alt="Schematic illustration of the types of thermoplastics."/>
Figure 2.3 Types of thermoplastics.
2.4.1(b) Thermosets
Thermosets are 3-D molecular structure and which tends to decompose on hardening. The composition of resins can leads to a change in their properties. To induce flexibility, the thermosets are retained for long period of time in a partially cured condition [9]. Generally, the condition of fiber material (chopped, aspect ratio) in resins decides the final application of thermosets. The thermosets are generally fabricated direct condensation polymerization followed by rearrangement reactions to form heterocyclic entities. The water as reactant product hinders the development of voidfree composites [10]. Figure 2.4 represents important kinds of thermosets:
2.4.2 Metal Matrix Materials
Metal matrices materials are known for their high stiffness, toughness, strength and their ability to withstand elevated temperature in corrosive environment. Most of the metals and alloys can be used as matrices, however, they often require compatible reinforcement materials which are stable over a range of temperature and also non-reactive [11]. Light metals form the matrix for temperature application and the reinforcements, besides the aforementioned reasons are characterized by high module. The high strength in metallic matrix materials can be achieved by utilizing high modulus reinforcements. In such composites materials, the strength-to-weight ratios can be higher than most alloys. The physico-chemical properties of the matrix materials determine the service temperature of the composites and so the choice of reinforcements becomes more important in application demanding higher melting temperature [12].
Figure 2.4 Classification of thermosets.
2.4.3 Ceramic Matrix Materials
Ceramics are the solid materials which exhibit very strong ionic bonding with partial covalent bonding. Ceramic-based matrix materials exhibits good corrosion resistance, high melting points, high compressive strength and stability at elevated temperatures and therefore can be utilized in high temperatures applications (~1500 °C) [13]. The addition of reinforcements in ceramic overcomes the problems related with high modulus of elasticity and low tensile strain to obtain strength improvement. The addition of reinforcements in adequate amount causes the ceramics to effectively transfer the load to them which reduces the possibilities of ceramics rupture at high stress levels [6]. However, this does not mean that the addition of high-strength fiber will result in strengthening a weaker ceramic. In such a case, the reinforcement possessing high modulus of elasticity can solve the problem. Usually, if the thermal expansion coefficient of ceramics is higher than reinforcement materials, the resultant composite may not be possessing good strength. In such a case, the tendency of forming microcracks while cooling increases and these microcracks extends from fiber to fiber within the matrix [9].
2.4.4 Carbon Matrices
Carbon based materials are generally known for high temperature withstanding properties which remain unaffected up to 2300 °C. The carboncarbon composite can be synthesized using compaction of carbon or multiple impregnations of porous frames with liquid carboniser precursors and subsequent pyrolization or through chemical vapor deposition of pyrolytic carbon [14]. These composites can retain their temperature stability upto 2400 °C along with dimensional stability and thus make them the oblivious choice in space research, military and aeronautics. The components which are regularly exposed to higher temperature and responsible for demands of high standard performance generally utilize carbon-carbon composites [15].
2.4.5 Glass Matrices
Composites with glass matrix are more reinforcement-friendly as compared to ceramics based composites. The methods of processing of polymers can be implied for glass matrices also. Composite based on glass matrix possess high strength and modulus and can withstand temperature up to 650 °C. Composites have low thermal expansion behavior with glass matrices and so are possess better dimensional properties then polymer or metal system.
2.5 Reinforcements
In composites, the reinforcements can be a whisker, fabrics particles or fibers. The whiskers possess a definite shape but are shorter in diameter and length of the fiber. The particles do not have any fixed shape or orientation whereas the fibers consist of a long axis and the two other axes in either circular or near circular shape [16]. Reinforcement addition in composites is generally done to increase the strength of the composite, however, they also serves other purpose of heat resistance or conduction, resistance to corrosion and provide rigidity. Reinforcements are responsible for either one all of the mentioned functions as per the requirements. The reinforcements are generally stronger than the matrix and are capable of changing failure mechanism of the composite. Figure 2.5 shows different types of reinforcements in composites.
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
