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Text A: «MECHANICAL PROPERTIES Of MATERIALS»

Materials Science and Technology is the study of ma­terials and how they can be fabricatedto meet the needsof modern technology. Using the laboratorytechniquesand knowledge of physics, chemistry, and metallurgy, scientists are finding new ways of using metals, plastics and other materials.

Engineers must know how materialsrespond to exter­nal forces, such astension, compression, torsion, bend­ing, and shear. All materials respond to these forces by elastic deformation. That is, the materials return their original size and form when theexternal force disap­pears. The materials may also havepermanent deforma­tion or they mayfracture. The results of external forces arecreep andfatigue.

Compression isa pressure causing adecrease in vol­ume. When a material is subjected to abending, shear­ing, or torsion (twisting) force, both tensile and compressive forces aresimultaneously at work. When a metal bar is bent, one side of it is stretched and subjected to a tensional force, and the other side is compressed.

Tension is a pulling force; for example, the force in a cable holding a weight. Under tension, a material usu­allystretches, returning to its original length if the force does notexceed the material'selastic limit. Under larger tensions, the material does not returncompletely to its original condition, and under greater forces the mate­rialruptures.

Fatigue is the growth of cracks under stress. It oc­curs when a mechanical part is subjected to a repeated or cyclic stress, such as vibration. Even when the maximum stress never exceeds the elastic limit, failure of the ma­terial can occur even after a short time. No deformation is seen during fatigue, but small localized cracks develop and propagate through the material until the remain­ing cross-sectional area cannot support the maximum stress of the cyclic force. Knowledge of tensile stress, elastic limits, and the resistance of materials to creep and fatigue are of basic importance in engineering.

Creep is a slow, permanent deformation that results from a steady force acting on a material. Materials at high temperatures usually suffer from this deformation. The gradualloosening of bolts and the deformation of components of machines and engines are all the exam­ples of creep. In many cases the slow deformation stops because deformationeliminates the force causing the creep. Creepextended over a long time finally leads to the rupture of the material.

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