For Lagrange and ALE subgrids, two of the principal directions lie in the XY plane. For planar symmetry the third principal direction is perpendicular to the XY plane, and for axial symmetry in Autodyn-2D the third principal direction is the hoop direction (this direction is termed as either TT or 33 in Autodyn-2D).

The first principal direction is defined as the angle, in degrees, between the principal direction and the global XY axes. This angle, which can be examined or plotted as a time-history, is called "P.ST.ANG". The second principal direction is orthogonal to the first principal direction in the XY plane. You can plot the principal directions using the "Direction" menu option.

You are allowed to define the initial orientation of the principal material axes in one of three ways. In all three cases, a rotation angle, β, can be specified. If this angle is non-zero, the first and second principal axes are additionally rotated anti-clockwise through this angle.