Plastic Deformation

What is 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

Slip

Twinning

Line defect is known as crystal slip

The surface defect grain boundary is known as twinning.

The atoms in the block move at the same distance during slip.

The atoms in each successive plane in a block move through distances which are proportional to their distance from the twinning plane at the time of twinning.

It is commonly observed in face-centered cubic (FCC) and body-centered cubic (BCC).

It is commonly observed in Hexagonal close packing (HCP) metals.

The crystal axis remains the same after slip.

The crystal axis is deformed in twinning.

The slipped crystal lattice has similar orientation.

In twinned crystal lattice is a minor image of original lattice.

Comparatively, less stress is required.

Comparatively, more stress is required.

The stress required to propagate slip is comparatively higher to the stress required to start slip.

The stress required to propagate twinning is comparatively lower to the stress required to start slip.

When viewed under a microscope, slip can be seen as thin lines.

When viewed under a microscope, slip can be seen as broad lines.

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.


  1. 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.

FAQ (Frequently Asked Questions)

1. Give one plastic deformation examples

A wet chewing gum is a plastic deformation example that can be stretched dozens of times to its original length. The plastic deformation is characterized by strain hardening region and necking region and finally fracture, also known as rupture.

2. Does severe plastic deformation help in improving the properties of a material?

To obtain a very fine crystalline structure in different metals and alloys, one of the methods is several plastic deformations (SPD), which also possess different crystallographic structures. The formation of micrometer and sub-micrometer sizes sub grains is caused by SPD, due to which enhanced mechanical performance is observed.

The mechanical properties of metal are improved due to the structural changes caused by SPD. The effects include the increased hardness and yield stress, both featuring tendency to saturation. The limited ductility of the ultrafine-grained structure is the drawback.

For SPD the common methods are

i. Equal channel angular pressing (ECAP)

ii. Accumulated roll bonding (ARB)

iii. High-pressure torsion (HPT)


3. What are the three types of deformation?

Three kinds of differential stress occur.

1. The stress which stretches rock is known as tensional stress (or extensional stress).

2. The stress which squeezes rock is known as compressional stress.

3. Slippage and translation are caused by shear stress.

4. What is the deformation caused by stress?

Strain is caused by stress, and it is adequate to overcome the strength of an object that is under stress. When forces are applied, it causes strain, which results in change in shape and size of the object. When rocks are placed under stress, they strain.