The beam element is made of a linear elastic material,
one which will return to it's original size and shape after
loading, once all applied loads are removed, if not stressed past
it's elastic or proportional limit. If stressed past it's
proportional limit (plastic range), the material will take a
permanent set and will not fully return to it's original size and
shape, when all loads are removed. Below the proportional limit,
an elastic material is in it's linear elastic range, where the
normal strain (
) varies
linearly with the applied load and the normal stress (
) varies linearly according
to Hooke's law: 
, where E is the modulus of elasticity,
which depends on the material of the beam. It is assumed that the
beam element is behaves in a linear elastic manner.
