Mechanical APDL Basic Analysis Guide - Ansys

Chapter 7:The General Postprocessor (POST1)
Caution
Certain element results data are always output in the element coordinate system regardless of
the active results coordinate system. These are miscellaneous result items that you would normally
expect to be interpreted only in the element coordinate system. They include forces, moments,
stresses, and strains for beam, pipe, and spar elements, and member forces and moments for
some shell elements.
In most circumstances, such as when working with a single load case or during linear combinations of multiple
load cases, rotating results data into the results coordinate system does not affect the final result values.
However, most modal combination techniques (PSD, CQC, SRSS, etc.) are performed in the solution coordinate
system and involve squaring operations. Since the squaring operation removes the sign associated with the
data, some combined results may not appear as expected after being rotated into the results coordinate
system. In these cases, RSYS,SOLU is on by default in order to keep the results data in the solution coordinate
systems. No other coordinate system may be used.
As an example of when you would need to change the results coordinate system, consider the case of a
cylindrical shell model, in which you may be interested in the tangential stress results. The SY stress contours
before and after results coordinate system transformation are shown below for such a case.
PLNSOL,S,Y ! Display a: SY is in global Cartesian system (default)
RSYS,1
PLNSOL,S,Y ! Display b: SY is in global cylindrical system
See the RSYS and PLNSOL command descriptions for further information.
Figure 7.22 SY in Global Cartesian and Cylindrical Systems
Plot 1 (top) illustrates SY in global Cartesian system. Plot 2 (bottom) illustrates SY in global
cylindrical system (note that nodes and elements are still in global Cartesian system).
In a large deformation analysis--for example, if you have issued the NLGEOM,ON command and the element
has large deflection capability--the element coordinate system is first rotated by the amount of rigid body
rotation of the element. Therefore, the component stresses and strains and other derived element data include
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