Oriented fibres

Difficulties in moulding large structural parts with no control on fibre orientation. 

For the next interface, z — —4 mm, the new values of , Sy and yxy can be calculated and hence the stresses in the global and local co-ordinates. / = 1 and f = 2 need to be analysed for this interface but there will be continuity across the interface because the orientation of the plies is the same in both cases. However, at z = —3 mm there will be a discontinuity of stresses in the global direction and discontinuity of stresses and strains in the local directions due to the difference in fibre orientation in plies 2 and 3. 

These long fibres give better product performance although injection moulding machine modifications may be necessary to prevent fibre damage and reduce undesirable fibre orientation effects in the mould. 

Modern representations of the virtual heart, therefore, describe structural aspects like fibre orientation in cardiac muscle, together with the distribution of various cell types, active and passive electrical and mechanical properties, as well as the coupling between cells. This then allows accurate reproduction of the spread of the electrical wave, subsequent contraction of the heart, and effects on blood pressure, coronary perfusion, etc. It is important to point out, here, that all these parameters are closely interrelated, and changes in any one of them influence the behaviour of all others. This makes for an exceedingly complex system. 

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). 

Fig. 16. Single voxel STEAM spectra of the SOL muscle (top) and the TA muscle (bottom). Different fibre orientation in those muscles results in clearly different patterns of the lines in the spectra In SOL (feathered muscle with oblique fibres), IMCL and EMCL signals show lower frequency separation than in TA (spindle-shaped muscle) due to bulk susceptibility effects. Furthermore, in SOL the Cr2 doublet merges into one resonance, the Cr3 triplet is less resolved, and TAU is shifted towards TMA.

In conclusion, muscular spectroscopy profits of the higher field strength. Clearly narrower IMCL resonances and nearly unchanged susceptibility based broad EMCL signals occur at higher field strength. Dipolar coupling effects especially observed in TA (with parallel muscle fibre orientation to Bg) have to be further studied in detail. 

Morton J. and Groves G.W. (1974). The cracking of composites consisting of discontinuous ductile fibers in a brittle matrix-effect of fibre orientation. J. Mater. Sci. 9, 1436-1445. 

Wood sheets, which are obtained by unrolling steamed wood rods with ablade, can be bound together to form thicker boards. The main characteristic of these sandwiches is the alternation of the fibre orientation by 90° in every layer. The anisotropy of the whole board is thus limited in the length and in the width. Three, five or seven layers are usually used in these panels, called plywood. Often, the inner layers of plywood are replaced by OSB (see below) to reduce costs. The two outer layers of wood sheets ensure the aesthetic appearance of the panel. 

Fig. 17. (a) 2D CSA-DECODER spectrum of polyethyleneterephthalate fibres oriented perpendicular to the rotor axis with a mixing time of 9.6 ms equivalent to a rotation of 125° (b) Simulation of the experimental data using a uniaxial distribution with FWHM = 25°. (Adapted from Lewis et al.260 with permission.)... 

However, the friction of the Duroid 5813 (about 0.34) was practically unaffected by fibre orientation, unlike the other two composites which gave the lowest friction when sliding was normal to the fibre orientation. This may indicate that in the Duroid 5813 the friction was to a considerable extent determined by the molybdenum disulphide, which was randomly oriented, or by the PTFE, whereas in the epoxy composites the friction was strongly influenced by the fibres themselves. Table 12.5 shows the relationship between fibre orientation and specific wear rate for the Duroid 5813. It should be noted that although the effect of fibre orientation is significant, ail three wear rates are of the same order of magnitude. 

Table 12.5 Relation Between Glass Fibre Orientation and Specific Wear Rate for Duroid 5813 (Ref.370 ...

Axis % of fibres oriented Specific wear rate... 

Sung N-H. and Suh, N.R., Effect of Fibre Orientation on Friction and Wear of Fibre-Reinforced Polymeric Composites, Wear, 53, 129, (1979). 

The influence of external variables, i.e. testing and environmental conditions such as rate of loading, temperature and moisture, on fracture mechanisms and hence on fracture toughness in short fibre polymer composites is investigated as a function of fibre orientation with respect to the fracture plane. Unidirectionally oriented materials with different polymer matrix (polyamide 6.6 and polyoxymethylene) and different glass fibre content (15wt% and 30wt%) are examined. 

All systems examined show the same fundamental result found in previous works the critical stress intensity factor, Kc, bears a bi-linear relationship with the factor characterising fibre orientation, with different slopes over different ranges of the orientation factor, suggesting a transition between different fracture mechanisms at a critical angle. 

The variation of Kc with fibre orientation and the transition between the two regimes at a critical angle depend on external variables as well as on the constitution of the material. 

Short fibre composites, fibre orientation, fracture toughness, fracture mechanisms, fibre pullout, fibre debonding, critical fibre angle. 

Short fibre polymer composites are being increasingly used as engineering materials because they provide mechanical properties superior to neat polymers and can be processed easily by the same fabrication methods, e.g. injection moulding. The mechanical properties of these materials are dependent on a complex combination of several internal variables, such as type of matrix, fibre-matrix interface, fibre content, fibre dimensions, fibre orientation, and external... 

The microstructure of all mouldings was characterised by determining fibre content (weight and volume fraction), average fibre dimensions (diameter and length), fibre length distribution and fibre orientation distribution. 

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