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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