How materials react to external forces

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

Engineers must know the mechanical properties of metals which determine their ability to resist the effect of mechanical forces (loads). They depend on the chemical composition of a metal, its structure, the method of metalworking and other factors. Knowing the mechanical properties engineers can judge the behaviour of a metal during its treatment and in the course of operation of the parts produced from this metal.

Metals work under three types of load: a static load which can be invariable or slowly growing in magnitude; a dynamic (impact) load which acts instantaneously; and a cyclic (alternating) load which changes either in magnitude or direction, or both. A load can cause deformations or strains of a metal. Since these strains can result in failure, it is important to know how materials respond to exter­nal forces, such as tension, 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 the external force disap­pears. The materials may also have permanent deforma­tion or they may fracture.

Compression is a pressure causing a decrease in vol­ume. When a material is subjected to a bending, shear­ing, or torsion (twisting) force, both tensile and com­pressive forces are simultaneously 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 usually stretches, returning to its original length if the force does not exceed the elastic limit of the material. Under larger tensions the material does not return completely to its original condition, and under greater forces the material ruptures.

The results of external forces are fatigue and creep. 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 is 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 gradual loosening 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 deformation eliminates the force causing the creep. Creep extended over a long time finally leads to the rupture of the material.

1. Find in the text English equivalents for the words and word-combinations given below.

Поступове ослаблення, поширюватися по матеріалу, площа поперечного перерізу, реагувати, судити про поведінку, первісний стан, ударне навантаження, призводить до зменшення обсягу, одночасно, що чергується, миттєво, відповідати потребам, хімічний склад, під час обробки.

1. Answer the following questions:

1. What do mechanical properties of metals determine?

2. What do mechanical properties of metals depend on?

3. Why is it important for engineers to know mechanical properties of metals?

4. Under what types of load do metals work?

5. What can a load cause?

6. What kinds of external forces do you know?

7. What is elastic deformation?

8. What forces act when a metal bar is bent?

9. When does fatigue occur?

10. What does creep result from?