No material is perfectly rigid. When they subjected to external force they change their size
and shape. This change in the shape or size of the body is defined as deformation. All
materials—steel, concrete, wood, and other structural materials—deform to some extent
under applied forces, but the deformations are usually too small to detect visually.
Figure 6 shows the deformation of a rod. Deformation usually denoted by the Greek letter
delta