Fibre arrangement

The fibre-optic-based collection scheme developed for conventional Raman spectroscopy [29] is equally valid for SORS. In the original concept, the collection fibres (arranged in a disk pattern) were re-organised onto the spectrograph slit into a linear pattern matching the geometry of the spectrograph... 

The fixed nature of the collection fibres in a conventional SORS experiment limits the range of spatial offsets available. An alternative approach (inverse SORS) swaps the laser and collection fibre geometries Raman light is collected through a group of fibres (arranged in a disc shape) contained within the centre of a probed area defined by a ring-shaped laser beam (see Fig. 3.5). 

Rockwool is an example of a stmcture consisting of fibres arranged more or less randomly in space. They are glued together at their contact points. The fibres have diameters a little below 10 pm. The air in between occupies about 95% of the volume, making this material a good thermal insulator. It is also fireproof Such a stmcture is known as a random mat. The stmcture of paper tissue is similar. 

There are two ways of pasting. One is to use wide pieces of the paper with the fibres arranged perpendicularly to the length(Fig.10 (A)). The other method uses long strips of the paper with the fibres parallel to the length(Fig.lO (B)). 

Fig. 5 (a) Schematic representation of the fibres arrangement across the thickness of the. tO wt% fibre-reinforced nylon, the melt filling direction is indicated view of the fibre orientation in (b) the skin and (c) the core regions. 

Table 8.6 Temperature range for softening and melting of fibres, arranged in order of increasing melting point or decomposition temperature.

Rowe, R. W. D. 1974. Collagen fibre arrangement in intramuscular connective tissue. Changes associated with muscle shortening and their possible relevance to raw meat toughness measurements. Journal of Food Technology 9 501-508. 

An instrament for single-point measurements is shown in Fig. 5.63. The excitation light is delivered to the sample via an optieal fibre. The fluoreseenee light is collected by a number of deteetion fibres arranged around the excitation fibre. The detection fibres are conneeted to the input slit plane of a polychromator. To match the polychromator input the bundle is flattened out at the polychromator end. Suitable fibre probes are available or ean be ordered to specification. 

Although off-angle and some multidirectionally reinforced FRP laminates may still have fibres aligned in one single direction or orientation in each ply (and just have different fibre orientations from ply to ply), there are more complex fibre arrangements that also need to be considered. 

Because of the virtually unlimited variations in fibre arrangements (short and continuous fibres, aligned and randomly distributed, laminated, woven, knitted or braided) there are too many materials available to discuss fully all the details of their fatigue behaviour. This chapter will therefore concentrate on some selected composites for demonstration purposes. We shall discuss both tension fatigue, which is the most important load situation in metals, and compression fatigue. The influence of harsh environments will be mentioned as well. 

Table 6.2 Fibre arrangements possible in fibre>reinforeed thermoset polymers...

Fibre reinforcement and resin are premixed, and other additives are added prior to compression moulding in a heated mould which triggers the hardening of the resin. Many forms of premix are available, making a variety of fibre arrangements possible ... 

Liquid precursors are mixed (one of them already containing short fibres) as they are injected into a closed mould (see Chapter 7). The resulting fibre arrangement is influenced by the flow of the simultaneously cross-linking polymer, but approximates to being random in three directions. 

What if the fibres are not uniaxially oriented Any misalignment of fibres causes j to fall below the value predicted by eqn 6.14. Suppose is defined as this maximum achievable modulus at any given fibre volume firaction. TWo examples of other fibre arrangements are found to give tensile moduli as follows. 

The fibre type, matrix type, volume fraction and fibre arrangement are all variables which may be selected to obtain the optimum performance requirements. These combine into a range of options which is literally infinite. 

The effect on tensile strength and stiffness when the fibre arrangement is changed is a reduction from unity for uni directional to half for bi directional and to three-eights for random (in plane) reinforcement (reference 1.5). 

In modelling the physical and the mechanical properties of textiles, there are usually two choices, i.e. whether to consider a discrete or a continuum model. A continuum model assumes that the property of any small part of the material can be considered equal to that of the whole volume. In order to model a textile structure as a continuum, its volume is divided into small parts termed unit cells or representative volume elements (RVE). RVEs model the material structure at a miaoscopic level, i.e. at the level of individual fibre arrangements. The mechanical properties of RVEs are modelled and then used at a macroscopic level, which is the level of yarn or fabric, under the assumption that the whole volume of the material can be re-constructed from a number of RVEs. 

Textile materials can often be characterized by preferential orientation and symmetry in fibre arrangement, so that they can be considered not as general anisotropic, but orthotropic or even transversely isotropic materials (Fig. 1.12). This simplifies the models development and experimental verification where there would be a smaller number of parameters to be measured. For example, the linear elastic behaviour of anisotropic material can be described in a matrix form as follows ... 

Fig, 2. Schematic of three standard concepts of fibre arrangements in C/C composites, ... 

Three-directional (3-D) orthogonal woven C/C composites, heat treated to 2200°C with a fibre volume fraction of 50 V/o in a fibre arrangement of 2, 2y, 3, have flexural strengths of 250 to 300 MN/m in the z direction.The strength is strongly dependent on the weave pattern and the fibre content in the x, y, and z directions and can be tailored from isotropic to anisotropic, according to special requirements, by means of the kind of weave pattern. 

A different kind of 3-D fibre arrangement is the reinforcement of carbon with short fibres or carbon felt. Composites with isotropic properties result, having flexural strengths ranging from 100 to 170... 

Further possibilities to vary the properties of C/C composites are given by so-called angle ply fibre arrangements. This is a fibre arrangement, where unidirectional C fibres are laminated in alternating angles from 0° to 90° bearing on the direction of the load attack. 

Fig. 11. Strength and modulus range of C/C composites with different fibre arrangements.

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