Are Principal Strains Always Tensile/Compressive or Can They Be Shear?

[kini.Amith]

Are principal strains necessarily tensile/compressive or can they be shear strains also? And are the principal strain planes parallel to principal stress planes at a point?

 

[boneh3ad]

By definition, principle stresses are normal in nature and not shear and are normal to the principal planes.

 

[kini.Amith]

I thought by definition principal strains were just the maximum strains at a point and the fact that they are normal to principal planes is not so obvious. My textbook even has a formal derivation just to prove that the principal strains are orthogonal.

 

[boneh3ad]

The idea is that in any compatible stress/strain field, you can find a coordinate system in which the only stresses are normal. These stresses are the principal stresses and are orthogonal. By definition, then, they are tensile rather than shear in nature.