We see that as the strain (length) of the material increases, a larger
amount of stress (force) is required. As the elongation is increased the sample
eventually breaks.
Stress ()
The tensile stress on a material is defined as the force per unit area as
the material is stretched. The cross-sectional area may change if the material
deforms as it is stretched, so the area used in the calculation is the original
undeformed cross-sectional area Ao.