When a force or load is applied to a solid object that is restrained from movement, the structure will deform. This deformation is a change in length in the same direction as the applied force. See the diagram at left for some examples of forces. A tensile load pulls a structure outward from both ends, while a compressive load pushes a structure inward from both ends.

Stress is a measure of force over cross-sectional area, with units of Newtons/meter squared, or Pascals. Stress can be thougth of as a measure of force intensity.

Strain is a measure of deformation (in length) over the original length of the structure. Because length divided by length is a dimensionless number, strain is typically expressed as a fraction, or percent length change. Forces that stretch a structure yield positive strains, and forces that squish a structure yield negative strains.