Answer:
The vectorial representation of principal stresses is possible if you display the angle alpha in the Results navigator under the stresses.
The values for Sigma‑1 and Sigma‑2 are then displayed separately for the stress in the direction of the principal axes on the positive/negative surface side, and the membrane stress in the direction of the principal axes in the vector form.
You can control the trajectory scale by the factor for trajectories in the control panel.
