This set of Forming Problems focuses on “Plastic Deformation and Yield Criteria”.
1. In plastic deformation of metal, the deformation is temporary and can regain its original shape.
a) True
b) False
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Explanation: Plastic deformation of any material means permanent deformation without the presence of external constraints. The plastic deformation of crystalline materials generally takes place by processes which are very different from that for the amorphous materials. In plastic deformation, material cannot regain its original shape.
2. Elastic deformation can disappear completely when the load is removed from the metal.
a) True
b) False
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Explanation: In general, deformation can be of two types, elastic or plastic. Elastic deformation can disappear completely when the load is removed from the metal, whereas plastic deformation is a permanent deformation which occurs when the elastic range of deformation exceeds. The plastic deformation takes place without any failure and does not disappear after removing the load.
3. The fluids reveal its flow behaviour when it is subjected to normal compressive stresses.
a) True
b) False
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Explanation: The fluids reveal their flow behaviour only when it is subjected to shearing stress. A little amount of shear stress can deform the flow and it never regains its original shape even after removing the shearing load. The compressive or tensile stress does not make any changes to the fluid behaviour.
4. In crystalline materials, the plastic deformation occurs when the group of atoms or molecules slides past each other.
a) True
b) False
View Answer
Explanation: Material always shows a non-recoverable deformation when it is subjected to stresses beyond certain minimum stress. Crystalline materials experience permanent deformation when slipping occurs along the definite crystalline planes. But in amorphous materials, permanent deformation occurs when the group of atoms or molecules slides past each other.
5. Metal forming operations can only be performed when the material undergoes plastic deformation.
a) True
b) False
View Answer
Explanation: In metal forming process, plastic deformation of the material is compulsory. It is a process in which the required shape of metal is achieved by plastic deformation. Stresses are induced in the metal which is greater than its yield strength but less than its fracture strength so that the metal can be deformed into the required shape.
6. The process of forming new grains in the material is known as re-crystallization.
a) True
b) False
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Explanation: The process of forming new grains in the material is known as re-crystallization. And the temperature at which the re-crystallization occurs is known as re-crystallization temperature. The plastic or permanent deformation at elevated temperature occurs at less stresses as the growing of new grains are much easier at high temperature.
7. For the plastic deformation, it is not necessary to have higher applied stress than the yield strength of the material.
a) True
b) False
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Explanation: In plastic deformation of metal, applied stress is the main parameter which must be higher than the yield strength of the material and less than the fracture strength. The plastic deformation increases with the applied stresses when these stresses present in the middle of yield strength and fracture strength.
8. In plastic deformation of metal, the metal strength decreases as the increase in its temperature.
a) True
b) False
View Answer
Explanation: In plastic deformation of metal, its strength decreases when the temperature is increased. The plasticity of material is better when the deformation temperature is greater than the re-crystallization temperature but should be below the melting point. Re-crystallization temperature is the temperature at which the metal shows enough plasticity for efficient deformation.
9. The yield point is very important for controlling the various manufacturing processes.
a) True
b) False
View Answer
Explanation: The yield point of a metal is defined as the point at which a metal starts to deform plastically. Study of yield point is very important for designing of any component as it shows a limit to the load which can be applied to that metal component. The yield point is very important for controlling the various manufacturing processes like forging, rolling, bending etc.
10. In the yield criteria of metal, von-mises criteria are also known as distortion energy theory.
a) True
b) False
View Answer
Explanation: In the yield criteria of any metal, von-mises criterion is also known as distortion energy theory. This theory shows that the plastic deformation of metal occurs when the value of shear strain energy per unit volume of the body becomes equals to the strain energy per unit volume at the elastic limit of the metal.
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