and/or metals alloys,) to give a unique blend of physico-chemical properties. The concept of composites was mimic from the Mother Nature. The naturally available, wood, is a composite material which is composed of cellulose fibers in a matrix of lignin which is also called as natural glue. Also, the shell of snails and oysters are also a composite material. Recently, the studies have revealed that the strength of spider’s web fibers is many-folds better than man-made processed fibers. The concept was also used in ancient civilizations wherein clay consisting of husks or straws was widely utilized to build houses that withstand for hundreds of years [1].
The composite consist of matrix materials (metals, plastics, or ceramics) which are formed by reinforcing materials (fibers, particulates, or whiskers) in a matrix resin (Figure 2.1(a)). The fibers can be of any dimension i.e. continuous, long, or short. The function of reinforcing material in a composite is to provide stiffness and mechanical strength, while matrix induces the rigidity and resistance to environmental effects [2]. The reinforcing materials can be of any type such as mat, short chopped fibers and a woven fabric. The style by which the fibers are fitted in the composites can induce different properties in the composite [3]. The composite materials composed of polymer matrix are easily processable and readily available and therefore widely used in various industries. In composite materials, the fiber acts as load carrier wherein its strength is most along the axis of the fiber. The fibers which are long and continuous can be aligned in the direction of the load thereby leading to the formation of composite with much enhanced properties than the pure matrix material. Figure 2.1(b) shows that the chopped fiber with continuous fibers shows better properties than short lengths fibers. Therefore, the targeted application together with the manufacturing method leads to the selection of the continuous fibers or long fibers (structural applications) and short fibers (nonstructural applications) [4]. The short fibers are needed in the process of injection and compression molding, whereas continuous fibers are required in the process of roll wrapping, pultrusion and filament winding.
2.2 Functions of Fibers and Matrix
A composite material consists of fiber and matrix material. A great estimation of the respective parts of these two components is highly necessary to estimate the composite behavior. The primary functions of the two components in a composite are:
1 a) The fibers in composite carries ~90% load.
2 b) They provide thermal stability, strength and stiffness in the composites.
3 c) Fibers are also responsible for the insulation and electrical conductivity in the composite.
4 d) The matrix material provides rigidity and shape to the composite materials by binding the fibers together and simultaneously transfers the load to the fibers.
5 e) The matrix acts as crack inhibitor by isolating the individual fibers.
6 f) The matrix is responsible for good surface finish.
7 g) The matrix protects reinforcing fibers against mechanical damage and chemical attack.
8 h) The failure of composite material directly depends upon the compatibility between the matrix material and the reinforcing fiber.
9 i) The matrix materials greatly influence the composite properties such as ductility, impact strength etc.
Figure 2.1 (a) Formation of a composite material using fibers and resin and (b) Continuous fiber and short fiber composites.
2.3 Classification of Composites
Composite materials are commonly classified according to matrix and fiber constituent as follows:
1 1. The matrix classes includes:(a) Organic Matrix Composites (OMCs):• Polymer Matrix Composites (PMCs)• Carbon matrix composites (carbon-carbon composites)(b) Metal Matrix Composites (MMCs)(c) Ceramic Matrix Composites (CMCs)
2 2. The fibers can be distributed under three categories:(a) Fiber Reinforced Composites: these are composed of fibers (discontinuous or continuous) embedded in matrix material. The properties of a discontinuous fiber composite vary with the fiber length.(b) Laminar Composites (e.g. sandwich structures): these are composed of layers of materials held together by matrix.(c) Particulate Composites (e.g. concrete and wood particle boards): these are composed of particles (in flakes or powder form) distributed or embedded in a matrix body.
2.4 Matrix-Based Composites
The matrix serves two main purposes in composites which includes (a) binding the reinforcement phases in place and (b) uniformly distributing the stresses among the constituent reinforcement materials in the event of an applied force. The matrix offer weight advantages and ease of handling. The metals, polymers and inorganic materials possesses high elastic strength and exhibit minimum failure strain, when loaded in tension and compression [5]. The fabrication method is selected considering the matrix properties and its related effect on the properties of reinforcements. Chemical inertness and non-reactivity is the primary condition for the processing of composites. The various type of matrices materials are illustrated in Figure 2.2.
2.4.1 Polymer Matrix Materials
Polymers are the materials of choice for their easier fabrication, lightweight and tunable mechanical properties. Two types of polymers are available viz. thermosets and thermoplastics:
Figure 2.2 Classification of matrices.
2.4.1(a) Thermoplastics
Thermoplastics have both 1-D and 2-D molecular structure and they tend to soften at an elevated temperature and show exaggerated melting point [6]. This process of softening can be reversed to regain its properties during cooling, which facilitates the processing of composite materials. The presence of reinforcements can minimize the tendency of shape loosening for the composite material. The thermoplastic materials do not require any chemical reactions for their synthesis, which causes unnecessary release of gases and heat. Figure 2.3 shows different kinds of thermoplastics materials.
Thermoplastics resins are generally used as molding compounds. The fibers are randomly dispersed in thermoplastics, and so the reinforcement is isotropic but directionality can be achieved using molding processes [7]. The heat resistance in thermoplastics can be increased by the addition of filler, however, a loss in strength at elevated temperature can occur. Because of their fascinating physico-chemical properties such as toughness, rigidity, and resistance to creep, thermoplastics have evolved as a material of choice in the fabrication of automotive control panels and electronic products encasement. The reinforced thermoplastics are readily available and can be easily transformed into desired shape, thereby facilitating the fabrication of bulky components [8].
