Structural Materials

Structural materials

Construction materials which, because of their ability to withstand external forces, are considered in the design of a structural framework.

Brick is the oldest of all artificial building materials. It is classified as face brick, common brick, and glazed brick. Face brick is used on the exterior of a wall and varies in color, texture, and mechanical perfection. Common brick consists of the kiln run of brick and is used behind whatever facing material is employed providing necessary wall thickness and additional structural strength. Glazed brick is employed largely for interiors where beauty, ease of cleaning, and sanitation are primary considerations. See Brick

Structural clay tiles are burned-clay masonry units having interior hollow spaces termed cells. Such tile is widely used because of its strength, light weight, and insulating and fire-protection qualities. See Tile

Architectural terra-cotta is a burned-clay material used for decorative purposes. The shapes are molded either by hand in plaster-of-paris molds or by machine, using the stiff-mud process.

Building stones generally used are limestone, sandstone, granite, and marble. Until the advent of steel and concrete, stone was the most important building material. Its principal use now is as a decorative material because of its beauty, dignity, and durability.

Concrete is a mixture of cement, mineral aggregate, and water, which, if combined in proper proportions, form a plastic mixture capable of being placed in forms and of hardening through the hydration of the cement. See Concrete, Prestressed concrete, Reinforced concrete

The cellular structure of wood is largely responsible for its basic characteristics, unique among the common structural materials. When cut into lumber, a tree provides a wide range of material which is classified according to use as yard lumber, factory or shop lumber, and structural lumber. Laminated lumber is used for beams, columns, arch ribs, chord members, and other structural members. Plywood is generally used as a replacement for sheathing, or as form lumber for reinforced concrete structures. See Wood products

Important structural metals are the structural steels, steel castings, aluminum alloys, magnesium alloys, and cast and wrought iron. Steel castings are used for rocker bearings under the ends of large bridges. Shoes and bearing plates are usually cast in carbon steel, but rollers are often cast in stainless steel. Aluminum alloys are strong, lightweight, and resistant to corrosion. The alloys most frequently used are comparable with the structural steels in strength. Magnesium alloys are produced as extruded shapes, rolled plate, and forgings. The principal structural applications are in aircraft, truck bodies, and portable scaffolding. Gray cast iron is used as a structural material for columns and column bases, bearing plates, stair treads, and railings. Malleable cast iron has few structural applications. Wrought iron is used extensively because of its ability to resist corrosion. It is used for blast plates to protect bridges, for solid decks to support ballasted roadways, and for trash racks for dams. See Structural steel

Composite materials are engineered materials that contain a load-bearing material housed in a relatively weak protective matrix. A composite material results when two or more materials, each having its own, usually different characteristics, are combined, producing a material with properties superior to its components. The matrix material (metallic, ceramic, or polymeric) bonds together the reinforcing materials (whiskers, laminated fibers, or woven fabric) and distributes the loading between them. See Composite material

Fiber-reinforced polymers (FRP) are a broad group of composite materials made of fibers embedded in a polymeric matrix. Compared to metals, they generally have relatively high strength-to-weight ratios and excellent corrosion resistance. They can be formed into virtually any shape and size. Glass is by far the most used fiber in FRP (glass-FRP), although carbon fiber (carbon-FRP) is finding greater application. Although complete FRP shapes and structures are possible, the most promising application of FRP in civil engineering is for repairing structures or infrastructure. FRP can be used to repair beams, walls, slabs, and columns.

Structural Materials

 

materials used to manufacture structural members of machines and buildings that absorb power loads. The determining parameters of structural materials are their mechanical properties; in this they differ from other industrial materials (optical, insulating, lubricating, paint-and-var-nish, decorative, and abrasive). Among the main quality criteria are the parameters of resistance to external loads, such as durability, toughness, reliability, and service life.

For a long time man used a limited number of materials, such as wood, stone, plant and animal fibers, fired clay, glass, bronze, and iron, to make tools and hunting equipment, utensils, and decorations. The industrial revolution of the 18th century and the subsequent development of technology, particularly the invention of the steam engine and the appearance in the late 19th century of the internal-combustion engine, electrical machines, and motor vehicles, complicated and differentiated the demands made of material for components, which began to operate under complex alternating loads and high temperatures. Metallurgical alloys using iron (pig iron and steel), copper (bronze and brass), lead, and tin became the main structural materials.

Wood plastics (plywood), low-alloyed steels, and aluminum and magnesium alloys were widely used in aircraft construction, where high specific strength is the major requirement. The subsequent development of aviation technology necessitated the creation of new cobalt- and nickel-based heat-resistant alloys, steels, and titanium, aluminum, and magnesium alloys suitable for long service at high temperatures. At each stage of development the improvement of technology presented new, increasingly complex requirements for structural materials (heat and wear resistance, electrical conductivity, and so on). For example, shipbuilding requires steels and alloys with good weldability and high corrosion resistance; and chemical engineering requires steels and alloys with high long-term resistance to aggressive mediums. The development of atomic power engineering is related to the use of structural materials that not only have sufficient strength and high corrosion resistance in various heat carriers but also satisfy a new requirement—low neutron capture cross section.

Structural materials are classified according to the type of substance, as metallic, nonmetallic, and composition materials (combining the positive properties of these and other materials); according to design, as deformable (rolled steel, forgings, stampings, and pressed profiles), cast, sintered, molded, glued, welded (fusion, explosive, diffusion, or joining welding); according to operating conditions, as low-temperature materials and materials resistant to heat, corrosion, scaling, wear, fuel, and oil; and according to durability, as low- and medium-durability materials, with large reserves of plasticity, and high-durability materials, with moderate reserves of plasticity.

The individual classes of structural materials, in turn, are divided into numerous groups. For example, metal alloys are distinguished according to system (aluminum, magnesium, titanium, copper, nickel, molybdenum, niobium, beryllium, and tungsten, as well as iron-based), type of hardening (tempered, refined, aged, case-hardened, cyanided, or nitrided), and according to structural composition (austenite and ferrite steels; brasses).

Nonmetallic structural materials are subdivided according to isomeric composition, technological design (pressed, woven, wound, molded, and so on), type of filler or reinforcing element, and nature of location and orientation of the filler. Certain structural materials (for example, steel and aluminum alloys) are used as building materials, and conversely, building materials (such as reinforced concrete) are used in engineering structures.

The technical and economic parameters of structural materials include engineering parameters, such as workability of metals by pressure and cutting, casting properties (fluidity and tendency to form hot cracks during casting), weldability, solderability, and speed of hardening and fluidity of polymeric materials at normal and high temperatures; and indices of economic efficiency (cost, labor consumption, shortage, and metal use index).

Metal structural materials include most types of steel produced by industry. Exceptions are steels not used in load-bearing elements: tool steels, steels for heating elements and filler wire (for welding), and certain other steels with special physical and technological properties. Steels account for most of the structural materials used in engineering. They are distinguished by a broad range of durability, from 200 to 3,000 meganewtons per sq m (MN/m2), or 20 to 300 kilograms-force per sq mm (kgf/mm2). Their plasticity is up to 80 percent; their viscosity, up to 3 megajoules per sq m (MJ/m2). Structural steels, including stainless steels, are smelted in converters and in open-hearth and electric furnaces. Argon scavenging and ladle processing with synthetic slag are used for further refining. Special-purpose steels, which require high dependability, are manufactured by vacuum-arc, vacuum-induction, and electroslag remelting, vacuum evacuation, and, in special cases, crystallization improvement (on units for continuous or semicontinuous pouring) through melt extraction.

Pig iron is widely used in machine building for the manufacture of frames, crankshafts, gears, and cylinders for internal-combustion engines and of components operating at temperatures up to 1200°C in oxidizing mediums. The strength of pig iron ranges from 110 MN/m2 for heat-resistant cast iron-aluminum alloy to 1,350 MN/m2 for spheroidal cast iron.

Nickel and cobalt alloys retain their strength up to 1000° -1100°C. They are smelted in vacuum-induction, vacuum-arc, plasma, and electron-beam furnaces. These alloys are used in aircraft and rocket engines, steam turbines, and units operating in aggressive mediums. The strength of deformed aluminum alloys is up to 750 MN/m2; that of cast aluminum alloys, up to 550 MN/m2. They far surpass steel in terms of specific rigidity. Aluminum alloys are used in the manufacture of frames of airplanes, helicopters, rockets, and ships of various types. Magnesium alloys have high specific volume (four times greater than that of steel); their strength is 400 MN/m2 and higher. They are used primarily as castings in structures of aircraft, in motor-vehicle construction, and in the textile and printing industries. Titanium alloys are beginning to compete successfully with steels and aluminum alloys in a number of branches of technology, surpassing them in specific strength, corrosion resistance, and rigidity. The strength of titanium alloys ranges up to 1,600 MN/m2. They are used in the manufacture of compressors for aircraft engines, equipment for the chemical and petroleum-refining industries, and medical instruments.

Structrual materials also include alloys based on copper, zinc, molybdenum, zirconium, chromium, and beryllium, which are used in various branches of engineering.

The main nonmetallic structural materials are plastics, thermoplastic polymeric materials, ceramics, refractory materials, glass, rubber, and wood. Plastics based on thermosetting, epoxy, phenol, organosilicon, and thermoplastic and polyfluoro-ethylene resins reinforced with glass, quartz, and asbestos fibers and by fabrics and tapes are used in structures of airplanes and rockets and in power and transportation engineering. Thermoplastic polymer materials—polystyrene, polymethylmethacrylate, polyamides, polyfluoroethylene resins, and reactive plastics —are used in components of electrical and radio equipment and in friction assemblies operating in various mediums, including chemically active mediums (fuel or oil).

Glass (silicate, quartz, organic, and laminated safety glass) is used for the windows of ships, airplanes, and rockets. Components operating at high temperatures are manufactured from ceramic materials. Resins based on various types of rubber and reinforced with corded fabrics are used for the production of casings and of one-piece wheels for airplanes and motor vehicles, as well as for various movable and fixed seals.

The development of technology places new and greater demands on structural materials, stimulating the creation of new materials. Laminated structures, combining lightness, rigidity, and durability, are used to reduce the weight of aircraft components. External reinforcement of hollow metal objects (spheres, flasks, or cylinders) with glass-fiber-reinforced plastic makes possible a significant reduction in their weight in comparison with metal structures. Materials that combine structural strength with good electrical, heat-resistance, optical, and other properties are required in many branches of technology.

Since almost all the elements of the Mendeleev periodic system have been used as components of structural materials, and methods of hardening that have become standard for metal alloys (a combination of specially selected alloying, high-quality smelting, and required thermal processing) are decreasing in effectiveness, the prospects for improving the properties of structural materials lie in the synthesis of materials from elements that have limiting values for properties (for example, durability, refractoriness, or thermal stability). Such materials make up a new class, composition structural materials. They incorporate highly durable elements (fibers, filaments, wires, filamentary crystals, granules, disperse high-rigidity and refractory compounds used as reinforcement or fillers), bound by a matrix of plastic and durable material (metal alloys or nonmetallic, primarily polymeric, materials). In terms of specific strength and specific elastic modulus, composition structural materials may surpass steel or aluminum alloys by 50–100 percent and provide savings in weight of 20–50 percent.

Along with the creation of compositional structural materials having an orientational (orthotropic) structure, regulation of the structure of traditional structural materials is a potential method of improving the quality of structural materials. Thus, cast components—for example, blades of gas turbines—consisting of crystals oriented relative to the principal stresses in such a way that the edges of the grains (the weak spots in heat-resistant alloys) are not loaded are produced by directed crystallization of steels and alloys. Directed crystallization makes possible a sever-alfold increase in plasticity and durability. An even more progressive method of creation of orthotropic structural materials is the production of single-crystal components with a specific crystallographic orientation relative to the effective stresses. Orientation methods are used very effectively in nonmetallic structural materials. Thus, the orientation of linear macromolecules of polymeric materials (orientation of polymethylmethacrylate glass) significantly increases their durability, toughness, and service life.

Attainments in the science of study of materials are used in the synthesis of composition structural materials and the creation of alloys and other materials.

REFERENCES

Kiselev, B. A. Stekloplastiki. Moscow, 1961.
Konstruktsionnye materialy, vols. 1–3. Moscow, 1963–65.
Tugoplavkie materialy ν mashinostroenii: Spravochnik. Edited by A. T. Tumanov and K. I. Portnoi. Moscow, 1967.
Konstruktsionnye svoistva plastmass. Moscow, 1967. (Translated from English.)
Rezina—konstruktsionnyi material sovremennogo mashinostroeniia: Sb. st. Moscow, 1967.
Materialy ν mashinostroenii. Vybor i primenenie: Spravochnik, vols. 1–5. Edited by I. V. Kudriavtsev. Moscow, 1967–69.
Khimushin, F. F. Zharoprochnye stali i splavy, 2nd ed. Moscow, 1969. Sovremennye kompozitsionnye materialy. Moscow, 1970. (Translated from English.)
Aliuminievye splavy: Sb. st., vols. 1–6. Moscow, 1963–69.

A. T. TUMANOV and N. S. SKLIAROV

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