Mechanics of Composites

The effect of dispersoids on the mechanical properties of metals has already been described in Section 5.1.2.2. In effect, these materials are composites, since the dispersoids are a second phase relative to the primary, metallic matrix. There are, however, many other types of composite materials, as outlined in Section 1.4, including laminates, random-fiber composites, and oriented fiber composites. Since the chemical nature of the matrix and reinforcement phases, as well as the way in which the two are brought together (e.g., random versus oriented), vary tremendously, we shall deal with specific types of composites separately. We will not attempt to deal with all possible matrix-reinforcement combinations, but rather focus on the most common and industrially important composites from a mechanical design point of view. 

The starting point for these descriptions will be the law of mixtnres, which was first introduced in Eq. (1.62) and which can be used to describe, to a first approximation, the composite property, P, that results from a combination of the reinforcement and matrix properties, Pr and Pm, respectively  

In the colloidal technique, the size and distribution of a dispersed thoria (Th02) phase is controlled to produce dispersion-strengthened alloys, primarily with nickel as the metallic phase. The so-called TD (thoria-dispersed) nickel has modest strength at room temperature, but retains this strength nearly to its melting point. TD nickel is 3 to 4 times stronger than pure nickel in the 870-1315°C range, and oxidation resistance of the alloy is better than that of nickel at 1100°C. 

As first described in Section 1.4.2, there are a number of ways of further classifying fiber-matrix composites, such as according to the fiber and matrix type—for example, glass-fiber-reinforced polymer composites (GFRP) or by fiber orientation. In this section, we utilize all of these combinations to describe the mechanical properties of some important fiber-reinforced composites. Again, not all possible combinations are covered, but the principles involved are applicable to most fiber-reinforced composites. We begin with some theoretical aspects of strength and modulus in composites. 

Fibers extend the entire length of the composite, so that at any section the area fractions occupied by fibers and matrix equal their respective volume fractions, Vf and = 1 - V/. The total stress, cti, must then equal the weighted sum of stresses in fibers and matrix, ct/i, and ct i, respectively  

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Cherepanov G.P. (1983) Fracture mechanics of composite materials. Nauka, Moscow (in Russian). 

R.M.J ones. Mechanics of Composite Materials, Sctipta Book Co., Washington, D.C., 1975. 

Powell, P.C. Engineering with Fibre-Polymer Laminates, Chapman and Hall, London (1994). Daniel, I.M. and Ishai, O. Engineering Mechanics of Composite Materials, Oxford University Press (1994). 

Stephen W. Tsai, Mechanics of Composite Materials, Part It, Theoretical Aspects, Air Force Materials Laboratory Technical Report AFML-TR-66-149, November 1966. 

C. W. Bert and J. N. Reddy, Mechanics of Bimodular Composite Structures, in Mechanics of Composite Materials - Recent Advances, Proceedings of the lUTAM... 

Symposium on Mechanics of Composite Materiais, Zvi Hashin and Cad T. Herakovich (Editors). Biacksburg, Virginia, 16-19 August 1982, Pergamon Press, New York, 1983, pp. 323-337. 

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