6 Composite Materials Definition and classification Composite materials are material systems that consist of a discrete constituent(the rein- forcement) distributed in a continuous phase(the matrix)and that derive their distinguishin characteristics from the properties and behavior of their constituents, from the geometry and arrangement of the constituents, and from the properties of the boundaries(interfaces)between the constituents. Composites are classified either on the basis of the nature of the continu ous(matrix) phase(polymer-matrix, metal-matrix, ceramic-matrix, and intermetallic-matrix composites), or on the basis of the nature of the reinforcing phase(particle reinforced, fiber reinforced, dispersion strengthened, laminated, etc. ) The properties of the composite can be tailored, and new combinations of properties can be achieved. For example, inherently brittle ceramics can be toughened by combining different types of ceramics in a ceramic-matrix com posite, and inherently ductile metals can be made strong and stiff by incorporating a ceramic reinforcement It is usually sufficient, and often desirable, to achieve a certain minimum level of reinforce nent content in a composite. Thus, in creep-resistant dispersion-strengthened composites, the reinforcement volume fraction is maintained below 15% in order to preserve many of the useful properties of the matrix. Other factors, such as the shape, size, distribution of the reinforcement, and properties of the interface, are also important. The shape, size, amount, and type of the rein- forcing phase to be used are dictated by the combination of properties desired in the composite For example, applications requiring anisotropic mechanical properties(high strength and high stiffness along one particular direction)employ directionally aligned, high-strength continuous fibers, whereas for applications where strength anisotropy is not critical and strength require ments are moderate, relatively inexpensive particulates can be used as the reinforcing phase. fIgure 6-1 shows some examples of continuous and discontinuous reinforcements developed Fi6 Composite Materials Definition and Classification Composite materials are material systems that consist of a discrete constituent (the rein￾forcement) distributed in a continuous phase (the matrix) and that derive their distinguishing characteristics from the properties and behavior of their constituents, from the geometry and arrangement of the constituents, and from the properties of the boundaries (interfaces) between the constituents. Composites are classified either on the basis of the nature of the continu￾ous (matrix) phase (polymer-matrix, metal-matrix, ceramic-matrix, and intermetallic-matrix composites), or on the basis of the nature of the reinforcing phase (particle reinforced, fiber reinforced, dispersion strengthened, laminated, etc.). The properties of the composite can be tailored, and new combinations of properties can be achieved. For example, inherently brittle ceramics can be toughened by combining different types of ceramics in a ceramic-matrix com￾posite, and inherently ductile metals can be made strong and stiff by incorporating a ceramic reinforcement. It is usually sufficient, and often desirable, to achieve a certain minimum level of reinforce￾ment content in a composite. Thus, in creep-resistant dispersion-strengthened composites, the reinforcement volume fraction is maintained below 15% in order to preserve many of the useful properties of the matrix. Other factors, such as the shape, size, distribution of the reinforcement, and properties of the interface, are also important. The shape, size, amount, and type of the rein￾forcing phase to be used are dictated by the combination of properties desired in the composite. For example, applications requiring anisotropic mechanical properties (high strength and high stiffness along one particular direction) employ directionally aligned, high-strength continuous fibers, whereas for applications where strength anisotropy is not critical and strength require￾ments are moderate, relatively inexpensive particulates can be used as the reinforcing phase. Figure 6-1 shows some examples of continuous and discontinuous reinforcements developed for use in modem engineered composites. 397