What are Stress and Strain?

In short

Stress is the force acting on each unit area of a material, and strain is the fractional change in its size caused by the stress. The ratio of the two is a constant called the modulus.

In simple words

For a stretched wire, stress is $\sigma = \dfrac{F}{A}$ in $\text{N/m}^2$ and strain is $\epsilon = \dfrac{\Delta L}{L}$ , which has no unit. Young's modulus is $Y = \dfrac{\sigma}{\epsilon}$ . On a stress-strain graph the line is straight up to the limit of proportionality, then it curves through the elastic limit and the yield point until the wire finally breaks.

Example

A steel wire of length $2.0\ \text{m}$ and area $1.0\ \text{mm}^2$ carries a load of $100\ \text{N}$ . The stress is $\dfrac{100\ \text{N}}{1.0 \times 10^{-6}\ \text{m}^2} = 1.0 \times 10^{8}\ \text{N/m}^2$ .

Also searched as: stress and strain curve, proportional limit stress strain.