Imagine a material in equilibrium and under a simple load (subjected to a force, F) as shown in Figure 7.1a. If an imaginary section is introduced at XX, normal to the plane of F, the total force carried must be equal to F and the distribution of F as internal forces of cohesion is called stress (σ). If the force is uniformly distributed over the section, then

σ =


, (7.1)

where A is the cross-sectional area of the material. In Figure 7.1a, the forces are tensile, but can be compressive as well. Strain (ε) is a measure of deformation produced in the material under F, de ned as elongation (x) divided by original materials length (l):

ε =

x l