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Types of Deformation

There are two main types of deformation: elastic (temporary) and plastic (permanent). Elastic deformation is recoverable upon removal of applied forces, while plastic deformation results in a permanent change to the object. Elastic deformation follows linear behavior in most materials like metals and ceramics, allowing small strains typically under 1%. Some materials like polymers exhibit large, nonlinear elastic deformation. Plastic deformation begins after elastic limits are exceeded and causes a permanent change in shape even after forces are removed. Ductile metals have large plastic ranges while materials like rubber have minimal plasticity.

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684 views2 pages

Types of Deformation

There are two main types of deformation: elastic (temporary) and plastic (permanent). Elastic deformation is recoverable upon removal of applied forces, while plastic deformation results in a permanent change to the object. Elastic deformation follows linear behavior in most materials like metals and ceramics, allowing small strains typically under 1%. Some materials like polymers exhibit large, nonlinear elastic deformation. Plastic deformation begins after elastic limits are exceeded and causes a permanent change in shape even after forces are removed. Ductile metals have large plastic ranges while materials like rubber have minimal plasticity.

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Types of deformation

Depending on the type of material, size and geometry of the object, and the forces applied, various
types of deformation may result. The image to the right shows the engineering stress vs. strain diagram
for a typical ductile material such as steel. Different deformation modes may occur under different
conditions, as can be depicted using a deformation mechanism map.
Permanent deformation is irreversible; the deformation stays even after removal of the applied forces,
while the temporary deformation is recoverable as it disappears after the removal of applied forces.
Temporary deformation is also called elastic deformation, while the permanent deformation is
called plastic deformation.

Elastic deformation

The study of temporary or elastic deformation in the case of engineering strain is applied to materials
used in mechanical and structural engineering, such as concrete and steel, which are subjected to very
small deformations. Engineering strain is modeled by infinitesimal strain theory, also called small strain
theory, small deformation theory, small displacement theory, or small displacement-gradient theory
where strains and rotations are both small.

For some materials, e.g. elastomers and polymers, subjected to large deformations, the engineering
definition of strain is not applicable, e.g. typical engineering strains greater than 1%,[1] thus other more
complex definitions of strain are required, such as stretch, logarithmic strain, Green strain, and Almansi
strain. Elastomers and shape memory metals such as Nitinol exhibit large elastic deformation ranges, as
does rubber. However, elasticity is nonlinear in these materials.

Normal metals, ceramics and most crystals show linear elasticity and a smaller elastic range.

Plastic deformation

This type of deformation is not undone simply by removing the applied force. An object in the plastic
deformation range, however, will first have undergone elastic deformation, which is undone simply be
removing the applied force, so the object will return part way to its original shape.
Soft thermoplastics have a rather large plastic deformation range as do ductile metals such
as copper, silver, and gold. Steel does, too, but not cast iron. Hard thermosetting plastics, rubber,
crystals, and ceramics have minimal plastic deformation ranges. An example of a material with a large
plastic deformation range is wet chewing gum, which can be stretched to dozens of times its original
length.
Under tensile stress, plastic deformation is characterized by a strain hardening region and
a necking region and finally, fracture (also called rupture). During strain hardening the material becomes
stronger through the movement of atomic dislocations. The necking phase is indicated by a reduction in
cross-sectional area of the specimen. Necking begins after the ultimate strength is reached. During
necking, the material can no longer withstand the maximum stress and the strain in the specimen
rapidly increases. Plastic deformation ends with the fracture of the material.

Source: Wikipedia

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