Elasticity - Stress, Strain, And Elastic Modulus, Other Elastic Deformations, Crystalline Materials, Elastomers, Sound Waves - Elastic limit, Elasticity on the atomic scale
shape substance called size
Elasticity is the ability of a material to return to its original shape and size after being stretched, compressed, twisted or bent. Elastic deformation (change of shape or size) lasts only as long as a deforming force is applied to the object, and disappears once the force is removed. Greater forces may cause permanent changes of shape or size, called plastic deformation.
In ordinary language, a substance is said to be "elas tic" if it stretches easily. Therefore, rubber is considered a very elastic substance, and rubber bands are even called "elastics" by some people. Actually, however, most substances are somewhat elastic, including steel, glass, and other familiar materials.
The greatest stress a material can undergo and still return to its original dimensions is called the elastic limit. When stressed beyond the elastic limit, some materials fracture, or break. Others undergo plastic deformation, taking on a new permanent shape. An example is a nail bent by excessive shear stress of a hammer blow.
The elastic modulus and elastic limit reveal much about the strength of the bonds between the smallest particles of a substance, the atoms or molecules it is composed of. However, to understand elastic behavior on the level of atoms requires first distinguishing between materials that are crystalline and those that are not.
Additional Topics
The simplest description of elasticity is Hooke's law, which states, "The stress is proportional to the strain." This relation was first expressed by the British scientist, Robert Hooke (1635-1702). He arrived at it through studies in which he placed weights on metal springs and measured how far the springs stretched in response. Hooke noted that the added length was always pr…
Metals are examples of crystalline materials. Solid pieces of metal contain millions of microscopically small crystals stuck together, often in random orientations. Within a single crystal, atoms are arranged in orderly rows. They are held by attractive forces on all sides. Scientists model the attractive force as a sort of a spring. When a spring is stretched, a restoring force tries to return it…
To explain the elastic behavior of materials like rubber requires a different model. Rubber consists of molecules, which are clusters of atoms joined by chemical bonds. Rubber molecules are very long and thin. They are polymers, long chain-like molecules built up by repeating small units. Rubber polymers consist of hundreds or thousands of atoms joined in a line. Many of the bonds are flexible, an…
Elasticity is involved whenever atoms vibrate. An example is the movement of sound waves. A sound wave consists of energy that pushes atoms closer together momentarily. The energy moves through the atoms, causing the region of compression to move forward. Behind it, the atoms spring further apart, as a result of the restoring force. The speed with which sound travels through a substance depends in…
The most direct way to determine the elastic modulus of a material is by placing a sample under increasing stresses, and measuring the resulting strains. The results are plotted as a graph, with strain along the horizontal axis and stress along the vertical axis. As long as the strain is small, the data form a straight line for most materials. This straight line is the "elastic region.�…
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