FEM: principal stress calculation, fix for direction of principal stress

src/Mod/Fem/feminout/importToolsFem.py

@@ -426,9 +426,9 @@ def calculate_von_mises(i):
 
 
 def calculate_principal_stress(i):
-    sigma = np.array([[i[0], i[3], i[4]],
-                      [i[3], i[1], i[5]],
-                      [i[4], i[5], i[2]]])
+    sigma = np.array([[i[0], i[3], i[5]],
+                      [i[3], i[1], i[4]],
+                      [i[5], i[4], i[2]]])  # https://forum.freecadweb.org/viewtopic.php?f=18&t=24637&start=10#p240408
     # compute principal stresses
     eigvals = list(np.linalg.eigvalsh(sigma))
     eigvals.sort()