Directional property anisotropic

Basically, birefringence is the contribution to the total birefringence of two-phase materials, due to deformation of the electric field associated with a propagating ray of light at anisotropically shaped phase boundaries. The effect may also occur with isotropic particles in an isotropic medium if they dispersed with a preferred orientation. The magnitude of the effect depends on the refractive index difference between the two phases and the shape of the dispersed particles. In thermoplastic systems the two phases may be crystalline and amorphous regions, plastic matrix and microvoids, or plastic and filler. See amorphous plastic coefficient of optical stress compact disc crystalline plastic directional property, anisotropic ... 

The wide choice available in plastics makes it necessary to select not only between TPs, TSs, reinforced plastics (RPs), and elastomers, but also between individual materials within each family of plastic types (Chapters 6 and 7). This selection requires having data suitable for making comparisons which, apart from the availability of data, depends on defining and recognizing the relevant plastics behavior characteristics. There can be, for instance, isotropic (homogeneous) plastics and plastics that can have different directional properties that run from the isotropic to anisotropic. Here, as an example, certain... 

Glass fibres dominate this field either as long continuous fibres (several centimetres long), which are hand-laid with the thermoset precursors, e.g., phenolics, epoxy, polyester, styrenics, and finally cured (often called fibre glass reinforcement plastic or polymer (FRP)). With thermoplastic polymers, e.g., PP, short fibres (less than 1 mm) are used. During processing with an extruder, these short fibres orient in the extrusion/draw direction giving anisotropic behaviour (properties perpendicular to the fibre direction are weaker). 

The industrial importance of preferred orientation lies in the effect, often very marked, which it has on the overall, macroscopic properties of materials. Given the fact that all single crystals are anisotropic, i.e., have different properties in different directions, it follows that an aggregate having preferred orientation must also have directional properties to a greater or lesser degree. Such properties may... 

LCPs being anisotropic, their properties are enhanced along with the direction of flow and decreased across the flow direction. This anisotropic ratio is reduced hy the presence of fillers, which is in contrast to conventional polymer composites. Thus the properties of fiher-filled LCPs, such as shrinkage, modulus, and creep, are less anisotropic than those of the unfilled LCP These LCPs also retain their excellent mechanical properties when used for long periods of time at elevated temperatures. 

When written in matrix form these equations relate the properties to the crystallographic directions. For ceramics and other crystals the piezoelectric constants are anisotropic. For this reason, they are expressed in tensor form. The directional properties are defined by the use of subscripts. For example, d i is the piezoelectric strain coefficient where the stress or strain direction is along the 1 axis and the dielectric displacement or electric field direction is along the 3 axis (i.e., the electrodes are perpendicular to the 3 axis). The notation can be understood by looking at Figure 31.19. 

An isotropic material has the same properties in all directions. Properties such as refractive index and Young s modulus are independent of direction, and if we wish to refer the properties to a set of rectangular cartesian co-ordinates, we can rotate the axes to be in any orientation without any preferoice. For an anisotropic material, where the properties differ with direction, it is usually convenient to choose coordinate systems which coincide with axes of S3rmmetry if this is possible. The material is then described by its properties referred to these principal directions, which affords considerable simplification. 

A very important factor in BM is the effective diameter swell of the parison. Ideally, the diameter swell is directly related to the weight swell and would require no further consideration. In actual practice, the existence of gravity, the finite parison drop time, and the anisotropic aspects (the parison has directional properties) of the BM operation prevent reliable prediction of parison diameter swell directly with the weight swell. After leaving the die, the melt—which has been under shear pressure—undergoes relaxation that causes cross-sectional deformation or swell. 

Quantitative predictions of the effects of fillers on the properties of the final product are difficult to make, considering that they also depend on the method of manufacture, which controls the dispersion and orientation of the filler and its distribution in the final part. Short-fiber- and flake-filled thermoplastics are usually anisotropic products with variable aspect ratio distribution and orientation varying across the thickness of a molded part. The situation becomes more complex if one considers anisotropy, not only in the macroscopic composite but also in the matrix (as a result of molecular orientation) and in the filler itself (e.g., graphite and aramid fibers and mica fiakes have directional properties). Thus, thermoplastic composites are not always amenable to rigorous analytical treatments, in contrast to continuous thermoset composites, which usually have controlled macrostructures and reinforcement orientation [8, 17]. 

Anisotropy n. The quality of being anisotropic having directionally dependent properties. Exhibiting different optical properties when tested along axes in different directions. G Anisotropic f, F anisotropic f, S.anisotropla f, I anisotropia f. See isotropic... 

Reaction gas-phase collisions should generate Newton spheres with isotropic distributions. However, very often these surface patterns are anisotropic (as the one shown in the figure) due to the existence of some directionality in the process. In photodissociation, this is often due to the use of a linearly polarized laser, which acts as the reference axis. In a bimolecular collision (see below), carried out in a crossed-beam experiment, the relative velocity vector introduces a reference axis to which the directional properties of the reaction products are referenced. 

In chemical crystallography, these two antonymic terms refer to the directional dependence of a physical property. Isotropic properties or features are directionally independent, while anisotropic properties or features vary according to direction. Properties to which these terms can apply include dimensions, thermal expansion, flexibility, transmission of light (see birefringence and pleochromism), tensile strength, second harmonic generation, and so on. 

Some intensive properties (e.g., refractive index and polarizability) can have directional characteristics. A uniform phase may be either isotropic, exhibiting the same values of these properties in all directions, or anisotropic, as in the case of some solids and liquid crystals. A vacuum is a uniform phase of zero density. 

Figure 2-37. An example of the thiee>dimensional or anisotropic directional properties of wood that can also occur in unreinforced or reinforced plastics.

It will be recalled from section3.1 that a fibre composite is inherently anisotropic because of the disparity between the properties of the fibre and the matrix. For GRP the ratios of the longitudinal to the transverse modulus and strength are of the order of 4 and 28 respectively, while similar figures for an ultra high modulus carbon fibre composite are 56 and 45, respectively. Materials with such highly directional properties can be hard to handle initially, particularly when coupled with the inability of thermoset matrix systems to undergo plastic deformation. 

Polymer crystals show very direction-dependent (anisotropic) properties. The Young s modulus of polyethylene at room temperature is approximately 300 GPa in the chain-axis direction and only 3 GPa in the transverse directions (Fig. 1.2). This considerable difference in modulus is due to the presence of two... 

Anisotropy A property of a material indicating directional nonuniformity. Anisotropic materials exhibit a property that varies depending on the direction of measurement. In an optically anisotropic material, more than one refractive index can be measured. 

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