Strain

$\epsilon = \frac{\Delta L}{L}$
$\gamma_{ij} = \frac {1}{2} \left( \frac{\partial u_i}{\partial x_j} + \frac{\partial u_i}{\partial x_j} \right)$
By geometry, one can deduce that $\gamma_{ij} = \gamma_{ji}$. Figure 1 shows normal and shear strain.