N.B.This is a ratio and as such does not have any units. Hookes Law states that stress is proportional to strain within the elastic limit of the material. The difference between stiffness and strength should now be clearer, and the following examples may help. We have been considering mechanical forces or energy applied to materials. An analagous situation applies for heat energy as well.
Materials expand and contract as they are heated and cooled.
Materials that have a regular, ordered structure are described as crystalline. Most metals are crystalline. However, some non-metals are partly crystalline.