G. Savage/ Engineering Failure Analysis 17(2010)92-115 3500 30 2500 500 500 0100200300400 Fig. 8. Relationship between flaw size and failure stress of a material [2]- ws therefore that the strength of a material can be enhanced by eliminating or minimising such imperfections. Cracks lying perpendicular to the applied loads are the most detrimental to the strength. Fibrous or filamentary materials thus exhibit high strength and stiffness along their lengths because in this direction the large flaws present in the bulk mate- rial are minimised. Fibres will readily support tensile loads but offer almost no resistance and buckle under compression. In order to be directly usable in engineering applications they must be embedded in matrix materials to form fibrous compos ites. The matrix serves to bind the fibres together, transfer loads to the fibres and protect them against handling damage and environmental attack. Composites can be divided into two classes: those with long fibres(continuous fibre reinforced composites )and those ith short fibres(discontinuous fibre reinforced composites). In a discontinuous fibre composite, the material properties are affected by the fibre length, whereas in a continuous fibre composite it is assumed that the load is transferred directly to the fibres and that the fibres in the direction of the applied load are the principal load-bearing constituent. Polymeric materials are the most common matrices for fibre reinforced composites. They can be subdivided into two distinct types thermosetting and thermoplastic Thermosetting polymers are resins which cross-link during curing into a glassy brittle so- id, examples being polyesters and epoxies Thermoplastic polymers are high molecular weight, long chain molecules which an either become entangled (amorphous)such as polycarbonate, or partially crystalline, such as nylon, at room temperature to provide strength and shape. In common with all structural applications of polymer matrix composites, Formula 1 is dom inated by those based on thermoset resins, particularly epoxies. The driving force for the increasing substitution of metal alloys is demonstrated in Table 1 Contrary to many a widely held belief, composites are not"wonder materials". Indeed their mechanical properties are roughly of the same order as their metal competitors. Furthermore they exhibit lower extensions to failure then metallic al- loys of comparable strength. What is important however is that they possess much lower densities Fibre reinforced com- posites thus exhibit vastly improved specific properties, strength and stiffness per unit mass for example. The higher specific properties enable the production of lower weight components. The weight savings obtained in practice are not Table 1 Comparison of mechanical properties of metallic and composite materials. Density (g cm) le strength, a(MPa) Tensile modulus, E(GPa) Specific strength(alp) gnetum HM carbon 1550It follows therefore that the strength of a material can be enhanced by eliminating or minimising such imperfections. Cracks lying perpendicular to the applied loads are the most detrimental to the strength. Fibrous or filamentary materials thus exhibit high strength and stiffness along their lengths because in this direction the large flaws present in the bulk mate￾rial are minimised. Fibres will readily support tensile loads but offer almost no resistance and buckle under compression. In order to be directly usable in engineering applications they must be embedded in matrix materials to form fibrous compos￾ites. The matrix serves to bind the fibres together, transfer loads to the fibres and protect them against handling damage and environmental attack. Composites can be divided into two classes: those with long fibres (continuous fibre reinforced composites) and those with short fibres (discontinuous fibre reinforced composites). In a discontinuous fibre composite, the material properties are affected by the fibre length, whereas in a continuous fibre composite it is assumed that the load is transferred directly to the fibres and that the fibres in the direction of the applied load are the principal load-bearing constituent. Polymeric materials are the most common matrices for fibre reinforced composites. They can be subdivided into two distinct types: thermosetting and thermoplastic. Thermosetting polymers are resins which cross-link during curing into a glassy brittle so￾lid, examples being polyesters and epoxies. Thermoplastic polymers are high molecular weight, long chain molecules which can either become entangled (amorphous) such as polycarbonate, or partially crystalline, such as nylon, at room temperature to provide strength and shape. In common with all structural applications of polymer matrix composites, Formula 1 is dom￾inated by those based on thermoset resins, particularly epoxies. The driving force for the increasing substitution of metal alloys is demonstrated in Table 1. Contrary to many a widely held belief, composites are not ‘‘wonder materials”. Indeed their mechanical properties are roughly of the same order as their metal competitors. Furthermore they exhibit lower extensions to failure then metallic al￾loys of comparable strength. What is important however is that they possess much lower densities. Fibre reinforced com￾posites thus exhibit vastly improved specific properties, strength and stiffness per unit mass for example. The higher specific properties enable the production of lower weight components. The weight savings obtained in practice are not as Fig. 8. Relationship between flaw size and failure stress of a material [2]. Table 1 Comparison of mechanical properties of metallic and composite materials. Material Density (g cm3 ) Tensile strength, r (MPa) Tensile modulus, E (GPa) Specific strength (r/q) Specific modulus Steel 7.8 1300 200 167 26 Aluminium 2.81 350 73 124 26 Titanium 4 900 108 204 25 Magnesium 1.8 270 45 150 25 E glass 2.10 1100 75 524 21.5 Aramid 1.32 1400 45 1060 57 IM carbon 1.51 2500 151 1656 100 HM carbon 1.54 1550 212 1006 138 96 G. Savage / Engineering Failure Analysis 17 (2010) 92–115