Question

From the stress against strain graph, the behaviour of the wire between elastic limit and yield point is :
A. Perfectly elastic
B. Formation of neck
C. Perfectly plastic
D. Elastic but with permanent deformation

Answer 1

Hint: When stress on the body is released, if the deformed body returns to its original shape and size then it is said to be perfectly elastic. The point where the body begins to lose its elasticity is called elastic limit. The yield point is the point on the stress strain curve until where the stress in the body is directly proportional to the strain in it. And the yield point is always before the elastic limit.

Complete answer:
Let us first understand what stress strain curves are. Before that let us understand what is meant by stress and strain.
When we apply a force on a body, there is a change in dimensions of the body. This change in dimensions of the body is called deformation of the body.
When the force is applied on the body, the body in return applies an internal force that is equal to the applied force in magnitude but its direction is opposite to the direction of the applied force.
For example, we stretch a rubber band, the band applies a force directed inwards and tries to bring the band to its original size.
This internal force is called the restoring force because it tries to bring the body to its original dimensions.
The restoring force per unit surface area is called stress in the body.
Strain is the ratio of the change in the dimension of the body to the original dimension.
When stress on the body is released, if the deformed body returns to its original shape and size then it is said to be perfectly elastic.
However, a body is perfectly elastic until a point (called elastic limit) on the stress-strain curve. Beyond this point, the body loses its elasticity.
The yield point is the point on the stress strain curve until where the stress in the body is directly proportional to the strain in it. And the yield point is always before the elastic limit.
Therefore, between the elastic limit and the yield point the body is perfectly elastic.

So, the correct answer is “Option A”.

Note:
When stress on the body is released, if the deformed body does not tend to return to its original shape and size then it is said to be perfectly plastic.
When the stress in the body begins to decrease with the increase of the strain, the curve is said to be forming a neck on the stress-strain graph.