Composites mechanical anisotropy

A high mechanical anisotropy is characterised by a small value of jlz. Application of the composite model to a fibre implies that Vc=l. For very long chains or b—>°o Eq. 45 is derived... 

Cai H, Bashar MT, Picot JJC (2004). Thermal and mechanical anisotropy in compression molded carbon fiber/resin composites. Polymer Composites 26 684-688. 

Anisotropy of mechanical properties at the increase of filler concentration [70, 78] rises significantly in the items produced from mineral media-filled thermoplasts. When a mineral filler is replaced for a mineralorganic, the dfect of mechanical anisotropy increase fades considerably [18]. At the same time cp values, durability of cold seal moulded products, made of kerogenes is higher than in products of mineral materials-filled compositions [18]. 

Despite its two-dimensional nature, theTakayanagi model is successful in explaining the main features of low strain mechanical anisotropy in oriented annealed crystalline polymers. Perhaps even more important it points the way to more sophisticated composite structure models which are at present being developed for crystalline polymers. 

Many efforts have been undertaken to overcome the mechanical anisotropy of in sim composites through processing [145-156]. In recent years, pre-extmded thermoplastic LCP sheets, strands, and fibers have been used to produce composites with a good balance in machine direction and transverse direction. Bassett and Yee [145] used a method consisting of spinning composite fibers... 

A composite s elastic constants serve further purposes. Their accurate measurement permits tests of theories relating composite properties to constituent properties. Their tensor character permits detecting fiber-induced mechanical anisotropy. Elastic constants may also facilitate quality control and incipient-failure detection. For some composites, empirical relationships exist between elastic constants and important mechanical properties. ... 

Guo F, Wang N, Wang L, Hou L, Ma L, Liu J, et al. An electrospun strong PCL/PU composite vascular graft with mechanical anisotropy and cyclic stability. J Mater Chem A 2015 3 4782-7. 

Mechanical anisotropy and strength of uniaxially aligned fibre composites... 

The theoretical considerations outlined above show that a uniaxially aligned fibre composite will show a high degree of mechanical anisotropy, i.e. > G -... 

Surface finish can be a problem with composites, although there are many well-known examples of painted composite surfaces which are judged to be of Class A surface finish. However, the most consistently under-appreciated problem with fibre-reinforced composites is anisotropy, i.e. the directional dependence of mechanical and dimensional properties. It arises from the orientation of the fibres, and consequently is most pronounced when a component has been shaped by a high speed melt flow process injection moulding is the prime example. It is best to assume that anisotropy is always present in a composite, unless isotropy has been designed into the material either by the use of a random glass mat, or by a deliberate layering process in which the orientation in different layers is balanced out. 

Broadline nmr, relating melt order with polymer composition and temperature, also provided evidence supporting the thermotropic nature of these materials. The physical properties of molded parts of certain compositions showed a definite mechanical anisotropy. Strength and modulus values in the machine direction were considerably higher than those values transverse to the machine direction, that is, perpendicular to the direction of flow. Again, facile and high molecular orientation was noted for these polymers in the flow direction and this became more pronounced as part thickness was reduced. Further, the melt viscosities of the X7G copolyesters, at about equivalent molecular weights, were... 

Although the detailed development of mechanical anisotropy in these particular filaments must depend on their exact chemical composition and subsequent processing, several... 

Mechanical Anisotropy of Polymer Composites 9.2.1 Mechanical Anisotropy of Lamellar Structures... 

Mechanical Anisotropy and Strength of Uniaxially Aligned Fibre Composites... 

Key results for the mechanical anisotropy of LDPE sheets have already been discussed in Section 9.5.3. Oriented and annealed sheets can be considered as composite solids, where the relaxation is an interlamellar shear process, consistent with its assignment by Boyd to an amorphous process. Figure 10.10 shows results for cold-drawn and annealed... 

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. 

The inherent anisotropy (most often only orthotropy) of composite materials leads to mechanical behavior characteristics that are quite different from those of conventional isotropic materials. The behavior of isotropic, orthotropic, and anisotropic materials under loadings of normal stress and shear stress is shown in Figure 1-4 and discussed in the following paragraphs. 

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