How materials react to external forces

From the earliest times, when people started to build it was found necessary to have information regarding the strength of structural' materials so that the rules for determining safe dimensions of members could be drawn up.

The term " Strength of Materials" used in its widest sense, has two sides, a mathematical one, which deals with the different kinds of stresses imposed on structures, and an experimental one, which deals with the mechanical properties of the materials of which these structures are composed.

The following deals only with the experimental side.

The application of forces to a solid body causes a deformation corresponding to the nature of the applied force, for instance, a pull causes an extension, while a push causes a shortening. If these forces be of sufficient magnitude, permanent deformation of the body may be seen when the forces are removed, even supposing these forces have not been sufficient to cause fracture. When these forces are of less intensity, the body may recover its original form. In order to define such strain we use very delicate instruments which are also required to measure permanent set. In the mathematical treatment of the subject, this property, which bodies posses, of recovering their original form after reaching a certain limit, forms the whole basis for the analysis. But if this fact can be proved experimentally it will be of great importance for the engineer.

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 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. The results of external forces are creep and fatigue.

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 com-pressive 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 material's elastic limit. Under larger tensions, the material does not return completely to its original condition, and under greater forces the material ruptures.

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 localised 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 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. Bending. Pure Bending. — Simplest case. —A prismatical bar under action of equal and opposite couples at ends is said to undergo pure bending. Considering the simplest case where the bar has a longitudinal plane of symmetry and the ex­ternal couples are acting in this plane, it is easy to see that bending will take place in this same plane. The solution of the problem for stress distribution and deflection of the bar can be obtained on the assumption that transverse sections of the bar originally plane, after bending, remain plane and normal to the longitudinal fibres.

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