Uniaxial loading

Consider here a tensile axial load P is applied to a straight rod.

Normal strains
The member shows a tensile normal component of strain in the axial x-direction and contractive normal strains in the transverse directions (due to the Poisson’s effect). The rod does NOT experience shear strains on the xyz faces of a stress element. A square painted on the side of the rod will retain its rectangular shape as a result of this strain, as shown with the red square below. Only the dimensions of the rectangle change.

 

Shear strains
For the same axial loading, consider a blue diamond-shaped image (a square rotated through a 45° angle) painted on the side of the rod. Under the action of the axial load,  the shape of the diamond is NOT retained, as the angles between the sides of the diamond change with the strains in the rod, as shown with the blue diamond below. The shear strain ϒ in this case is NOT zero; the components of normal strain are also not zero.

CONCLUSION: A change in the orientation of the stress element exposes differ normal and shear components of strain.