2.5.1 Fiber Reinforcement
Fibers reinforcements are important as they provide the desired functionalities as per the application requirement and transfer the strength to the matrix constituent by altering the properties of composites as per the requirements. The glass fibers are the earliest known fibers used to reinforce materials. Later, the ceramic and metal fibers were discovered and utilized to prepare stiffer composites. However, the fibers also suffer from certain limitations. Generally, the performance of fiber reinforcement is judged by its material, length, shape/orientation and the mechanical properties of the matrix. The strength of composite is highest at it’s longitudinal alignment, and so even a slightest shift in the angle of loading can reduce the strength of the overall composite material.
Figure 2.5 Types of reinforcements in composites.
In a 2-D composite, the strength remains only one-third to the strength of a unidirectional fiber-stressed in the direction of fibers, however, for a 3-D composite, less than one-fifth of the strength is obtained [17]. The fiber composites can be either continuous or short fibers. It is generally observed that the continuous fibers exhibit better orientation in matrix. The aspect ratio (length/diameter) of fibers is very high and so they can be processed easily using continuous process. Due to their high strengths and low densities, the fiber’s length greatly affects the mechanical properties and processing of composites. Also with the proper orientation of shorter fibers in composites comprising of glass, the multi-purpose fibers can be processed with higher strength. Filament winding process utilizes the continuous fiber constituent of a composite [18]. The short-length fibers are comparatively cheaper but are less efficient when incorporated by the open moulding process. The solid fibers are mostly used for their easy handling and production. However, hollow fibers and non-conventional shapes can improve the mechanical qualities of the composites. The high aspect ratio of fiber induces great strength in the composite due to minimization of surface of surface defects. Organic and inorganic fibers can also be used as reinforcement in composite materials. Organic fibers own low density, flexibility, and elasticity while, the inorganic fibers displays better modulus, thermal stability and superior rigidity in comparison to the organic fibers [19]. The frequently used fibers includes the fibers made of glass, silicon carbide, aluminina, metal, graphite, boron, aramid and multiphase. These fibers are discussed in detail as follows:
2.5.1(a) Glass Fiber
Glass fibers constitute the major proportion (> 95%) in reinforced plastics. They are inexpensive, possesses low density, chemicals resistant, offers good insulation, easy to process with high strength/stiffness than the plastics with which they are reinforced [3]. However, they are more likely to breakage under high tensile stress for a prolonged duration. Other factors affecting the strength of glass fibers include temperature, moisture and period of loading. However, the facile fabrication processes (die-moulding and filament winding) make them a material of choice in various applications. Commercially, glass fibers are available in the form of continuous and chopped filaments, cloth, mates, roving, tapes, and yarns. The further addition of chemicals to silica sand while making glass can also yield different types of glasses.
2.5.1(b) Metals Fibers
The metal fibers possess several advantages including easier fabrication, easier handling (than glass fibers), high strengths, more ductility, better temperature resistance and less sensitive to surface damage [1]. Metal fibers when amalgamated with ceramics material tends to improve their thermal and impact resistance properties [7]. The metal fibers reinforced plastic composites exhibits good flexural properties and demonstrate much improved strength than neat glass fibers, yet, they also suffer from poor high temperature tolerance and the variations in the thermal expansion coefficient with the resins limit their application.
2.5.1(c) Alumina Fibers
Alumina oxide fibers are generally used in metal matrices composite. They offer better compressive strength than tensile strength. The fibers can sustain high melting point (2000 °C) and the composite can be utilized up to about 1000 °C. Generally, the magnesium and aluminum matrices use alumina fiber reinforcements as they do not damage the fiber even in the liquid state.
2.5.1(d) Boron Fibers
In these composites, the boron is coated on the tungsten substrate. Borontungsten fibers are obtained when hot tungsten filament is allowed to pass through a mixture of gases and boron with certain thickness gets deposited on tungsten substrate. The thickness of tungsten remains constant in the process. The properties of boron fibers depend upon their diameter due to the changing ratio of boron and tungsten and also with the associated surface defects. The boron fiber possesses good stiffness and strength while their tensile modulus is ~5 times better that of glass fibers. The boron coated carbons fibers are quite cheaper than boron tungsten fiber, however, they suffer from low modulus of elasticity [6].
2.5.1(e) Silicon Carbide Fibers
The room temperature tensile strength of silicon carbide fibers is high and comparable to that of boron-tungsten, yet, the advantages of silicon carbide-tungsten fibers is more than the uncoated boron tungsten fibers, e.g. they only possess 35% loss of strength at 1350 °C. Both silicon carbide-tungsten and silicon carbide-carbon have very high stress-breakage at 1100 °C and 1300 °C, respectively. The uncoated boron-tungsten fibers are non-reactive to molten aluminum and can withstand high temperatures for their applications in hot-press titanium matrices. Owing to the fact that the silicon carbide-tungsten fibers are dense and prone to the surface damage, a careful handling is utmost required during fabrication of the composite [20]. The weakening reactions between tungsten and silicon carbide occur above 930 °C which further requires delicate handling in high-temperature matrix formations. The silicon carbide offers several advantages on carbon substrates including non-reactivity at high temperature, light-weight, better tensile strengths and modulus than those of silicon carbide-tungsten and boron fibers.
2.5.1(f) Aramid Fibers
Aramid fibers are generally fabricated from aromatic polyamides which are long polymeric chains and aromatic rings. They consist of rings of six carbon atoms bonded to each other. These fibers have high tensile strength, high modulus and low weight and so they find excellent use in reinforcement of automobile tires, fabrication of bullet proof vests and power boats. The density of aramid fibers is less than that of glass and graphite fibers, therefore high impact-resistant structures can be produced from them [13]. They are fire and high-temperature resistant and remain unaffected by organic solvents fuels. They can be easily woven into matrices through simple processes. The coefficient of thermal expansion of aramid fibers is negative in the fiber direction which protects it from failure.
2.5.1(g) Quartz and Silica Fibers
The glass contains approximately 50 to 70% silica. Silica glass is one of the fiber types which are processed by treating fiberglass in an acid bath. The quartz fibers are made from natural quartz crystals which contain