Plastic Deformation

When enough load is applied to a material or a structure, it will cause the material to change shape. This change in shape is known as deformation. As per the material science theory, when sufficient stress is applied to cause permanent deformation to the metal, it is called plastic deformation.

Also, the involvement of breaking of a limited number of atomic bonds by the movement of dislocations is known as plastic deformation. The force required is so huge to break the bonds of all the atoms in a crystal plane.

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The process due to which an object changes its size or shape due to applied force, that is irreversible, is known as plastic deformation. It can be observed in many objects like

• Concrete

• Plastics

• Soils

• Metals

• Rocks

Example: steel rod bending.

Plastic Deformation Meaning

Plastic Deformation – Even after the removal of the applied forces when the deformation stays, it is irreversible.

• The property of the material to undergo enduring the deformation under pressure is known as plasticity. It is the state or quality of plastic material, especially the molding and altering capacity.

• The malleability and ductility of a material are directly proportional to plasticity of the material. For an ideal plasticity property of material, it is to undergo irreversible deformation without any increase in load or stress.

• Fracture or rupture of the material may be caused by plasticity. Plasticity also leads to plastic deformation, which happens in many metal forming processes like forging, pressing, rolling, swaging, etc.

Plastic Deformation of Metals

In experiments, plastic deformation is studied with the help of springs. Here, Hooke's law is explained to differentiate between plastic and elastic materials. There are many mechanisms that cause plastic deformation. Dislocation plasticity is caused in metals, whereas for brittle materials like concrete, rock, and bone plasticity is caused due to slippage of microcracks.

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Even when the initiating stress is removed, the plastic deformation and plastic strain is a dimensional change that doesn't disappear. Deformation to the body will be permanent if the load exceeds the limit even after its removal. When applied stress exceeds the elastic limit or yield stress, the deformation of the material body occurs. This is due to the result of slip or dislocation mechanism at the atomic level.

Slip and Twinning

Plastic deformation in a metal has two prominent mechanisms, and they are:

• Slip Twinning Slip and

• Twinning

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1. A prominent mechanism of deformation in metal is slip. It occurs in crystallographic planes or slip planes and involves the sliding of blocks of crystal over one another

2. Whereas in twining, a portion of crystals takes up orientation related to the rest of the untwined lattice in a symmetrical and definite way.

A comparison between slip and twinning can be done based on defect type, change in crystal axis, visibility, seen in, stress requirement, and occurrence threshold value of stress.

Differences between Slip and Twinning

Difference between Plastic and Elastic Deformation

The difference between elastic and plastic deformation is given here below:

1. Elastic Deformation

• When the material recovers its original dimension from a deformed body after the load is removed; it is known as elastic deformation.

• Elastic limit is the limiting load beyond which the material no longer behaves elastically.

• When a force applied to the body increases, there is more change in an object's shape or size.

• E.g., when a rubber band is released, it regains its original shape.

2. Plastic Deformation

• When the elastic limit of a body is exceeded, it will experience a permanent deformation or set when the applied load is removed. It is known as plastic deformation in which an object is permanently deformed.

• It happens when bonds between atoms are broken, and new ones are formed, making the reversal to original shape impossible.

• How much force is applied on the object (stress) is directly proportional to the object’s dimensions (strain).

• E.g., a hanger doesn't regain its original shape when it is bent.