Composite materials are formed from two or more dissimilar materials, each of which contributes to the final properties. Unlike metallic alloys, the materials in a composite remain distinct from each other at the macroscopic level. Most engineering composites consist of two materials: a reinforcement called a filler and a matrix. The filler provides stiffness and strength; the matrix holds the material together and serves to transfer load among the discontinuous reinforcements. The
most common reinforcements, illustrated in Fig. 2–14, are continuous fibers, either straight or woven, short chopped fibers, and particulates. The most common matrices are various plastic resins although other materials including metals are used.
Metals and other traditional engineering materials are uniform, or isotropic, in nature. This means that material properties, such as strength, stiffness, and thermal conductivity, are independent of both position within the material and the choice of coordinate system. The discontinuous nature of composite reinforcements, though, means that material properties can vary with both position and direction. For example, an epoxy resin reinforced with continuous graphite fibers will have very high strength and
stiffness in the direction of the fibers, but very low properties normal or transverse to the fibers. For this reason, structures of composite materials are normally constructed of multiple plies (laminates) where each ply is oriented to achieve optimal structural stiffness and strength performance.
High strength-to-weight ratios, up to 5 times greater than those of high-strength steels, can be achieved. High stiffness-to-weight ratios can also be obtained, as much as 8 times greater than those of structural metals. For this reason, composite materials are becoming very popular in automotive, aircraft, and spacecraft applications where weight is a premium.
The directionality of properties of composite materials increases the complexity of structural analyses. Isotropic materials are fully defined by two engineering constants:
Young’s modulus E and Poisson’s ratio ν. A single ply of a composite material, however, requires four constants, defined with respect to the ply coordinate system. The constants are two Young’s moduli (the longitudinal modulus in the direction of the fibers, E1, and the transverse modulus normal to the fibers, E2), one Poisson’s ratio (ν12, called the major Poisson’s ratio), and one shear modulus (G12). A fifth constant,
the minor Poisson’s ratio, ν21, is determined through the reciprocity relation, ν21/E2 = ν12/E1. Combining this with multiple plies oriented at different angles makes structural analysis of complex structures unapproachable by manual techniques. For this reason, computer software is available to calculate the properties of a laminated composite construction.
Mechanical Engineering
McGraw−Hill Primis
ISBN: 0−390−76487−6
Text:
Shigley’s Mechanical Engineering Design,
Eighth Edition
Budynas−Nisbett
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