Composite Materials

metallic or nonmetallic matrices (bases) with a specific distribution within them of reinforcement (fibers, dispersed particles, and so on); in such materials the individual properties of the components are effectively used.

According to the nature of their structure, composite materials are classified as fibrous materials, which are reinforced by continuous fibers and filamentary (whisker) crystals; precipitation-strengthened materials, which are produced by introducing dispersed reinforcing particles into the metallic matrix; and laminated materials, which are produced by pressing or rolling heterogeneous materials. Alloys with directed crystallization of eutectic structures are also composite materials. By using various combinations of proportions of the components, it is possible to produce materials for various purposes, with the required strength, heat resistance, elastic modulus, and abrasion resistance and to create compositions with the necessary magnetic, dielectric, radio-absorption, and other special properties.

Fibrous composite materials reinforced with filamentary crystals and continuous fibers of refractory compounds and elements used stone slabs reinforced with iron strips. Prototypes of composite materials are the widely known reinforced concrete, which is a combination of concrete under compression and steel reinforcement rods under tension, and laminated materials produced by rolling in the 19th century.

The successful development of modern composite materials has been furthered by the development and use in construction of glass-fiber-reinforced plastics with high specific strength (1940–50); the discovery of the extremely high strength (approaching the theoretical strength) of filamentary crystals and proof of the possibility of using them to strengthen metallic and nonmetallic materials (1950–60); and the development of new reinforcing materials, such as the high-strength and high-modulus continuous fibers of boron, carbon, A1₂O₃, SiC, and the fibers of other refractory inorganic compounds, as well as metal-based strengtheners (1960–70).

Fibrous composite materials reinforced by high-strength and high-modulus continuous fibers, in which the reinforcing elements bear the main load and the matrix transmits the stresses to the fibers, are widely used in industry. Fibrous composite materials are usually anisotropic. Their mechanical properties (see Table 1) are determined not only by the properties of the fibers themselves (see Table 2) but also by their orientation and cubic content and the ability of the matrix to transmit the applied load to the fibers. The continuous fibers of carbon, boron, and the refractory compounds (B₄C, SiC, and so on) are usually 100–150 microns (μ) in diameter.

(SiC, A1₂O₃, boron, carbon, and so on) constitute a new class of materials. However, the principles of reinforcing for strength have been known since ancient times. In Babylonia, reeds were used to reinforce clay in building houses, and in ancient Greece marble columns were strengthened with iron rods in building palaces and temples. In the construction of St. Basil’s Cathedral in Moscow (1555—60), the Russian architects Barma and Postnik

Fibrous composite materials, unlike monolithic alloys, have high fatigue strength σ-1. For example, σ-1 (base 10⁷ cycles) for aluminum alloys is 130–150 meganewtons per sq m (MN/m²), or 13–15 kilograms-force per sq mm (kgf/mm²), and for aluminum composite materials reinforced with boron fibers, σ-1 is about 500 MN/m² (with the same base). The ultimate strength and elastic modulus of composite materials based on

aluminum reinforced with boron fiber are approximately double those of the V-95 and AK4–1 aluminum alloys.

The most important methods of production of composite materials are impregnation of the reinforcing fibers with the matrix material; the forming in a press mold of strips of the strengthener and matrix produced by winding; cold pressing of both components, with subsequent baking; electrochemical application of coatings to the fibers, with subsequent pressing; precipitation of the matrix onto the strengthener by plasma spraying, with subsequent compression; diffusion-pack-welding of single-layer strips of the components; and joint rolling of the reinforcing elements with the matrix.

In structural assemblies requiring maximum strength, the reinforcement fibers are arranged in the direction of the applied load. Cylindrical products and other solids of revolution (for example, high-pressure vessels) are reinforced by longitudinal and transverse fibers. An increase in strength and reliability of operation of cylindrical casings, as well as a decrease in weight, is achieved by external reinforcement of structural assemblies with high-strength and high-modulus fibers, which makes possible an increase in specific structural stength by a factor of 1.5–2.0 in comparison with all-metal casings. The strengthening of materials by fibers of refractory substances considerably raises their heat resistance. For example, reinforcement of nickel alloy with tungsten fiber (wire) doubles its heat resistance at 1100°C.

Composite materials reinforced by filamentary crystals (whiskers) of ceramics, polymers, and other materials are very promising. The whiskers are usually fractions of a micron to several microns in diameter and approximately 10–15 mm long.

Composite materials with special properties are being developed; among them are radiotransparent and radioabsorbent materials, materials for the thermal insulation of orbital spacecraft, and materials with a low thermal coefficient of linear expansion and a high specific elastic modulus. The properties of composite materials based on aluminum and magnesium (strength, elastic modulus, fatigue strength, durability) are more than double those of conventional alloys at temperatures of up to 500°C. Composite materials based on nickel and cobalt increase the operating temperature level from 1000°C to 1200°C; those based on refractory metals and compounds raise it to 1500°-2000°C. The increased strength and elastic properties of materials make possible substantial reduction of the weight of structural members, and the higher operating temperatures of such materials make possible an increase in the power of engines, machines, and units.

Composite materials have numerous fields of application: in addition to the aerospace industry and rocket technology, they can be used successfully in power turbine construction; the automotive industry (for parts of engines and motor-vehicle bodies), machine building (for machine casings and parts), mining (for drilling tools and machines), metallurgy (as refractory materials for lining furnaces, shells, and other furnace equipment, as well as thermocouple terminals), construction (for bridge spans and truss supports and panels for tall prefabricated structures), the chemical industry (for autoclaves, tanks, and equipment used in the production of sulfuric acid and for vessels used for storing and transporting petroleum products), the textile industry (for parts of spinning machines and looms), agricultural machine building (for the cutting parts of plows, disk mowers, and tractor parts), and domestic equipment (washing-machine parts, frames of racing bicycles, and parts of radio sets).

The use of composite materials in a number of cases calls for the development of new methods of manufacturing parts and changes in the principles of constructing parts and structural assemblies.