Uni-directional composites

That is, if the local tensile, compressive or shear stresses exceed the materials tensile, compressive or shear strength then failure will occur. Some typical values for the strengths of uni-directional composites are given in Table 3.5. 

The fibre volume fraction depends heavily on the method of manufacture. A uni directional composite may have a fibre volume fraction as high as 75%. However, this can only be achieved if all the fibres are highly aligned and closely packed. A more typical fibre volume fraction for uni directional composites is 65%. If the fibre configuration is changed to put fibres in other directions, then the maximum fibre packing is reduced further. A typical fibre volume fraction for bi-directional reinforcement (woven fibre) is 40% and a typical volume fraction for random in-plane reinforcement (chopped strand mat) is 20%. 

Uni-directional composites produced from Toray T300 or similar carbon fibre reinforcement have typical properties as follows ... 

The specific modulus of carbon fibre composites (i.e. the ratio of elastic modulus to density) is their most important characteristic. A uni-directional composite of Torayca... 

This method is based on the simplified assumption of uniform stress or strain and is a conservative method for predicting the elastic properties of a uni directional composite... 

The following techniques are useful for analysis and charaterization for the sizing on the fibres SEM, IR and 6PC of extracts, IGC and DMA (T ) for the fibre-matrix bond micro bond, micro failure with SEM [2], confocal laser scanning microscopy [3] DMA, flexural strength, ILSS and impact of the uni-directional composites (fixed fibre length, orientation and volume fraction distributions). After this detailed analysis, the fibres are tested in their application, which is an injection moulded compound for thermoplasts, where the microstructure (fibre length, fibre orientation and fibre-fibre distance distributions) as well as the fibre- matrix adhesion determine the mechanical properties. 

The majority of work done on VGCF reinforced composites has been carbon/carbon (CC) composites [20-26], These composites were made by densifying VGCF preforms using chemical vapor infiltration techniques and/or pitch infiltration techniques. Preforms were typically prepared using furfuryl alcohol as the binder. Composites thus made have either uni-directional (ID) fiber reinforcement or two-directional, orthogonal (0/90) fiber reinforcement (2D). Composite specimens were heated at a temperature near 3000 °C before characterization. Effects of fiber volume fraction, composite density, and densification method on composite thermal conductivity were addressed. The results of these investigations are summarized below. 

Ramsleiner F. and Theysohn R. (1979). Tensile and impact strength of uni-directional short fiber reinforced thermoplastics. Composites 10, 111-119. 

Figure 52. Schematic diagram of reinforced structure of foamed composites. (1) Unidirectional, (2) Continuous-strand mat, (3) Chopped-strand mat, (4) Uni-directional skin layer, (5) Three-dimensional.

Within UML, domain entities are represented through classes and their instances (objects). Classes can be hierarchically ordered and further characterized by means of attributes. Additionally, binary relations, called associations in UML, can be introduced between classes. Associations can be either uni-directional or bi-directional. The number of objects participating in an association can be indicated by cardinality constraints. Additionally, UML introduces two special tjrpes of associations aggregation and composition. 

In fibre-reinforced glass matrix composites, 3-D parts offer greater potential than uni-directionally reinforced composites (in 1-D or 2-D fibre lay-up), discussed in Chapter 19, because of their potentially better interlaminar shear strength and isotropic properties. A... 

The use of S2—or R-glass improves the composite modulus to about 60 kN/mm for uni-directional and 25 kN/mm for non-crimped bi-directional constructions. However, they are both more expensive than E-glass and are only available in a fairly limited range of material types and resin compatibilities. Probably the most important virtue of S2— and R glass is that the strength is considerably higher than that of E-glass. 

The properties of the composite are affected by fibre and matrix type, the relative quantity of each in the composite and the directionality of the fibres. A composite which has all the fibres aligned in one direction, i.e. is uni-directional, is stiff and strong in that direction, but in the transverse direction it will have low modulus and low strength. 

A composite with equal amounts of fibre in the longitudinal and transverse directions has equal strength and stiffness in the two directions. However, neither would be as high as in the uni-directional case. In theory, if that same amount of fibre was randomly laid in-plane (isotropically), then the resulting composite would have equal strength and stiffness in all in-plane directions but less than in the bi-directional case (in the axes of the fibres). The directionality of the reinforcement has a significant effect on the amount of reinforcement which can be packed into a composite. 

A uni-directional glass fibre composite, with 65% fibres, will have a tensile strength of about 700 N/mm and a tensile modulus of about 40 kN/mml... 

A uni-directional aramid fibre composite with a similar volume of fibres will have a tensile strength of about 1400 N/mm However, its compressive yield strength is about one-skth of this at 230 N/mml This also affects the flexural performance, giving a value of about 300 N/mml... 

M46 has a modulus of 255 kN/mm which compares with about 210 kN/mm for steel. The specific density of composites of the two fibres is about 1.6, giving specific moduli as shown in Table 1.4 together with those for uni directional E-glass and steel. 

Uni directional carbon FRP composites are relatively insensitive to tension fatigue when loaded in the fibre direction. Therefore they have excellent fatigue performance when compared with metals and other composites even at very high stress levels (reference 1.8). 

Although aramid fibres have high inherent tensile strength, particularly in uni directional construction, in composites they tend to have creep rates very much higher than similar glass or carbon composites (reference 1.9). 

Reinforcements can be uni directional, cross ply, angle ply or random in their arrangement. In any one direction, the mechanical properties of the composites will be proportional to the volume fraction of the fibres oriented in that direction and the components due to other fibre directions. 

A single lamina with uni directional reinforcement has high mechanical properties along its longitudinal axis, and low-to-moderate properties along its transverse axis. This is the primary difference from a structural analysis and design standpoint between composites and metals. 

Fabric composites have mechanical properties similar to those of laminates made from orthogonal uni-directional layers. However, fibre curvature arising from yarn twist and weave crimp makes fabric reinforcement less efficient than in the case of aligned straight fibres. 

Marline, E.A., Long term tensile creep and stress rupture evaluation of uni-directional fibre glass reinforced composites. Paper 94, SPI 48th Annual Conference, Cincinnati, USA, 1993,... 

Hamada H, Oya N, Yamashita K, Maekawa Zl, Tensile strength and its scatter of uni-directional carbon fibre reinforced composites, J Reinf Plastics Composites, 16(2), 119-130, 1997. 

Yoshida H, Miyata N, Naito K, Ishikawa S, Yamagishi C, Influence of orientation angle of fibre on mechanical properties in uni-directional and two-directional carbon fibre reinforced SiC composites, J Ceramic Soc Japan, 102(11), 1016-1021, 1994. 

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