Mechanical properties anisotropy

The mechanical piopeities of stmctuial foams and thek variation with polymer composition and density has been reviewed (103). The variation of stmctural foam mechanical properties with density as a function of polymer properties is extracted from stress—strain curves and, owkig to possible anisotropy of the foam, must be considered apparent data. These relations can provide valuable guidance toward arriving at an optimum stmctural foam, however. 

Mechanical Properties and Structural Performance. As a result of the manufacturing process, some cellular plastics have an elongated cell shape and thus exhibit anisotropy in mechanical, thermal, and expansion properties (35,36). Efforts are underway to develop manufacturing techniques that reduce such anisotropy and its effects. In general, higher strengths occur for the paraHel-to-rise direction than in the perpendicular-to-rise orientation. Properties of these materials show variabiUty due to specimen form and position in the bulk material and to uncertainty in the axes with respect to direction of foam rise. Expanded and molded bead products exhibit Httie anisotropy. 

Densification to what is practically hiU density is achievable for most materials. The resulting mechanical properties are equivalent to wrought parts in similar condition. In some materials, the properties of the HIP product are superior because of reduced anisotropy. 

The introduction of large gas phase volumes into the polymer alters the physical characteristics of the material volume weight, permeability to fluids and gases, and physico-mechanical properties. Moreover, the properties of the polymer matrix itself are changed (owing to orientation effects, supermolecular structure of the polymer in the walls, ribs and tension bars of cells), which drives up the value of specific strength on impact, and results in anisotropy of elasticity. 

Shrinkage can influence product performances such as mechanical properties. Anisotropy directional property can be used when referring to the way a material shrinks during processing, such as in injection molding (Fig. 2-62) and extrusion. Shrinkage is an important consideration when fabricating... 

The present review shows how the microhardness technique can be used to elucidate the dependence of a variety of local deformational processes upon polymer texture and morphology. Microhardness is a rather elusive quantity, that is really a combination of other mechanical properties. It is most suitably defined in terms of the pyramid indentation test. Hardness is primarily taken as a measure of the irreversible deformation mechanisms which characterize a polymeric material, though it also involves elastic and time dependent effects which depend on microstructural details. In isotropic lamellar polymers a hardness depression from ideal values, due to the finite crystal thickness, occurs. The interlamellar non-crystalline layer introduces an additional weak component which contributes further to a lowering of the hardness value. Annealing effects and chemical etching are shown to produce, on the contrary, a significant hardening of the material. The prevalent mechanisms for plastic deformation are proposed. Anisotropy behaviour for several oriented materials is critically discussed. 

For density values g > 0.92 g/cm3 the deformation modes of the crystals predominate. The hard elements are the lamellae. The mechanical properties are primarily determined by the large anisotropy of molecular forces. The mosaic structure of blocks introduces a specific weakness element which permits chain slip to proceed faster at the block boundaries than inside the blocks. The weakest element of the solid is the surface layer between adjacent lamellae, containing chain folds, free chain ends, tie molecules, etc. 

Why do we perform tear testing on polymeric films What creates anisotropy in a film s mechanical properties at a molecular level Why is this anisotropy so much more important in films than in thicker parts ... 

Wollastonite is a preferred filler in some instances due to its fibrous form. While not as effective in improving the mechanical properties as glass fibers, it will give more strength than spherical fillers and less anisotropy than longer glass fibers. 

The essential difference between treatments of chemical processes in the solid state and those in the fluid state is (aside from periodicity and anisotropy) the influence of the unique mechanical properties of a solid (such as elasticity, plasticity, creep, and fracture) on the process kinetics. The key to the understanding of most of these properties is the concept of the dislocation which is defined and extensively discussed in Chapter 3. In addition, other important structural defects such as grain boundaries, which are of still higher dimension, exist and are unknown in the fluid state. 

Hardness determination methods find wide uses in basic research on the mechanical properties of minerals and their deformation. In the face of the rapid development of industrial uses of natural minerals, as well as manmade, in monocrystal or grain form or as polymineral materials, there is a definite need for more comprehensive crystallomechanical investigations. Apart from the above aspects, hardness determination should furnish valuable information on the genesis of minerals. These authors consider that it would well serve the purpose to examine the mechanical properties of all minerals so as to obtain their allround crystallomechanical characterization and to investigate into their anisotropy and relationship to the structure and composition of minerals. By determining the typomorphism of the mechanical parameters of minerals and its involvement in the conditions of their formation, and also by investigating the specificity of occurrence of deformations in minerals under natural conditions and of the deformative mechanism, it should be possible to develop a general theory of mechanical properties of crystals. 

Zhitaru R. P., Radautsan S. I., Tezlevan V. E., Buzhor V. P., 1978, Ob anizotropii mekhanicheskikh svoistv monokristallov Cd Cr2 Se4 v Kristallicheskie i stekloobra-znye poluprovodniki (On the Anisotropy of Mechanical Properties of CdCr2Se4 Monocrystals) Izd. Shtiintsa, Kishinev. 

Grabko et al. (1977) studying the anisotropy of the mechanical properties of admixed ZnTe monocrystals used the scratch method to reveal the reorientation of blocks in crystals ZnTe Mg, and ZnTe Be (Fig. 4.3.7). The minimum-hardness direction within one block would change... 

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