Shear strain is caused by the shear stress t_{xy} · Then

Shear stress / Shear strain = G

where G is called shear modulus or modulus of rigidity.

t_{xy }/ ᵞxy = G

t_{xy } = G . ᵞxy

In Fig. 2.15 is shown a rectangular block ABCD fixed at one face AC and subjected to force F. After application of force, it distorts through an angle a. and occupies new :position AB’CD’. The shear strain ‘Yxy is given by

ᵞxy = DD / CD = tan a

= a [ as a is very small]

By the previous definition,

ᵞxy = 90^{0} – θ = a ,