Strengthening fibres

If the failure strain in the matrix is larger than in the fibre, the fibres fracture before the matrix fails. This is frequently the case in composites with metallic or polymeric matrix. Figure 9.3 shows the resulting stress-strain diagram. It is assumed that the matrix yields plastically before the fibre breaks. The material deforms elastically until the matrix yields. On further increasing the strain, the strengthening fibres fracture, and the stress-strain curve drops to a small stress value that lies below that of the pure matrix material because of the reduced volume. Eventually, failure by fracturing of the matrix occurs. The fracture strain is smaller than in a pure matrix material. This is due... 

How does your calculation of the tensile strength change if the strengthening fibres are as short as 5 mm, much smaller than the dimension of the component Assume that the short fibres are still perfectly aligned The interfacial strength between fibre and matrix is 30 MPa, the fibre diameter is 8 pm. 

The word "composites" has a modern ring. But using the high strength of fibres to stiffen and strengthen a cheap matrix material is probably older than the wheel. The Processional Way in ancient Babylon, one of the lesser wonders of the ancient world, was made of bitumen reinforced with plaited straw. Straw and horse hair have been used to reinforce mud bricks (improving their fracture toughness) for at least 5000 years. Paper is a composite so is concrete both were known to the Romans. And almost all natural materials which must bear load - wood, bone, muscle - are composites. 

Fig. 25.3. The variafion of peak stress with volume fraction of fibres. A minimum volume fraction (VJ ) is needed to give any strengthening.

In addition it may be seen that the strengthening effect of the fibres is only observed (i.e. a u > volume fraction is greater than a certain... 

R. Hill, Theory of Mechanical Properties of Fibre-Strengthened Materials - III. Self-Consistent Model, Journal of the Mechanics and Physics of Solids, August 1965, pp. 189-198. 

More recently, Stanicioiu, Chinta Hartner (1959) attempted to reinforce the cement with glass fibres, but this was not successful. The most serious study on the reinforcement of dental silicate cement was made by J. Aveston (in Wilson et al., 1972). Silicon carbide whiskers, carbon fibres and alumina powder were introduced into the cement mix. Unfortunately, the glass powder/liquid ratio had to be reduced, and the strength gained by reinforcement was thereby lost. It is clear that dental silicate cement cannot be strengthened by fibre or particulate reinforcement. 

Adsorption might be expected to create additional fibre-to-fibre bonds and/or to strengthen existing bonds, but there is evidence that the increase in tensile strength of paper is caused primarily by an increase in the bond strength per unit of optically bonded area, rather than by an increase in the extent of the relative bonded area itself (Figure 7.13). 

When brought near to a flame, natural (ordinary and wild or tussah) silk bums with an odour of burnt hom and leaves a spongy carbonaceous residue (except with heavily weighted silk) artificial silks with a cellulose basis bum rapidly, almost without odour or residue, while " strengthened " 1 silks leave an ash in the form of the fibre artificial silks with a gelatine basis (A. Millar s silk, Vandura silk) bum like animal fibres. 

During spinning and weaving of textiles (cotton, wool, bast fibres, etc.) breakdown of the textile yarn is frequently observed17. This has an influence not only on the quality of the textile product but also on the production cost, since the machines have to be stopped every time that the yarn is broken. To avoid or reduce the incidence of these harmful consequences, the textile yarn is strengthened with a layer of cellulose. Less than 5% of the breakages that occur with an untreated yarn still occur when the yarn is treated with cellulose. 

Section 18.2 gives a very brief introduction to sialons. Section 18.3 outlines the challenges to be overcome in order to make toughened and strengthened sialon products. Progress in developing sialon composites forms the main part of this chapter and is discussed in section 18.4, which deals with a/p-sialon composites, particle/whisker-reinforced sialons, and fibre-reinforced sialons. In the final section, several conclusions and suggestions for future work are summarised. 

Reprinted from J. Eur. Ceram. Soc., 22(2), Yu Z B, Thompson D P and Bhatti A R, Synergistic roles of carbon fibres and Zr02 particles in strengthening and toughening Li-a-sialon composites, 225-235 (2002). Copyright 2002, with permission of Elsevier. 

Products made from sisal are being developed rapidly, such as furniture and wall tiles made of resonated sisal. A recent development has even expanded the range to car parts for cabin interiors. Other products developed from sisal fibre include spa products, cat scratching posts, lumbar support belts, rugs, slippers and cloths. In recent years sisal has been utilized as a strengthening agent to replace asbestos and glass fibre, as well as an environmentally friendly component in the automobile industry. 

Industrial separation membranes and ion-exchange resins can be made from chitin, especially for water purification. Chitin is also used industrially as an additive to thicken and stabilize foods and pharmaceuticals. Since it can be shaped into fibres, the textile industry has used chitin, especially for socks, as it is claimed that chitin fabrics are naturally antibacterial and antiodour (www.solstitch.net). Chitin also acts as a binder in dyes, fabrics and adhesives. Some processes to size and strengthen paper employ chitin. 

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