This deformation is not shear alone, but shear accompanied by rotation.
Shear can be considered as produced in another way.
Take the square ABCD ([fig. 25]), and suppose that it is pulled out from along one of its diagonals both ways, and proportionately compressed along the other diagonal. It will assume the shape in [fig. 26].
This compression and expansion along two lines at right angles is what is called shear; it is equivalent to the sliding illustrated above, combined with a turning round.
Fig. 25.
Fig. 26.
In pure shear a body is compressed and extended in two directions at right angles to each other, so that its volume remains unchanged.
Now we know that our material bodies resist shear—shear does violence to the internal arrangement of their particles, but they turn as wholes without such internal resistance.