All materials deform elastically, although some materials
only deform elastically at imperceptibly low strains. After a material has
reached its elastic limit, it will begin to yield which means that it will
deform more without any extra stress being added to it. This is plastic
If a material is deformed elastically, it will return to
its original shape when the load is removed. If a material is deformed
plastically, it is deformed permanently and will not return to its original
shape when unloaded.
In a ductile material, such
as steel, once it reaches the plastic limit a hinge is formed. In a brittle
material, such as concrete, it simply cracks.
is usually considered when calculating the deflection of an element, whilst
plastic deformation is usually considered when calculating the load required
for an element to fail.