Engineering materials and structures are exposed to forces that involve stress and strain and these forces are analysed by stress and strain analysis. To understand how these analyses are carried, it is important to understand what is stress? What is the strain? What is the stress-strain curve?

**What is stress?**

Stress is defined as the ratio of the internal force F on an area A of a substance which results in the deformation of the substance. The SI unit of stress is Newton per square meter (Nm-2) and its dimensional formula is ML-1T-2. There are two types of stress and they are:

- Normal stress and tangential stress. Normal stress is the restoring force per unit area which is perpendicular to the surface of the substance which results in the elongation of the material. Stress can be further divided as a tensile stress and compressive stress.
- Tensile stress: Tensile stress is defined as the external stretching force acting per unit area of the material whose SI unit is Nm-2. For a material with high load, tensile stress is also high.
- Compressive stress: Compressive stress is defined as the force that is applied to make the material squeeze resulting in shortening of the material.

- Tangential stress is the restoring force per unit area which is parallel to the surface of the substance.

**What is the strain?**

A strain is defined as the ratio of change in the shape of an object to its original shape. This is a dimensionless quantity. There are two types of strain and they are:

**Longitudinal strain:**A material that undergoes deformation such that there is a change in length, such strain is known as longitudinal strain.**Volumetric strain:**A material that undergoes deformation such that there is a change in the volume, such strain is known as volumetric strain.

**Hooke’s law**

Hooke’s law explains how much amount of force is needed to be applied to either elongate or to compress the material to cause the deformation is proportional to the distance.

**Application of stress and strain**

When force is applied on a tennis ball, the ball starts to deform temporarily, the force experienced on the thumb while applying the force is the internal force. This is known as stress. And if the deformation happens permanently it is known as strain.

**Stress-strain curve**

Stress-strain curve explains the relationship between the stress and the strain that is displayed by the material. The curve varies from material to material which is studied by the amount of deformation (strain) at distant intervals of load (stress).

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