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Plastic deformation can be defined as a process in which an object changes size or shape due to applied force in a way that cannot be reversed.
- Deformation refers to the change in the shape of an object whenever the load is applied to it.
- Plastic deformation can be observed in a variety of objects, including plastics, metals, and soils.
- The deformation produced by an elastic deformation is reversible, and the deformation disappears as the applied forces are removed.
- Plastic deformation is studied through spring experiments, and Hooke's law is used to distinguish between plastic and elastic materials.
NCERT Solutions of: Class 11 Physics Chapter 9 MECHANICAL PROPERTIES OF SOLIDS
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Key Terms: Plastic deformation, irreversible, reversible, stress and strain, shape, malleability, ductility, atomic bonding, atomic distances
What is Plastic Deformation?
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Whenever a particular amount of stress is applied on a metal, it will change its shape permanently.
- This is called plastic deformation. It involves breakage of atomic bonds.
- The force required to break the bonds of all atoms is very large.
- Whenever an object is undergoing the deformation due to the pressure then it is having the property called plasticity.
- Plasticity is directly proportional to malleability and ductility.
- It will undergo irreversible deformation which means when the object doesn’t come to its original shape even after removing the pressure.
Stress
In the field of physics, stress is defined as the force acting per the unit area of a material. Due to the stress the material or the body can undergo different types of changes. It is also called as the restoring force per unit area of the body. It is measured in pascal or N/m2. There are six types of stress, they are:
- Tensile stress
- Compressive stress
- Bulk stress or volume stress
- Shearing stress or tangential stress
- Normal stress
- Longitudinal stress
Strain
It is defined as the amount of deformation experienced by the body when a force is applied and also the deformation occurring in the direction of the force applied divided by the dimensions of the original material or the original body. This will occur due to the uneven heating or applied forces. There are two types of strain we can observe:
- Tensile strain
- Compressive strain
Based on the stress and strain we can draw a stress-strain curve. This gives us the information about the stages in deformation in a material which is under the pressure. The different stages we observe are:
- Proportional limit: this is the area where the curve shows the behaviour of hooke’s law. This is the limit area where we can get the ratio of stress and strain as a proportional constant. This proportional constant is called young’s modulus.
- Elastic limit: this is the stage at which the body which is under pressure gets to its original position whenever the load or pressure is removed.
- Yield point: here in this stage the body starts to deform permanently. That is the material will be deformed plastically.
- Ultimate stress point: here in this stage, we can find out the maximum stress on the object.
- Fracture or breaking point: here at this point the failure will occur.
Slip
- Here the orientation across the slip plane is same.
- The atomic movements are equal to atomic distances.
- Atoms are moving in only one plane.
- It takes place in milli seconds.
- It takes place at low strain rates.
- In this no sound is created.
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Twinning
- Here the orientation across the twin plane is different.
- The atomic movements are lesser than the atomic distances.
- Atoms are moving in all planes in the region of twin.
- It takes place in micro seconds.
- It takes place at high strain rates.
- In this sound is created.
Elastic Deformation
When a certain pressure is applied on the metal or object it will undergo the deformation.
- But after the releasing of the pressure, it will come back into its original shape.
- This is called as elastic deformation.
- This elasticity remains up to a certain pressure limit only.
- After the limit metal will not behave elastically.
- It will not get its own shape back.
- To maintain elasticity, we must maintain the pressure or load in its limit.
Plastic deformation occurs whenever the elastic limit has exceeded.
- Here the bond between the atoms is broken.
- The bonding will not be formed again when we release the pressure.
- Thus, it is irreversible. Here getting its original shape is impossible.
- It will permanently change its shape in plastic deformation.
There are three categories in plastic deformation they are:
- Elongation
- Contraction
- Expansion
Elongation
In this process, when the pressure is applied the change of shape of object or metal is observed as, the length of the object will increase and the thickness of the object will decrease. This takes place in the material whose stress is very low.
Contraction
In this process, when the pressure is applied the change of shape of object or metal is observed as, the length of the object will decrease and the thickness of the object will decrease. This will make the size of the object very small. It will get contracted and shrink.
Expansion
In this process, when the pressure is applied the change of shape of object or metal is observed as, the length of the object will increase and the thickness of the object will increase. This will make the size of the object very large.
These types of plastic deformation are observed in the objects like soil, rocks, metals, rubber etc.
What is Deformation?
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It is the process in which when a load or pressure is applied on an object, it changes its shape. It is called as the deformation. Based on the material science theory, whenever a particular amount of stress is applied on a metal, it will change its shape. This is called deformation.
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Things to Remember
- In the field of physics, stress is defined as the force acting per the unit area of a material.
- Strain is defined as the amount of deformation experienced by the body when a force is applied and also the deformation occurring in the direction of the force applied divided by the dimensions of the original material or the original body.
- Deformation is the process in which when a load or pressure is applied on an object, it changes its shape.
- Whenever a particular amount of stress is applied on a metal, it will change its shape permanently. This is called plastic deformation.
- In the elastic deformation when a certain pressure is applied on the metal or object it will undergo the deformation. But after the releasing of the pressure, it will come back into its original shape.
Sample Questions
Ques. What is the cause of plastic deformation? (5 marks)
Ans. When an object is kept under tensile, compressive stress then it will start to change its shape. When the limit of the elasticity of the object is crossed, then it will undergo plastic deformation. Whenever a particular amount of stress is applied to a metal, it will change its shape permanently. This is called plastic deformation. It involves the breakage of atomic bonds. The force required to break the bonds of all atoms is very large. whenever an object is undergoing deformation due to pressure then it has the property called plasticity.
Ques. What is plastic deformation? (2 Marks)
Ans. According to material science theory, plastic deformation occurs when enough stress is given to a metal to produce permanent deformation.
Ques. What is plasticity? (2 Marks)
Ans. Plasticity refers to the ability of a substance to withstand deformation under pressure. It refers to the state or quality of a plastic substance, particularly its molding and altering capacity.
Ques. What is the SI unit for the stress of a material? (2 Marks)
a. N m-2
b. N
c. N m
d. N m2
Ans. The correct answer is a. N m-2
Explanation:Internal forces acting in a material are termed stress. Shear, tensile, yield, and ultimate tensile strength are the many types of stress. These stresses are all measured in Newton per square meter.
Ques. The process of increasing the strength of a material by changing grain size is known as (2 Marks)
a. Work hardening
b. Precipitation hardening
c. Grain boundary strengthening
d. Solid solution hardening
Ans. The correct answer is c. Grain boundary strengthening
Explanation: Grain-boundary strengthening is a method for enhancing material strength by modifying the average crystallite size. This is also known as the Hall-Petch approach.
Ques. What is meant by twinning? (2 Marks)
Ans. In twinning, the section of crystals imbibes an orientation that is definite and symmetrically related to the orientation of the remaining untwined lattice.
Ques. What is meant by elastic deformation? (2 Marks)
Ans. Elastic deformation is reversible, and it disappears when the applied forces are removed. A common example is the stretching of a spring.
Ques. What is deformation theory? (2 Marks)
Ans. Many analytical reasons exist for plasticity. Deformation theory is one of the primary descriptions. In this case, the Cauchy stress tensor (of order d-1 in d dimensions) is a direct function of the strain tensor. Although this theory is valid when a tiny area of matter is subjected to increased stress (such as strain loading), it does not explain irreversibility.
Ques. Give one difference between slip and twinning. (2 Marks)
Ans. Slip occurs in discrete atomic spacing multiples. Atomic motion is reduced in twinning compared to atomic spacing.
Ques. What is meant by stress? (2 Marks)
Ans. Stress is defined as the force applied to a material's unit area. Strain is the term used to describe the effect of stress on the body. Stress can cause physical deformities.
Ques. What is meant by slip? (2 Marks)
Ans. Slip is an essential deformation process in metals. Slip occurs when a crystal body is stretched beyond its elastic barrier. It extends slightly, and a step forms on the surface, indicating that one piece of the crystal has shifted.
Ques. What is the importance of plastic deformation? (3 marks)
Ans. Plastic deformation is widely used in the manufacturing of goods. For example, the glass items that we use are deformed permanently from their original shape to the required shape. Iron items and steel items, gold products all will undergo plastic deformation to fulfill the needs of humans.
Ques. What are the modes of plastic deformation? (3 marks)
Ans. Slip: Here the orientation across the slip plane is the same. The atomic movements are equal to atomic distances. Atoms are moving in only one plane. It takes place in milliseconds. It takes place at low strain rates. In this, no sound is created.
Twinning: Here the orientation across the twin plane is different. The atomic movements are lesser than the atomic distances. Atoms are moving in all planes in the region of the twin. It takes place in microseconds. It takes place at high strain rates. In this sound is created.
Ques. What are the types of plastic deformation? (3 marks)
Ans. Ductile: The process in which the object undergoes plastic deformation by the stress greater than the yield stress is called ductile.
Irreversible plastic deformation: The plastic deformation is said to be irreversible when the original shape of the object is not regained after the removal of the pressure.
Ques. What are the ways to prevent plastic deformation? (3 marks)
Ans.
- Decrease mild temperature.
- extending the cooling time.
- decreasing the material temperature.
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