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# What is Plasticity?

## Plasticity

Plasticity can define as the property of the material by virtue of which it does not regain its original shape when the external force acting on it is removed.

It is also call as plastic deformation. Because of the ability of a solid material to undergo permanent deformation. Under the effect applied forces an irreversible change of shape occurs. For example Paraffin wax, wet clay are the closest approach to perfectly plastic bodies.

## Yielding

We can define Yielding as the transition from elastic behavior to plastic behavior.

## Plasticity –  Microscopic Phenomenon

The solid acquires a permanent change in its structure. Because on restoring the external environment (under which the deformation process has taken place) to the initial configuration, the solid fails to restore to the initial structure. The quantifiers for such structural changes can be the shape of the body (at a macroscopic level of observation) or a rearrangement of defects (at a microscopic level of observation). Unlike elasticity, plasticity is fundamentally a microscopic phenomenon.

## Illustration of the phenomenon of Plasticity

We consider a simple example of stretching a metal wire (with cylindrical cross-section) under an isothermal environment. Thus we have measured following two quantities

• The force per unit cross-sectional area (stress).
• The change in length of the element with respect to some fixed reference state (strain).

Below a certain critical value of stress and strain, the element under observation returns to its original state of stress and strain upon removal of the external mechanism for stretching. The collection of all such stress/strain values forms the elastic range associated with the material. As long as the stress/strain values are restricted to the elastic range, the deformation remains elastic.

For values beyond the elastic range, the element will undergo permanent structural changes. The boundary of the elastic range signifies the onset of plastic deformation and therefore can be appropriately termed as the yield limit for the transition from an elastic to a plastic deformation process.

Because of the gradual nature of the transition from elastic to plastic behavior. It become difficult to measure an accurate value of the yield limit. It then becomes a matter of convention to choose an appropriate yield limit for such materials.

Plastic deformation occurs in many metal-forming processes and in geologic processes.

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