When a crystalline body is subjected to an applied force, it will, under most circumstances, undergo a change in shape. If the forces are small, the deformation will be elastic, i.e., the material will revert to its former shape as soon as the external forces are released. If, however, the forces are sufficiently large, the material will only partly revert to its original shape when they are released. Such deformation is referred to as plastic.

At high enough stresses, irreversible processes accompany deformation. Brittle materials break into separate pieces (i.e., fracture or mechanically fail) at the point at which the applied stress exceeds the value at which there is no further plastic deformation.Ductile materials, on the other hand, exhibit a time-dependent extension (i.e., one that depends on how fast the material is stressed) that is not recovered when the stress is removed. The ductility of a material is characterized by the strain (or elongation) at fracture; the more ductile a material is, the easier it is to draw that material into a wire. The metals tungsten and copper both exhibit very high ductilities. Here strain is defined as the relative change in dimensions or shape in a body as the result of an applied stress (it is a dimensionless quantity); stress is the magnitude of the applied force per unit area (usually measured in units of pounds per square inch or pascals).

Certain types of metals undergo a transition from ductile to brittle fracture when the temperature is decreased sufficiently, the strain rate is increased sufficiently, and/or the surface of the metal has been notched. The ductile to brittle transition temperature is typically measured by the change in energy absorbed, the change in ductility, or the change in the fracture appearance.