STATIC STRESS-STRAIN PROPERTIES IN TENSION AND COMPRESSION

(IV)

Other factors that designers must consider in selecting ma­terials are durability, appearanсe and cost. Appearance is an important consideration in constructions of various kinds that are not hidden from view as, for example, bridges, buildings, aircraft, trains, and domestic equipment. The cost of a mate­rial may often be a deciding factor in its selection. In consi­dering cost, it is not only the initial cost that is referred to but also the maintenance and replacement costs of the part. By durability of a material is meant its resistance to internal or external destructive conditions. These destructive conditions may be chemical, electrical, thermal and mechanical in nature or combinations of these conditions. Sometimes heat or light is de­structive. Variations of temperatures, as in the alternating processes of thawing and freezing, will produce damage as, for example, in the weathering of concrete. Corrosion of metal struc­tures by air, water and various chemicals represents an important destructive condition. In machinery, the wearing of rotat­ing parts by friction is another common type of destructive con­dition. The foregoing discussion indicates that there are many and varied factors that the designer must consider in selecting the best material for a given application.

N 8

STATIC STRESS-STRAIN PROPERTIES IN TENSION AND COMPRESSION

(V)

Tension and compression are the most common macroscopic me­chanical tests since they are usually the simplest static tests. Although the quantitative values of the material properties based on these tests may differ considerably from the property values of the actual structural or machine member, the results obtained provide qualitative values of the properties for va­rious possible materials. Tension tests are most commonly made for both ferrous and nonferrous metals where the stresses pro­duced include tension, while compression tests are performed on many nonmetallic materials such as concrete, timber, and brick, where the material is most commonly used in compression. Tension and compression tests are made not only to determine the properties of materials, based on samples of the materials, but they are also used as tests of the fabricated member or pro­duct Fоr example, tension test are conducted to determine the strength of wire, rods, tubing, reinforcing rods, fabrics, anchor chains, crane hooks, and eyebars. In the same way, com­pression tests are performed on tile, masonry blocks, building blocks, cast iron and concrete pipe, and concrete columns.

N 9

Date: 2015-01-02; view: 1026