The mechanical properties are those that indicate how the material is expected to behave when subjected to varying conditions of load and environment. These characteristics are determined by standardized destructive and nondestructive test methods outlined by the American Society for Testing and Materials (ASTM). A thorough understanding of material properties permits the designer to determine the size, shape, and method of manufacturing mechanical components. Durability denotes the ability of a material to resist destruction over long periods of time. The destructive conditions may be chemical, electrical, thermal, or mechanical in nature or combinations of these conditions. The relative ease with which a material may be machined, or cut with sharp-edged tools, is termed its machinability. Workability represents the ability of a material to be formed into required shape. Usually, malleability is considered a property that represents the capacity of a material to withstand plastic deformation in compression without fracture. We see in Section 2.10 that hardness may represent the ability of a material to resist scratching, abrasion, cutting, or penetration. Frequently, the limitations imposed by the materials are the controlling factors in design. Strength and stiffness are main factors considered in the selection of a material. However, for a particular design, durability, malleability, workability, cost, and hardness of the materials may be equally significant. In considering the cost, attention focuses on not only the initial cost but also the maintenance and replacement costs of the part. Therefore, selecting a material from both its functional and economic standpoints is vitally important. An elastic material returns to its original dimensions on removal of applied loads. This elastic property is called elasticity. Usually, the elastic range includes a region throughout which stress and strain have a linear relationship. The elastic portion ends at a point called the proportional limit. Such materials are linearly elastic. In a viscoelastic solid, the state of stress is function of not only the strain but the time rates of change of stress and strain as well. A plastically deformed member does not return to its initial size and shape when the load is removed. A homogenous solid displays identical properties throughout. If properties are the same in all directions at a point, the material is isotropic. A composite material is made up of two or more distinct constituents. A no isotropic, or anisotropic, solid has direction-dependent properties. Simplest among them is that the material properties differ in three mutually perpendicular directions. A material so described is orthotropic. Some wood material may be modeled by orthotropic properties. Many manufactured materials are approximated as orthotropic, such as corrugated and rolled metal sheet, plywood, and fiber-reinforced concrete. The capacity of a material to undergo large strains with no significant increase in stress is called ductility. Thus, a ductile material is capable of substantial elongation prior to failure. Such materials include mild steel, nickel, brass, copper, magnesium, lead, and Teflon. The converse applies to a brittle material. A brittle material exhibits little deformation before rupture, for example, concrete, stone, cast iron, glass, ceramic materials, and many metallic alloys. A member that ruptures is said to fracture. Metals with strains at rupture in excess of 0.05 in./in. in the tensile test are sometimes considered to be ductile [5]. Note that, generally, ductile materials fail in shear, while brittle materials fail in tension. Further details on material property definitions are found in Sections 2.12 through 2.14, where description of metal alloys, the numbering system of steels, typical nonmetallic materials and material selection are included.
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