Stress and strain

In your daily life, you might have come across objects that can be stretched and objects that cannot be stretched. For example, consider an eraser. Can you stretch it? Yes, What about an iron rod? Not possible, right?

The difference in the physical nature of a material is what enables it to stretch or not stretch.

When you stretch or press a solid object, some amount of force is going in, thereby causing deformation of that object. The force that causes the deformation of an object is called stress.

It can be defined as the force applied per unit area.

As mentioned earlier, stress causes the deformation of an object. Strain, on the other hand, can be defined as the ratio of the amount of deformation to its original dimension on applying force.

Stress

Stress is the force applied per unit area of an object.

It is given by the formula,

σ =F/A

Where σ is the stress, F is the force and A is the area.

So, the unit of stress can be given by N/m2.

Types of stress

Stress can mainly be divided into two types-

• Tensile stress
• Compressive stress

Tensile stress

Tensile stress is the force that results in the elongation or stretching of a material. Objects undergoing tensile stress tend to be longer and thinner.

It is the force applied per unit area of an object.

E.g., the Cable of a crane.

Compressive stress 

It is the force that results in the compression or shortening of an object. The compressing force that acts on an object makes it shorter and thicker.

E.g., concrete pillars that are used to support buildings.

Tensile stress and compressive stress are both different in that one leads to the stretching of an object, and the other leads to the compression of an object.

Strain

Strain is the ratio of the amount of deformation of an object from its original dimensions.

In the formula, it can be given as the amount of deformation experienced by the body divided by its original dimensions.

The equation of strain is as follows

 ϵ = LL

Where ϵ is the strain when stress is applied, L is the change in length of the body, and L is the original length of the body.

Elasticity

Elasticity is the property of a material to regain its original shape and size when the applied force is removed.

Types of strain

Depending on the amount of stress applied, strain can be divided into two types.

• Tensile strain
• Compressive strain

Tensile strain

The strain developed as a result of applying tensile stress is called tensile strain. Tensile strain results in a change in the length of the body.

Compressive strain

Compressive strain is the change in the length or area of the body due to the application of compressive stress.

Stress-strain curve

Understanding the relationship between the stress and strain of an object is significant. It gives us an idea about the elastic properties of an object and also helps us to determine the strength of an object to support the weight of other bodies.

The relation between stress and strain can be explored experimentally.

Take spring and stretch it by applying force; as a result, the spring will get stretched and increase in length. Note down this change in length. Now, change the force applied and again note down the difference in length. Repeat this a few times by increasing the force applied and noting down the difference in the length of the spring, i.e., strain.

Plot the values of stress and strain on a graph, and this graph is called the stress-strain curve.

Proportional limit

The region in the stress-strain curve is called proportional limit which obeys Hooke’s law.

Hooke’s law

Hooke’s law states that the strain developed in a solid is directly proportional to the stress applied on an object within the elastic limit of that object. This means that with increasing stress on an object, the strain developed also increases. But the object soon regains its original position as soon as the force is released because, in the elastic region, the solid behaves like an elastic body.

Elastic limit

The region in the stress-strain curve is not linear because the body regains its original position when the force is removed. The point in the graph is called the elastic limit, and in this limit maximum stress is applied before the material gets permanent deformation. 

Yield point

The yield point is the point in the stress-strain curve at which material gets permanent deformation due to the application of stress.

Breaking stress

This is the point where the maximum tensile strength is experienced beyond which breakage occurs. 

Fracture point

On further applying stress, breakage of the object occurs. This point is known as Fracture or breaking point.

Elastic region

It is the region where the material can be deformed, and when force is released, it returns to its original position.

Plastic region

The region where the object deforms permanently is called a plastic region.

Applications

• In the construction field, it can be used in designing bridges and skyscrapers that are stable and safe.
• Cranes that are used to lift load use ropes that can endure maximum stress without breaking.
• It is also used in designing beams and pillars that remain safe and stable despite carrying a maximum load.

Conclusion

Every matter possesses different properties. Understanding these properties of matter can make our lives so much easier. Applying the theory of elasticity, one can design stable and safe man-made bridges and skyscrapers that are an architectural marvel. You have seen the countless applications of a single property of matter. Just like that, learning more about the matter and its properties can help us create a better world.