Question:
Is this true?
(and by that, I mean, both the statement and the example)
The directions of principle stress and the directions of principal
strain are not necessary the same. A case in point would be
an orthotropic material.
Answer:
It is true that the directions of principal stress and strain are not
necessarily the same. I believe that an orthotropic material does not
fit this bill, because it has preferred directions. However, an
anisotropic material does, because there are terms coupling normal
stresses with shear strains.
For isotropic materials the directions of principle stress and the
directions of principal
strain ARE the same. This is not necessarily true for anisotropic
(including orthotropic) materials.
This is only true for isotropic elastic materials. Not so for isotropic
inelastic materials (e.g. elastoplastic).
