Composite materials can be classified on the basis of the Among the thermoset resins, polyester resins are lower in matrix material used for their fabrication polymer matrix composites(PMc) matrix composites are used in the fabrication of boat hulls metal matrix composites(MMC) structural panels and parts for automobiles and aircrafts ceramic matrix composites(CMc) building panels and beams, electrical appliances, water Theoretically, a multitude of materials can come under tanks, pressure vessels, etc. Epoxy resins, in addition these categories. In the following a brief description of have a lower shrinkage after cure allowing for highe some of the PMc, MMc and CMc composites most fabrication accuracy. Epoxy matrix is used commonly in ly used for industrial applications is reported CFRP and AFRP composites for aerospace applications. 2.1 Polymer matrix composites(PMc) military equipments, satellite antennae, sports The most common types of reinforcement used for PMC equipments, medical prostheses, etc Thermoplastic polymers consists of flexible line A are placed before the acronym FrP to specify the strength and modulus but quite high ductility. Among the thermoplastic resins, polyamide and peek resins are used nature of the reinforcing fibres glass, carbon or aramid fibres. The fibres can be long (continuous)or short as matrix materials in FRP composites for applications in (discontinuous). Long fibres can be unidirectional(al the aerospace industry due to their superior mechanical properties and high glass transition temperature fibres parallel to each other) or woven into a fabric or Maximum service temperatures for FRP composites are cloth.Unidirectional fibres provide for the highest relatively low, as the matrix material is prone to softening. mechanical properties in a composite Glass fibre reinforced plastics(GFRP)are by far the most chemical decomposition or degradation at moderate temperatures. The same temperature limitations apply to commonly used materials in view of their high specifi mechanical properties and low cost. Carbon fibre the machining of FRP composites Table 1 reports the main mechanical properties of some einforced plastics( CFRP)and aramid fibre reinforced FRP composite materials lastics(AFRP)provide higher specific strength, higher specific stiffness and ligher weight. They are, however 2.2 Metal matrix composites(MMc) expensive and are used only for those applications where MMC are used for applications requiring higher operating performance and not cost is the major consideration emperatures than are possible with PMc materials AFRP is used instead of CFRP where strength, lightness Most of these composites are developed for the and toughness are major considerations, and stiffness and aerospace industry, but new applications are found in the high temperature performance are not automotive industry such as in automobile engine parts The common matrix materials for FRP composites ar making use of continuous fibre, discontinuous fibre or thermoset polymers(e.g. polyester, epoxy) particle reinforced MMC. Continuous fibres provide for the thermoplastic polymers(e.g polyamide, peek highest stiffness and strength properties obtainable in Thermoset polymers remain rigid when heated and consist MMc materials of a highly cross-linked three-dimensional network; they Boron-aluminium composites are one of the earliest are quite strong and stiff and have poor ductility layers of boron fibres between aluminium foils, so that the TensileElastic foils deform around the fibres and bond to each other [1] FRP odulus By reinforcing with boron, the tensile strength can be material ou(MPa)E(MPa) failure, d(g/ Increas ed by a factor of three to five while the elastic modulus can be tripled GFRP Further reinforcing materials for MMc are silicon carbide alumina and graphite in the form of particles, short fibres (whiskers) or long fibres. Aluminium, magnesium and itanium alloys are the most common matrix materials coth*10030010000200 used in MMc materials Table 2 reports the main mechanical properties of some 50-2006000-12000 13-2.1 MMC materials hort fibres Figure 1 is a plot of specific strength versus specific stiffness for various composites and conventional m materials. It can be seen that composites, in general, have compound* 1.3-19 higher specific strength and specific modulus over conventional steel, Al, Ti, and Mg alloys and mMc have properties superior to PMc compos Applications of continuous fibre reinforced use of B-Al for the fuselage of the space shuttle (V=60%) 145000 0.9 1.6 SiC-Al for the vertical tail section of advanced e orbiter Discontinuous fibre and particle reinforced MMc are low (V=60%) 0.3 1.6 cost MMC that provide higher strength and stiffness and 220000 better dimensional stability over the corresponding unreinforced alloys. Small additions of reinforcement(V 20%)moderately increase the base alloy strength and stiffness. They also increase the wear resistance and Unidir 75000 1.6 contribute toward the difficulty in machining these materials. These MMc are used for sport equipments Table 1: Mechanical properties of FRP composite automobile engine parts(pistons cylinder liners, brake "For these materials: v+= 20%-50% drums), missile guidance parts, etcComposite materials can be classified on the basis of the matrix material used for their fabrication: - polymer matrix composites (PMC); - metal matrix composites (MMC); - ceramic matrix composites (CMC). Theoretically, a multitude of materials can come under these categories. In the following, a brief description of some of the PMC, MMC and CMC composites most commonly used for industrial applications is reported. 2.1 Polymer matrix composites (PMC) The most common types of reinforcement used for PMC are strong and brittle fibres incorporated into a soft and ductile polymeric matrix. In this case, PMC are referred to as fibre reinforced plastics (FRP). Capital letters G, C and A are placed before the acronym FRP to specify the nature of the reinforcing fibres: glass, carbon or aramid fibres. The fibres can be long (continuous) or short (discontinuous). Long fibres can be unidirectional (all fibres parallel to each other) or woven into a fabric or cloth. Unidirectional fibres provide for the highest mechanical properties in a composite. Glass fibre reinforced plastics (GFRP) are by far the most commonly used materials in view of their high specific mechanical properties and low cost. Carbon fibre reinforced plastics (CFRP) and aramid fibre reinforced plastics (AFRP) provide higher specific strength, higher specific stiffness and ligher weight. They are, however, expensive and are used only for those applications where performance and not cost is the major consideration. AFRP is used instead of CFRP where strength, lightness and toughness are major considerations, and stiffness and high temperature performance are not. The common matrix materials for FRP composites are: - thermoset polymers (e.g. polyester, epoxy) - thermoplastic polymers (e.g. polyamide, peek). Thermoset polymers remain rigid when heated and consist of a highly cross-linked three-dimensional network; they are quite strong and stiff and have poor ductility. FRP material Strain to Density failure, d (g/cmS; Tensile Elastic strength, modulus, ail (MPa) E (MPa) Eu (%) GFRP I Unidirectional (Vr = 60 %) Woven cloth* Chopped looo 45000 2.3 2.1 100-300 10000-20000 - 1.5-2.1 .. roving* (short fibres) Sheet 50-200 6000-12000 - 1.3-2.1 modulus AFRP molding compound* (short fibres) CFRP Unidirectional (Vr = 60 %) High strength Unidirectional High (Vr = 60 %) 10-20 500-2000 1.3-1.9 1200 145000 0.9 1.6 800 220000 0.3 1.6 Table 1: Mechanical properties of FRP composite *For these materials: Vf = 20% - 50%. Unidirectional (Vr = 60 %) Among the thermoset resins, polyester resins are lower in cost and are not as strong as epoxy resins. Polyester matrix composites are used in the fabrication of boat hulls, structural panels and parts for automobiles and aircrafts, building panels and beams, electrical appliances, water tanks, pressure vessels, etc. Epoxy resins, in addition, have a lower shrinkage after cure allowing for higher fabrication accuracy. Epoxy matrix is used commonly in CFRP and AFRP composites for aerospace applications, military equipments, satellite antennae, sports equipments, medical prostheses, etc. Thermoplastic polymers consists of flexible linear molecular chains that are tangled together and, as the name indicates, soften when heated; they have lower strength and modulus but quite high ductility. Among the thermoplastic resins, polyamide and peek resins are used as matrix materials in FRP composites for applications in the aerospace industry due to their superior mechanical properties and high glass transition temperature. Maximum service temperatures for FRP composites are relatively low, as the matrix material is prone to softening, chemical decomposition or degradation at moderate temperatures. The same temperature limitations apply to the machining of FRP composites. Table 1 reports the main mechanical properties of some FRP composite materials. 2.2 Metal matrix composites (MMC) MMC are used for applications requiring higher operating temperatures than are possible with PMC materials. Most of these composites are developed for the aerospace industry, but new applications are found in the automotive industry, such as in automobile engine parts, making use of continuous fibre, discontinuous fibre, or particle reinforced MMC. Continuous fibres provide for the highest stiffness and strength properties obtainable in MMC materials. Boron-aluminium composites are one of the earliest developed MMC material types. It is made by hot pressing layers of boron fibres between aluminium foils, so that the foils deform around the fibres and bond to each other [I]. By reinforcing with boron, the tensile strength can be increased by a factor of three to five while the elastic modulus can be tripled. Further reinforcing materials for MMC are silicon carbide, alumina and graphite in the form of particles, short fibres (whiskers) or long fibres. Aluminium, magnesium and titanium alloys are the most common matrix materials used in MMC materials. Table 2 reports the main mechanical properties of some MMC materials. Figure 1 is a plot of specific strength versus specific stiffness for various composites and conventional metal materials. It can be seen that composites, in general, have higher specific strength and specific modulus over conventional steel, Al, Ti, and Mg alloys, and MMC have properties superior to PMC composites. Applications of continuous fibre reinforced MMC include use of B-AI for the fuselage of the space shuttle orbiter, SIC-AI for the vertical tail section of advanced fighter planes, SIC-TiAI for hypersonic aircraft, etc. Discontinuous fibre and particle reinforced MMC are low cost MMC that provide higher strength and stiffness and better dimensional stability over the corresponding unreinforced alloys. Small additions of reinforcement (Vr = 20%) moderately increase the base alloy strength and stiffness. They also increase the wear resistance and contribute toward the difficulty in machining these materials. These MMC are used for sport equipments, automobile engine parts (pistons, cylinder liners, brake drums), missile guidance parts, etc. looo 75000 1.6 1.4