Class 11 Physics Mechanical Properties of Solids Stress- Strain Curve

Stress- Strain Curve • It is a curve between stress and strain.
• A graph is plotted between the stress (which is equal in magnitude to the applied force per unit area) and the strain produced.
• The graph helps us to understand how a given material deforms with increasing loads.
• The curve between O and A, is a straight line. This means stress is directly ∝ to strain. In this region Hooke’s Law is applicable.
• In this region the material behaves like an elastic body.
• In the region from A to B, stress and strain are not directly ∝. But still the material returns to its original dimension after the force is removed. They exhibit elastic properties.
• The point B in the curve is known as yield point (also known as elastic limit) which means till this point the material will be elastic in behaviour and the stress corresponding to point B is known as yield strength (Sy) of the material.
• The region between O and B is called as Elastic region.
• From point B to point D we can see that strain increases rapidly even for small change in stress.
• Even if we remove the force the material does not come back to its original position. At this point stress is zero but strain is not zero as body has changed its shape.
• The material has undergone plastic deformation.
• The material is said to be permanent set.
• The point D on the graph is known as ultimate tensile strength (Su) of the material.
• From D to E we can see that stress decreases even if strain increases.
• Finally at point E fracture occurs. This means the body breaks.
• Conclusion:-
• An object is brittle if D and E are very close. This means fracture point is near to tensile strength.
• For example:-Glass which is brittle. Glass Jar

If Glass jar is dropped it will easily break into pieces. An object is ductile if D and E are very far apart from each other. This means fracture point is far away from tensile strength.

For example:-Metals, Gold and silver etc. Elastic substances like rubber have larger elastic region.

For example:-spring, catapult, tissue of aorta etc. Below is the graph between stress-strain for the elastic tissue of aorta, present in the heart.

We can see from the graph even though elastic region is very large, the material does not obey Hooke’s law over most of the region. .