Physics · Mechanical Properties Of Solids · NEET
They are linked but not identical. Plastic deformation is the process: stretching the material past its elastic limit so it deforms without recovering. Permanent set is the result you measure: the strain that is left in the body after you remove the load. In NCERT terms, when you unload at a point C (between B and D on the curve), the stress becomes zero but the strain does not return to zero. That leftover strain is the permanent set, and the deformation that caused it is plastic deformation.
Below the elastic limit, atoms are only pulled slightly out of position, so when the force is removed they spring back and strain returns to zero. Beyond the elastic limit, the layers of atoms actually slide over each other into new positions. Removing the force cannot pull them back to the old positions, so a fixed amount of deformation stays behind. This is why the unloading line does not return to the origin, and the material has a non-zero strain at zero stress.
It begins just after the yield point (elastic limit), marked B on the NCERT curve. From the origin to B the behaviour is elastic (recoverable). The region from B to D is the plastic region: any unloading here leaves a permanent set. Point D is the ultimate tensile strength, and fracture happens at point E. So: O to B = elastic, B to D = plastic, E = breaks.
Not by simply removing the load. The material will not return to its original shape on its own, exactly like putty or mud after you press them. This is the whole point of the word permanent. In practice you would need to physically reshape or reprocess the material. For NEET, treat permanent set as non-recoverable strain that stays after unloading past the elastic limit.
Do not mix them up. Plastic deformation is a behaviour that even metals like steel show once you cross their elastic limit. An elastomer (like rubber or aorta tissue) is a material that stretches a lot but has almost no clear plastic region and largely returns to shape. Putty and mud are close to ideal plastics because they deform permanently very easily and barely recover at all.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
It is the amount of stretch or deformation that stays in a body after the stretching force is removed, once the body has been loaded beyond its elastic limit. The body does not go back to its original size.
It is deformation that is not recovered when the load is removed. It happens when a material is stressed beyond its elastic limit. Putty and mud are close to ideal plastics because they deform this way very easily.
No. Unlike elastic deformation, plastic deformation is not reversible. When you remove the force, the material keeps a permanent set instead of returning to its original shape.
The region between the yield point (elastic limit, B) and the ultimate tensile strength point D. Unloading anywhere in this region leaves a permanent set. Fracture occurs at point E.
NEET asks conceptual questions on the stress-strain curve: identifying the elastic limit, yield point, permanent set, ultimate strength, and fracture point, and telling elastic from plastic behaviour. Knowing exactly where permanent set begins prevents common trap mistakes.
