Engineering of materials

Mechanical properties

The mechanical properties of materials depend on their composition and microstructure. Material's composition, nature of bonding, crystal structure and defects (dislocations, grain boundaries...) have a profound influence on the strenght and ductility of metallic materials and other factors, such as how lower temperatures can cause many metals and plastics to become brittle.

There are diferent types of forces that are encountered in dealing with mechanical properties of materials. In general, we define stress as the force acting per unit area over which the force is applied. Compressive, tensile and shear stresses are illustrated.
 

Strain is defined as the change in dimension per unit length. Stress is expressed in Pa (Pascal). Strain has no dimensions and is often expressed as cm/cm.

Elastic strain is defined as fully recoverable strain resulting from an applied stress. A material subjected to an elastic strain does not show any permanent deformation, it returns to its original shape after the force or stress is removed.

In many materials, elastic stress and elastic strain are linearly related. The slope of a tensile stress-strain curve in the linear regime defines the Young´s modulus or modulus of elasticity (E) of a material.

When a material deforms elastically, the amount of deformation likewise depends on the size of the material, but the strain for a given stress is always the same and the two are related by Hooke´s Law (stress is directly proportional to strain):  σ = . E ε

A viscous material is one in which the strain develops over a period of time and the material does not return to its original shape after the stress is removed. A viscoelastic or anelastic material can be thought of as a material with a response between that of a viscous material and an elastic material. The term anelastic is typically used for metals, while the term viscoelastic is usually associated with polimeric materials.