Isostatic lines (principal stress lines) inside a solid?

[Joseph_Oster]

DUPLICATE THREAD alert!!!

How to get isostatic lines (principal stress lines) in 3D?

Hello, I would like to calculate the principal stress lines in a 3D volume. I tried the calculation on a cube but it only calculates the lines on the faces’ surfaces and the cube stays empty inside. Is there a way to calculate them inside the volume? Thank you in advance. [cube-lines]