Why is it that 1st principal stress vector's don't line up with 1st principal normal strain vector plots? I was always under the impression these should line up.

Why is it that 1st principal stress vector's don't line up with 1st principal normal strain vector plots? I was always under the impression these should line up.

- Make sure you change the stress plot to elemental values (like strain is by default), then they should line up.
That doesn't help. I created a purely tensile simulation to better understand this but it has only left me more confused. I attached the simulation to this.

I thought about this further and I was incorrect in my earlier statement about element vs nodal values.

Based on Generalized Hooke's Law (http://www.engrasp.com/doc/etb/mod/stat1/hooke/hooke_help.html), the strain and stress may not line up if Poisson's ratio is a non-zero quantity. If you make nuxy in your material definition zero, the two vectors will line up.

- I was looking under the section "strain transformation" in the link you provided and I was trying to understand how you concluded that poisson's ratio had an effect on principal normal strain.
- The strain and stress transformations are done separately; principal strains are calculated from the normal and shear strains, and principal stresses from the normal and shear stresses. If you want to relate strain to stress, you need to look at the section titled Stress-Strain Relationship, and specifically at the Stiffness matrix.