Comparative Properties of Composite Materials

There are a large and increasing number of materials that fall in each of the four types of composites, making generalization difficult. However, as a class of materials, composites tend to have the following characteristics: high strength; high modulus; low density; excellent resistance to fatigue, creep, creep rupture,  corrosion, and wear; and low coefficient of thermal expansion (CTE). As for monolithic materials, each of the four classes of composites has its own particular attributes. For example, CMCs tend to have particularly good resistance to corrosion, oxidation, and wear, along with high-temperature capability. 

For applications in which both mechanical properties and low weight are important,

useful figures of merit are specific strength (strength divided by specific gravity or density) and specific stiffness (stiffness divided by specific gravity or density). Figure 2 presents specific stiffness and specific tensile strength of conventional structural metals (steel, titanium, aluminum, magnesium, and beryllium), two engineering ceramics (silicon nitride and alumina), and selected composite materials. The composites are PMCs reinforced with selected continuous fibers—carbon, aramid, E-glass, and boron—and an MMC, aluminum containing silicon carbide particles. Also shown is beryllium–aluminum, which can be considered a type of metal matrix composite, rather than an alloy, because the mutual solubility of the constituents at room temperature is low.

The carbon fibers represented in Fig. 2 are made from several types of precursor materials: 

polyacrylonitrile (PAN), petroleum pitch, and coal tar pitch. Characteristics of the

two types of pitch-based fibers tend to be similar but very different from those made from

PAN. Several types of carbon fibers are represented: standard-modulus (SM) PAN, ultrahighstrength

(UHS) PAN, ultrahigh-modulus (UHM) PAN, and UHM pitch. These fibers are

discussed in Section 2. It should be noted that there are dozens of different kinds of commercial

carbon fibers, and new ones are continually being developed.

Because the properties of fiber-reinforced composites depend strongly on fiber orientation,

fiber-reinforced polymers are represented by lines. The upper end corresponds to the

axial properties of a unidirectional laminate, in which all the fibers are aligned in one direction.

The lower end represents a quasi-isotropic laminate having equal stiffness and approximately

equal strength characteristics in all directions in the plane of the fibers.

As Figure 2 shows, composites offer order-of-magnitude improvements over metals in

both specific strength and stiffness. It has been observed that order-of-magnitude improvements

in key properties typically produce revolutionary effects in a technology. Consequently,

it is not surprising that composites are having such a dramatic influence in engineering

applications.

READ MORE.......

Carl Zweben

Devon, Pennsylvania

Mechanical Engineers’ Handbook: Materials and Mechanical Design, Volume 1, Third Edition.

Edited by Myer Kutz

Copyright  2006 by John Wiley & Sons, Inc.

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