Fibre continued short

Over the last decade, considerable efforts have been committed to the toughening of sialons and substantial progress has been achieved using various reinforcements. According to the form of reinforcement, sialon composites can be classified as either particle reinforced, discontinuous fibre (whiskers/ short fibres) reinforced, or continuous fibre reinforced. 

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. 

This is the well-known rule-of-mixture which describes a rather idealised situation and can predict the modulus only for continuous fibre-reinforced composites where there is sufficient stress transfer from the matrix to the fibre. However, short fibres are usually much shorter than the specimen length. For short fibres we must consider the matrix-fibre stress transfer. When the matrix is under stress, the maximum stress transferred to the fibre is described by the interfacial stress transfer (t). The stress transfer depends on the fibre length (/), so that at some critical length, /, the stress transferred is large enough to break the fibre. The stress transferred to the fibre builds up to its maximum value (o that which causes breakage) over a distance 1 from the end of the fibre. This means that the long fibres carry load more efficiently than short fibres. 

Steel fibres continue to have a wide range of apphcations in civil engineering materials. There are some structural applications where they have been used in concrete without any conventional reinforcing bars. These have been short span, elevated slabs, for example a parking garage at Heathrow Airport... 

In composite materials thin dispersed fibres, either short or continuous, are applied and distributed randomly or regularly, and also in the form of mesh or fabrics. 

Reinforcing fibres have diameters varying from 7 /im to 1(X) /im. They may be continuous or in the form of chopped strands (lengths 3 mm-50 mm). When chopped strands are used, the length to diameter ratio is called the Aspect Ratio. The properties of a short-fibre composite are very dependent on the aspect ratio - the greater the aspect ratio the greater will be the strength and stiffness of the composite. 

In order to understand the effect of discontinuous fibres in a polymer matrix it is important to understand the reinforcing mechanism of fibres. Fibres exert their effect by restraining the deformation of the matrix as shown in Fig. 3.28. The external loading applied through the matrix is transferred to the fibres by shear at the fibre/matrix interface. The resultant stress distributions in the fibre and matrix are complex. In short fibres the tensile stress increases from zero at the ends to a value ([Pg.226]

It may be seen from Fig. 3.29 that due to the ineffective end portions of short fibres, the average stress in the fibre will be less than in a continuous fibre. The exact value of the average stress will depend on the length of the fibres. Using the stress distributions shown in Fig. 3.29(b) the fibre stresses may be analysed as follows. 

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). 

In the manufacture of man-made and synthetic textiles the base materials are extruded through tiny orifices producing continuous lengths of very fine fibres or filaments. The textiles are mostly used in this continuous filament form, but for some purposes the filaments may be cut up into short fibres and spun into a yam. 

Man-made fibres produced as continuous filaments and then cut into short lengths to match those of some natural fibres such as cotton or wool. The staple of cotton, wool, staple fibre, etc., is an indication of the average fibre length. Stark Rubber... 

Filament yams can exist in an almost twistless form, but this is not the case for staple fibre yams. The twisting of fibres together is still the most practical method of making short fibres into long continuous strands of yam suitable for weaving. See S-Twist and Z-Twist. 

The brain does not send a continuous current through the nerve, but short spurts . We call them impulses, which transfer between nerve fibres within the synapses of cells (see Figure 7.16). The cell floats within an ionic solution called plasma. The membrane separating the synapse from the solution with which the nerve fibre is in contact surrounding the cell is the axon, and is essential to the nerve s operation. 

Composites are obtained by using short, long or continuous fibres. 

As for glass fibres, reinforcement with continuous fibres leads to the highest performances. Compared to short glass fibres, short carbon fibres yield higher reinforcement ratios for the modulus and tensile strength but the impact strength decreases. 

Obtaining electrical conductivity. Short fibres are added to thermoplastics to obtain EMI, ESD or conductive grades. In certain composites, long or continuous fibres partially replace glass fibres. 

Traditional processes can be modified to better industrialize the manufacturing of medium- or short-run manufacturing. In the thermoplastic composite field, the Pressure Diaphorm Process allows the processing of continuous fibre reinforced thermoplastics with low pressures. The press and the moulds (wood, composite or aluminium) can be about 70% cheaper. The process is convenient for short and medium runs in the range of 1000 up to 100000 parts. 

HTPC (Hybrid ThermoPlastic Composite) bumper beams made by Plastic Omnium are used by General Motors on the Pontiac Montana, Chevrolet Venture and Oldsmobile Silhouette. Continuous woven fibres are overmoulded with a long or short fibre reinforced polypropylene to save weight (6 kg), enhance impact resistance (20-40%) and integrate numerous functions such as reinforcement ribs. The process is fully automated. 

Intuitively, however, the RVE for an amorphous polymer may be expected to be no more than 10 nanometres or so. In an aligned fibre composite with continuous fibres we need to take a sample sufficiently large as to contain perhaps 30 to 50 fibres in a lateral dimension and in this case the coefficient of variation of stress as between fibres would enable a measure to be decided. In a random array of fibres however (chopped strand mat) the RVE must be of the order of a fibre length at least as is also the case for an aligned short fibre composite. 

Reinforcements in the form of continuous fibres, short fibres, whiskers or particles are available commercially. Continuous ceramic fibres are very attractive as reinforcements in high-temperature structural materials. They provide high strength and elastic modulus with high temperature-resistant capability and are free from environmental attack. Ceramic reinforcement materials are divided into oxide and non-oxide categories, listed in Table 3.1. The chemical compositions of some commercially available oxide and non-oxide reinforcements are given in Table 3.2 and Table 3.3. 

A higher stiffness is also obtained by the incorporation of hard particles or short fibres. Particles are responsible for a 2 to 2.5 fold increase in E, while with short glass fibres a factor of 3 to 5 can be obtained. Long fibres, forming a continuous reinforcing phase, produce a much stronger effect here the fibres practically carry the whole stress, while the matrix polymer has hardly any influence on the stiffness. 

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