In the physical sciences and engineering, a number that describes the relative deformation of elastic, plastic, and fluid materials under applied forces.
It arises throughout the material as the particles of the material are displaced from their usual position. Normal strain is caused by forces perpendicular to planes or cross sections of the material, such as in a volume that is under pressure on all sides. Shear strain is caused by forces that are parallel to, and lie in, planes or cross sections, such as in a short metal tube that is twisted about its longitudinal axis. See also deformation and flow .