Elastic deformation
describes the behaviour of a material that returns to its original size,
shape and form once the deforming force has been removed.
Plastic deformation
occurs when the material is taken beyond its elastic
limit and changes its size, shape and form permanently once the
deforming force is removed.
A material that can be permanently deformed using
compressive forces to press it stamp it without cracking or rupturing is
known as a malleable
material. Lead is a very malleable material
A material that can undergo extensive plastic
deformation using tensile forces to stretch or 'draw' it without
fracture is known as a ductile
material. Copper, aluminium and silver are all ductile materials.
Materials which are not capable of plastic deformation
are known as brittle
materials. Plastic deformation occurs more easily with the application
of heat. Some forming processes rely on the application of heat to
enable material forming to take place. Acrylic is quite a brittle
material at low room temperatures but becomes softer and more flexible
as heat is applied. Most thermoplastic
materials required heating prior to forming.
Most cold forming processes use metal tools to press,
stamp or 'draw'. These tools need to withstand the sudden impacts and
shock loads caused by the large compressive forces exerted. Toughness
is the material property required of press tools.
Timber is a flexible material and has a fairly high
degree of elasticity. To form timber it needs to be cut into thin sheets
to which adhesive is applied and held in its distorted position until
the adhesive sets. This process is known as laminating.
With the exception of laminating wood, forming a
material
