If we consider two adjacent faces AB and AC which are perpendicular to each other and also perpendicular to x and y axis. After deformation the side AB and AC take positions to A’B’ and A’C’ respectively. LB’ A’C’ will in general not be 90°. Shear strain will be defined as the reduction of the angle from 90°. Shear strain, Y_{xy} = 90° – θ, (Fig 2.13).

It we draw the figure as shown in Fig. 2.14., the displacement C to C’ is in y direction

and B to B’ in x direction. The shear strain Y_{xy} will be

ᵞxy = 90^{0} – θ

= (a_{1} + a_{2})