferential orientation of the fibers is not important. 

From an analysis of the intensity distribution of the amorphous halo it is concluded that in maximally stretched EB-11 fibres (///o= 4.5), half of the amorphous phase is oriented at random while the other half responds to stretch-alignment and is preferentially oriented in the same direction as the oriented crystalline fraction, although with a very broad di,stribution. The increase in crystallinity upon stretching is due to nucleation of new crystallites rather than growth of preexisting ones. Similar results were arrived at in a more detailed study by means of a four-circle diffiactometer... 

From a combination of X-ray difiraction, isochronous modulus and dynamic mechanical studies Wardand coworkers explained the anomalous behaviour of (0) at low draw ratio (see Fig. 7) in terms of shear parallel to oriented crystalline chains. ° Thus, at low draw ratio, the crystalline units become preferentially oriented with their c-axes conically disposed about the fibre axis, the half-angle of the cone being approxi-... 

Due to the forward motion of the fluid it is possible to detect a preferential orientation of the fibres in the plane of the sheet along the machine direction (MD). The anisotropy is on average 2 1 (MD/cross direction (CD)) but increases in regions of lower density due to fibre-fibre interactions. This anisotropy can also easily be detected by trying to tear paper apart in different directions. 

The structure of poly(3-hydroxybutyrate-co-3-hydroxyvalerate) fibres exhibit three crystalline structures two orthorhombic, and one pseudo-hexagonal. The orthorhombic crystals with the c-axis preferentially orientate perpendicular to the fibre axis, and pseudo-hexagonal crystals are preferentially formed in the drawing process (114). 

When the material contains short glass fibres, or other reinforcements, their orientation will also be determined the flow pattern, figure 83 shows a section through an injection-moulded part made fiom glass-reinforced polypropylene near the surface, the fibres are oriented preferentially in the flow direction, whilst in the central region they are aligned in the direction transverse to flow. A full e q>lanatk>n of these effects is beyond the scope of this book. 

Textile materials can often be characterized by preferential orientation and symmetry in fibre arrangement, so that they can be considered not as general anisotropic, but orthotropic or even transversely isotropic materials (Fig. 1.12). This simplifies the models development and experimental verification where there would be a smaller number of parameters to be measured. For example, the linear elastic behaviour of anisotropic material can be described in a matrix form as follows ... 

At 25 C, the powder precipitated from zinc sulphate solution was very heterogeneous, presenting some traditional morphologies, like half ellipsoids (250 nm x 350 nm) and ellipsoids (250 nm x 800 nm), both constituted by crystallites grouped in a preferential orientation, as well as 40-nm-diameter fibres of various lengths (Fig. 3a). The fibre structures for ZnO particles are usually synthesised by a thermal evaporation process [27, 28]. 

In Eq. (11.1) (cosx) is the average cosine that the poles of a given family of planes make with the preferred (fibre axis) direction and (P2(cosx)) the second order Legendre polynomial of argument (cos ). Equation (11.1) is also known as Herman s orientation parameter, where P2(cosj ) = 1 corresponds to an ideal case of perfect alignment of the poles toward a preferential direction, P2(cosx) = 0 corresponds to isotropic case and P2(cosx) = 0.5 corresponds to an ideal... 

These 900 C fibres have been shown to contain bimodal pore diameter distributions in the mescpcsre range, with the smaller pxores being randomly oriented and large pores being preferentially oriented parallel to the fibre axes (previously examined 5% silica dcped alumina did not possess any of these lOOnm axial pores). On... 

---